T cell

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/T_cell 

 

T lymphocyte cell
Scanning electron micrograph of a human T cell
Scanning electron micrograph of a red blood cell (left), a platelet (center), and a T lymphocyte (right)
Details
System	Immune system
Identifiers
Latin	lymphocytus T
MeSH	D013601
TH	H2.00.04.1.02007
FMA	62870

3D rendering of a T cell

 

A T cell is a type of lymphocyte, which develops in the thymus gland (hence the name) and plays a central role in the immune response. T cells can be distinguished from other lymphocytes by the presence of a T-cell receptor on the cell surface. These immune cells originate as precursor cells, derived from bone marrow, and develop into several distinct types of T cells once they have migrated to the thymus gland. T cell differentiation continues even after they have left the thymus.

Groups of specific, differentiated T cells have an important role in controlling and shaping the immune response by providing a variety of immune-related functions. One of these functions is immune-mediated cell death, and it is carried out by T cells in several ways: CD8+ T cells, also known as "killer cells", are cytotoxic - this means that they are able to directly kill virus-infected cells as well as cancer cells. CD8+ T cells are also able to utilize small signalling proteins, known as cytokines, to recruit other cells when mounting an immune response. A different population of T cells, the CD4+ T cells, function as "helper cells". Unlike CD8+ killer T cells, these CD4+ helper T cells function by indirectly killing cells identified as foreign: they determine if and how other parts of the immune system respond to a specific, perceived threat. Helper T cells also use cytokine signalling to influence regulatory B cells directly, and other cell populations indirectly. Regulatory T cells are yet another distinct population of these cells that provide the critical mechanism of tolerance, whereby immune cells are able to distinguish invading cells from "self" - thus preventing immune cells from inappropriately mounting a response against oneself (which would by definition be an "autoimmune" response). For this reason these regulatory T cells have also been called "suppressor" T cells. These same self-tolerant cells are co-opted by cancer cells to prevent the recognition of, and an immune response against, tumour cells.

Development

Origin, early development and migration to the thymus

All T cells originate from c-kit⁺Sca1⁺ haematopoietic stem cells (HSC) which reside in the bone marrow. In some cases, the origin might be the fetal liver during embryonic development. The HSC then differentiate into multipotent progenitors (MPP) which retain the potential to become both myeloid and lymphoid cells. The process of differentiation then proceeds to a common lymphoid progenitor (CLP), which can only differentiate into T, B or NK cells. These CLP cells then migrate via the blood to the thymus, where they engraft. The earliest cells which arrived in the thymus are termed double-negative, as they express neither the CD4 nor CD8 co-receptor. The newly arrived CLP cells are CD4^-CD8^-CD44⁺CD25^-ckit⁺ cells, and are termed early thymic progenitors (ETP) cells. These cells will then undergo a round of division and downregulate c-kit and are termed DN1 cells.

TCR-Beta selection

At the DN2 stage (CD44⁺CD25⁺), cells upregulate the recombination genes RAG1 and RAG2 and re-arrange the TCRβ locus, combining V-D-J and constant region genes in an attempt to create a functional TCRβ chain. As the developing thymocyte progresses through to the DN3 stage (CD44^-CD25⁺), the T cell expresses an invariant α-chain called pre-Tα alongside the TCRβ gene. If the rearranged β-chain successfully pairs with the invariant α-chain, signals are produced which cease rearrangement of the β-chain (and silences the alternate allele). Although these signals require this pre-TCR at the cell surface, they are independent of ligand binding to the pre-TCR. If the pre-TCR forms, then the cell downregulates CD25 and is termed a DN4 cell (CD25^-CD44^-). These cells then undergo a round of proliferation and begin to re-arrange the TCRα locus.

Positive selection

Double-positive thymocytes (CD4⁺/CD8⁺) migrate deep into the thymic cortex, where they are presented with self-antigens. These self-antigens are expressed by thymic cortical epithelial cells on MHC molecules on the surface of cortical epithelial cells. Only those thymocytes that interact with MHC-I or MHC-II will receive a vital "survival signal". All that cannot (if they do not interact strongly enough) will die by "death by neglect" (no survival signal). This process ensures that the selected T cells will have an MHC affinity that can serve useful functions in the body (i.e., the cells must be able to interact with MHC and peptide complexes to effect immune responses). The vast majority of developing thymocytes will die during this process. The process of positive selection takes a number of days.

A thymocyte's fate is determined during positive selection. Double-positive cells (CD4⁺/CD8⁺) that interact well with MHC class II molecules will eventually become CD4⁺ cells, whereas thymocytes that interact well with MHC class I molecules mature into CD8⁺ cells. A T cell becomes a CD4⁺ cell by down-regulating expression of its CD8 cell surface receptors. If the cell does not lose its signal, it will continue downregulating CD8 and become a CD4⁺, single positive cell.

This process does not remove thymocytes that may cause autoimmunity. The potentially autoimmune cells are removed by the process of negative selection, which occurs in the thymic medulla (discussed below).

Negative selection

Negative selection removes thymocytes that are capable of strongly binding with "self" MHC peptides. Thymocytes that survive positive selection migrate towards the boundary of the cortex and medulla in the thymus. While in the medulla, they are again presented with a self-antigen presented on the MHC complex of medullary thymic epithelial cells (mTECs). mTECs must be AIRE⁺ to properly express self-antigens from all tissues of the body on their MHC class I peptides. Some mTECs are phagocytosed by thymic dendritic cells; this allows for presentation of self-antigens on MHC class II molecules (positively selected CD4⁺ cells must interact with MHC class II molecules, thus APCs, which possess MHC class II, must be present for CD4⁺ T-cell negative selection). Thymocytes that interact too strongly with the self-antigen receive an apoptotic signal that leads to cell death. However, some of these cells are selected to become Treg cells. The remaining cells exit the thymus as mature naïve T cells (also known as recent thymic emigrants). This process is an important component of central tolerance and serves to prevent the formation of self-reactive T cells that are capable of inducing autoimmune diseases in the host. 

β-selection is the first checkpoint, where the T cells that are able to form a functional pre-TCR with an invariant alpha chain and a functional beta chain are allowed to continue development in the thymus. Next, positive selection checks that T cells have successfully rearranged their TCRα locus and are capable of recognizing peptide-MHC complexes with appropriate affinity. Negative selection in the medulla then obliterates T cells that bind too strongly to self-antigens expressed on MHC molecules. These selection processes allow for tolerance of self by the immune system. Typical T cells that leave the thymus (via the corticomedullary junction) are self-restricted, self-tolerant, and single positive.

Thymic output

About 98% of thymocytes die during the development processes in the thymus by failing either positive selection or negative selection, whereas the other 2% survive and leave the thymus to become mature immunocompetent T cells. The thymus contributes fewer cells as a person ages. As the thymus shrinks by about 3% a year throughout middle age, a corresponding fall in the thymic production of naïve T cells occurs, leaving peripheral T cell expansion and regeneration to play a greater role in protecting older people.

Types of T cell

T cells are grouped into a series of subsets based on their function. CD4 and CD8 T cells are selected in the thymus, but undergo further differentiation in the periphery to specialized cells which have different functions. T cell subsets were initially defined by function, but also have associated gene or protein expression patterns. 

Depiction of the various key subsets of CD4-positive T cells with corresponding associated cytokines and transcription factors.

Conventional Adaptive T cells

Helper CD4+ T cells

T helper cells (T_H cells) assist other lymphocytes, including maturation of B cells into plasma cells and memory B cells, and activation of cytotoxic T cells and macrophages. These cells are also known as CD4⁺ T cells as they express the CD4 on their surfaces. Helper T cells become activated when they are presented with peptide antigens by MHC class II molecules, which are expressed on the surface of antigen-presenting cells (APCs). Once activated, they divide rapidly and secrete cytokines that regulate or assist the immune response. These cells can differentiate into one of several subtypes, which have different roles. Cytokines direct T cells into particular subtypes.

CD4+ Helper T cell subsets

Cell type	Cytokines Produced	Key Transcription Factor	Role in immune defence	Role in autoimmunity
Th1	IFNγ	Tbet	Produce an inflammatory response, key for defense against intracellular bacteria, viruses and cancer.	MS, Type 1 diabetes
Th2	IL-4	GATA-3	Aid the differentiation and antibody production by B cells	Asthma and other allergic diseases
Th17	IL-17	RORγt	Defense against gut pathogens and at mucosal barriers	MS, Rheumatoid Arthritis, Psoriasis
Th9	IL-9	IRF4, PU.1	Defense against helminths (parasitic worms)	Multiple Sclerosis
Tfh	IL-21, IL-4	Bcl-6	Help B cells produce antibody	Asthma and other allergic diseases

Cytotoxic CD8+ T cells

Superresolution image of a group of cytotoxic T cells surrounding a cancer cell

Cytotoxic T cells (T_C cells, CTLs, T-killer cells, killer T cells) destroy virus-infected cells and tumor cells, and are also implicated in transplant rejection. These cells are defined by the expression of CD8⁺ on the cell surface. These cells recognize their targets by binding to short peptides (8-11AA) associated with MHC class I molecules, present on the surface of all nucleated cells. CD8+ T cells also produce the key cytokines IL-2 and IFNγ, which influence the effector functions of other cells, in particular macrophages and NK cells.

Memory T cells

Antigen-naïve T cells expand and differentiate into memory and effector T cells, after they encounter their cognate antigen within the context of an MHC molecule on the surface of a professional antigen presenting cell (e.g. a dendritic cell). Appropriate co-stimulation must be present at the time of antigen encounter for this process to occur. Historically, memory T cells were thought to belong to either the effector or central memory subtypes, each with their own distinguishing set of cell surface markers (see below). Subsequently, numerous new populations of memory T cells were discovered including tissue-resident memory T (Trm) cells, stem memory TSCM cells, and virtual memory T cells. The single unifying theme for all memory T cell subtypes is that they are long-lived and can quickly expand to large numbers of effector T cells upon re-exposure to their cognate antigen. By this mechanism they provide the immune system with "memory" against previously encountered pathogens. Memory T cells may be either CD4⁺ or CD8⁺ and usually express CD45RO.

Memory T cell subtypes:

• Central memory T cells (T_CM cells) express CD45RO, C-C chemokine receptor type 7 (CCR7), and L-selectin (CD62L). Central memory T cells also have intermediate to high expression of CD44. This memory subpopulation is commonly found in the lymph nodes and in the peripheral circulation. (Note- CD44 expression is usually used to distinguish murine naive from memory T cells).
• Effector memory T cells (T_EM cells and T_EMRA cells) express CD45RO but lack expression of CCR7 and L-selectin. They also have intermediate to high expression of CD44. These memory T cells lack lymph node-homing receptors and are thus found in the peripheral circulation and tissues. T_EMRA stands for terminally differentiated effector memory cells re-expressing CD45RA, which is a marker usually found on naive T cells.
• Tissue resident memory T cells (T_RM) occupy tissues (skin, lung, etc..) without recirculating. One cell surface marker that has been associated with T_RM is the intern αeβ7, also known as CD103.
• Virtual memory T cells differ from the other memory subsets in that they do not originate following a strong clonal expansion event. Thus, although this population as a whole is abundant within the peripheral circulation, individual virtual memory T cell clones reside at relatively low frequencies. One theory is that homeostatic proliferation gives rise to this T cell population. Although CD8 virtual memory T cells were the first to be described, it is now known that CD4 virtual memory cells also exist.

Regulatory CD4+ T cells

Regulatory T cells are crucial for the maintenance of immunological tolerance. Their major role is to shut down T cell-mediated immunity toward the end of an immune reaction and to suppress autoreactive T cells that escaped the process of negative selection in the thymus. 

Two major classes of CD4⁺ T_reg cells have been described — FOXP3⁺ T_reg cells and FOXP3⁻ T_reg cells. 

Regulatory T cells can develop either during normal development in the thymus, and are then known as thymic Treg cells, or can be induced peripherally and are called peripherally derived Treg cells. These two subsets were previously called "naturally occurring", and "adaptive" or "induced", respectively. Both subsets require the expression of the transcription factor FOXP3 which can be used to identify the cells. Mutations of the FOXP3 gene can prevent regulatory T cell development, causing the fatal autoimmune disease IPEX. 

Several other types of T cell have suppressive activity, but do not express FOXP3. These include Tr1 cells and Th3 cells, which are thought to originate during an immune response and act by producing suppressive molecules. Tr1 cells are associated with IL-10, and Th3 cells are associated with TGF-beta. Recently, Treg17 cells have been added to this list.

Innate-like T cells

Natural killer T cell

Natural killer T cells (NKT cells – not to be confused with natural killer cells of the innate immune system) bridge the adaptive immune system with the innate immune system. Unlike conventional T cells that recognize peptide antigens presented by major histocompatibility complex (MHC) molecules, NKT cells recognize glycolipid antigen presented by CD1d. Once activated, these cells can perform functions ascribed to both T_h and T_c cells (i.e., cytokine production and release of cytolytic/cell killing molecules). They are also able to recognize and eliminate some tumor cells and cells infected with herpes viruses.

Mucosal associated invariant

MAIT cells display innate, effector-like qualities. In humans, MAIT cells are found in the blood, liver, lungs, and mucosa, defending against microbial activity and infection. The MHC class I-like protein, MR1, is responsible for presenting bacterially-produced vitamin B metabolites to MAIT cells. After the presentation of foreign antigen by MR1, MAIT cells secretes pro-inflammatory cytokines and are capable of lysing bacterially-infected cells. MAIT cells can also be activated through MR1-independent signaling. In addition to possessing innate-like functions, this T cell subset supports the adaptive immune response and has a memory-like phenotype. Furthermore, MAIT cells are thought to play a role in autoimmune diseases, such as multiple sclerosis, arthritis and inflammatory bowel disease, although definitive evidence is yet to be published.

Gamma delta T cells

Gamma delta T cells (γδ T cells) represent a small subset of T cells which possess a γδ TCR rather than the αβ TCR on the cell surface. The majority of T cells express αβ TCR chains. This group of T cells is much less common in humans and mice (about 2% of total T cells) and are found mostly in the gut mucosa, within a population of intraepithelial lymphocytes. In rabbits, sheep, and chickens, the number of γδ T cells can be as high as 60% of total T cells. The antigenic molecules that activate γδ T cells are still mostly unknown. However, γδ T cells are not MHC-restricted and seem to be able to recognize whole proteins rather than requiring peptides to be presented by MHC molecules on APCs. Some murine γδ T cells recognize MHC class IB molecules. Human γδ T cells which use the Vγ9 and Vδ2 gene fragments constitute the major γδ T cell population in peripheral blood, and are unique in that they specifically and rapidly respond to a set of nonpeptidic phosphorylated isoprenoid precursors, collectively named phosphoantigens, which are produced by virtually all living cells. The most common phosphoantigens from animal and human cells (including cancer cells) are isopentenyl pyrophosphate (IPP) and its isomer dimethylallyl pyrophosphate (DMPP). Many microbes produce the highly active compound hydroxy-DMAPP (HMB-PP) and corresponding mononucleotide conjugates, in addition to IPP and DMAPP. Plant cells produce both types of phosphoantigens. Drugs activating human Vγ9/Vδ2 T cells comprise synthetic phosphoantigens and aminobisphosphonates, which upregulate endogenous IPP/DMAPP.

Activation

The T lymphocyte activation pathway: T cells contribute to immune defenses in two major ways; some direct and regulate immune responses; others directly attack infected or cancerous cells.

 

Activation of CD4⁺ T cells occurs through the simultaneous engagement of the T-cell receptor and a co-stimulatory molecule (like CD28, or ICOS) on the T cell by the major histocompatibility complex (MHCII) peptide and co-stimulatory molecules on the APC. Both are required for production of an effective immune response; in the absence of co-stimulation, T cell receptor signalling alone results in anergy. The signalling pathways downstream from co-stimulatory molecules usually engages the PI3K pathway generating PIP3 at the plasma membrane and recruiting PH domain containing signaling molecules like PDK1 that are essential for the activation of PKCθ, and eventual IL-2 production. Optimal CD8⁺ T cell response relies on CD4⁺ signalling. CD4⁺ cells are useful in the initial antigenic activation of naïve CD8 T cells, and sustaining memory CD8⁺ T cells in the aftermath of an acute infection. Therefore, activation of CD4⁺ T cells can be beneficial to the action of CD8⁺ T cells.

The first signal is provided by binding of the T cell receptor to its cognate peptide presented on MHCII on an APC. MHCII is restricted to so-called professional antigen-presenting cells, like dendritic cells, B cells, and macrophages, to name a few. The peptides presented to CD8⁺ T cells by MHC class I molecules are 8–13 amino acids in length; the peptides presented to CD4⁺ cells by MHC class II molecules are longer, usually 12–25 amino acids in length, as the ends of the binding cleft of the MHC class II molecule are open.

The second signal comes from co-stimulation, in which surface receptors on the APC are induced by a relatively small number of stimuli, usually products of pathogens, but sometimes breakdown products of cells, such as necrotic-bodies or heat shock proteins. The only co-stimulatory receptor expressed constitutively by naïve T cells is CD28, so co-stimulation for these cells comes from the CD80 and CD86 proteins, which together constitute the B7 protein, (B7.1 and B7.2, respectively) on the APC. Other receptors are expressed upon activation of the T cell, such as OX40 and ICOS, but these largely depend upon CD28 for their expression. The second signal licenses the T cell to respond to an antigen. Without it, the T cell becomes anergic, and it becomes more difficult for it to activate in future. This mechanism prevents inappropriate responses to self, as self-peptides will not usually be presented with suitable co-stimulation. Once a T cell has been appropriately activated (i.e. has received signal one and signal two) it alters its cell surface expression of a variety of proteins. Markers of T cell activation include CD69, CD71 and CD25 (also a marker for Treg cells), and HLA-DR (a marker of human T cell activation). CTLA-4 expression is also up-regulated on activated T cells, which in turn outcompetes CD28 for binding to the B7 proteins. This is a checkpoint mechanism to prevent over activation of the T cell. Activated T cells also change their cell surface glycosylation profile.

The T cell receptor exists as a complex of several proteins. The actual T cell receptor is composed of two separate peptide chains, which are produced from the independent T cell receptor alpha and beta (TCRα and TCRβ) genes. The other proteins in the complex are the CD3 proteins: CD3εγ and CD3εδ heterodimers and, most important, a CD3ζ homodimer, which has a total of six ITAM motifs. The ITAM motifs on the CD3ζ can be phosphorylated by Lck and in turn recruit ZAP-70. Lck and/or ZAP-70 can also phosphorylate the tyrosines on many other molecules, not least CD28, LAT and SLP-76, which allows the aggregation of signalling complexes around these proteins. 

Phosphorylated LAT recruits SLP-76 to the membrane, where it can then bring in PLC-γ, VAV1, Itk and potentially PI3K. PLC-γ cleaves PI(4,5)P2 on the inner leaflet of the membrane to create the active intermediaries diacylglycerol (DAG), inositol-1,4,5-trisphosphate (IP3); PI3K also acts on PIP2, phosphorylating it to produce phosphatidlyinositol-3,4,5-trisphosphate (PIP3). DAG binds and activates some PKCs. Most important in T cells is PKCθ, critical for activating the transcription factors NF-κB and AP-1. IP3 is released from the membrane by PLC-γ and diffuses rapidly to activate calcium channel receptors on the ER, which induces the release of calcium into the cytosol. Low calcium in the endoplasmic reticulum causes STIM1 clustering on the ER membrane and leads to activation of cell membrane CRAC channels that allows additional calcium to flow into the cytosol from the extracellular space. This aggregated cytosolic calcium binds calmodulin, which can then activate calcineurin. Calcineurin, in turn, activates NFAT, which then translocates to the nucleus. NFAT is a transcription factor that activates the transcription of a pleiotropic set of genes, most notable, IL-2, a cytokine that promotes long-term proliferation of activated T cells. 

PLCγ can also initiate the NF-κB pathway. DAG activates PKCθ, which then phosphorylates CARMA1, causing it to unfold and function as a scaffold. The cytosolic domains bind an adapter BCL10 via CARD (Caspase activation and recruitment domains) domains; that then binds TRAF6, which is ubiquitinated at K63. This form of ubiquitination does not lead to degradation of target proteins. Rather, it serves to recruit NEMO, IKKα and -β, and TAB1-2/ TAK1. TAK 1 phosphorylates IKK-β, which then phosphorylates IκB allowing for K48 ubiquitination: leads to proteasomal degradation. Rel A and p50 can then enter the nucleus and bind the NF-κB response element. This coupled with NFAT signaling allows for complete activation of the IL-2 gene.

While in most cases activation is dependent on TCR recognition of antigen, alternative pathways for activation have been described. For example, cytotoxic T cells have been shown to become activated when targeted by other CD8 T cells leading to tolerization of the latter.

In spring 2014, the T-Cell Activation in Space (TCAS) experiment was launched to the International Space Station on the SpaceX CRS-3 mission to study how "deficiencies in the human immune system are affected by a microgravity environment".

T cell activation is modulated by reactive oxygen species.

Antigen discrimination

A unique feature of T cells is their ability to discriminate between healthy and abnormal (e.g. infected or cancerous) cells in the body. Healthy cells typically express a large number of self derived pMHC on their cell surface and although the T cell antigen receptor can interact with at least a subset of these self pMHC, the T cell generally ignores these healthy cells. However, when these very same cells contain even minute quantities of pathogen derived pMHC, T cells are able to become activated and initiate immune responses. The ability of T cells to ignore healthy cells but respond when these same cells contain pathogen (or cancer) derived pMHC is known as antigen discrimination. The molecular mechanisms that underlie this process are controversial.

Clinical significance

Deficiency

Causes of T cell deficiency include lymphocytopenia of T cells and/or defects on function of individual T cells. Complete insufficiency of T cell function can result from hereditary conditions such as severe combined immunodeficiency (SCID), Omenn syndrome, and cartilage–hair hypoplasia. Causes of partial insufficiencies of T cell function include acquired immune deficiency syndrome (AIDS), and hereditary conditions such as DiGeorge syndrome (DGS), chromosomal breakage syndromes (CBSs), and B-cell and T-cell combined disorders such as ataxia-telangiectasia (AT) and Wiskott–Aldrich syndrome (WAS).

The main pathogens of concern in T cell deficiencies are intracellular pathogens, including Herpes simplex virus, Mycobacterium and Listeria. Also, fungal infections are also more common and severe in T cell deficiencies.

Cancer

Cancer of T cells is termed T-cell lymphoma, and accounts for perhaps one in ten cases of non-Hodgkin lymphoma. The main forms of T cell lymphoma are:

• Extranodal T cell lymphoma
• Cutaneous T cell lymphomas: Sézary syndrome and Mycosis fungoides
• Anaplastic large cell lymphoma
• Angioimmunoblastic T cell lymphoma

Exhaustion

T cell exhaustion is a state of dysfunctional T cells. It is characterized by progressive loss of function, changes in transcriptional profiles and sustained expression of inhibitory receptors. At first cells lose their ability to produce IL-2 and TNFα followed by the loss of high proliferative capacity and cytotoxic potential, eventually leading to their deletion. Exhausted T cells typically indicate higher levels of CD43, CD69 and inhibitory receptors combined with lower expression of CD62L and CD127. Exhaustion can develop during chronic infections, sepsis and cancer. Exhausted T cells preserve their functional exhaustion even after repeated antigen exposure.

During chronic infection and sepsis

T cell exhaustion can be triggered by several factors like persistent antigen exposure and lack of CD4 T cell help. Antigen exposure also has effect on the course of exhaustion because longer exposure time and higher viral load increases the severity of T cell exhaustion. At least 2–4 weeks exposure is needed to establish exhaustion. Another factor able to induce exhaustion are inhibitory receptors including programmed cell death protein 1 (PD1), CTLA-4, T cell membrane protein-3 (TIM3), and lymphocyte activation gene 3 protein (LAG3). Soluble molecules such as cytokines IL-10 or TGF-β are also able to trigger exhaustion. Last known factors that can play a role in T cell exhaustion are regulatory cells. Treg cells can be a source of IL-10 and TGF-β and therefore they can play a role in T cell exhaustion. Furthermore, T cell exhaustion is reverted after depletion of Treg cells and blockade of PD1. T cell exhaustion can also occur during sepsis as a result of cytokine storm. Later after the initial septic encounter anti-inflammatory cytokines and pro-apoptotic proteins take over to protect the body from damage. Sepsis also carries high antigen load and inflammation. In this stage of sepsis T cell exhaustion increases. Currently there are studies aiming to utilize inhibitory receptor blockades in treatment of sepsis.

During transplantation

While during infection T cell exhaustion can develop following persistent antigen exposure after graft transplant similar situation arises with alloantigen presence. It was shown that T cell response diminishes over time after kidney transplant. These data suggest T cell exhaustion plays an important role in tolerance of a graft mainly by depletion of alloreactive CD8 T cells. Several studies showed positive effect of chronic infection on graft acceptance and its long-term survival mediated partly by T cell exhaustion. It was also shown that recipient T cell exhaustion provides sufficient conditions for NK cell transfer. While there are data showing that induction of T cell exhaustion can be beneficial for transplantation it also carries disadvantages among which can be counted increased number of infections and the risk of tumor development.

During cancer

During cancer T cell exhaustion plays a role in tumor protection. According to research some cancer-associated cells as well as tumor cells themselves can actively induce T cell exhaustion at the site of tumor. T cell exhaustion can also play a role in cancer relapses as was shown on leukemia. Some study even suggested that it is possible to predict relapse of leukemia based on expression of inhibitory receptors PD-1 and TIM-3 by T cells. In recent years there is a lot of experiments and clinical trials with immune checkpoint blockers in cancer therapy. Some of them were approved as valid therapies and are now used in clinics. Inhibitory receptors targeted by those medical procedures are vital in T cell exhaustion and blocking them can reverse these changes.

Side effect

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Side_effect

In medicine, a side effect is an effect, whether therapeutic or adverse, that is secondary to the one intended; although the term is predominantly employed to describe adverse effects, it can also apply to beneficial, but unintended, consequences of the use of a drug. Developing drugs is a complicated process, because no two people are exactly the same, so even drugs that have virtually no side effects, might be difficult for some people. Also, it is difficult to make a drug that targets one part of the body but that doesn't affect other parts, the fact that increases the risk of side effects in the untargeted parts.

Occasionally, drugs are prescribed or procedures performed specifically for their side effects; in that case, said side effect ceases to be a side effect, and is now an intended effect. For instance, X-rays were historically (and are currently) used as an imaging technique; the discovery of their oncolytic capability led to their employ in radiotherapy (ablation of malignant tumours).

Frequency of side effects

The probability or chance of experiencing side effects are characterised as : 

• Very common, ≥ ¹⁄₁₀
• Common (frequent), ¹⁄₁₀ to ¹⁄₁₀₀
• Uncommon (infrequent), ¹⁄₁₀₀ to ¹⁄₁₀₀₀
• Rare, ¹⁄₁₀₀₀ to ¹⁄₁₀₀₀₀
• Very rare, < ¹⁄₁₀₀₀₀

Examples of therapeutic side effects

Possible side effects of nicotine.

• Bevacizumab (Avastin), used to slow the growth of blood vessels, has been used against dry age-related macular degeneration, as well as macular edema from diseases such as diabetic retinopathy and central retinal vein occlusion.
• Buprenorphine has been shown experimentally (1982–1995) to be effective against severe, refractory depression.
• Bupropion (Wellbutrin), an anti-depressant, is also used as a smoking cessation aid; this indication was later approved, and the name of the smoking cessation product is Zyban. In Ontario, Canada, smoking cessation drugs are not covered by provincial drug plans; elsewhere, Zyban is priced higher than Wellbutrin, despite being the same drug. Therefore, some physicians prescribe Wellbutrin for both indications.
• Carbamazepine is an approved treatment for bipolar disorder and epileptic seizures, but it has side effects useful in treating attention-deficit hyperactivity disorder (ADHD), schizophrenia, phantom limb syndrome, paroxysmal extreme pain disorder, neuromyotonia, and post-traumatic stress disorder.
• Dexamethasone and betamethasone in premature labor, to enhance pulmonary maturation of the fetus.
• Doxepin has been used to treat angiodema and severe allergic reactions due to its strong antihistamine properties.
• Gabapentin, approved for treatment of seizures and postherpetic neuralgia in adults, has side-effects which are useful in treating bipolar disorder1, essential tremor, hot flashes, migraine prophylaxis, neuropathic pain syndromes, phantom limb syndrome, and restless leg syndrome.
• Hydroxyzine, an antihistamine, is also used as an anxiolytic.
• Magnesium sulfate in obstetrics for premature labor and preeclampsia.
• Methotrexate (MTX), approved for the treatment of choriocarcinoma, is frequently used for the medical treatment of an unruptured ectopic pregnancy.
• The SSRI medication sertraline is approved as an antidepressant but delays conjugal climax in men, and thus may be supplied to those in which climax is premature.
• Sildenafil was originally intended for pulmonary hypertension; subsequently, it was discovered that it also produces erections, for which it was later marketed.
• Terazosin, an α₁-adrenergic antagonist approved to treat benign prostatic hyperplasia (enlarged prostate) and hypertension, is (one of several drugs) used off-label to treat drug induced diaphoresis and hyperhidrosis (excessive sweating).

Examples of undesirable/unwanted side effects

• Echinacea – more than 20 different types of reactions have been reported, including asthma attacks, loss of pregnancy, hives, swelling, aching muscles and gastrointestinal upsets.
• Feverfew – pregnant women should avoid using this herb, as it can trigger uterine contractions which could lead to premature labour or miscarriage.
• Asteraceae plants – which include feverfew, echinacea, dandelion and chamomile. Side effects include allergic dermatitis and hay fever.

Antimalarial medication

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Antimalarial_medication 

Antimalarial medications or simply antimalarials are a type of antiparasitic chemical agent, often naturally derived, that can be used to treat or to prevent malaria, in the latter case, most often aiming at two susceptible target groups, young children and pregnant women. As of 2018, modern treatments, including for severe malaria, continued to depend on therapies deriving historically from quinine and artesunate, both parenteral (injectable) drugs, expanding from there into the many classes of available modern drugs. Incidence and distribution of the disease ("malaria burden") is expected to remain high, globally, for many years to come; moreover, known antimalarial drugs have repeatedly been observed to elicit resistance in the malaria parasite—including for combination therapies featuring artemisinin, a drug of last resort, where resistance has now been observed in Southeast Asia. As such, the needs for new antimalarial agents and new strategies of treatment (e.g., new combination therapies) remain important priorities in tropical medicine. As well, despite very positive outcomes from many modern treatments, serious side effects can impact some individuals taking standard doses (e.g., retinopathy with chloroquine, acute haemolytic anaemia with tafenoquine).

Specifically, antimalarial drugs may be used to treat malaria in three categories of individuals, (i) those with suspected or confirmed infection, (ii) those visiting a malaria-endemic regions who have no immunity, to prevent infection via malaria prophylaxis, and (iii) or in broader groups of individuals, in routine but intermittent preventative treatment in regions where malaria is endemic via intermittent preventive therapy. As of this date, practice in treating cases of malaria is most often based on the concept of combination therapy (e.g., using agents such as artemether and lumefantrine against chloroquine-resistant Plasmodium falciparum infection), since this offers advantages including reduced risk of treatment failure, reduced risk of developed resistance, as well as the possibility of reduced side-effects. Prompt parasitological confirmation by microscopy, or alternatively by rapid diagnostic tests, is recommended in all patients suspected of malaria before treatment is started. Treatment solely on the basis of clinical suspicion is considered when a parasitological diagnosis is not possible.

Medications

It is practical to consider antimalarials by chemical structure since this is associated with important properties of each drug, such as mechanism of action.

Quinine and related agents

Quinine has a long history stretching from Peru, and the discovery of the cinchona tree, and the potential uses of its bark, to the current day and a collection of derivatives that are still frequently used in the prevention and treatment of malaria. Quinine is an alkaloid that acts as a blood schizonticidal and weak gametocide against Plasmodium vivax and Plasmodium malariae. As an alkaloid, it is accumulated in the food vacuoles of Plasmodium species, especially Plasmodium falciparum. It acts by inhibiting the hemozoin biocrystallization, thus facilitating an aggregation of cytotoxic heme. Quinine is less effective and more toxic as a blood schizonticidal agent than chloroquine; however, it is still very effective and widely used in the treatment of acute cases of severe P. falciparum. It is especially useful in areas where there is known to be a high level of resistance to chloroquine, mefloquine, and sulfa drug combinations with pyrimethamine. Quinine is also used in post-exposure treatment of individuals returning from an area where malaria is endemic.

The treatment regimen of quinine is complex and is determined largely by the parasite's level of resistance and the reason for drug therapy (i.e. acute treatment or prophylaxis). The World Health Organization recommendation for quinine is 20 mg/kg first times and 10 mg/kg every eight hours for five days where parasites are sensitive to quinine, combined with doxycycline, tetracycline or clindamycin. Doses can be given by oral, intravenous or intramuscular routes. The recommended method depends on the urgency of treatment and the available resources (i.e. sterilised needles for IV or IM injections).

Use of quinine is characterised by a frequently experienced syndrome called cinchonism. Tinnitus (a hearing impairment), rashes, vertigo, nausea, vomiting and abdominal pain are the most common symptoms. Neurological effects are experienced in some cases due to the drug's neurotoxic properties. These actions are mediated through the interactions of quinine causing a decrease in the excitability of the motor neuron end plates. This often results in functional impairment of the eighth cranial nerve, resulting in confusion, delirium and coma. Quinine can cause hypoglycaemia through its action of stimulating insulin secretion; this occurs in therapeutic doses and therefore it is advised that glucose levels are monitored in all patients every 4–6 hours. This effect can be exaggerated in pregnancy and therefore additional care in administering and monitoring the dosage is essential. Repeated or over-dosage can result in renal failure and death through depression of the respiratory system. 

Quinimax and quinidine are the two most commonly used alkaloids related to quinine in the treatment or prevention of malaria. Quinimax is a combination of four alkaloids (quinine, quinidine, cinchoine and cinchonidine). This combination has been shown in several studies to be more effective than quinine, supposedly due to a synergistic action between the four cinchona derivatives. Quinidine is a direct derivative of quinine. It is a distereoisomer, thus having similar anti-malarial properties to the parent compound. Quinidine is recommended only for the treatment of severe cases of malaria. 

Warburg's tincture was a febrifuge developed by Carl Warburg in 1834, which included quinine as a key ingredient. In the 19th-century it was a well-known anti-malarial drug. Although originally sold as a secret medicine, Warburg's tincture was highly regarded by many eminent medical professionals who considered it as being superior to quinine (e.g. Surgeon-General W. C. Maclean, Professor of Military Medicine at British Army Medical School, Netley). Warburg's tincture appeared in Martindale: The complete drug reference from 1883 until about 1920. The formula was published in The Lancet 1875.

Chloroquine

Chloroquine was, until recently, the most widely used anti-malarial. It was the original prototype from which most methods of treatment are derived. It is also the least expensive, best tested and safest of all available drugs. The emergence of drug-resistant parasitic strains is rapidly decreasing its effectiveness; however, it is still the first-line drug of choice in most sub-Saharan African countries. It is now suggested that it is used in combination with other antimalarial drugs to extend its effective usage. Popular drugs based on chloroquine phosphate (also called nivaquine) are Chloroquine FNA, Resochin and Dawaquin. 

Chloroquine is a 4-aminoquinolone compound with a complicated and still unclear mechanism of action. It is believed to reach high concentrations in the vacuoles of the parasite, which, due to its alkaline nature, raises the internal pH. It controls the conversion of toxic heme to hemozoin by inhibiting the biocrystallization of hemozoin, thus poisoning the parasite through excess levels of toxicity. Other potential mechanisms through which it may act include interfering with the biosynthesis of parasitic nucleic acids and the formation of a chloroquine-haem or chloroquine-DNA complex. The most significant level of activity found is against all forms of the schizonts (with the obvious exception of chloroquine-resistant P. falciparum and P. vivax strains) and the gametocytes of P. vivax, P. malariae, P. ovale as well as the immature gametocytes of P. falciparum. Chloroquine also has a significant anti-pyretic and anti-inflammatory effect when used to treat P. vivax infections, and thus it may still remain useful even when resistance is more widespread. According to a report on the Science and Development Network website's sub-Saharan Africa section, there is very little drug resistance among children infected with malaria on the island of Madagascar, but what drug resistance there is exists against chloroquinine.

Children and adults should receive 25 mg of chloroquine per kg given over three days. A pharmacokinetically superior regime, recommended by the WHO, involves giving an initial dose of 10 mg/kg followed 6–8 hours later by 5 mg/kg, then 5 mg/kg on the following two days. For chemoprophylaxis: 5 mg/kg/week (single dose) or 10 mg/kg/week divided into six daily doses is advised. Chloroquine is only recommended as a prophylactic drug in regions only affected by P. vivax and sensitive P. falciparum strains. Chloroquine has been used in the treatment of malaria for many years and no abortifacient or teratogenic effects have been reported during this time; therefore, it is considered very safe to use during pregnancy. However, itching can occur at intolerable level and Chloroquinine can be a provocation factor of psoriasis.

Amodiaquine

Amodiaquine is a 4-aminoquinolone anti-malarial drug similar in structure and mechanism of action to chloroquine. Amodiaquine has tended to be administered in areas of chloroquine resistance while some patients prefer its tendency to cause less itching than chloroquine. Amodiaquine is now available in a combined formulation with artesunate (ASAQ) and is among the artemisinin-combination therapies recommended by the World Health Organization. Combination with sulfadoxine=pyrimethamine is not recommended.

The drug should be given in doses between 25 mg/kg and 35 mg/kg over three days in a similar method to that used in chloroquine administration. Adverse reactions are generally similar in severity and type to that seen in chloroquine treatment. In addition, bradycardia, itching, nausea, vomiting and some abdominal pain have been recorded. Some blood and hepatic disorders have also been seen in a small number of patients.

Pyrimethamine

Pyrimethamine is used in the treatment of uncomplicated malaria. It is particularly useful in cases of chloroquine-resistant P. falciparum strains when combined with sulfadoxine. It acts by inhibiting dihydrofolate reductase in the parasite thus preventing the biosynthesis of purines and pyrimidines, thereby halting the processes of DNA replication, cell division and reproduction. It acts primarily on the schizonts during the erythrocytic phase, and nowadays is only used in concert with a sulfonamide.

Proguanil

Proguanil (chloroguanide) is a biguanide; a synthetic derivative of pyrimidine. It was developed in 1945 by a British Antimalarial research group. It has many mechanisms of action but primarily is mediated through conversion to the active metabolite cycloguanil. This inhibits the malarial dihydrofolate reductase enzyme. Its most prominent effect is on the primary tissue stages of P. falciparum, P. vivax and P. ovale. It has no known effect against hypnozoites therefore is not used in the prevention of relapse. It has a weak blood schizonticidal activity and is not recommended for therapy of acute infection. However it is useful in prophylaxis when combined with atovaquone or chloroquine (in areas where there is no chloroquine resistance). 3 mg/kg is the advised dosage per day, (hence approximate adult dosage is 200 mg). The pharmacokinetic profile of the drugs indicates that a half dose, twice daily maintains the plasma levels with a greater level of consistency, thus giving a greater level of protection. The proguanil- chloroquine combination does not provide effective protection against resistant strains of P. falciparum. There are very few side effects to proguanil, with slight hair loss and mouth ulcers being occasionally reported following prophylactic use. Proguanil hydrochloride is marketed as Paludrine by AstraZeneca.

Sulfonamides

Sulfadoxine and sulfamethoxypyridazine are specific inhibitors of the enzyme dihydropteroate synthetase in the tetrahydrofolate synthesis pathway of malaria parasites. They are structural analogs of p-aminobenzoic acid (PABA) and compete with PABA to block its conversion to dihydrofolic acid. Sulfonamides act on the schizont stages of the erythrocytic (asexual) cycle. When administered alone sulfonamides are not efficacious in treating malaria but co-administration with the antifolate pyrimethamine, most commonly as fixed-dose sulfadoxine-pyrimethamine (Fansidar), produces synergistic effects sufficient to cure sensitive strains of malaria. 

Sulfonamides are not recommended for chemoprophylaxis because of rare but severe skin reactions experienced. However it is used frequently for clinical episodes of the disease.

Mefloquine

Mefloquine was developed during the Vietnam War and is chemically related to quinine. It was developed to protect American troops against multi-drug resistant P. falciparum. It is a very potent blood schizonticide with a long half-life. It is thought to act by forming toxic heme complexes that damage parasitic food vacuoles. Mefloquine is effective in prophylaxis and for acute therapy. It is now used solely for the prevention of resistant strains of P. falciparum (usually combined with Artesunate) despite being effective against P. vivax, P. ovale and P. marlariae. Chloroquine/proguanil or sulfa drug-pyrimethamine combinations should be used in all other plasmodia infections. 

The major commercial manufacturer of mefloquine-based malaria treatment is Roche Pharmaceuticals, which markets the drug under the trade name "Lariam". Lariam is fairly expensive at around three € per tablet (pricing of the year 2000). 

A dose of 15–25 mg/kg is recommended, depending on the prevalence of mefloquine resistance. The increased dosage is associated with a much greater level of intolerance, most noticeably in young children; with the drug inducing vomiting and esophagitis. It was not recommended for use during the first trimester, although considered safe during the second and third trimesters; nevertheless, in October 2011, the Centers for Disease Control and Prevention (CDC) changed its recommendation and approved use of Mefloquine for both prophylaxis and treatment of malaria in all trimesters, after the Food and Drug Administration (FDA) changed its categorization from C to B. Mefloquine frequently produces side effects, including nausea, vomiting, diarrhea, abdominal pain and dizziness. Several associations with neurological events have been made, namely affective and anxiety disorders, hallucinations, sleep disturbances, psychosis, toxic encephalopathy, convulsions and delirium. Cardiovascular effects have been recorded with bradycardia and sinus arrhythmia being consistently recorded in 68% of patients treated with mefloquine (in one hospital-based study). 

Mefloquine can only be taken for a period up to six months due to side effects. After this, other drugs (such as those based on paludrine/nivaquine) again need to be taken.

Atovaquone

Atovaquone is available in combination with proguanil under the name Malarone, albeit at a price higher than Lariam. It is commonly used in prophylaxis by travelers and used to treat falciparum malaria in developed countries. A liquid oral suspension of Atovaquone is available under the name Mepron.

Primaquine

Primaquine is a highly active 8-aminoquinolone that is effective against P. falcipaum gametocytes but also acts on merozoites in the bloodstream and on hypnozoites, the dormant hepatic forms of P. vivax and P. ovale. It is the only known drug to cure both relapsing malaria infections and acute cases. The mechanism of action is not fully understood but it is thought to block oxidative metabolism in Plasmodia. It can also be combined with methylene blue.

For the prevention of relapse in P. vivax and P. ovale 0.15 mg/kg should be given for 14 days. As a gametocytocidal drug in P. falciparum infections a single dose of 0.75 mg/kg repeated seven days later is sufficient. This treatment method is only used in conjunction with another effective blood schizonticidal drug. There are few significant side effects although it has been shown that primaquine may cause anorexia, nausea, vomiting, cramps, chest weakness, anaemia, some suppression of myeloid activity and abdominal pains. In cases of over-dosage granulocytopenia may occur.

Artemisinin and derivatives

Artemisinin is a Chinese herb (qinghaosu) that has been used in the treatment of fevers for over 1,000 years, thus predating the use of Quinine in the western world. It is derived from the plant Artemisia annua, with the first documentation as a successful therapeutic agent in the treatment of malaria is in 340 AD by Ge Hong in his book Zhou Hou Bei Ji Fang (A Handbook of Prescriptions for Emergencies). Ge Hong extracted the artemesinin using a simple macerate, and this method is still in use today. The active compound was isolated first in 1971 and named artemisinin. It is a sesquiterpene lactone with a chemically rare peroxide bridge linkage. It is thought to be responsible for the majority of its anti-malarial action, although the target within the parasite remains controversial. At present  it is strictly controlled under WHO guidelines as it has proven to be effective against all forms of multi-drug resistant P. falciparum, thus every care is taken to ensure compliance and adherence together with other behaviors associated with the development of resistance. It is also only given in combination with other anti-malarials.

• Artemisinin has a very rapid action and the vast majority of acute patients treated show significant improvement within 1–3 days of receiving treatment. It has demonstrated the fastest clearance of all anti-malarials currently used and acts primarily on the trophozite phase, thus preventing progression of the disease. Semi-synthetic artemisinin derivatives (e.g. artesunate, artemether) are easier to use than the parent compound and are converted rapidly once in the body to the active compound dihydroartemesinin. On the first day of treatment 20 mg/kg is often given, and the dose then reduced to 10 mg/kg per day for the six following days. Few side effects are associated with artemesinin use. However, headaches, nausea, vomiting, abnormal bleeding, dark urine, itching and some drug fever have been reported by a small number of patients. Some cardiac changes were reported during a clinical trial, notably non specific ST changes and a first degree atrioventricular block (these disappeared when the patients recovered from the malarial fever).
• Artemether is a methyl ether derivative of dihydroartemesinin. It is similar to artemesinin in mode of action but demonstrates a reduced ability as a hypnozoiticidal compound, instead acting more significantly to decrease gametocyte carriage. Similar restrictions are in place, as with artemesinin, to prevent the development of resistance, therefore it is only used in combination therapy for severe acute cases of drug-resistant P. falciparum. It should be administered in a 7-day course with 4 mg/kg given per day for three days, followed by 1.6 mg/kg for three days. Side effects of the drug are few but include potential neurotoxicity developing if high doses are given.
• Artesunate is a hemisuccinate derivative of the active metabolite dihydroartemisin. Currently it is the most frequently used of all the artemesinin-type drugs. Its only effect is mediated through a reduction in the gametocyte transmission. It is used in combination therapy and is effective in cases of uncomplicated P. falciparum. The dosage recommended by the WHO is a five or seven day course (depending on the predicted adherence level) of 4 mg/kg for three days (usually given in combination with mefloquine) followed by 2 mg/kg for the remaining two or four days. In large studies carried out on over 10,000 patients in Thailand no adverse effects have been shown.
• Dihydroartemisinin is the active metabolite to which artemesinin is reduced. It is the most effective artemesinin compound and the least stable. It has a strong blood schizonticidal action and reduces gametocyte transmission. It is used for therapeutic treatment of cases of resistant and uncomplicated P. falciparum. 4 mg/kg doses are recommended on the first day of therapy followed by 2 mg/kg for six days. As with artesunate, no side effects to treatment have thus far been recorded. Arteether is an ethyl ether derivative of dihydroartemisinin. It is used in combination therapy for cases of uncomplicated resistant P. falciparum. The recommended dosage is 150 mg/kg per day for three days given by IM injections. With the exception of a small number of cases demonstrating neurotoxicity following parenteral administration no side effects have been recorded.

Halofantrine

Halofantrine is a relatively new drug developed by the Walter Reed Army Institute of Research in the 1960s. It is a phenanthrene methanol, chemically related to Quinine and acts acting as a blood schizonticide effective against all Plasmodium parasites. Its mechanism of action is similar to other anti-malarials. Cytotoxic complexes are formed with ferritoporphyrin XI that cause plasmodial membrane damage. Despite being effective against drug resistant parasites, halofantrine is not commonly used in the treatment (prophylactic or therapeutic) of malaria due to its high cost. It has very variable bioavailability and has been shown to have potentially high levels of cardiotoxicity. It is still a useful drug and can be used in patients that are known to be free of heart disease and are suffering from severe and resistant forms of acute malaria. A popular drug based on halofantrine is Halfan. The level of governmental control and the prescription-only basis on which it can be used contributes to the cost, thus halofantrine is not frequently used.

A dose of 8 mg/kg of halofantrine is advised to be given in three doses at six-hour intervals for the duration of the clinical episode. It is not recommended for children under 10 kg despite data supporting the use and demonstrating that it is well tolerated. The most frequently experienced side-effects include nausea, abdominal pain, diarrhea, and itch. Severe ventricular dysrhythmias, occasionally causing death are seen when high doses are administered. This is due to prolongation of the QTc interval. Halofantrine is not recommended for use in pregnancy and lactation, in small children, or in patients that have taken mefloquine previously.

Lumefantrine

Lumefantrine is a relative of halofantrine that is used in some combination antimalarial regimens.

Doxycycline

Probably one of the more prevalent antimalarial drugs prescribed, due to its relative effectiveness and cheapness, doxycycline is a tetracycline compound derived from oxytetracycline. The tetracyclines were one of the earliest groups of antibiotics to be developed and are still used widely in many types of infection. It is a bacteriostatic agent that acts to inhibit the process of protein synthesis by binding to the 30S ribosomal subunit thus preventing the 50s and 30s units from bonding. Doxycycline is used primarily for chemoprophylaxis in areas where chloroquine resistance exists. It can also be used in combination with quinine to treat resistant cases of P. falciparum but has a very slow action in acute malaria, and should not be used as monotherapy. 

When treating acute cases and given in combination with quinine; 100 mg of doxycycline should be given per day for seven days. In prophylactic therapy, 100 mg (adult dose) of doxycycline should be given every day during exposure to malaria.

The most commonly experienced side effects are permanent enamel hypoplasia, transient depression of bone growth, gastrointestinal disturbances and some increased levels of photosensitivity. Due to its effect of bone and tooth growth it is not used in children under 8, pregnant or lactating women and those with a known hepatic dysfunction.

Tetracycline is only used in combination for the treatment of acute cases of P. falciparum infections. This is due to its slow onset. Unlike doxycycline it is not used in chemoprophylaxis. For tetracycline, 250 mg is the recommended adult dosage (it should not be used in children) for five or seven days depending on the level of adherence and compliance expected. Oesophageal ulceration, gastrointestinal upset and interferences with the process of ossification and depression of bone growth are known to occur. The majority of side effects associated with doxycycline are also experienced.

Clindamycin

Clindamycin is a derivative of lincomycin, with a slow action against blood schizonticides. It is only used in combination with quinine in the treatment of acute cases of resistant P. falciparum infections and not as a prophylactic. Being more toxic than the other antibiotic alternatives, it is used only in cases where the Tetracyclines are contraindicated (for example in children). 

Clindamycin should be given in conjunction with quinine as a 300 mg dose (in adults) four times a day for five days. The only side effects recorded in patients taking clindamycin are nausea, vomiting and abdominal pains and cramps. However these can be alleviated by consuming large quantities of water and food when taking the drug. Pseudomembranous colitis (caused by Clostridium difficile) has also developed in some patients; this condition may be fatal in a small number of cases.

Resistance

Anti-malarial drug resistance has been defined as: "the ability of a parasite to survive and/or multiply despite the administration and absorption of a drug given in doses equal to or higher than those usually recommended but within tolerance of the subject. The drug in question must gain access to the parasite or the infected red blood cell for the duration of the time necessary for its normal action." Resistance to antimalarial drugs is common. In most instances this refers to parasites that remain following on from an observed treatment; thus, it excludes all cases where anti-malarial prophylaxis has failed. In order for a case to be defined as resistant, the patient in question must have received a known and observed anti-malarial therapy while the blood drug and metabolite concentrations are monitored concurrently; techniques used to demonstrate this include in vivo, in vitro, and animal model testing, and more recently developed molecular techniques.

Drug resistant parasites are often used to explain malaria treatment failure. However, they are two potentially very different clinical scenarios. The failure to clear parasitemia and recover from an acute clinical episode when a suitable treatment has been given is anti-malarial resistance in its true form. Drug resistance may lead to treatment failure, but treatment failure is not necessarily caused by drug resistance despite assisting with its development. A multitude of factors can be involved in the processes including problems with non-compliance and adherence, poor drug quality, interactions with other pharmaceuticals, poor absorption, misdiagnosis and incorrect doses being given. The majority of these factors also contribute to the development of drug resistance.

The generation of resistance can be complicated and varies between Plasmodium species. It is generally accepted to be initiated primarily through a spontaneous mutation that provides some evolutionary benefit, thus giving the anti-malarial used a reduced level of sensitivity. This can be caused by a single point mutation or multiple mutations. In most instances a mutation will be fatal for the parasite or the drug pressure will remove parasites that remain susceptible, however some resistant parasites will survive. Resistance can become firmly established within a parasite population, existing for long periods of time. 

The first type of resistance to be acknowledged was to chloroquine in Thailand in 1957. The biological mechanism behind this resistance was subsequently discovered to be related to the development of an efflux mechanism that expels chloroquine from the parasite before the level required to effectively inhibit the process of haem polymerization (that is necessary to prevent buildup of the toxic byproducts formed by haemoglobin digestion). This theory has been supported by evidence showing that resistance can be effectively reversed on the addition of substances which halt the efflux. The resistance of other quinolone anti-malarials such as amiodiaquine, mefloquine, halofantrine and quinine are thought to have occurred by similar mechanisms.

Plasmodium have developed resistance against antifolate combination drugs, the most commonly used being sulfadoxine and pyrimethamine. Two gene mutations are thought to be responsible, allowing synergistic blockages of two enzymes involved in folate synthesis. Regional variations of specific mutations give differing levels of resistance. 

Atovaquone is recommended to be used only in combination with another anti-malarial compound as the selection of resistant parasites occurs very quickly when used in mono-therapy. Resistance is thought to originate from a single-point mutation in the gene coding for cytochrome-b.

Spread of resistance

There is no single factor that confers the greatest degree of influence on the spread of drug resistance, but a number of plausible causes associated with an increase have been acknowledged. These include aspects of economics, human behaviour, pharmacokinetics, and the biology of vectors and parasites.

The most influential causes are examined below:

1. The biological influences are based on the parasites ability to survive the presence of an anti-malarial thus enabling the persistence of resistance and the potential for further transmission despite treatment. In normal circumstances any parasites that persist after treatment are destroyed by the host's immune system, therefore any factors that act to reduce the elimination of parasites could facilitate the development of resistance. This attempts to explain the poorer response associated with immunocompromised individuals, pregnant women and young children.
2. There has been evidence to suggest that certain parasite-vector combinations can alternatively enhance or inhibit the transmission of resistant parasites, causing 'pocket-like' areas of resistance.
3. The use of anti-malarials developed from similar basic chemical compounds can increase the rate of resistance development, for example cross-resistance to chloroquine and amiodiaquine, two 4-aminoquinolones and mefloquine conferring resistance to quinine and halofantrine. This phenomenon may reduce the usefulness of newly developed therapies prior to large-scale usage.
4. The resistance to anti-malarials may be increased by a process found in some species of Plasmodium, where a degree of phenotypic plasticity was exhibited, allowing the rapid development of resistance to a new drug, even if the drug has not been previously experienced.
5. The pharmacokinetics of the chosen anti-malarial are key; the decision of choosing a long half-life over a drug that is metabolised quickly is complex and still remains unclear. Drugs with shorter half-life's require more frequent administration to maintain the correct plasma concentrations, therefore potentially presenting more problems if levels of adherence and compliance are unreliable, but longer-lasting drugs can increase the development of resistance due to prolonged periods of low drug concentration.
6. The pharmacokinetics of anti-malarials is important when using combination therapy. Mismatched drug combinations, for example having an 'unprotected' period where one drug dominates can seriously increase the likelihood of selection for resistant parasites.
7. Ecologically there is a linkage between the level of transmission and the development of resistance, however at present this still remains unclear.
8. The treatment regime prescribed can have a substantial influence on the development of resistance. This can involve the drug intake, combination and interactions as well as the drug's pharmacokinetic and dynamic properties.

Prevention

The prevention of anti-malarial drug resistance is of enormous public health importance. It can be assumed that no therapy currently under development or to be developed in the foreseeable future will be totally protective against malaria. In accordance with this, there is the possibility of resistance developing to any given therapy that is developed. This is a serious concern, as the rate at which new drugs are produced by no means matches the rate of the development of resistance. In addition, the most newly developed therapeutics tend to be the most expensive and are required in the largest quantities by some of the poorest areas of the world. Therefore, it is apparent that the degree to which malaria can be controlled depends on the careful use of the existing drugs to limit, insofar as it is possible, any further development of resistance.

Provisions essential to this process include the delivery of fast primary care where staff are well trained and supported with the necessary supplies for efficient treatment. This in itself is inadequate in large areas where malaria is endemic thus presenting an initial problem. One method proposed that aims to avoid the fundamental lack in certain countries' health care infrastructure is the privatisation of some areas, thus enabling drugs to be purchased on the open market from sources that are not officially related to the health care industry. Although this is now gaining some support there are many problems related to limited access and improper drug use, which could potentially increase the rate of resistance development to an even greater extent.

There are two general approaches to preventing the spread of resistance: preventing malaria infections, and preventing the transmission of resistant parasites.

Preventing malaria infections developing has a substantial effect on the potential rate of development of resistance, by directly reducing the number of cases of malaria thus decreasing the need for anti-malarial therapy. Preventing the transmission of resistant parasites limits the risk of resistant malarial infections becoming endemic and can be controlled by a variety of non-medical methods including insecticide-treated bed nets, indoor residual spraying, environmental controls (such as swamp draining) and personal protective methods such as using mosquito repellent. Chemoprophylaxis is also important in the transmission of malaria infection and resistance in defined populations (for example travelers). 

A hope for future of anti-malarial therapy is the development of an effective malaria vaccine. This could have enormous public health benefits, providing a cost-effective and easily applicable approach to preventing not only the onset of malaria but the transmission of gametocytes, thus reducing the risk of resistance developing. Anti-malarial therapy also could be diversified by combining a potentially effective vaccine with current chemotherapy, thereby reducing the chance of vaccine resistance developing.

Combination therapy

The problem of the development of malaria resistance must be weighed against the essential goal of anti-malarial care; that is to reduce morbidity and mortality. Thus a balance must be reached that attempts to achieve both goals while not compromising either too much by doing so. The most successful attempts so far have been in the administration of combination therapy. This can be defined as, 'the simultaneous use of two or more blood schizonticidal drugs with independent modes of action and different biochemical targets in the parasite'. There is much evidence to support the use of combination therapies, some of which has been discussed previously, however several problems prevent the wide use in the areas where its use is most advisable. These include: problems identifying the most suitable drug for different epidemiological situations, the expense of combined therapy (it is over 10 times more expensive than traditional mono-therapy), how soon the programmes should be introduced and problems linked with policy implementation and issues of compliance.

The combinations of drugs currently prescribed can be divided into two categories: non-artemesinin-based combinations and artemesinin based combinations. It is also important to distinguish fixed-dose combination therapies (in which two or more drugs are co-formulated into a single tablet) from combinations achieved by taking two separate antimalarials.

Non-artemisinin based combinations

Components	Description	Dose
Sulfadoxine-pyrimethamine (SP) (Fansidar)	This fixed-dose combination has been used for many years, causes few adverse effects, is cheap and effective in a single dose, thus decreasing problems associated with adherence and compliance. In technical terms Fansidar is not generally considered a true combination therapy since the components do not possess independent curative activity. Fansidar should no longer be used alone for treatment of falciparum malaria.	25 mg/kg of sulfadoxine and 1.25 mg/kg of pyrimethamine.
SP plus chloroquine	High levels of resistance to one or both components means this combination is effective in few locations and it is not recommended by the World Health Organization (WHO).	Chloroquine 25 mg/kg over three days with a single dose of SP as described above.
SP plus amodiaquine	This combination has been shown to produce a faster rate of clinical recovery than SP and chloroquine, but is clearly inferior to artemisinin-based combinations (ACTs) for the treatment of malaria.	10 mg/kg of Amodiaquine per day for three days with a single standard dose of SP.
SP plus mefloquine (Fansimef)	This single dose pill offered obvious advantages of convenience over more complex regimes but it has not been recommended for use for many years owing to widespread resistance to the components.
Quinine plus tetracycline/doxycycline	This combination retains a high cure rate in many areas. Problems with this regime include the relatively complicated drug regimen, where quinine must be taken every eight hours for seven days. Additionally, there are significant side effects with quinine ('cinchonism') and tetracyclines are contraindicated in children and pregnant women (these groups should use clindamycin instead). With the advent of artemisinin-combination therapies, quinine-based treatment is less popular than previously.	Quinine 10 mg/kg doses every eight hours and tetracycline in 4 mg/kg doses every six hours for seven days.

Artemisinin-based combination therapies should be used in preference to amodiaquine plus sulfadoxine-pyrimethamine for the treatment of uncomplicated P. falciparum malaria.

Artemisinin-based combination therapies (ACTs)

Artemesinin has a very different mode of action than conventional anti-malarials (see information above), which makes it particularly useful in the treatment of resistant infections. However, to prevent the development of resistance to this drug it is only recommended in combination with another non-artemesinin based therapy. It produces a very rapid reduction in the parasite biomass with an associated reduction in clinical symptoms and is known to cause a reduction in the transmission of gametocytes thus decreasing the potential for the spread of resistant alleles. At present there is no known resistance to Artemesinin (though some resistant strains may be emerging) and very few reported side-effects to drug usage, however this data is limited.

Components	Description	Dose
Artesunate and amodiaquine (Coarsucam or ASAQ)	This combination has been tested and proved to be efficacious in many areas where amodiaquine retains some efficacy. A potential disadvantage is a suggested link with neutropenia. It's recommended by the WHO for uncomplicated falciparum malaria.	Dosage is as a fixed-dose combination (ASAQ) recommended as 4 mg/kg of Artesunate and 10 mg/kg of Amodiaquine per day for three days.
Artesunate and mefloquine (Artequin or ASMQ)	This has been used as an efficacious first-line treatment regimen in areas of Thailand for many years. Mefloquine is known to cause vomiting in children and induces some neuropsychiatric and cardiotoxic effects. These adverse reactions seem to be reduced when the drug is combined with artesunate, it is suggested that this is due to a delayed onset of action of mefloquine. This is not considered a viable option to be introduced in Africa due to the long half-life of mefloquine, which potentially could exert a high selection pressure on parasites. It's recommended by the WHO for uncomplicated falciparum malaria.	The standard dose required is 4 mg/kg per day of Artesunate plus 25 mg/kg of Mefloquine as a split dose of 15 mg/kg on day two and 10 mg/kg on day three.
Artemether and lumefantrine (Coartem Riamet, Faverid, Amatem, Lonart or AL)	This combination has been extensively tested in 16 clinical trials, proving effective in children under five and has been shown to be better tolerated than artesunate plus mefloquine combinations. There are no serious side effects documented but the drug is not recommended in pregnant or lactating women due to limited safety testing in these groups. This is the most viable option for widespread use and is available in fixed-dose formulas thus increasing compliance and adherence. It's recommended by the WHO for uncomplicated falciparum malaria.
Artesunate and sulfadoxine/pyrimethamine (Ariplus or Amalar plus)	This is a well tolerated combination but the overall level of efficacy still depends on the level of resistance to sulfadoxine and pyrimethamine thus limiting is usage. It is recommended by the WHO for uncomplicated falciparum malaria.	It is recommended in doses of 4 mg/kg of Artesunate per day for three days and a single dose of 25 mg/kg of SP.
Dihydroartemisinin-piperaquine (Duo-Cotecxin, or Artekin)	Has been studied mainly in China, Vietnam and other countries in SEAsia. The drug has been shown to be highly efficacious (greater than 90%). It's recommended by the WHO for uncomplicated falciparum malaria.
Artesinin/piperaguine/primaquine (Fast Elimination of Malaria through Source Eradication (FEMSE))	This protocol involves three doses of Artequick, spaced a month apart. The first dose is accompanied by one of primaquine. An experimental program in the Comoros islands employed the protocol. At the outset, more than 90% of the inhabitants of some villages had malaria. On one island the number of cases fell by 95%. In 2012, on the second island, the number of cases fell by 97%.
Pyronaridine and artesunate (Pyramax)	Pyramax developed by Shin Poong Pharmaceutical and Medicines for Malaria Venture (MMV). This is a first fixed-dose artemisinin-based combination therapy to be granted a positive scientific opinion for efficacy, safety and quality from European Medicines Agency (EMA) under Article 58 for the treatment of P. falciparum and P. vivax in adults and children over 20 kg based on five multi-centre phase III trials conducted in Africa and South-East Asia. Pyramax has been shown to be highly efficacious (greater than 97%) in both species and only ACT approved by stringent regulatory authority for treatment of both P. falciparum and P vivax by now.

Other combinations

Several other anti-malarial combinations have been used or are in development. For example, Chlorproguanil-dapsone and artesunate (CDA) appears efficacious but the problem of haemolysis in patients with glucose-6-phosphate dehydrogenase (G6PD) deficiency is likely to prevent widespread use.

By type of malaria

Antimalarial drugs and combinations may also be sorted according to the type of malaria in which they are used.

Falciparum malaria

Artemisinin-based combination therapies (ACTs) are the recommended antimalarial treatments for uncomplicated malaria caused by P. falciparum. The choice of ACT in a country or region will be based on the level of resistance to the constituents in the combination. For pregnant women, the recommended first-line treatment during the first trimester is quinine plus clindamycin to be given for seven days. In second and third trimesters, it is recommended to give ACTs known to be effective in the country/region or artesunate plus clindamycin for seven days, or quinine plus clindamycin to be given for seven days. Lactating women should receive standard antimalarial treatment (including ACTs) except for dapsone, primaquine and tetracyclines. In infants and young children, it is recommended to give ACTs for first-line treatment, with attention to accurate dosing and ensuring the administered dose is retained.

In severe falciparum malaria, it is recommended that rapid clinical assessment and confirmation of the diagnosis is made, followed by administration of full doses of parenteral antimalarial treatment without delay with whichever effective antimalarial is first available. For adults, intravenous (IV) or intramuscular (IM) artesunate is recommended. Quinine is an acceptable alternative if parenteral artesunate is not available. Parenteral antimalarials should be administered for a minimum of 24 h in the treatment of severe malaria, irrespective of the patient's ability to tolerate oral medication earlier. Thereafter, it is recommended to complete treatment by giving a complete course of any of the following:

• an ACT
• artesunate plus clindamycin or doxycycline;
• quinine plus clindamycin or doxycycline.

Vivax malaria

Chloroquine remains the treatment of choice for vivax malaria, except in Indonesia's Irian Jaya (Western New Guinea) region and the geographically contiguous Papua New Guinea, where chloroquine resistance is common (up to 20% resistance).