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The T helper cells (Th cells) are a type of T cell that play an important role in the immune system, particularly in the adaptive immune system. They help the activity of other immune cells by releasing T cell cytokines. These cells help suppress or regulate immune responses. They are essential in B cell antibody class switching, in the activation and growth of cytotoxic T cells, and in maximizing bactericidal activity of phagocytes such as macrophages.
Mature Th cells express the surface protein CD4 and are referred to as CD4+ T cells. CD4+ T cells are generally treated as having a pre-defined role as helper T cells within the immune system. For example, when an antigen-presenting cell expresses an antigen on MHC class II, a CD4+ cell will aid those cells through a combination of cell to cell interactions (e.g. CD40 and CD40L) and through cytokines. Nevertheless, there are rare exceptions; for example, sub-groups of regulatory T cells, natural killer cells, and cytotoxic T cells express CD4. All of the latter CD4+ T cell groups are not considered T helper cells.
The importance of helper T cells can be seen from HIV, a virus that infects CD4+ cells. Towards the end of an HIV infection, once the viral load has increased enough, the number of functional CD4+ T cells falls, which leads to the symptomatic stage of infection known as the acquired immunodeficiency syndrome (AIDS); if the HIV virus is detected in bodily fluids, such as the blood, early enough, continuous therapy, if it is warranted, can delay the time at which this happens and better manage the course of AIDS if and when it occurs. There are also some rare disorders that result in the absence or dysfunction of CD4+ T cells. These disorders produce similar symptoms, and many of these are fatal.
Following T cell development, matured, naïve T cells leave the thymus and begin to spread throughout the body, including the lymph nodes. (Naïve T cells are those T cells that have never been exposed to the antigen that they are programmed to respond to). Like all T cells, they express the T cell receptor-CD3 complex. The T cell receptor (TCR) consists of both constant and variable regions. The variable region determines what antigen the T cell can respond to. CD4+ T cells have TCRs with an affinity for Class II MHC, and CD4 is involved in determining MHC affinity during maturation in the thymus. Class II MHC proteins are generally only found on the surface of specialised antigen-presenting cells (APCs). Specialised antigen presenting cells are primarily dendritic cells, macrophages and B cells, although dendritic cells are the only cell group that expresses MHC Class II constitutively (at all times). Some APCs also bind native (or unprocessed) antigens to their surface, such as follicular dendritic cells, but unprocessed antigens do not interact with T cells and are not involved in their activation. The antigens that bind to MHC proteins are always short peptides, 8-10 amino acids long for MHC Class I, and up to 25 or so for MHC Class II.
During an immune response, professional antigen-presenting cells (APCs) endocytose foreign material (typically bacteria or viruses), which undergoes processing, then travel from the infection site to the lymph nodes. Once at the lymph nodes, the APC begin to present antigen peptides that are bound to Class II MHC, allowing CD4+ T cells that express the specific TCRs against the peptide/MHC complex to activate.
When a Th cell encounters and recognises the antigen on an APC, the TCR-CD3 complex binds strongly to the peptide-MHC complex present on the surface of professional APCs. CD4, a co-receptor of the TCR complex, also binds to a different section of the MHC molecule. These interactions bring these proteins closer together, allowing the intracellular kinases present on the TCR, CD3 and CD4 proteins to activate each other via phosphorylation. With the assistance of a phosphatase present on the intracellular section of CD45 (common leukocyte antigen), these molecules activate major Th cell intracellular pathways. These active pathways are known as Signal 1 of T cell activation, as it is the first and primary pro-activation signal in a Th cell. Upon subsequent encounters with a given antigen, memory T cells are re-activated using the same TCR pathways.
The binding of the antigen-MHC to the TCR complex and CD4 may also help the APC and the Th cell adhere during Th cell activation, but the integrin protein LFA-1 on the T cell and ICAM on the APC are the primary molecules of adhesion in this cell interaction.
It is unknown what role the relatively bulky extracellular region of CD45 plays during cell interactions, but CD45 has various isoforms that change in size depending on the Th cell's activation and maturation status. For example, CD45 shortens in length following Th activation (CD45RA+ to CD45RO+), but whether this change in length influences activation is unknown. It has been proposed that the larger CD45RA+ may decrease the accessibility of the T cell receptor for the antigen-MHC molecule, thereby necessitating an increase in the affinity (and specificity) of the T cell for activation. Once the activation has occurred however, CD45 shortens, allowing easier interactions and activation as an effector T helper cell.[citation needed]
Having received the first TCR/CD3 signal, the naïve T cell must activate a second independent biochemical pathway, known as Signal 2. This verification step is a protective measure to ensure that a T cell is responding to a foreign antigen. If this second signal is not present during initial antigen exposure, the T cell presumes that it is auto-reactive. This results in the cell becoming anergic (anergy is generated from the unprotected biochemical changes of Signal 1). Anergic cells will not respond to any antigen in the future, even if both signals are present later on. These cells are generally believed to circulate throughout the body with no value until they undergo apoptosis.
The second signal involves an interaction between CD28 on the CD4+ T cell and the proteins CD80 (B7.1) or CD86 (B7.2) on the professional APCs. Both CD80 and CD86 activate the CD28 receptor. These proteins are also known as co-stimulatory molecules.
Although the verification stage is necessary for the activation of naïve helper T cells, the importance of this stage is best demonstrated during the similar activation mechanism of CD8+ cytotoxic T cells. As naïve CD8+ T cells have no true bias towards foreign sources, these T cells must rely on the activation of CD28 for confirmation that they recognise a foreign antigen (as CD80/CD86 is only expressed by active APC's). CD28 plays an important role in decreasing the risk of T cell auto-immunity against host antigens.
Once the naïve T cell has both pathways activated, the biochemical changes induced by Signal 1 are altered, allowing the cell to activate instead of anergise. The second signal is then obsolete; only the first signal is necessary for future activation. This is also true for memory T cells, which is one example of learned immunity. Faster responses occur upon reinfection because memory T cells have already undergone confirmation and can produce effector cells much sooner.
Once the two-signal activation is complete the T helper cell (Th) then allows itself to proliferate. It achieves this by releasing a potent T cell growth factor called interleukin 2 (IL-2) which acts upon itself in an autocrine fashion. Activated T cells also produce the alpha sub-unit of the IL-2 receptor (CD25 or IL-2R), enabling a fully functional receptor that can bind with IL-2, which in turn activates the T cell's proliferation pathways.
The autocrine or paracrine secretion of IL-2 can bind that same Th cell or neighboring Th's via the IL-2R thus driving proliferation and clonal expansion. The Th cells receiving both signals of activation and proliferation will then become Th0 cells (T helper 0) cell that secrete IL-2, IL-4 and interferon gamma (IFN-γ). The Th0 cells will then differentiate into Th1 or Th2 cells depending on cytokine environment. IFN-γ drives Th1 cell production while IL-10 and IL-4 inhibit Th1 cell production. Conversely, IL-4 drives Th2 cell production and IFN-γ inhibits Th2 cells. It should be noted that these cytokines are pleiotropic and carry out many other functions of the immune response.
After many cell generations, the Th cell's progenitors differentiate into effector Th cells, memory Th cells, and regulatory Th cells.
Helper T cells are capable of influencing a variety of immune cells, and the T cell response generated (including the extracellular signals such as cytokines) can be essential for a successful outcome from infection. In order to be effective, helper T cells must determine which cytokines will allow the immune system to be most useful or beneficial for the host. Understanding exactly how helper T cells respond to immune challenges is currently of major interest in immunology, because such knowledge may be very useful in the treatment of disease and in increasing the effectiveness of vaccination.
Proliferating helper T cells that develop into effector T cells differentiate into two major subtypes of cells known as Th1 and Th2 cells (also known as Type 1 and Type 2 helper T cells, respectively).
Th1 helper cells are the host immunity effectors against intracellular bacteria and protozoa. They are triggered by IL-12, IL-2 and their effector cytokine is IFN-γ. The main effector cells of Th1 immunity are macrophages as well as CD8 T cells, IgG B cells, and IFN-γ CD4 T cells. The key Th1 transcription factors are STAT4 and T-bet. IFN-γ secreted by CD4 T cells can activate macrophages to phagocytose and digest intracellular bacteria and protozoa. In addition, IFN-γ can activate iNOS to produce NOx free radicals to directly kill intracellular bacteria and protozoa. Th1 overactivation against autoantigens will cause Type4 delayed-type hypersensitivity. Tuberculin reaction or Type 1 diabetes belong to this category of autoimmunity.[1]
Th2 helper cell are the host immunity effectors against extracellular parasites including helminths. They are triggered by IL-4 and their effector cytokines are IL-4, IL-5, IL-9, IL-10 and IL-13. The main effector cells are eosinophils, basophils, and mast cells as well as B cells, and IL-4/IL-5 CD4 T cells. The key Th2 transcription factors are STAT6 and GATAs. IL-4 is the positive feedback cytokine for Th2 cells differentiation. Besides, IL-4 stimulates B-cells to produce IgE antibodies, which in turn stimulate mast cells to release histamine, serotonin, and leukotriene to cause broncho-constriction, intestinal peristalsis, gastric fluid acidification to expel helminths. IL-5 from CD4 T cells will activate eosinophils to attack helminths. IL-10 suppresses Th1 cells differentiation and function of dendritic cells. Th2 overactivation against autoantigen will cause Type1 IgE-mediated allergy and hypersensitivity. Allergic rhinitis, atopic dermatitis, and asthma belong to this category of autoimmunity.[1]
| Type 1/ Th1 | Type 2/ Th2[1] | |
|---|---|---|
| Main partner cell type | Macrophage, CD8+ T cell | B-cell, eosinophil, mast cell |
| Cytokines produced | interferon-γ and TNF-β. Interleukin-2 interleukin-10 production has been reported in activated Th1 cell.[3] | interleukin-4, interleukin-5, interleukin-6, interleukin-9, interleukin-10, interleukin-13 |
| Immune stimulation promoted | Cellular immune system. Maximizes the killing efficacy of the macrophages and the proliferation of cytotoxic CD8+ T cells. Also promotes the production of opsonizing antibodies. (IgG, IgM and IgA but not IgE antibodies) | Humoral immune system. Stimulates B-cells into proliferation, to induce B-cell antibody class switching, and to increase neutralizing antibody production. (IgG, IgM and IgA as well as IgE antibodies) |
| Other functions | The Type 1 cytokine IFN-γ increases the production of interleukin-12 by dendritic cells and macrophages, and via positive feedback, IL-12 stimulates the production of IFN-γ in helper T cells, thereby promoting the Th1 profile. IFN-gamma also inhibits the production of cytokines such as interleukin-4, an important cytokine associated with the Type 2 response, and thus it also acts to preserve its own response. | The Type 2 response promotes its own profile using two different cytokines. Interleukin-4 acts on helper T cells to promote the production of Th2 cytokines (including itself; it is auto-regulatory), while interleukin-10 (IL-10) inhibits a variety of cytokines including interleukin-2 and IFN-γ in helper T cells and IL-12 in dendritic cells and macrophages. The combined action of these two cytokines suggests that once the T cell has decided to produce these cytokines, that decision is preserved (and also encourages other T cells to do the same). |
While we know about the types of cytokine patterns helper T cells tend to produce, we understand less about how the patterns themselves are decided. Various evidence suggests that the type of APC presenting the antigen to the T cell has a major influence on its profile. Other evidence suggests that the concentration of antigen presented to the T cell during primary activation influences its choice. The presence of some cytokines (such as the ones mentioned above) will also influence the response that will eventually be generated, but our understanding is nowhere near complete.
Th17 helper cells mediate host immunity against extracellular bacteria and fungi. This effector cell subtype is triggered by IL-6 and TGF beta or IL-23 and IL-1beta. Its main effector cytokines are IL-17a, IL-21, and IL-22. The main Th17 effector cells are neutrophils as well as IgM/IgA B cells, and IL-17 CD4 T cells. The key Th17 transcription factors are STAT3 and RORg. The Th17 cells can alter their differentiation program ultimately giving rise to either protective or pro-inflammatory pathogenic cells. The protective and non-pathogenic Th17 cells induced by IL-6 and TGF beta are termed as Treg17 cells. The pathogenic Th17 cells are induced by IL-23 and IL-1 beta.[4] TNF alpha can activate neutrophils to kill extracellular bacteria and fungi. Besides, IL-6 can upregulate the complement system to directly kill extracellular bacteria and fungi. Th17 overactivation against autoantigen will cause type 3 immune complex and complement-mediated hypersensitivity. Rheumatoid arthritis or Arthus reaction belong to this category.[5]
Bone erosion caused by mature osteoclast cells is common in patients with rheumatoid arthritis. Activated T helper cells such as Th1, Th2, and Th17 are found in the synovial cavity during the time of inflammation due to rheumatoid arthritis. The known mechanisms associated with the differentiation of osteoclast precursors into mature osteoclasts involve the signaling molecules produced by immune-associate cells, as well as the direct cell to cell contact of osteoblasts and osteoclast precursors. However, it has been suggested that Th17 can also play a more major role in osteoclast differentiation via cell to cell contact with osteoclast precursors.[6][7]
THαβ helper cells provide the host immunity against viruses. Their differentiation is triggered by IFN alpha/beta or IL-10. Their key effector cytokine is IL-10. Their main effector cells are NK cells as well as CD8 T cells, IgG B cells, and IL-10 CD4 T cells. The key THαβ transcription factors are STAT1 and STAT3 as well as IRFs. IL-10 from CD4 T cells activate NK cells' ADCC to apoptose virus-infected cells and to induce host as well as viral DNA fragmentation. IFN alpha/beta can suppress transcription to avoid virus replication and transmission. Overactivation of THαβ against autoantigen will cause type 2 antibody-dependent cytotoxic hypersensitivity. Myasthenia gravis or Graves' disease belong to this category.[8]
The interactions between cytokines from the Th1/Th2 model can be more complicated in some animals. For example, the Th2 cytokine IL-10 inhibits cytokine production of both Th subsets in humans. Human IL-10 (hIL-10) suppresses the proliferation and cytokine production of all T cells and the activity of macrophages, but continues to stimulate plasma cells, ensuring that antibody production still occurs. As such, hIL-10 is not believed to truly promote the Th2 response in humans, but acts to prevent over-stimulation of helper T cells while still maximising the production of antibodies.
There are also other types of T cells that can influence the expression and activation of helper T cells, such as natural regulatory T cells, along with less common cytokine profiles such as the Th3 subset of helper T cells. Terms such as "regulatory" and "suppression" have become ambiguous after the discovery that helper CD4+ T cells are also capable of regulating (and suppressing) their own responses outside of dedicated regulatory T cells.
One major difference between regulatory T cells and effector T cells is that regulatory T cells typically serve to modulate and deactivate the immune response, while effector T cell groups usually begin with immune-promoting cytokines and then switch to inhibitory cytokines later in their life cycle. The latter is a feature of Th3 cells, which transform into a regulatory subset after its initial activation and cytokine production.
Both regulatory T cells and Th3 cells produce the cytokine transforming growth factor-beta (TGF-β) and IL-10. Both cytokines are inhibitory to helper T cells; TGF-β suppresses the activity of most of the immune system. There is evidence to suggest that TGF-β may not suppress activated Th2 cells as effectively as it might suppress naive cells, but it is not typically considered a Th2 cytokine.
The characterisation of another novel T helper subtype, T helper 17 cells (Th17)[5] has cast further doubt on the basic Th1/Th2 model. These IL-17 producing cells were initially described as a pathogenic population implicated in autoimmunity but are now thought to have their own distinct effector and regulatory functions. Of note, recent evidence suggest that functional plasticity is an intrinsic capacity of T helper cells. Indeed, a study in mice demonstrated that Th17 cells transform into Th1 cells in vivo.[9] A subsequent study furthermore showed that extensive T helper cell plasticity is also prominent in man.[10]
Many of the cytokines in this article are also expressed by other immune cells (see individual cytokines for details), and it is becoming clear that while the original Th1/Th2 model is enlightening and gives insight into the functions of helper T cells, it is far too simple to define its entire role or actions. Some immunologists question the model completely, as some in vivo studies suggest that individual helper T cells usually do not match the specific cytokine profiles of the Th model, and many cells express cytokines from both profiles.[11] That said, the Th model has still played an important part in developing our understanding of the roles and behaviour of helper T cells and the cytokines they produce during an immune response.
Recent scientific studies by Stockinger et al. revealed that another T helper subset may exist. Th9 cells are claimed to be an IL9 (interleukin 9)–producing T cell subset focused on defending helminth infections.[12]
Considering the diverse and important role helper T cells play in the immune system, it is not surprising that these cells often influence the immune response against disease. They also appear to make occasional mistakes, or generate responses that would be politely considered non-beneficial. In the worst-case scenario, the helper T cell response could lead to a disaster and the fatality of the host. Fortunately this is a very rare occurrence.
The immune system must achieve a balance of sensitivity in order to respond to foreign antigens without responding to the antigens of the host itself. When the immune system responds to very low levels of antigen that it usually shouldn't respond to, a hypersensitivity response occurs. Hypersensitivity is believed to be the cause of allergy and some auto-immune disease.
Hypersensitivity reactions can be divided into four types:
Other cellular hypersensitivities include cytotoxic T cell mediated auto-immune disease, and a similar phenomenon; transplant rejection. Helper T cells are required to fuel the development of these diseases. In order to create sufficient auto-reactive killer T cells, interleukin-2 must be produced, and this is supplied by CD4+ T cells. CD4+ T cells can also stimulate cells such as natural killer cells and macrophages via cytokines such as interferon-gamma, encouraging these cytotoxic cells to kill host cells in certain circumstances.
The mechanism that killer T cells use during auto-immunity is almost identical to their response against viruses, and some viruses have been accused of causing auto-immune diseases such as Type 1 Diabetes mellitus. Cellular auto-immune disease occurs because the host antigen recognition systems fail, and the immune system believes, by mistake, that a host antigen is foreign. As a result, the CD8+ T cells treat the host cell presenting that antigen as infected, and go on to destroy all host cells (or in the case of transplant rejection, transplant organ) that express that antigen.
Some of this section is a simplification. Many auto-immune diseases are more complex. A well-known example is rheumatoid arthritis, where both antibodies and immune cells are known to play a role in the pathology. Generally the immunology of most auto-immune diseases is not well understood.
Perhaps the best example of the importance of CD4+ T cells is demonstrated with human immunodeficiency virus (HIV) infection. HIV targets cells that express CD4, and can infect macrophages, dendritic cells (both groups express CD4 at low levels) and CD4+ T cells.
It has been proposed that during the non-symptomatic phase of HIV infection, the virus has a relatively low affinity towards T cells (and has a higher affinity for macrophages), resulting in a slow kill rate of CD4+ T cells by the immune system[citation needed]. This is initially compensated for via the production of new helper T cells from the thymus (originally from the bone marrow). Once the virus becomes lymphotropic (or T-tropic) however, it begins to infect CD4+ T cells far more efficiently (likely due to a change in the co-receptors it binds to during infection), and the immune system is overwhelmed.
At this point, functional CD4+ T cell levels begin to decrease, eventually to a point where the CD4+ T cell population is too small to recognize the full range of antigens that could potentially be detected. The lack of full antigen cover results in the core symptoms of acquired immune deficiency syndrome (AIDS). CD4 T cell depleted to the cell count of less than 200cell/μL in blood during AIDS allows various pathogens to escape T cell recognition, thus allowing opportunistic infections that would normally elicit a helper T cell response to bypass the immune system.[13] While these complete bypass situations only occur when the helper T cell response is absolutely necessary for infection clearance, most infections increase in severity and/or duration because the immune system's helper T cells provide a weaker contribution to a less efficient immune response.
Two components of the immune system are particularly affected in AIDS, due to its CD4+ T cell dependency:
If the patient does not respond to (or does not receive) HIV treatment they will succumb usually to either cancers or infections; the immune system finally reaches a point where it is no longer coordinated or stimulated enough to deal with the disease.
Inhibition of CD4 T-cell expansion during HIV infection is also due to microbial translocation in an IL-10-dependent way. Indeed, triggering PD-1, expressed on monocytes and up-regulated upon monocytes activation, by its ligand PD-L1 induces IL-10 production which inhibits CD4 T-cell function.[14]
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