Immune Tolerance and Immune Recognition

• The great diversity of T-cell receptors make it possible for any antigen present on a pathogen to be recognised by a T-cell.

  However, if there are T-cells with TCRs complimentary to peptide-MHC complexes derived from self proteins, then normal healthy cells and tissue can be attacked and damaged.
• Immunological tolerance is what prevents the body from damaged itself. Tolerance refers to a system that has the potential to respond and so something, but instead makes the decision to not respond.

  Immune tolerance therefore prevents autoreactivity harm to healthy self tissues, but permits a vigorous and rapid immune response to non-self antigens/tissues.
• T-cells are called T-cells because they develop and mature in the thymus. T-cell precursors are seeded from the bone marrow into the thymus, which is a multi-lobular organ that contains specialised organised zones and different cell types that are involved in developing T-cells so that they work correctly. The process of T-cells developing and learning how to behave in the thymus is known as thymic education of T-cells.
• The thymocytes from the bone marrow are seeded into the thymus where they start to express a functional TCR. They also learn which accessory molecule, CD4 or CD8, is more appropiate for that particular T-cell when its individual TCR is expressed. Then finally, the T-cells learn how to use their TCRs safely within the laws of MHC restriction.
• The 1st step of thymic education is the rearrangement and pairing of the TCR genes to form an alpha-beta-TCR. Only T-cells that are capable of forming a functional TCR that makes it to the cell surface will survive. T-cells that aren't able to have a functional TCR on their cell surface will die by apoptosis.

  At this point, cells with a functional alpha-beta-TCR start to express both of the co-receptors CD4 and CD8.
• The 2nd step of thymic education is called positive selection. This is where the TCRs are checked to see whether they will bind to MHC class 1 or MHC class 2 molecules with a weak affinity.

  Positive selection takes place in the thymic cortex on specialised APCs that express both MHC 1 and MHC 2 molecules with a very restricted set of self peptides within the peptide-binding groove of the MHC molecules.
• In positive selection, if the T-cell has a TCR that binds with weak affinity to a peptide-MHC 2 complex, then this T-cell will stop expressing CD8 and retain expression of CD4. This T-cell will become a CD4 T-cell that is capable of recognising peptides presented by MHC class 2 molecules.

  Likewise, if the T-cell has a TCR that binds with weak affinity to a peptide-MHC 1 complex, the T-cell will stop expressed CD4 and retain expression of the CD8 receptor. This is now a CD8 T-cell capable of recognising peptides presented by MHC class 1 molecules.
• During positive selection, there are some T-cells that won't have any affinity to peptides presented by either MHC 1 or MHC 2 molecules, despite having a TCR on their cell surface. Therefore, these T-cells will be of no use in the periphery (since they can't recognise anything), and so will die by apoptosis.
• The 3rd and final stage of thymic education is known as negative selection. Negative selection is where TCRs are checked to see whether they have a high affinity for self-peptide-MHC complexes. If so, then these T-cells are eliminated.

  Negative selection takes place in the thymic medulla on specialised antigen presenting cells known as thymic medullary epithelial cells (TMECs). TMECs can express both peptide-MHC 1 and peptide-MHC 2 complexes.
• In negative selection, if a T-cell demonstrates a low or moderate affinity to the peptide-MHC complexes displayed by the TMECs, then it will be free to enter the circulation.

  If however, a T-cell demonstrated a high affinity to the peptide-MHC complexes displayed by the TMECs, then it will die by activation-induced cell death.
• TMECs are able to present a wide variety of peptides to T-cells because they are capable of expressing a whole variety of antigens, known as tissue-restricted antigens (TRAs). TMECs possess special transcription factors that allow them to express proteins that would normally only be expressed by a very restricted/specialised set of tissues. For example, TMECs can express insulin, which would only normally be expressed by the Islets of Langerhans.
• The property of TMECs to express TRAs has clinical importance, as there are some individuals that aren't able to express some TRAs in the thymus due to genetic mutations. This can lead to a whole range of autoimmune diseases, such as APS (autoimmune polyglandular syndrome) type 1.
• Thymic education is sometimes referred to as a mechanism of central tolerance, or a central tolerance mechanism, because it all takes place at a single site: the thymus.

  However, although thymic education is highly effective, there are still some T-cells that end up recognising self antigens with high affinity. In order to deal with these T-cells, there are other mechanisms in place called peripheral mechanisms of immune tolerance. These mechanisms include two things: anergy, and the action of regulatory T-cells.
• Anergy happens when T-cells receive (activation) signal 1 without signal 2. Under steady state circumstances - when APCs aren't activated by infection or inflammation - the APC doesn't express high levels of co-stimulatory molecules. Under these conditions, the T-cell will remain in the peripheral circulation, but its very unresponsive to any future stimulation.
• Anergy is important for 3 reasons:

  • it establishes a tolerance to antigens not expressed in the thymus. If T-cells consistently see the same antigens expressed in APCs without any inflammation or infection, and hence, with no co-stimulatory molecules/signal 2, then the T-cells will ignore it.
  • It builds a tolerance to non-self antigens and peptides that are taken into the body via food.
  • builds a tolerance to non-self antigens expressed on the surface of friendly microorganisms, such as commensal bacteria, that reside in the body and are vital for human health.
• Anergy is a more passive mechanism for peripheral immune tolerance, so there is another more active mechanism of maintaining self-tolerance via the action of CD4 regulatory T-cells.

  CD4 regulatory T-cells control or supress the action of effector T-cells. These regulatory T-cells don't eliminate pathogens; instead, they control the cells whose job it is to eliminate pathogens.
• CD4 regulatory T-cells work in 2 main ways:

  • they directly alter the function of effector T-cells. They can stop them from proliferating, prevent them from making their effector cytokines, and prevent them from performing their killing functions.
  • They stop the message from reaching the effector T-cells by reducing co-stimulation, or altering the cytokine production by the APCs. Therefore, they alter the quality and quantity of signal that will be received by the effector T-cells, and subsequently, maintain immune tolerance.
• There are at least 2 main types of of regulatory T-cells:

  • Natural regulatory T-cells (nTreg)
  • Adaptive/induced T-cells (aTreg)
• Natural Regulatory T-cells (nTregs) are a naturally occuring population of T-cells which are produced in the thymus and leave the thymus with the function of regulating immune responses. They often respond preferentially to self antigens, and are very important for protection from auto-immune diseases.
• Adaptive/induced regulatory T-cells (aTregs) develop from naïve CD4 T-cells in the periphery. They develop (for example) by constant low-level exposure to antigens; an almost flipside or consequence of anergy. They are very important for protecting the organism from auto-immunity, and particularly, regulation of responses against non-self antigens, such as antigens from food or commensal bacteria.
• Individuals who can't make nTregs tend to develop a range of autoimmune disease, and this is known as a syndrome called IPEX. Individuals with a range of autoimmune diseases such as type 1 diabetes or multiple sclerosis tend to have impaired regulatory T-cell function.

  As a result, regulatory T-cells are targeted heavily in immunotherapy trials which aim to slow down the progression or prevent autoimmune diseases. On the flipside, if an anti-tumour response or a powerful vaccination response is to be induced, then regulatory T-cell function may need to be reduced instead.
• Alongside pathogen associated molecular patterns, there are also damage associated molecular patterns (DAMPs). DAMPs are endogenous activators that can be produced by dead, dying, or stressed cells that can also lead to the activation of APCs.
• After activation of the APCs by DAMPs, the dendritic cells migrate from the tissue to the local draining lymph node, and here they secretion a combination of cytokines and chemokines that lead to up-regulation of adhesion molecules on the high endothelial venules that line the arterioles going into the lymph node. This, alongside the chemokines that they secrete, lead to an increase in the migration of naïve T-cells into this lymph node.
• The signals that allow T-cells to migrate out of a lymph node are blocked, and alongside the signals that cause T-cells to enter the lymph node, there is a subsequent increase in the size and cellularity of the lymph node that are associated with inflammation.

  The purpose of bringing all the cells together to one place at one time s to allow the naïve T-cells to encounter peptide-MHC complexes presented on the surface of APCs.
• The cytokines produced by a particular APC that direct naïve CD4 T-cells into different differentiation pathways:

  • CD4-Th1 result from intracellular pathogens. The APC will secrete IL-12.
  • CD4-Th2 result from extracellular pathogens. The APC with antigens from such pathogens will secret IL-4.
  • CD4-Th17 result from extracellular bacteria. The APC with antigens from these sources will secrete TGF-beta and IL-6.
  • CD4-Treg result in immune tolerance. An APC that hasn't encountered any foreign antigens will end up producing cytokines such as TGF-beta, IL-2 and IL-10.
• In the lymph node, the differentiated T-cells will perform their effector function. For instance, a CD4-Th1 cell will release cytokines (IFN-gamma and IL-2) to allow the CD8 T-cell to differentiate into a cytotoxic T-cell, proliferate, and then gain effector function.

  Isolated in the lymph node, the differentiated T-cells don't just gain their effector function, but they also increase in number.
• Once the T-cells have finished proliferating and gaining effector function in the draining lymph nodes, they'll eventually pass into the peripheral circulation, Here, if they encounter inflamed tissue, they will cross over from the bloodstream into the tissue. If in the inflamed tissue the T-cells don't find any APCs bearing the peptide-MHC complexes they're specific for, they'll return back to the lymphatics.
• If the T-cells that have passed out of the lymph node, into the peripheral circulation, and then migrate into the inflamed tissue do encounter APCs presenting their corresponding peptide-MHC complex, then they begin their effector function. If the T-cells are CD4 T-cells, they will secrete cytokines that provide help. If the T-cells are CD8 T-cells, if they see any cells in the inflamed tissue that are presenting peptide-MHC 1 complexes, then they'll kill those cells.
• When the pathogen is eventually removed, inflammation is cleared and the immune effectors are removed. The apoptotic cells that remove the T-cells will be removed by macrophages. Some T-cells however, will migrate out of the inflamed tissue and will become the cells that maintain immunological memory.
• Immunological memory, and the cells that maintain memory, are retained at 2 sites: Secondary lymphoid organs, such as bone marrow, lymph node, and spleen, and the tissue themselves.
• The cells of immunological memory differ both in the frequency and functional capability they have.

  Plasma cells are retained, and they can secret high affinity antibodies and live for years.

  Memory B-cells are of a higher frequency than naïve B-cells, and they can rapidly develop into plasma cells to secrete more antibodies upon a secondary encounter with an infection.

  Memory T-cells are of a very high frequency once an infection has been encountered compared to before. They can undergo immediate effector function, and don't have to develop and proliferate in the lymph node again.