Tolerance and immunoregulation

How does innate and adaptive immunity work?

Innate immunity is encoded in the germline and recognise via PRRs which have evolved over time, whereas adaptive immunity is somatically generated and recognise via BCRs and TCRs. Any individual lymphocyte is clonal and makes only 1 receptor. Adaptive immunity works by producing so many receptors that they can recognise almost anything, generating broad immunity but also the potential for autoimmunity.

Why is self-reactivity important?

Self reactivity is a normal component of a healthy immune system but is restrained by mechanisms of tolerance. Broadly speaking, tolerance mechanisms include limiting the production of self-reactive T and B cells during their development and preventing unwanted destructive response by any clones produced that enter the periphery.

What is tolerance?

Tolerance is antigen specific. Self tolerance is the failure to respond to intrinsic self antigens. Tolerance can also develop against external antigens like harmless self antigens (fetomaternal Ags etc) and therapeutically relevant antigens (transplantation etc).

How was tolerance discovered?

Self tolerance is acquired, not inherited. A study showed that non-identical twin claves sharing a common placenta become tolerant of each other by showing there was chimerism of the blood systems. Later, tolerance was suggested to be induced to Ags that are shared as a foetus, and that the cows can tolerate skin grafts from each other. On the basis of this, a study showed that donor cells delivered to a foetus mouse in utero allowed it to accept skin grafts from the same donor strain as an adult - acquired tolerance to donor Ags exposed in utero. 

Around the same time, clonal selection theory was developing - mature lymphocytes with specific receptor to Ag are selected to proliferate. It was suggested that immune tolerance is generated through clonal deletion - immature cells that can recognise Ag are deleted. This is not the only case as it would generate blind spots, so self tolerance must remove dangerous self-reactivity without impairing the capacity for broad recognition and effective defence. 

What are the 2 types of tolerance?

Tolerance is established as a series of checkpoints. Central tolerance occurs in the primary lymphoid organs (thymus, bone marrow) during lymphocyte development and is the removal of highly self reactive clones via deletion, editing, and anergy. This removes the most dangerous but is not absolute so some self reactive cells will enter the periphery. Peripheral tolerance occurs in organs, tissues and secondary lymphoid organs (spleen, lymph nodes) and consists of multiple mechanisms to limit reactivity against self and harmless Ags.

What is central tolerance of B cells?

Central tolerance of B cells occurs during development in the bone marrow. B cell development occurs in steps in which there are 2 checkpoints. At the large pre-B Cell stage where the VDJ is arranged and the μ chain is transiently at the surface as part of the receptor (mainly intracellular), there is a quality checkpoint. It is Ag independent, and detects whether the heavy chain can fold correctly to join to the not yet formed light chain and make it to the surface. If it does, it signals back to induce proliferation and inactivation of RAG genes (?). The second checkpoint is the tolerance checkpoint and occurs in the next 2 stages (small pre-B cell and Immature B cell). These stages are VJ rearrangement, and in the second stage IgM gets expressed on the cell surface. It detects whether the heavy and light chain can join to make a functional receptor. 

What are the fates of B cells during central tolerance?

The B cells fate at this point is determined by whether it can interact with self antigen in the bone marrow. The strength of interaction is influenced by concentration of Ag and its ability to cross link surface receptors. 

• If there is no self reaction, the B cell migrates to the periphery and now expresses IgD in addition to IgM.

• If there are multivalent cross reactions with self antigen, this suggests danger. The cells are removed by clonal deletion (apoptosis) or their receptors are edited (non-harmful B cell). Receptor editing is when RAG genes stay on to induce further recombination at the light chain locus. This can rescue the cell, or if it does not work, clonal deletion occurs.

• If there is binding to soluble self molecule, there is not enough signal to cause self deletion so it migrates to the periphery. There, it is induced to become anergic (low surface IgM, normal IgD - unresponsive) so are unable to compete for survival signals so die rapidly.

• If there is low affinity, non-cross-linking binding to self molecules, it migrates to the periphery where they are ignorant mature B cells (potential to bind -not strong enough reaction or not enough Ag).

What is central tolerance of T cells?

Central tolerance of T cells occurs in the thymus during development. The common lymphoid progenitor moves through the blood to enter the thymus where abt 95% become alpha/beta T cells. There are several stages of development; early thymocytes do not express CD4 or CD8 (therefore denoted double negative). As they pass along, they start to express both receptors so they are double positive. Then, they undergo a lineage commitment phase where they express one or the other (single positive). The DN phase is split into 4.

What are the fates of T cells during central tolerance?

During T cell development, TCR gene rearrangement can give rise to the following scenarios;

• T cells that fail to make surface expressed TCR - useless.

• TCR that cannot recognise self-MHC molecules - useless.

• TCR that recognises self-MHC:self-antigens too strongly - harmful. Checkpoints remove these 3 T cell clones.

• TCR that recognises self-MHC and does not bind to self-MHC:self-antigens too strongly - useful. Only about 2% of the cells that make a structural TCR make it through thymic selection to enter the periphery.

What are the checkpoints and locations of T cell central tolerance?

The first checkpoint occurs in DN3 and is an Ag independent quality checkpoint that checks whether beta VDJ arrangement is sufficient to bind to surrogate alpha chain. Failure to do this leads to reduced survival signalling, leading to apoptosis.

The different development stages occur in different regions of the thymus which facilitates different selection criteria at different checkpoints. DN1 occurs in the medulla, DN2-4 occurs outwards in the cortex. After DN3, the cells start to migrate towards the medulla again. The second checkpoint occurs closer to the coritco-medullary junction with immature DP thymocytes and cortical epithelial cells. This is a positive selection. The ones that survive enter the medulla where there is a negative selection checkpoint that checks interactions between mature SP thymocytes and medullary epithelial cells and dendritic cells.

How does positive and negative selection occur during T cell development?

Positive selection detects whether the rearranged TCR interacts with self-peptide:MHC on the cortical epithelial cells. If they can, they receive a survival signal and move to the medulla as SP thymocytes. If they cannot, then there is receptor editing of the alpha genes to allow them to enter the medulla or if they cannot be recovered then die by apoptosis (3-4 days). 

Negative selection detects whether the TCR recognise the self-MHC:self-peptide too strongly. In the medullary region, the cells speed up to scan as fast as they can. Those that bind too strongly are deleted. Medullary epithelial cells express transcription regulators: autoimmune regulator (AIRE) and Fezf2 which allow for promiscuous expression of genes not normally expressed in the thymus.

Diagram of T cell affinity-based hypothesis.

How does peripheral tolerance of B cells occur?

T cell responses are determined by integration of 3 signals. This is also relevant to B cells since they need Th cells to mount an Ab response. Peripheral tolerance in B cells is mainly due to a lack of T cell help. For instance, a B cell will uptake self Ag and present it on its surface but there will be no T cell for the self Ag so no B cell activation. 

How does peripheral tolerance of T cells occur?

Signalling from dendritic cells determines T cell activation but can also direct inactivation and tolerance. Ag must be presented on DC in an activation context (costimulation) to initiate naive T cell activation. Presentation in the absence of danger (no costimulation) is tolerogenic. Signal 1 alone renders T cell anergic - unresponsive. There can also be regulation through Ag with an immunosuppressive signal 3 (cytokines) which drives T cells to become peripheral-derived Tregs (Fox3p + or -). Thymic-derived Treg (Foxp3+) recognise self Ag, pTreg recognise self Ag and harmless non-self Ag.

Tregs regulate through the production of antiinflammatory cytokines, modulation of DCs, outcompete effector T cells for resources (IL2), direct killing of T cells and exosome transfer of miRNAs.

How does the immune system differ between tissues?

Not all tissues are immunologically equivalent. For example, the immune system in the gut is tuned to be more tolerogenic since the gut is exposed to many non-harmful Ags like gut microbiota and food Ags. How an Ag is encountered determines the response. For example, oral tolerance (eating) exposes the Ag firstly to the more tolerant environment of the gut which can train the immune system to avoid more harmful reactions to the Ag. For example, American Indians ingested small amounts of poison ivy leaves to avoid allergic skin reactions following contact.

What is immune privilege?

Some regions of the body have evolved to protect vital organs like the brain, eye etc. These are known as immune privilege sites and Ag in these areas do not induce immune attack - grafts in these sites are not rejected. Generally, these sites are enclosed by a physical barrier (limited lymphatic drainage; selective entry), low MHC-I, rich in suppressive cytokines and express FasL (apoptose T cells if response). However, they can inadvertently become targets for autoimmune disorders. For example, sympathetic ophthalmia is when there is injury to the eye causing Ag to drain into the lymph nodes and activation of T cells - effector T cells return via the bloodstream to attack both eyes.

How do tumours avoid immune recognition?

Recruitment of immune suppression can allow tumours to attain immune privilege.