In immunology, central tolerance (also known as negative selection) is the process of eliminating any developing T or B lymphocytes that are autoreactive, i.e. reactive to the body itself. Through elimination of autoreactive lymphocytes, tolerance ensures that the immune system does not attack self peptides. Lymphocyte maturation (and central tolerance) occurs in primary lymphoid organs such as the bone marrow and the thymus. In mammals, B cells mature in the bone marrow and T cells mature in the thymus.
Central tolerance is not perfect, so peripheral tolerance exists as a secondary mechanism to ensure that T and B cells are not self-reactive once they leave primary lymphoid organs. Peripheral tolerance is distinct from central tolerance in that it occurs once developing immune cells exit primary lymphoid organs (the thymus and bone-marrow), prior to their export into the periphery.
Function Central tolerance is essential to proper immune cell functioning because it helps ensure that mature B cells and T cells do not recognize self-antigens as foreign microbes. More specifically, central tolerance is necessary because T cell receptors (TCRs) and B cell receptors (BCRs) are made by cells through random somatic rearrangement. This process, known as V(D)J recombination, is important because it increases the receptor diversity which increases the likelihood that B cells and T cells will have receptors for novel antigens. Junctional diversity occurs during recombination and serves to further increase the diversity of BCRs and TCRs. The production of random TCRs and BCRs is an important method of defense against microbes due to their high mutation rate. This process also plays an important role in promoting the survival of a species, because there will be a variety of receptor arrangements within a species – this enables a very high chance of at least one member of the species having receptors for a novel antigen.
While the process of somatic recombination is essential to a successful immune defense, it can lead to autoreactivity. For example, lack of functional RAG1/2, enzymes necessary for somatic recombination, has been linked to development of immune cytopenias in which antibodies are produced against the patient's blood cells. Due to the nature of a random receptor recombination, there will be some BCRs and TCRs produced that recognize self antigens as foreign. This is problematic, since these B and T cells would, if activated, mount an immune response against self if not killed or inactivated by central tolerance mechanisms. Therefore, without central tolerance, the immune system could attack self, which is not sustainable and could result in an autoimmune disorder.
Mechanism The result of central tolerance is a population of lymphocytes that do not mount immune response towards self-antigens. These cells use their TCR or BCR specificity to recognize foreign antigens, in order to play their specific roles in immune reaction against those antigens.
In this way, the mechanisms of central tolerance ensure that lymphocytes that would recognise self-antigens in a way that could endanger the host, are not released into the periphery.
It is of note that T cells, despite tolerance mechanisms, are at least to some extent self-reactive. TCR of conventional T cells must be able to recognize parts of major histocompatibility complex (MHC) molecules (MHC class I in case of CD8+ T cells or MHC class II in case of CD4+ T cells) to create proper interaction with antigen-presenting cell. Furthermore, TCRs of regulatory T cells (Treg cells) are directly reactive towards self-antigens (although their self-reactivity is not very strong) and use this autoreactivity to regulate immune reactions by suppressing immune system when it should not be active. Importantly, lymphocytes can only develop tolerance towards antigens that are present in the bone marrow (for B cells) and thymus (for T cells).
T cell T cell progenitors (also called thymocytes) are created in the bone marrow and then migrate to the thymus where they continue their development. During this development, the thymocytes perform the V(D)J recombination and some of the developing T cell clones produce TCR that is completely unfunctional (unable to bind peptide-MHC complexes) and some produce TCR that is self-reactive and could therefore promote autoimmunity. These "problematic" clones are therefore removed from the pool of T cells by specific mechanisms.
First, during "positive selection" the thymocytes are tested, whether their TCR works properly and those with unfunctional TCR are removed by apoptosis. The mechanism has its name because it selects for survival only those thymocytes whose TCRs do interact with peptide-MHC complexes on antigen presenting cells in the thymus.
During the late stage of positive selection, another process called "MHC restriction" (or lineage commitment) takes place. In this process the thymocytes whose TCR recognize MHCI (MHC class I) molecules become CD4- CD8+ and thymocytes whose TCR recognize MHCII (MHC class II) become CD4+ CD8-.
Subsequently, the positively selected thymocytes go through "negative selection" which tests the thymocytes for self-reactivity. The cells that are strongly self-reactive (and therefore prone to attacking the host cells) are removed by apoptosis. Thymocytes that are still self-reactive, but only slightly develop into T regulatory (Treg) cells. Thymocytes that are not self-reactive become mature naïve T cells. Both the Treg and mature naïve T cells subsequently migrate to the secondary lymphoid organs. The negative selection has its name because it selects for survival only those thymocytes whose TCRs do not interact (or interact only slightly) with peptide-MHC complexes on antigen presenting cells in the thymus.
Two other terms - recessive and dominant tolerance are also important regarding the T cell central tolerance. Both the terms refer to two possible ways of tolerance establishment towards particular antigen (typically self antigen). The "recessive tolerance" means that the antigen is tolerated via deletion of those T cells that would facilitate immune response against the antigen (deletion of autoreactive cells in negative selection). The "dominant tolerance" means that the T cell clones specific for the antigen are deviated into Treg cells and therefore suppress the immune response against the antigen (Treg selection during the negative selection).
Steps of T cell tolerance Development of T cell progenitors T cell precursors originate from bone marrow (BM). Population of the earliest hematopoietic progenitors do not bear markers of differentiated cells (for that they are called Lin- "lineage negative") but express molecules such as SCA1 (stem cell antigen) and KIT (receptor for stem cell factor SCF). Based on these markers the cells are called LSKs (Lineage-SCA1-KIT). This population can be further divided, based on expression of markers such as CD150 and FMS-related tyrosine kinase 3 (FLT3), into CD150+ FLT3-hematopoietic stem cells (HSCs) and CD150- FLT3low multipotent progenitors (MPPs). The HSCs are "true hematopoietic stem cells" because they have the ability of self-renewal (generating new HSCs) and also have the potential to differentiate into all blood cell types. The direct descendants of HSCs are the more mature multipotent progenitors (MPPs) that highly proliferate, can differentiate into all blood cell types but are not capable of self-renewal (do not have the ability to indefinitely generate new MPPs and therefore HSCs are needed for generation of new MPPs). Some of the MPPs further upregulate expression of FLT3 (becoming CD150- FLT3high) and start to upregulate genes specific for lymphoid lineage (for example Rag1) (but remain Lin-). These progenitors (still belong to the LSK cells) consist of two similar populations termed lymphoid-primed MPPs (LMPPs) and early lymphoid progenitors (ELPs). The LMPPs/ELPs subsequently give rise to common lymphoid progenitors (CLPs). These cells (FLT3high LIN- KITlow) do not belong to LSK pool, are more mature and more prone towards the lymphoid lineage, meaning that under normal circumstances they will ultimately give rise to T or B cells or other lymphocytes (NK cells). But since they are only progenitors, their cell fate is not strictly predetermined and they still have the ability to differentiate into other lineages. Migration into the thymus Progenitors from bone marrow (BM), even the HSCs, have the ability to randomly exit the BM to the bloodstream and thus can be readily detected there. Therefore, after being generated, the T cell progenitors exit the BM and are randomly carried by blood throughout the body. At the moment they reach postcapillary venules in the thymic cortico-medullary junction, they start slowing down and rolling on the endothelium, because all the progenitors, including LSK cells, express on their surface glycoprotein PSGL1, which is a ligand for P-selectin, expressed on the thymic endothelium. But out of all the aforementioned T cell progenitors, only the LMPPs/ELPs and CLPs express chemokine receptors CCR7 and CCR9 that enable them to enter the thymus. The thymic endothelium express chemokines CCL19 and CCL21, which are ligands for CCR7 and CCL25 which is a ligand for CCR9. The final part of thymic entry is not yet fully understood. Suggested model is that receptor sensing of chemokines by the progenitors activates their integrins (suggested integrins are VLA-4 and LFA-1) which engage with ligands on the endothelium. This interaction stops the rolling, leads to cellular arrest and finally to transmigration along the chemokine gradient inside thymus. Therefore, all the progenitors will be rolling on the thymic endothelium, but only the LMPPs/ELPs and CLPs will enter the thymus because only they have the proper receptor equipment to do so. The mechanism is highly similar to the transmigration, which is used by leukocytes to enter lymph nodes or inflamed tissues. Early thymic development From the moment LMPPs/ETPs and CLPs enter the thymus in the corticomedullary junction, they are referred to as thymus settling progenitors (TSPs). The TSPs highly proliferate and start to migrate to the subcapsullar zone of the thymus. It is not celar what signals drive the migration. One possibility is that they migrate along chemokine gradients, using CXCR4, CCR7 and CCR9 receptors but the migration can be also driven only by interactions of integrins and other cells and ECM (extra-cellular matrix) without direct involvement of chemokines. As they migrate towards the subcapsular zone, the TSPs further continue in their differentiation, which is driven mainly by the thymic microenvironment. Out of many signals the TSPs and other subsequent precursors receive from the microenvironment, the Notch signalling is especially important to drive their differentiation fate. The precursors express Notch1 receptor which is activated by ligands present in the thymic tissue. The subsequent activation of Notch pathway leads to gradual loss of the progenitors capability to generate other cell lineages and they ultimately become only capable to create T cells but this comes at the later stages of the differentiation. At the stage of TSPs, the progenitors still retain the capacity to create both lymphoid and myeloid cells. Given their capability to generate other cell lineages (mainly in vitro) it is even debated that they can physiologically, at least partially contribute to generation of other cell types, present in the thymus, mainly plasmacytoid dendritic cells (pDCs). But this has not yet been clearly proven.