Premed · Premed · Immunology
Lecture 22: Tolerance and Autoimmunity I: Mechanisms
Immunology
Learning Objectives
By the end of this lecture, students will be able to:
- Define immunological tolerance and distinguish central from peripheral tolerance
- Describe the mechanisms of T cell central tolerance in the thymus (positive and negative selection, AIRE)
- Describe the mechanisms of peripheral tolerance (anergy, regulatory T cells, deletion, ignorance)
- Explain B cell tolerance mechanisms (central and peripheral)
- Describe how breakdown of tolerance leads to autoimmunity and identify predisposing factors
Lecture Content
I. Overview of Immunological Tolerance
Tolerance is the specific unresponsiveness of the immune system to an antigen, most critically to self-antigens. Tolerance is not a passive state -- it is an active process through which the immune system learns to refrain from attacking self. Because the adaptive immune system randomly generates antigen receptors through V(D)J recombination, many of the resulting TCRs and BCRs inevitably recognize self-antigens. Tolerance mechanisms exist to eliminate or control these self-reactive lymphocytes. These mechanisms are divided into two categories: central tolerance, which operates during lymphocyte development in primary lymphoid organs (the thymus for T cells and the bone marrow for B cells), and peripheral tolerance, which acts on mature lymphocytes that have escaped central tolerance and entered the periphery. Autoimmunity results when one or more tolerance mechanisms fail, allowing the immune system to mount an attack on self-tissues.
II. T Cell Central Tolerance
T cell central tolerance occurs in the thymus during T cell development (reviewed in detail in Lecture 14). Positive selection takes place in the cortex, where double-positive (CD4+CD8+) thymocytes must recognize self-MHC on cortical thymic epithelial cells (cTECs). Cells that fail to recognize self-MHC die by neglect, ensuring that all surviving T cells are MHC-restricted.
Negative selection occurs at the corticomedullary junction and in the medulla. Thymocytes that bind self-peptide-MHC with high affinity are deleted through apoptosis, mediated by medullary thymic epithelial cells (mTECs) and thymic DCs. The transcription factor AIRE (autoimmune regulator), expressed by mTECs, drives ectopic expression of thousands of tissue-restricted antigens (TRAs) in the thymus -- proteins such as insulin, thyroglobulin, and myelin proteins that are normally found only in specific peripheral tissues. This allows the thymus to screen developing T cells against a broad catalog of self-antigens. Mutations in AIRE cause autoimmune polyendocrinopathy-candidiasis-ectodermal dystrophy (APECED/APS-1), a multi-organ autoimmune disease resulting from the failure to delete TRA-reactive T cells. A second transcription factor, Fezf2, controls expression of an additional set of TRAs independently of AIRE.
Some thymocytes with intermediate self-reactivity -- stronger than positive selection but weaker than the deletion threshold -- are diverted to become thymic regulatory T cells (tTregs). These CD4+CD25+FOXP3+ natural Tregs exit the thymus and serve as critical mediators of peripheral tolerance.
<image>A diagram of T cell central tolerance in the thymus. The thymic cortex is shown at the top with double-positive (CD4+CD8+) thymocytes interacting with cortical thymic epithelial cells (cTECs). Positive selection: thymocytes with TCRs that recognize self-MHC with moderate affinity survive (green checkmark), while those that fail to recognize MHC die by neglect (gray cells undergoing apoptosis). Surviving cells migrate to the medulla. In the medulla, medullary thymic epithelial cells (mTECs) express AIRE, which drives expression of tissue-restricted antigens (shown as a list: insulin, thyroglobulin, myelin basic protein, etc.). Negative selection: thymocytes with high-affinity TCRs for self-peptide-MHC are deleted (red X, apoptosis). Thymocytes with intermediate self-reactivity are shown being diverted into the FOXP3+ Treg lineage (purple cells). The final output at the bottom shows mature single-positive T cells (CD4+ or CD8+) that are self-MHC restricted but not strongly self-reactive, plus tTregs, exiting to the periphery.</image>
III. Peripheral T Cell Tolerance
Because not all self-antigens are represented in the thymus, some self-reactive T cells inevitably escape to the periphery. Multiple redundant mechanisms maintain peripheral tolerance.
Anergy occurs when a T cell receives Signal 1 (TCR-peptide-MHC engagement) without adequate Signal 2 (co-stimulation). This situation arises when self-antigens are presented by non-professional APCs, such as tissue cells, that lack B7 (CD80/CD86). The T cell survives but cannot produce IL-2 or proliferate upon restimulation. Molecularly, anergy involves upregulation of E3 ubiquitin ligases (GRAIL, Cbl-b, and Itch) that degrade key signaling molecules, along with epigenetic silencing of the IL-2 locus. CTLA-4 promotes anergy by competing with CD28 for B7 binding with higher affinity, delivering inhibitory signals, and stripping B7 from APC surfaces through trans-endocytosis.
Regulatory T cells (Tregs) constitute the most important mechanism of active peripheral tolerance. CD4+CD25+FOXP3+ Tregs arise from two sources: thymic Tregs (tTregs/nTregs), generated in the thymus, and peripheral Tregs (pTregs/iTregs), induced from naive CD4+ T cells by TGF-beta and IL-2, especially at mucosal surfaces. Tregs suppress through multiple mechanisms: secretion of inhibitory cytokines (IL-10, TGF-beta, IL-35), granzyme B-mediated killing of effector T cells, metabolic disruption through consumption of IL-2 via high-affinity CD25, modulation of DCs by CTLA-4-mediated stripping of B7 and induction of IDO, and adenosine production via CD39/CD73. The essential role of Tregs is demonstrated by FOXP3 mutations, which cause IPEX syndrome -- severe multi-organ autoimmunity due to Treg absence or dysfunction.
Peripheral deletion (activation-induced cell death, AICD) eliminates chronically activated self-reactive T cells. Repeated stimulation upregulates FasL, leading to Fas-FasL-mediated apoptosis. Bim-mediated intrinsic apoptosis also occurs after cytokine withdrawal.
Ignorance describes the situation in which self-reactive T cells exist in the periphery but never encounter their cognate self-antigen in an immunogenic context -- because the antigen is sequestered (such as eye lens proteins or intracellular antigens), present at too low a concentration, or located in immune-privileged sites. Ignorance is not a true tolerance mechanism, as it can be broken if antigen is released or presented in the context of danger signals.
IV. B Cell Tolerance
Central tolerance in B cells operates in the bone marrow. Immature B cells that strongly bind self-antigens undergo receptor editing (re-expression of RAG genes to rearrange a new light chain, changing specificity -- the primary mechanism), clonal deletion (apoptosis if editing fails), or anergy (functional unresponsiveness with reduced surface IgM for low-avidity self-antigen interactions).
Peripheral tolerance in B cells relies on several mechanisms. Self-reactive B cells may be excluded from lymphoid follicles, losing access to BAFF survival signals and dying by neglect. Chronic exposure to soluble self-antigen induces anergy. Critically, self-reactive B cells depend on T cell help to mount effective responses, and since self-reactive T cells are largely deleted or suppressed, this T cell help dependence serves as a crucial checkpoint. The inhibitory receptor FcgammaRIIB further dampens BCR signaling when co-engaged with immune complexes.
V. Breakdown of Tolerance: Pathways to Autoimmunity
Autoimmunity results from the failure of one or more tolerance mechanisms, driven by a combination of predisposing factors. Genetic susceptibility is paramount, with HLA associations representing the strongest genetic risk factors: HLA-B27 with ankylosing spondylitis, HLA-DR4 with rheumatoid arthritis, and HLA-DR3/DR4 with type 1 diabetes. Polymorphisms in tolerance genes (CTLA-4, PTPN22, IL-2R-alpha) modify risk, and monogenic defects can cause severe autoimmune syndromes: AIRE mutations (APECED), FOXP3 mutations (IPEX), FAS mutations (ALPS), and complement deficiencies (C1q/C2/C4 predisposing to SLE).
Environmental triggers initiate autoimmunity in genetically susceptible individuals. Molecular mimicry occurs when microbial antigens share structural similarity with self-antigens, leading infection-induced immune responses to cross-react with self -- as in Streptococcus-triggered rheumatic fever and Campylobacter-associated Guillain-Barre syndrome. Bystander activation occurs when inflammation from infection nonspecifically activates self-reactive lymphocytes. Epitope spreading describes the phenomenon in which tissue damage from an initial immune response releases new self-antigens, expanding the autoimmune response to additional self-epitopes. Superantigens can polyclonally activate T cells, potentially activating self-reactive clones.
Additional factors include dysbiosis (altered gut microbiome composition linked to IBD, T1D, and MS) and sex, with autoimmune diseases being more common in females due to hormonal influences and X-chromosome gene dosage effects.
<image>A comprehensive diagram showing the mechanisms of peripheral T cell tolerance and how their failure leads to autoimmunity. Four panels arranged in a grid. Top-left (Anergy): A T cell recognizes self-antigen on a tissue cell lacking B7 co-stimulatory molecules. Signal 1 without Signal 2 leads to anergy. The anergic T cell is shown with upregulated CTLA-4 and ubiquitin ligases (GRAIL, Cbl-b). Below, a "failure" scenario shows an APC aberrantly expressing B7 (due to inflammation) activating the self-reactive T cell. Top-right (Tregs): A Treg cell (FOXP3+) is shown suppressing an autoreactive effector T cell via IL-10, TGF-beta, IL-2 consumption, and CTLA-4-mediated B7 stripping from a DC. Below, "failure" shows FOXP3 mutation (IPEX) with absent Treg function and unchecked effector T cell activity attacking a target organ. Bottom-left (Deletion/AICD): Repeated self-antigen stimulation leads to Fas-FasL-mediated apoptosis. "Failure" shows Fas mutation (ALPS) with lymphoproliferation. Bottom-right (Ignorance): A sequestered antigen (e.g., eye lens protein) behind a barrier is shown. "Failure" shows tissue damage releasing the antigen, which is then presented by an APC with danger signals, activating the previously ignorant T cell → autoimmune attack on the eye.</image>
VI. Immune-Privileged Sites
Certain anatomical sites have reduced immune surveillance to protect delicate tissues from inflammatory damage. The eye (anterior chamber) maintains privilege through TGF-beta, alpha-MSH, FasL expression on corneal cells, and lack of lymphatic drainage, a phenomenon known as anterior chamber-associated immune deviation (ACAID). The brain is protected by the blood-brain barrier, which limits lymphocyte entry, along with microglial maintenance of tolerance and low MHC expression. The testis is shielded by Sertoli cells expressing FasL and the blood-testis barrier. The placenta and fetus are protected by trophoblast expression of HLA-G (a non-classical MHC molecule that inhibits NK cells), FasL, the enzyme IDO, and the accumulation of Tregs at the maternal-fetal interface. Immune privilege can be broken by trauma or infection, releasing sequestered antigens and triggering autoimmune responses -- as exemplified by sympathetic ophthalmia, in which trauma to one eye can precipitate an autoimmune attack on both eyes.
