1. Lecture outline Self-tolerance: concept, significance
Mechanisms of central and peripheral tolerance: deletion, anergy, regulatory T cells
Pathogenesis of autoimmunity: roles of susceptibility genes and environmental factors

2. The immunological equilibrium: balancing lymphocyte activation and control
Effector and memory T cells
Tolerance
Regulatory T cells
Normal: reactions against pathogens
Pathologic: inflammatory
disease, e.g. caused by reactions against self
No response to self
Controlled response to pathogens

3. The problem of self-nonself discrimination
The immune system responds to many foreign (microbial) antigens but not to self antigens
Developing lymphocytes express a large number of antigen receptors, not biased by specificity
Therefore, all individuals produce lymphocytes with the ability to recognize self antigens
Self antigens have access to the immune system
Therefore, self-reactive lymphocytes must be selected against (eliminated or inactivated) to prevent autoimmunity

4. Immunological tolerance
Definition:
specific unresponsiveness to an antigen that is induced by exposure of lymphocytes to that antigen (implies antigen specificity, in contrast to “non-specific immunosuppression”)
Significance:
All individuals are tolerant of their own antigens (self-tolerance); breakdown of self-tolerance results in autoimmunity
Therapeutic potential: Inducing tolerance may be exploited to prevent graft rejection, treat autoimmune and allergic diseases, and prevent immune responses in gene therapy, perhaps stem cell transplantation

5. Central and peripheral tolerance
The principal fate
of lymphocytes that
recognize self antigens
in the generative organs
is death (deletion), BUT:
Some B cells may change
their specificity (called
“receptor editing”)
Some T cells may
differentiate into
regulatory (suppressor)
T lymphocytes

6. Mechanisms of unresponsiveness to self antigens
Central tolerance: Immature self-reactive lymphocytes that recognize self antigens in generative (“central”) lymphoid organs die by apoptosis; other fates
Peripheral tolerance: Mature self-reactive lymphocytes that recognize self antigens in peripheral tissues are inactivated (anergy), killed (deletion) or suppressed
“Clonal ignorance”: Mature self-reactive lymphocyte clones do not encounter or respond to self antigens
In normal individuals it is not known which self antigens induce tolerance by which mechanism

7. Central T cell tolerance

8. What self antigens are seen in the thymus?
Ubiquitous cell-associated and circulating proteins
The thymus has a special mechanism for displaying peripheral tissue antigens in thymic medullary epithelial cells, where they signal self-reactive thymocytes for death

9. Consequences of AIRE mutation
Human disease: autoimmune polyendocrinopathy with candidiasis and ectodermal dysplasia (APECED), also called autoimmune polyendocrine syndrome (APS-1)
Associated gene identified by positional cloning, named AIRE (“autoimmune regulator”)
Mouse knockout: autoantibodies against multiple endocrine organs, retina
Failure to express many self antigens in the thymus --> failure of negative selection

10. Deletion of self-reactive T cells in the thymus:
how are self antigens expressed in the thymus?
AIRE (autoimmune regulator) is a regulator of gene transcription
that stimulates thymic expression of many self antigens which are largely restricted to peripheral tissues
Discovered as the genetic cause of a human autoimmune disease (APS-1)

11. Central tolerance: fates of immature self-reactive lymphocytes
Induced by antigen in generative lymphoid organs (thymus for T cells, bone marrow for B cells), and high-affinity (“strong”) recognition of the antigens
Immature lymphocytes undergo apoptosis upon encounter with antigens (negative selection)
Eliminates high-affinity self-reactive (potentially most dangerous) lymphocytes
Some self-reactive T cells that encounter self antigens in the thymus develop into regulatory T cells and immature B cells in the bone marrow change their receptors (rendered harmless)

12. Peripheral tolerance

13. Peripheral tolerance Costimulation (signal 2)
Immunogenic antigen (microbe, vaccine)
APC
Naïve
T cell
TCR
Antigen (peptide + HLA): signal 1
Effector and memory cells
Tolerogenic antigen (e.g. self)
Tolerance: functional inactivation or cell death, or sensitive to suppression

14. T cell anergy

15. T cell anergy (“clonal anergy”)
Induced by self antigens that are displayed to the immune system without inflammation or innate immune responses (prolonged signal 1, i.e. antigen, inadequate signal 2)
Recognition of such antigens may lead to signaling block and/or engagement of inhibitory receptors
Role of anergy in self-tolerance in humans is unclear; therapeutic potential?

16. Regulatory T cells

17. Regulatory T cells
Regulatory T cells are CD4+ cells that express high levels of CD25 (IL-2 receptor a chain)
Generated by self antigen recognition in the thymus or peripheral tissues
Generation requires a transcription factor called Foxp3 (mutations in Foxp3 are the cause of a severe autoimmune disease in humans and mice)

18. Mechanism of action: may be multiple
Regulatory T cells
Regulatory T cells are CD4+ cells that express high levels of CD25 (IL-2 receptor a chain)
Mechanism of action: may be multiple
Secretion of immune-suppressive cytokines
CTLA-4 on Tregs blocks B7 on APCs
Significance for self-tolerance:
Some autoimmune diseases may be associated with defects in regulatory T cells or resistance of responding cells to suppression
Therapeutic potential of cellular therapy (autoimmune diseases, graft rejection, etc)

19. “Activation-induced cell death”: death of mature
T cells upon recognition of self antigens

20. Deletion (activation-induced cell death)
Stimulation of T cells by self antigen triggers apoptosis by engagement of death receptors (“death receptor pathway”) or imbalanced expression of pro-apoptotic proteins (“mitochondrial pathway”)
Evidence for the importance of AICD in maintenance of self-tolerance:
Mice with mutations in Fas or Fas ligand develop a lupus-like autoimmune disease
Humans with mutations in Fas or enzymes involved in death receptor-induced apoptosis (caspases): the autoimmune lymphoproliferative syndrome (ALPS)
Eliminating both death pathways in mice --> “spontaneous” systemic autoimmune disease

21. Tolerance in B lymphocytes
Central tolerance:
Deletion of immature cells by high-affinity antigen recognition in the bone marrow
Some immature cells may change their antigen receptors when they encounter antigens in the bone marrow (“receptor editing”)
Peripheral tolerance:
Anergy
Exclusion from lymphoid follicles, death because of loss of survival signals
Engagement of inhibitory receptors

22. Autoimmunity
Definition: immune response against self (auto-) antigen, by implication pathologic
General principles:
Pathogenesis: The development of autoimmunity reflects a combination of susceptibility genes and environmental triggers (usually infections)
Different autoimmune diseases may be systemic or organ-specific; may be caused by different types of immune reactions (antibody- or T cell-mediated)

23. Autoimmunity
Definition: immune response against self (auto-) antigen
General principles:
Pathogenesis: The development of autoimmunity reflects a combination of susceptibility genes and environmental triggers (usually infections)
Different autoimmune diseases may be systemic or organ-specific; may be caused by different types of immune reactions
Challenges in understanding pathogenesis of human autoimmune diseases:
Failure to identify target antigens, heterogeneous disease manifestations, disease may present long after initiation
Recent advances: identifying self antigens (MS, type 1 diabetes); genetic analyses; improved methods for studying immune system of patients

24. Pathogenesis of autoimmunity

25. Genetic basis of autoimmunity -- 1
Genetic predisposition of autoimmune diseases
Increased incidence in twins (more in monozygotic than in dizygotic twins)
Identification of disease-associated genes by breeding and genomic approaches
Multiple genes are associated with autoimmunity
Most human autoimmune diseases are multigenic
Single gene mutations and mouse knockouts reveal critical pathways

26. Genetic basis of autoimmunity -- 2
MHC genes
Major genetic association with autoimmune diseases (relative risk of disease in individuals with particular HLA haplotypes)
Disease-associated alleles are present in normal individuals
Non-MHC genes:
Many gene variants identified by genome-wide association and linkage studies
Many of these variant genes individually have small effects on disease susceptibility, may influence disease when present in combination
Interactions with environmental factors difficult to define

27. Genetic basis of autoimmunity -- 3
Genome wide association studies are revealing genetic polymorphisms associated with autoimmune diseases
Crohn’s disease:
NOD-2: microbial sensor in intestinal epithelial and other cells
IL-23 receptor: involved in TH17 responses
Rheumatoid arthritis, others:
PTPN-22 (tyrosine phosphatase): may control kinase-dependent lymphocyte activation
Multiple sclerosis, others:
CD25 (IL-2 receptor): role in T cell activation and maintenance of regulatory T cells

28. Genetics of autoimmunity: challenges
Difficult to relate complex genotypes to phenotypic and functional abnormalities, to better understand pathogenesis
Limitations of GWAS: detects frequency of common variants, misses rare mutations
Identified disease-associated polymorphisms have small effects, therefore little predictive value
Because of small effects of any one gene, targeting these genes therapeutically is unlikely to have significant benefit

29. Infections predispose to autoimmunity
Genes encoding antigen receptor
specific for a myelin antigen
Transgenic mouse with
myelin-specific T cells
Normal mouse colony
Pathogen-free mouse colony
CNS disease
No disease

30. Infections and autoimmunity
Infections trigger autoimmune reactions
Clinical prodromes, animal models
Autoimmunity may develop after infection is eradicated (i.e. the autoimmune disease is precipitated by infection but is not directly caused by the infection)
Some autoimmune diseases are prevented by certain infections
Type 1 diabetes, multiple sclerosis, others? -- increasing incidence in developed countries
Mechanism unknown; the “hygiene hypothesis”

31. Mechanisms by which infections may promote autoimmunity

32. Other environmental influences
Hormones
Gender bias of autoimmune diseases
Mechanisms still not defined
UV exposure
SLE

33. Understanding autoimmunity
Experimental models have been very valuable for defining pathways of normal and abnormal immune responses, BUT they may have limited value for understanding human diseases
Need technologies for studying patients
Emphasis should be on antigen-specific immune responses