Lecture outline Self-tolerance: concept, significance Mechanisms of central and peripheral tolerance: deletion, anergy, regulatory T cells Pathogenesis.

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

ActivationEffector T cells

Normal: reactions against pathogens

Pathologic: inflammatory

disease, e.g. caused by reactions against self

ToleranceRegulatory T cells

No response to selfControlled response

to pathogens

The immunological equilibrium: balancing lymphocyte activation

and control

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

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

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

Central and peripheral tolerance

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

Central T cell tolerance

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

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)

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)

APC TCR

T cellCD28

ActivatedT cells

APC TCR

Functionalunresponsiveness

Normal T cellresponse

Anergy

Apoptosis(activation-induced

cell death)APC

Deletion

APC

Block in

activationSuppression

RegulatoryT cell

Peripheral tolerance

Off signals

ActivatedT cell

APCTCR

NaïveT cell

Immunogenic antigen

(microbe, vaccine)

Tolerogenic antigen

(e.g. self)

Effector and memory cells

Tolerance: functional inactivation or cell

death, or sensitive to suppression

Antigen (peptide +

HLA): signal 1

Costimulation (signal 2)

Peripheral tolerance 12

T cell anergy

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?

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

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

Regulatory T cells

Abbas, Lichtman and Pillai. Cellular and Molecular Immunology, 7th edition, 2011 c Elsevier

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)

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)

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

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)

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

Pathogenesis of organ-specific autoimmunity

Abbas, Lichtman and Pillai. Cellular and Molecular Immunology, 7th edition, 2011 c Elsevier

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

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 loci identified by genome-wide

association and linkage studies– Most are chromosomal locations; actual

genes and roles in disease are largely unknown

– Recent discoveries: NOD-2, PTPN-22, CD25

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

Genetics of autoimmunity: challenges

• Difficult to relate complex genotypes to phenotypic and functional abnormalities, to better understand pathogenesis

• Limitations of GWAS: 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

Genes encoding antigen receptor specific for a myelin antigen

Transgenic mouse withmyelin-specific T cells

Normal mouse colony

CNS disease

Pathogen-free mouse colony

No disease

Infections predispose to autoimmunity

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 infections (type 1 diabetes, multiple sclerosis, others? -- increasing incidence in developed countries): mechanism unknown; similar protection suggested for asthma• The “hygiene hypothesis”

Mechanisms by which infections may promote autoimmunity

Pathogenesis of autoimmunity

Immune-mediated inflammatory diseases

• Chronic diseases in which inflammation is a prominent component and the immune system reacts excessively against one or more tissues

• Immune-mediated inflammatory diseases develop because the normal controls on immune responses fail; typically due to autoimmunity but may be excessive reactions to microbes – MS, type 1 diabetes, RA: autoimmunity– Crohn’s: reaction against gut microbes?