Peripheral tolerance and Immunoregulation. Dr. C. Piccirillo Canada Research Chair Department of Microbiology & Immunology McGill University MIMM-414A Lecture 1- Oct. 20, 2006

Why T cell regulation? How does the immune system prevent self- reactivity while maintaining reactivity to invaders/non-self? T cell regulation in the absence of regulatory T cells? 1.Suppression of autoreactive T cells 2.Control immunity to enteric bacteria 3.Limit histopathology

Self/non-self discrimination Turn of the 20th century 1901: Ehrlich and Morgenroth –Immunized goats with RBC from another goat to conclude that host immune responses respond to foreign antigens. –They coined the Latin phrase: horror autoxicus –Why did the goats develop autoAb to their own RBC? Traub induced tolerance by injecting LCMV in utero into mice producing an infection that was long-lived in life Macfarlane Burnet: postulated that the age of the animal at the time of the first encounter with antigen was the critical determinant in the induction of tolerance. 1953: Medawar induced immune tolerance to skin allografts in mice by neonatal injection of allogeneic cells. Prevention or failure to mount response to self-components?

Immunological tolerance A state of functional unresponsiveness for a particular antigen. May occur in the context of a non-inflammatory immune response. Tolerance is an active process.

Thymic deletional processes are inefficient. Moderate-high affinity TCR/pMHC interactions --> clonal deletion Low-moderate TCR/pMHC interactions -> selection No interaction- neglect and death BMDC involved in clonal deletion (cortico-medullary junction) T cell maturational state is important (nature,site and how) Role of AIRE (AutoImmune Regulator) Autoreactive T cells leave thymus and exist in periphery. Absence of disease suggests mechanism of active suppression. Central tolerance largely controls self / non-self discrimination.

Multiple mechanisms ensure peripheral T cell tolerance. Ignorance: anatomy, lymphatics, Ag crypticity, privilege Deletion : cross-presentation of Ag by BMDC results in death (eg;CD8+) Anergy: Insufficient co-stimulation on self-tissues Clonal exhaustion: CD8+ T cells in chronic viral infections Immune deviation: shift from inflammatory to anti- inflammatory cytokine production (eg; Th1-Th2). Activation-induced cell death (AICD): Fas/FasL (IL-2) = Death Peripheral tolerance is an adaptable process.

Tolerance to Self Antigen Antigen sequestrationAntigen sequestration (lens of eye, spermatozoa) Low MHC expressionLow MHC expression (i.e. hepatocytes) Immune Response to Foreign Antigens Influence of Antigen Dose (low zone, high zone), timing/duration of exposure, routes, nature of antigen, protein > CHO >> lipids, presence of adjuvants Influence of Antibody feedback inhibition (IgG inhibits IgM), differential antigen binding affinity Factors favoring tolerance Age, neurological and endocrine factors, Nutritional status, MHC Haplotypes

Acquired Tolerance In many cases, experimental unresponsiveness may be mediated by suppressor (T) cells, which actively prevent an immune response. 1.Original experiments of Gershon and Kondo (1970) showed that T cells were required for tolerance induction. Moreover, T cells from tolerant mice suppressed B cells from normal mice. 2.Active suppression by T cells also seen in some responses under Ir gene control and in the regulation of IgE responses (Tada). 3.Suppression commonly induced by systemic administration of antigen- coupled to self cells; route of injection is important (i.v. favors; intradermal gives contact sensitization). 4.Suppressor T cells produce factors. These factors may act directly on T/B targets. 5.Can be adaptively transferred.

Infamous Suppressor T cells 1970’s-early 80’s Gershon, Kondo, Tada….. Taboo of T “suppressor” cells –CD8+ –Complex regulatory networks mapping to I-J (within the MHC) –Molecular cloning of TCR and MHC disappointed in T cell hybridomas secreting “suppressor factors” –No identifiable cell surface marker –No clone or cell lines with suppressor activity –No antigen specific suppressor factor gene identified –Transient phenotype Suppressor or Regulatory lymphocytes Patrol periphery, silencing self-reactive T cells and maintain tolerance.

Questions ? Answers The reincarnation of the suppressor T cell More questions than answers Present 1 n Number of publications *

CD25 GITR CTLA-4 Foxp3 Activated Effector T cell APC _ _ TCR + Peripheral differentiation signals Thymic CD4 + T cell pool A network of CD4 + regulatory T cells control immune reponses. Thymically-derived naturally-occurring CD4 + CD25 + Treg cells (nTreg ) Peripherally-induced CD4 + Treg cells ( iTreg ) Autoimmunity Transplantation Tumor Immunity Infectious disease TCR Foxp3 + GITR + CTLA-4 + CD25 + CD25 +/- GITR +/- CTLA-4 +/- Foxp3 +/- Piccirillo et al. Trends in Immunol