Immunologic Tolerance

Contents Part Ⅰ Introduction Part Ⅱ Development of Immunologic Tolerance Part Ⅲ Mechanism of Immunologic Tolerance Part Ⅳ Immunologic Tolerance and Clinic Medicine

Part Ⅰ Introduction Definition: A type of specific unresponsiveness to an antigen induced by the exposure of specific lymphocytes to that antigen, but response to other antigens normally. Tolerogens: antigens that induce tolerance

General features of Immunologic tolerance Tolerance is antigenic specific and results from the recognition of antigens by specific lymphocytes. Normal individuals are tolerant of their own antigens(self antigen)----- Self-tolerance. Foreign antigens may be administered in ways that preferentially inhibit immune response by inducing tolerance in specific lymphocytes---antigen induction.

Immunologic features of tolerance It is an antigen-induced, active process Like immunologic memory, it is antigen specific Like immunologic memory, it can exist in B cells, T cells or both Like immunologic memory, its easier to induce and lasts longer in T cells than in B cell Induction of tolerance is very similar to induction of an immune response. Immunologic features of tolerance: Tolerance is different from non-specific immunosuppression, and immunodeficiency. It is an active antigen dependent process in response to the antigen. Like immune response tolerance is specific and like immunological memory, it can exist in T-cell, B cells or both and like immunological memory, tolerance at the T cell level is longer lasting than tolerance at the B cell level.

Difference of Immuologic tolerance & immunodeficiency, immunosuppression Immunodeficiency: any condition in which there is deficiency in the production of humoral and /or cell-mediated immunity---non-specificity to Ag Immunosuppression: The suppression of immune responses to antigens. This can be achieved by various means, including physical, chemical----non-specificity to Ag

Part Ⅱ Development of Immunologic Tolerance

1. Induction of immunologic tolerance to antigen in fetal period and neonate period Owen first observed immunologic tolerance to allogenic antigen in fetal period in 1945

cattle of dizygotic twin Need for tissue and organ graft drove the curiosity to understand the mechanisms of tolerance. The observations a zoologist, Owens, that Dizygotic bovine twins could accept grafts from each other but their siblings from other pregnancies could not tolerate such grafts led Medawar to perform a series of experiments to induce tolerance in mice.

Experiment of Medawar on immunologic tolerance He and his collaborators Brent and Billingham under the hypothesis that early exposure to foreign antigens might facilitate tolerance induction performed this experiment.

2. Induction of immunologic tolerance to antigen in adult Antigen and immunologic tolerance: Concentration of antigens Type of antigen: monomer, aggregates Pathway of antigen entering body Features of determinant: tolerogenic epitope Variation of antigen

High and low dose tolerance

Tolerance in T and B cells Induction of tolerance in T cells is easier and requires relatively smaller amounts of tolerogen than tolerance in B cells. Maintenance of immunological tolerance requires persistence of antigen. Tolerance can be broken naturally (as in autoimmune diseases) or artificially (as shown in experimental animals, by x‑irradiation, certain drug treatments and by exposure to cross reactive antigens). Tolerance may be induced to all epitopes or only some epitopes on an antigen and Tolerance to a single antigen may exist at B cell level or T cells level or at both levels.

Factors affecting tolerance role of antigen Factors which affect response Favor immune response Favor tolerance Physical form of antigen Route of injection Dose of antigen Large, aggregated, complex molecules, properly processed Subcutaneous or intramuscular Optimal dose soluble, aggregate-free, simple small molecules, not processed Oral or, sometimes, intravenous Very large or very small dose Tolerance to soluble antigens: A state of tolerance to a variety of T-dependent and T-independent antigens has been achieved in various experimental models. Based on these observations it is clear that a number of factors determine whether an antigen will stimulate an immune response or tolerance (Table 1).

Individual and immunologic tolerance: Heredity, Age, Gender, Health

Factors affecting tolerance the role of host Factors that affect response Favor immune response Favor tolerance Age of responding animal Differentiation state of cells Fully differentiated; memory T & B cells Older, immuno-logically mature Newborn (mice), immuno-logically immature Relative undifferentiated B cell with only IgM, T cells in the thymic cortex

Host age and antigen dose affect tolerance newborn adult

Part Ⅲ Mechanism of Immunologic Tolerance

1. Central tolerance: Central tolerance occurs in the central lymphoid organs as a consequence of immature self-reactive lymphocytes recognizing ubiquitous self-antigen. 2. Peripheral tolerance: tolerance was induced in peripheral organs as a result of mature self-reactive lymphocytes encountering tissue-specific self antigens under particular conditions

1. Central tolerance Clonal deletion (apoptotic cell death) During maturation of lymphocytes in the thymus for T cell or in the bone marrow for B maturation, immature lymphocytes that recognize ubiquitous self-antigen with high affinity are deleted by negative selection

Clonal deletion: negative selection of T cells in the thymus Clonal deletion: Functionally immature cells of a clone encountering antigen undergo a programmed cell death. For example, auto-reactive T-cell are eliminated in the thymus following interaction with self antigen during their differentiation (negative selection). Clonal deletion has been shown to occur also in the periphery. B cells expressing only IgM (no IgD) on their surface when exposed to antigen are eliminated.

Central Tolerance

Negative selection of B cells in bone marrow Clonal deletion: Functionally immature cells of a clone encountering antigen undergo a programmed cell death. For example, auto-reactive T-cell are eliminated in the thymus following interaction with self antigen during their differentiation (negative selection). Clonal deletion has been shown to occur also in the periphery. B cells expressing only IgM (no IgD) on their surface when exposed to antigen are eliminated.

2. Peripheral tolerance ① clonal deletion and clonal ignorance: large tissue specific antigen delete specific T cells. self-reactive lymphocytes remain viable and functional but do not react to the self antigens in any detectable way. ② Clonal anergy and inactivation: functional inactivation without cell death: lack co-stimulatory signal

Clonal anergy in T cells Clonal anergy: Auto-reactive T cells when exposed to antigenic peptides that do not possess co-stimulatory molecules (B7-1 or B7-2) become anergic to the antigen.

Clonal anergy in B cells Also, B cells when exposed to large amounts of soluble antigen down regulate their surface IgM and become anergic and short lived. These cells also up regulate Fas molecules on their surface. An interaction of these B cells with Fas-ligand bearing cells result in their death via apoptosis.

③ Action of Suppressor lymphocyte (Ts) ④ Action of cytokines: TGF- , IL-10 ⑤ Holdback in signal tranduction ⑥ Immunologically privileged sites anatomic barrier: clonal ignorance

Pathways to Peripheral Tolerance Activated T cells Normal Response CD28 B7 Proliferation & differentiation Antigen Recognition without co-stimulation Anergy CTLA4 B7 Functionally Unresponsive CTL4-B7 interaction Fas Activation induced cell death Adapted From:   Van Parijs: Science, Volume 280(5361).April 10, 1998.243-248 Fas-FasL interaction Apoptosis FasL Inhibition of proliferation & effector action Cytokine-mediated suppression Cytokine regulation cytokines

The Two Signal Hypothesis for T-cell Activation Mature Dendritic cell APC Activated TH cell TH cell MHC II TCR CD28 B7 Signal 2

Hypothetical mechanism of tolerance in mature T cells Signal 1 CD28 Resting B-cell APC TH0 cell Tolerant T cell Tolerance (anergy or apoptosis) from lack of signal 2

Summary: Lack of co-stimulation can lead to tolerance (anergy) Activated T cells CD28 B7 Proliferation & differentiation Normal Response Antigen Recognition without co-stimulation Anergy

Functionally Unresponsive (Anergic) T cell Regulation by CTLA-4 CTLA4 CTLA4-B7 interaction Functionally Unresponsive (Anergic) T cell B7 Activated T cell

Functionally Unresponsive T cell Regulatory T cells Production of IL-10 or TGF-b Functionally Unresponsive T cell Regulatory T cell

Pathways to Peripheral Tolerance

Inhibition by Antibody Feedback Passively administered antibody can prevent an antibody response Antibody produced during an immune responses leads to elimination of antigen (stimulus) Less antigen available to stimulate specific cells Immune complexes can bind to inhibitory receptors Application: RhoGam for Erythroblastosis Fetalis

Major Immune Inhibitory Receptors B cells FcgRII T cells CTLA4 NK cells KIR (killer cell Ig-like receptors),

Anti-Idiotypes and Immune Regulation Definition anti-idiotype response-antibody produced against immunoglobulin or TCR idiotypes that serve to down-regulate immune response The epitope for an responsive anti-idiotype molecule (antibody, BCR, or TCR) is the internal image formed by the CDR region of the respective epitopes antigen receptor

Idiotype/Anti-idiotype network

Part Ⅳ Immunologic Tolerance and Clinic Medicine

1. To induce immunologic tolerance Prevent the rejection of organ allografts and xenografts Treat autoimmune diseases Treat allergic diseases

2. To terminate immunologic tolerance To treat tumor: enhance first signal or second signal To treat infection diseases

And now for a clinical case….

Inhibitor Impact Inhibitors have a major impact on treatment outcome. When inhibitors are present, hemostasis is often difficult to control. Therefore normal therapy typically fails, which leads to increased morbidity and mortality. The presence of inhibitors is also associated with the decreased ability to perform needed or elective surgery. FIX inhibitors pose specific problems in treatment. (See next slide.)

Patient Presentation 6 year old male, ER with unexplained bruising associated with minor trauma Patient has minimal clotting activity FVIII levels <1% of normal Patient given i.v. FVIII concentrate i.v. and released but returns in two weeks with same problem Repeated FVIII treatment However, FVIII is ineffective.

Issues Coagulation factor inhibitors (anti-FVIII activity) Basis? Lack of tolerance. Why? Prevalence/impact 20-30% FVIII, less FIX Treatment/problems FVIII concentrate or rFVIII Inhibitors develop that neutralize FVIII Therapy? Porcine FVIII with less cross-reactivity Tolerance (high dose) Gene therapy Overview Inhibitors are antibodies that counteract the therapeutic actions of factor concentrates currently used to treat hemophilia bleeding episodes. Inhibitor development therefore represents one of the greatest obstacles for physicians caring for persons with coagulation disorders. In the first part of today’s presentation, the prevalence and impact of coagulation factor inhibitors will be addressed. In addition, the advantages and disadvantages of current inhibitor treatment options will be presented. The second part of the presentation will discuss recombinant factor VIIa, a new therapeutic agent for managing bleeds in patients with inhibitors to FVIII or FIX. The mechanism of action and production of rFVIIa as well as preclinical and clinical rFVIIa data will also be presented. The final part of the presentation will address future challenges concerning the use of rFVIIa in the treatment and maintenance of bleeding episodes in inhibitor patients.

What are Inhibitors? IgG; commonly subclass 4, mixed 1 & 4 Occur in Congenital factor deficiency = alloimmune Previously unaffected = autoimmune Associated with pregnancy, autoimmunity, malignancy, multi-transfusion, advanced age etc. Inhibitors Inhibitors in persons with hemophilia A or hemophilia B are almost always alloantibodies of the IgG immunoglobulin class and are of the IgG1 and IgG4 subtypes. Inhibitors occur either in congenital deficiencies (alloimmune states) or they are acquired in previously unaffected individuals (autoimmune states).

Summary Definition of immunologic tolerance Features of immunologic tolerance Induction of immunologic tolerance Mechanism of immunologic tolerance Clinical application of immunologic tolerance