Lack of response to a given antigen , i.e., specific immunological unresponsiveness
Individuals normally do not respond to their "self" antigens , i.e., they are "tolerant" to self. This is the basis for self - non self discrimination by the immune system. Loss of self tolerance can lead to autoimmunity.
Inability of the immune system to respond to a broad range of antigens.
I mmunosuppression :
A disorder or condition where the immune response is reduced or absent.
Sufficient for protection; prompt downregulation; does not persist after antigenic challenge resolved. Keep memory, eliminate effectors.
A healthy immune system is a balanced immune system, not a strong one!!
Sir Frank Macfarlane Burnet (The Function of Antibodies, 1949)
Embryonic Ag exposure suppress future response
4. Peter Medawar
(experimentally acquired tolerance, 1953)
Freemartins Sterile XX calves whose external genitalia appear to be masculinized due to the hormones from the XY twin Found when the affected cow is one of a pair of twins, the other twin being XY. Lillie, F. R. 1917. J. Exp. Zool. 23: 371 - 452 Cattle Placenta Institut national de recherche pédagogique Timing
Ray Owen, 1945 Dizygotic cattle twins that shared the same placenta were blood cell chimaeras, ie contained blood cells of two different haplotypes in adults and, - these two haplotypes co-existed in the adults without mutual rejection. Such chimaeric cattle will exchange skin grafts without rejection. Normally, if cells of one of the two haplotypes were transferred to an adult cattle of the other haplotype, these were promptly rejected. Thus, tolerance in the dizygotic twins resulted from the sharing of the placenta in fetal life, ie from exposure to the antigen during the development of the immune system.
Peter Medawar (shared a Nobel Prize with Burnet. 1960):
Autografts succeed, but allografts failed after an initial take.
The destruction of the foreign epidermis was brought about by a mechanism of active immunization
Newborn mice of CBA strain received splenocytes from A strain. Once immunological mature, they were unable to reject a graft from the A strain but remained perfectly able to reject an equivalent graft from a third strain.
Self tolerance- “A Learning Experience” How does the immune system learn to discriminate between self and non-self during development ? The primary repertoire of T (and B) cells is enormous as a result of combinatorial diversity and imprecisional joining of VDJ or VJ segments. This repertoire contains self-reactive BCRs and TCRs and yet a normal immune system does not exhibit self-reactivity. In the lecture on T cell development mechanisms, we have been explained T cell tolerance for self antigen by negative selection. Through positive and negative selection, the resulting repertoire is therefore one of T cells that react with non-self antigens in association with self-MHC (slightly react to self-antigen plus self-MHC). However, negative selection is not complete and other mechanisms are needed in the peripheral.
Passively administered antibody can prevent an antibody response (von Behring). This principle is utilized in the prevention of erythroblastosis fetalis by RHOGAM (Many women are given RhoGAM around the 28th week of pregnancy)
Only self-reactive CTL are suppressed by veto cells.
Upon recognition of class I on the veto cell by TCR on the effector T cell, Fas is up-regulated on effector cells, allowing for FasL on the veto CTL to induce apoptosis Reich-Zeliger et al. 2004 J. Immunol. 173:6660
Th3 From Weiner, Microbes and Infection , 3:947
Idiotypic Regulation The idiotypes expressed by T and B cells can be recognized by the immune system. 1. Anti-idiotypic responses may serve to down-regulate an ongoing immune response. 2. Since some anti-idiotypes (Ab2) may look like antigen, they can be used as "vaccines” 3. V region genes code for idiotype 4. Anti-idiotype response can initiate control loops How important is the idiotypic network?
DC1 IL-12 Stat 4 ERM T-bet TRAIL CD95L ? DC2 Thp Th2 Th1 Stat 6 GATA-3 c-Maf IRS IL-4 FLIP Suicide Fratricide Fratricide TCR MHC+Ag MHC+Ag A Model Depicting the Role of Apoptosis in Th1 and Th2 Balance
DEFINITION: Tolerance may be defined as a specific unresponsive state induced by prior exposure to an antigen. Therefore, tolerance is a learned process and is specific.
1. Tolerance is manifested in both T and B cells.
2. The kinetics of tolerance differ in B vs T cells.
a. T cells exhibit more rapid tolerance induction than B cells.
b. Tolerance is lost in B cells more rapidly than in T cells.
c. T cells are tolerized by lower doses of antigen (tolerogen) than B cells.
Waning reflects the differentiation of new T and B cells in the absence of
sufficient tolerogen, and can be prevented with more antigen.
4. Tolerance can be broken
a. New clones of T and B cells appear in the absence of antigen
b. Viral infection can create a cytokine milieu to turn on quiescent (anergic) cells
c. New epitopes are introduced either by viral modification or i a trogenically
d. C ross-reactive or modified antigens. New epitopes are introduced either by viral
modification or i a trogenically
5. Autoimmunity results when tolerance is lost or broken. Often the response is of low avidity to self.
Factors Affecting Tolerance Induction A. Age: Young immunologically immature animals show tolerance antigen exposure. 1. Immature B cells lack surface IgD and fail to resynthesize IgM receptors after capping. 2. Antigen is poorly localized and presented in immature animals B. Route of exposure: i.v. or oral exposure favors tolerance. S.c. or intradermal favors immunity. Intramuscular favors Th2. C. Dose of antigen: High doses favor tolerance; however, repeated low doses can also cause tolerance D. Associated antigens: Coupling of antigens to self Ig or self cells enhances tolerogenicity. 1. Coupling nucleosides to self carriers can prevent anti-DNA in genetically autoimmune mice.