Alpha beta TCR+ cells are a minimal fraction of peripheral CD8+ pool in MHC class I-deficient mice

MHC-dependent desensitization of intrinsic anti-self reactivity

The survival of naive T cells is compromised in the absence of molecules encoded by the major histocompatibility complex (MHC) while antigen-experienced T cells survive. We hypothesized that survival pressures in an in vivo, MHC-deficient environment would permit enrichment of less frequent antigen-experienced autoreactive cells at the expense of the majority of antigen naive T cells. To test this hypothesis, we generated MHC class I- and class II-deficient mice in NOD and C57Bl/6 (B6) backgrounds, and examined the capacity of adoptively transferred autoimmune-prone NOD T cells, or non-autoimmune prone naive B6 T cells, respectively, to reject transplanted wild-type pancreatic islets or transplantable tumors in the MHC-deficient mice. In the MHC-deficient environment, CD4 T cells acquired self-hostile properties (islet rejection and tumor invasion) that were independent from their genetic propensity for autoreactivity, while CD8 T cells required appropriate prior exposure to antigen in order to survive and function (reject tumor) in this environment; however, disengagement of Tob1, a negative regulator of proliferation, led to a reverse phenotype with regard to persistence of CD4 and CD8 T cells in the MHC-deficient environment. Our data suggest that self-peptide/MHC interactions have dual roles to facilitate survival and restrain autoreactivity, thus acting as integral components of an intrinsic network of negative regulation that maintains tolerance.

Self-peptide/MHC and TCR antagonism: physiological role and therapeutic potential

TCR antagonists are peptides that bind MHC molecules and can specifically inhibit T cell activation induced by antigens. Studying TCR antagonism has taken an important place in immunology for both theoretical and practical reasons. Deciphering the mechanism(s) of action of TCR antagonists can yield important information about interactions of the TCR with ligands, T cell development, and TCR signaling. Moreover, microorganisms may employ TCR antagonism to elude the attention of the immune system. Finally, specificity of inhibition makes TCR antagonists an ideal tool to seek antigen-specific immunomodulation. Present state of knowledge on these topics is reviewed.

Memory phenotype of CD8+ T cells in MHC class Ia-deficient mice

B6.K(b-)D(b-) mice are devoid of class Ia but express normal levels of class Ib molecules. They have low levels of CD8 T cells in both the thymus as well as peripheral T cell compartments. Although the percentage of splenic CD8 alpha alpha T cells is increased in these animals, approximately 90% of CD8 T cells are CD8 alpha beta. In contrast to B6 animals, most of the CD8 T cells from these mice have a memory phenotype (CD44(high)CD122(high) CD62L(low)) including both CD8 alpha beta and CD8 alpha alpha subsets. In the thymus of B6.K(b-)D(b-) animals, there is a decrease in the percentage of SP CD8 T cells, although most are CD44(low), similar to that seen in B6 mice. The spleens from day 1-old B6 and B6.K(b-)D(b-) mice have a relatively high proportion of CD44(high)CD62L(low) CD8 T cells. However, by day 28 most CD8 T cells in B6 mice have a naive phenotype while in B6.K(b-)D(b-) mice the memory phenotype remains. Unlike CD44(high) cells that are found in B6 animals, most CD44(high) cells from B6.K(b-)D(b-) mice do not secrete IFN-gamma rapidly upon activation. The paucity of CD8 T cells in B6.K(b-)D(b-) mice might be due in part to their inability to undergo homeostatic expansion. Consistent with this, we found that CD8 T cells from these animals expand poorly in X-irradiated syngeneic hosts compared with B6 CD8 T cells that respond to class Ia Ags. We examined homeostatic expansion of B6 CD8 T cells in single as well as double class Ia knockout mice and were able to estimate the fraction of cells reactive against class Ia vs class Ib molecules.

Contribution of T cells to mortality in neurovirulent Sindbis virus encephalomyelitis

Intranasal inoculation of C57BL/6 mice with a neurovirulent strain of Sindbis virus (SV) results in fatal encephalomyelitis. Mice with selective immune deficiencies were studied to determine the role of the immune response in fatal outcome. Mortality was decreased in mice deficient in alphabeta, but not gammadelta, T cells demonstrating a contribution of alphabeta T cells. Mice lacking either CD4+ or CD8+ T cells also had reduced mortality and mice lacking interferon (IFN)-gamma were completely protected. Clearance of infectious virus was identical in mice without T cells or IFN-gamma, but clearance of viral RNA was delayed compared to normal mice. Mice unable to produce antibody, perforin, Fas, TNF-alpha receptor1, IL-6 or IL-12 were not protected. These data suggest that T cells contribute to fatal acute viral encephalomyelitis through the production of IFN-gamma.

Factors influencing the patterns of T lymphocyte allorecognition

BACKGROUND

Strong alloreactive T cell responses are a menace in transplantation surgery and their menagement requires understanding the basis of alloreactivity. Alloantigen recognition can be peptide independent, peptide specific, or peptide dependent. The mechanisms influencing each recognition pattern are largely unknown.

METHODS

Peptide dependence was examined in vitro by adding peptides to antigen processing-deficient cell line used as target in cytotoxic T cell assays. Responses to major histocompatibility complex (MHC) alleles most homologous to self were recently shown to be more peptide dependent than to those with lesser homology to self. Hence, peptide reactivity in vivo was estimated based on relative strengths of alloreactive responses to more homologous and less homologous MHC alleles.

RESULTS

Alloreactive CD8+ TCR repertoire in beta2-microglobulin-deficient mice is preferentially peptide independent. The peptide-specific component is acquired as a function of wild-type thymic epithelium grafting. Irrespective of the presence of the peptide-specific component, in vivo alloantigenic priming was associated with a greater sensitivity to the MHC structure than was in vitro priming.

CONCLUSIONS

Thymic positive selection and the mode of alloreactivity induction are the major independent factors determining the patterns of alloantigen recognition.