Mechanisms of peripheral CD4+ T cell tolerance

The immune system is charged with the daunting task of discriminating self-antigens from an endless array of non-self antigens. Although T lymphocytes undergo selection in the thymus directed toward the elimination of self-reactive clones, T cells capable of recognizing self-antigen escape to the periphery, necessitating peripheral tolerance mechanisms for the regulation of these self-reactive cells. Three major forms of tolerance have been observed, deletion, anergy, and suppression, all of which may be critical to the avoidance of autoimmunity. Our laboratory has developed a mouse model of peripheral tolerance utilizing a T cell receptor (TCR) transgenic animal that expresses the Vβ8.1 TCR component on the majority of peripheral CD4+ T cells. The CD4+ T cells in this mouse can be rendered tolerant by the administration of antigen presenting cells (APC) expressing the minor lymphocyte stimulating-1a (Mls-1a) gene product. CD4+ T cell tolerance in this model system has been shown to involve deletion, anergy, and suppression. In CD4+ T cells isolated from tolerant mice, we have previously described a proliferative defect and decreased tyrosine phosphorylation of signaling intermediates, and we now identify these hypophosphorylated molecules as Cbl-b, ERK, and SLP-76. The 6C10 antibody has previously been identified as a marker of T cell responsiveness in the Vβ8.1 TCR transgenic tolerance system. Here, we demonstrate that 6C10 staining has functional significance in both tolerant and naive CD4+ T cell populations, identifying T cells with defects in proliferation, phosphotyrosine signaling, and immunologic synapse formation. The nature of the epitope recognized by 6C10 is further defined as a population of Thy-1 molecules with a low degree of sialylation. Furthermore, 6C10 binding serves a marker for the overall sialylation state of the CD4+ T cell in both the naive and tolerant animal, with tolerant T cells displaying more complete sialylation on Winked glycans. Perhaps most significantly, enzymatic desialylation of CD4+ T cells, in vivo or in vitro, leads to the reversal of tolerance as measured by proliferation and phosphotyrosine signaling