Regulation of T cell immunity and tolerance in vivo by CD4

Previous experiments showed that peptides corresponding to a major CD4-binding site on the β2 domain of MHC class II molecules, IAβ134-148, enhance responses by CD4⁺ T lymphocytes to antigen, allo-antigen and bacterial superantigen in vitro, and to soluble protein in vivo. To determine whether peptide IAβ134-148 acted by inhibiting antigen-induced T cell tolerance, ovalbumin-specific CD4⁺ lymph node (LN) T cells from TCR transgenic DO.11.10 mice were adoptively transferred into H-2 syngeneic BALB/c recipients. Tolerance was then induced by injecting antigen i.v. When peptide IAβ134-148 was used to interfere with CD4-MHC class II interactions, accumulation of clonotype-positive T lymphocytes in the LN and induction of T cell tolerance in vivo were delayed. The mechanism by which peptide IAβ134-148 inhibited T cell tolerance included the peptide's ability to block activation-induced cell death. Further, antigen-specific splenic T lymphocytes were not tolerized in IAβ134-148-treated mice, providing a reservoir of T cells that could respond to a secondary immunization. The results reported here suggest that participation of the T cell co-receptor, CD4, in TCR signaling differentially affected both T cell migration and the induction of antigen-specific tolerance. Therefore, in this in vivo model system, the combined strength of all signals received (e.g. via TCR, co-receptors and co-stimulators) determined whether T cell immunity or apoptosis and tolerance resulted from antigenic stimulation. These findings are potentially important for the development of reagents to enhance vaccine efficacy and tumor immunity.