CD8 blockade promotes the expansion of antigen-specific CD4+ FOXP3+ regulatory T cells in vivo.

Treatment with a cocktail of CD4 and CD8-specific monoclonal antibodies (mAb) induces long-term transplantation tolerance and regulatory CD4(+) T cells that induce tolerance in non-tolerant T cells. In contrast, treatment with a CD4-specific mAb alone fails to induce long-term tolerance. The current study was designed to test the hypothesis that CD8 blockade plays a role in promoting the development of CD4(+) regulatory T cells. Using the DO11.10 CD4(+) TCR transgenic mouse model we show that treatment with a CD4/CD8-specific mAb cocktail induces antigen-specific tolerance to OVA, measured by a significant decrease in OVA-specific IgG, on challenge with antigen. Although treatment with OVA and the CD4-specific mAb alone also induces a significant decrease in OVA-specific antibody, the number of DO11.10 cells is significantly greater in mice treated with the CD4/CD8-specific mAb cocktail, and this is associated with a significant increase in proliferation of DO11.10 cells in response to specific antigen. DO11.10 cells sorted from mice made tolerant to OVA with the CD4/CD8-specific mAb cocktail promote an OVA-specific IgG1 (Th2-type) response but not an OVA-specific IgG3 (Th1-type) response on transfer into new syngeneic recipients, suggesting their ability to regulate the type of antigen-specific immune response that ensues after priming with antigen. In addition, DO11.10 cells from tolerant mice express markers that are characteristic of CD4(+) regulatory cells, including FOXP3, GITR and CTLA4, but not CD25. Taken as a whole, these data suggest that CD8 blockade promotes CD4(+) FOXP3(+) regulatory CD4(+) T cells by promoting their proliferation in tolerant mice.