CD4+CD25+ regulatory T cells suppress the immune responses of mouse embryo fibroblasts to murine cytomegalovirus infection

Cytomegaloviruses (CMVs) cause common viral infectious diseases and are difficult for the host immune system to eliminate, which leads to persistent or chronic infection. To investigate the T cell immune response stimulated by murine cytomegalovirus (MCMV) infection and the role of CD4+CD25+Foxp3+ T regulatory cells (Tregs) in this process, T cells containing various proportions of Tregs were co-cultured with MCMV-infected mouse embryo fibroblasts (MEFs). MCMV infection stimulated proliferation of effector T cells as well as differentiation to Tregs, which consequently increased the expression of TGF-β and IL-10. The proliferation of Tc1 (CD3+CD8+IFN-γ+), Th1 (CD3+CD4+IFN-γ+), and Tc2 (CD3+CD8+IL-4+) subsets was significantly suppressed with an increased proportion of Tregs in the co-culture system. Treg-depleted T cells inhibited viral load when co-cultured with MCMV-infected MEFs, however, this inhibitory effect was diminished when an increased proportion of Tregs was introduced. The suppressing effects of Tregs on effector T cells were attenuated by the addition of monoclonal antibody to TGF-β, but not the one to IL-10, suggesting that TGF-β is a major messenger involved in the immune suppressing effect of Tregs.