A major role for myeloid-derived suppressor cells and a minor role for regulatory T cells in immunosuppression during Staphylococcus aureus infection.

Staphylococcus aureus can cause difficult-to-treat chronic infections. We recently reported that S. aureus chronic infection was associated with a profound inhibition of T cell responses. In this study, we investigated the mechanisms responsible for the suppression of T cell responses during chronic S. aureus infection. Using in vitro coculture systems, as well as in vivo adoptive transfer of CFSE-labeled OT-II cells, we demonstrated the presence of immunosuppressive mechanisms in splenocytes of S. aureus-infected mice that inhibited the response of OT-II cells to cognate antigenic stimulation. Immunosuppression was IL-10/TGF-ß independent but required cell-cell proximity. Using DEREG and Foxp3(gfp) mice, we demonstrated that CD4(+)CD25(+)Foxp3(+) regulatory T cells contributed, but only to a minor degree, to bystander immunosuppression. Neither regulatory B cells nor tolerogenic dendritic cells contributed to immunosuppression. Instead, we found a significant expansion of granulocytic (CD11b(+)Ly6G(+)Ly6C(low)) and monocytic (CD11b(+)Ly6G(-)Ly6C(high)) myeloid-derived suppressor cells (MDSC) in chronically infected mice, which exerted a strong immunosuppressive effect on T cell responses. Splenocytes of S. aureus-infected mice lost most of their suppressive activity after the in vivo depletion of MDSC by treatment with gemcitabine. Furthermore, a robust negative correlation was observed between the degree of T cell inhibition and the number of MDSC. An increase in the numbers of MDSC in S. aureus-infected mice by adoptive transfer caused a significant exacerbation of infection. In summary, our results indicate that expansion of MDSC and, to a minor degree, of regulatory T cells in S. aureus-infected mice may create an immunosuppressive environment that sustains chronic infection.