CD4⁺ CD25⁺ Foxp3^- T-regulatory cells produce both gamma interferon and interleukin-10 during acute severe murine spotted fever rickettsiosis

Spotted fever group rickettsiae cause life-threatening human infections worldwide. Until now, the immune regulatory mechanisms involved in fatal rickettsial infection have been unknown. C3H/HeN mice infected with 3 × 10⁵ PFU of Rickettsia conorii developed an acute progressive disease, and all mice succumbed to this infection. A sublethal infection induced protective immunity, and mice survived. Compared to splenic T cells from sublethally infected mice, splenic T cells from lethally infected mice produced significantly lower levels of interleukin-2 (IL-2) and gamma interferon (IFN-γ) and a higher level of IL-10, but not of IL-4 or transforming growth factor β, and there was markedly suppressed CD4⁺ T-cell proliferation in response to antigen-specific stimulation with R. conorii. Furthermore, lethal infection induced significant expansion of CD4⁺ CD25⁺ Foxp3^- T cells in infected organs compared to the levels in naïve and sublethally infected mice. In a lethal infection, splenic CD4⁺ CD25⁺ Foxp3^- T cells, which were CTLA-4^high T-bet⁺ and secreted both IFN-γ and IL-10, suppressed the proliferation of and IL-2 production by splenic CD4⁺ CD25^- Foxp3^- T cells in vitro. Interestingly, depletion of CD25⁺ T cells in vivo did not change the disease progression, but it increased the bacterial load in the lung and liver, significantly reduced the number of IFN-γ-producing Th1 cells in the spleen, and increased the serum levels of IFN-γ. These results suggested that CD4⁺ CD25 ⁺ T cells generated in acute murine spotted fever rickettsiosis are Th1-cell-related adaptive T-regulatory cells, which substantially contribute to suppressing the systemic immune response, possibly by a mechanism involving IL-10 and/or cytotoxic T-lymphocyte antigen 4.