Description: Pathogenic Yersinia species modulate host immune responses through the activity of a plasmid-encoded type III secretion system and its associated effector proteins. One effector, YopM, is a leucine-rich-repeat-containing protein that is important for virulence in murine models of Yersinia infection. Although the mechanism by which YopM promotes virulence is unknown, we previously demonstrated that YopM was required for the induction of high levels of the immunosuppressive cytokine interleukin-10 (IL-10) in sera of C57BL/6J mice infected with Yersinia pseudotuberculosis . To determine if IL-10 production is important for the virulence function of YopM, C57BL/6J or congenic IL-10 –/– mice were infected intravenously with wild-type or yopM mutant Y. pseudotuberculosis strains. Analysis of cytokine levels in serum and bacterial colonization in the spleen and liver showed that YopM is required for IL-10 induction in C57BL/6J mice infected with either the IP32953 or the 32777 strain of Y. pseudotuberculosis , demonstrating that the phenotype is conserved in the species. In single-strain infections, the ability of the 32777 yopM mutant to colonize the liver was significantly increased by the delivery of exogenous IL-10 to C57BL/6J mice. In mixed infections, the competitive advantage of a yopM + 32777 strain over an isogenic yopM mutant to colonize spleen and liver, as observed for C57BL/6J mice, was significantly reduced in IL-10 –/– animals. Thus, by experimentally controlling IL-10 levels in a mouse infection model, we obtained evidence that the induction of this cytokine is an important mechanism by which YopM contributes to Y. pseudotuberculosis virulence.