Candida -induced granulocytic myeloid-derived suppressor cells are protective against polymicrobial sepsis.

Polymicrobial intra-abdominal infections (IAI) can lead to life-threatening sepsis with significant morbidity and mortality, especially when pathogenic fungi are involved. We have employed an established clinically relevant mouse model of fungal/bacterial IAI and shown that immunization with low-virulence Candida species, that is, Candida dubliniensis , can induce responses that protect against sepsis via the suppression of lethal inflammation. This protection is dependent on long-lived Gr-1 + polymorphonuclear leukocytes that display characteristics consistent with myeloid-derived suppressor cells (MDSCs) and trained innate immunity. Here we aimed to functionally and phenotypically characterize these protective Gr-1 + leukocytes. Compared to nonimmunized control mice, we observed increased levels of CD11b + Gr-1 + cells systemically and locally in the peritoneal cavity of immunized mice. Isolated peritoneal Gr-1 + cells displayed hallmark MDSC phenotypes including increased T-cell suppressor activity and increased MDSC effector activity. Furthermore, we observed increased levels of the anti-inflammatory MDSC cytokine interleukin (IL)-10 in the peritoneal cavity of immunized mice and, in contrast, increased inflammatory responses when Gr-1 + leukocytes were depleted from immunized mice prior to challenge. Flow cytometric analysis revealed that Ly6G + granulocytic MDSCs (G-MDSCs) were preferentially increased over Ly6C + monocytic MDSCs (M-MDSCs) in immunized mice. Importantly, G-MDSCs, but not M-MDSCs, as well as IL-10 production, are required for full protection against lethal sepsis. From these data, we conclude that the Gr-1 + leukocytes that protect against polymicrobial sepsis are bona fide MDSCs and hypothesize that the mechanism of MDSC-mediated protection includes abrogation of lethal inflammation by IL-10.IMPORTANCEPolymicrobial intra-abdominal infections are serious clinical infections that can lead to life-threatening sepsis, which is difficult to treat in part due to the complex and dynamic inflammatory responses involved. Our prior studies demonstrated that immunization with low-virulence Candida species can provide strong protection against lethal polymicrobial sepsis challenge in mice. This long-lived protection was found to be mediated by trained Gr-1 + polymorphonuclear leukocytes with features resembling myeloid-derived suppressor cells (MDSCs). Here we definitively characterize these cells as MDSCs and demonstrate that their mechanism of protection involves the abrogation of lethal inflammation, in part through the action of the anti-inflammatory cytokine interleukin (IL)-10. These studies highlight the role of MDSCs and IL-10 in controlling acute lethal inflammation and give support for the utility of trained tolerogenic immune responses in the clinical treatment of sepsis.