Induction of myelopoiesis by <i>Candida dubliniensis</i> drives protective trained immunity against sepsis in a Card9-dependent manner

Trained immunity is a long-term functional reprogramming of innate cells that allows for the formation of immune memory akin to adaptive responses, but with a more rapid nonspecific response. We recently identified an anti-inflammatory trained immunity response, termed trained tolerogenic immunity (TTI), induced by low virulence <i>Candida dubliniensis,</i> that protects against acute lethal polymicrobial sepsis via myeloid-derived suppressor cells (MDSCs). Trained immunity and MDSCs arise following hematopoietic stem and progenitor cell (HSPC) expansion in the bone marrow. Here, we show that <i>C. dubliniensis</i> induces inoculum-dependent protection and HSPC expansion, whereas depletion of HSPCs following <i>C. dubliniensis</i> immunization abrogates protection. We further show that myeloid progenitors and putative MDSCs are increased in the bone marrow following <i>C. dubliniensis</i> immunization, and that HSPC depletion results in reduced MDSC influx into the peritoneal cavity following lethal challenge. Bone marrow cytokine analysis revealed that mediators associated with MDSC development and trafficking, including G-CSF, CXCL2, and CCL2, are upregulated following <i>C. dubliniensis</i> immunization. Finally, we find that the C-type lectin receptor adaptor protein Card9 is required for <i>C. dubliniensis</i>-induced protection and HSPC expansion, but not fungal trafficking to the bone marrow. Taken together, these results suggest that Card9-dependent fungal recognition in the bone marrow drives myelopoiesis and the induction of immunosuppressive MDSCs that are protective against lethal inflammation and sepsis.IMPORTANCECells of the innate immune system can be "trained" by inducers to have enhanced memory responses, a phenomenon known as trained immunity. We recently identified an anti-inflammatory training response that is induced by low virulence fungal species (i.e., <i>Candida dubliniensis</i>) and is protective against acute lethal polymicrobial sepsis. Trained immunity inducers, including <i>C. dubliniensis,</i> can access the bone marrow and direct hematopoietic responses. Here, we demonstrate that protection is correlated with <i>C. dubliniensis</i>-induced bone marrow expansion, which directs a myeloid bias in the bone marrow and ultimately results in the expansion of protective myeloid-derived suppressor cells. Involvement of the C-type lectin receptor adaptor protein Card9 in the protective response suggests fungal recognition in the bone marrow drives this response. These findings offer new insights into how trained immunity inducers direct differential outcomes, which will inform the development of novel immunotherapeutics to exploit the full spectrum of trained immune responses.