Candida albicans-stimulated hematopoietic stem and progenitor cells generate trained neutrophils with enhanced mitochondrial ROS production that defend against infection.

Central trained immunity, induced via reprogramming of hematopoietic stem and progenitor cells (HSPCs), mediates sustained heightened responsiveness of mature myeloid cells to secondary challenges. We have previously demonstrated that HSPCs use TLR2 and Dectin-1 to sense Candida albicans to induce the production of trained monocytes/macrophages to fight against secondary infection. Neutrophils play an important role in innate immunity and are critical for clearance of C. albicans. In this work, we used an in vitro model of mouse HSPC differentiation to investigate the functional phenotype of neutrophils derived from HSPCs exposed to various PAMPs and C. albicans cells. We found that neutrophils derived from HSPCs stimulated by a TLR2 agonist exhibit reduced inflammatory cytokine production (tolerized neutrophils) whereas neutrophils generated from a Dectin-1 agonist or C. albicans stimulated HSPCs produce higher amounts of cytokines (trained neutrophils). We further demonstrated that a transient exposure of HSPCs to live C. albicans cells is sufficient to induce a trained phenotype of the neutrophils they produce in a Dectin-1- and TLR2-dependent manner. These trained neutrophils exhibited higher phagocytosis and microbicidal capacity than control neutrophils. Additionally, their adoptive transfer was sufficient to reduce fungal burden during invasive candidiasis. Mechanistically, we demonstrated that trained neutrophils use mitochondrial ROS (mtROS) to enhance their ability to kill C. albicans cells, as they produce higher amounts of mtROS and scavenging mtROS with MitoTEMPO attenuated their yeast-killing ability to match that of control neutrophils. Altogether, these data suggest that infection-experienced HSPCs contribute to trained immunity by providing a source of trained neutrophils with enhanced antimicrobial activity which may confer prolonged protection from infection. The tailored manipulation of this mechanism might offer new therapeutic strategies for controlling fungal infections by harnessing neutrophils.