Protection against lethal sepsis following immunization with Candida species varies by isolate and inversely correlates with bone marrow tissue damage

Protection against lethal Candida albicans (Ca)/Staphylococcus aureus (Sa) intra-abdominal infection (IAI)-mediated sepsis can be achieved by a novel form of trained innate immunity (TII) involving Gr-1+ myeloid-derived suppressor cells (MDSCs) that are induced by inoculation (immunization) with low virulence Candida species [i.e., Candida dubliniensis (Cd)] that infiltrate the bone marrow (BM). In contrast, more virulent Candida species (i.e., C. albicans), even at sub-lethal inocula, fail to induce similar levels of protection. The purpose of the present study was to test the hypothesis that the level of TII-mediated protection induced by Ca strains inversely correlates with damage in the BM as a reflection of virulence. Mice were immunized by intraperitoneal inoculation with several parental and mutant strains of C. albicans deficient in virulence factors (hyphal formation and candidalysin production), followed by an intraperitoneal Ca/Sa challenge 14 d later and monitored for sepsis and mortality. Whole femur bones were collected 24 h and 13 d after immunization and assessed for BM tissue/cellular damage via ferroptosis and histology. While immunization with standard but not sub-lethal inocula of most wild-type C. albicans strains resulted in considerable mortality, protection against lethal Ca/Sa IAI challenge varied by strain was usually less than that for C. dubliniensis, with no differences observed between parental and corresponding mutants. Finally, levels of protection afforded by the Ca strains were inversely correlated with BM tissue damage (R (2) = −0.773). TII-mediated protection against lethal Ca/Sa sepsis induced by Candida strain immunization inversely correlates with BM tissue/cellular damage as a reflection of localized virulence.