Immune Protection against Lethal Fungal-Bacterial Intra-Abdominal Infections

Abstract

Polymicrobial intra-abdominal infections (IAIs) are clinically prevalent and cause significant morbidity and mortality, especially those involving fungi. Our laboratory developed a mouse model of IAI and demonstrated that intraperitoneal inoculation with <i>Candida albicans</i> or other virulent non-<i>albicans Candida</i> (NAC) species plus <i>Staphylococcus aureus</i> resulted in 70 to 80% mortality in 48 to 72 h due to robust local and systemic inflammation (sepsis). Surprisingly, inoculation with <i>Candida dubliniensis</i> or <i>Candida glabrata</i> with <i>S. aureus</i> resulted in minimal mortality, and rechallenge of these mice with lethal <i>C. albicans</i>/<i>S. aureus</i> (i.e., coninfection) resulted in >90% protection. The purpose of this study was to define requirements for <i>C. dubliniensis</i>/<i>S. aureus</i>-mediated protection and interrogate the mechanism of the protective response. Protection was conferred by <i>C. dubliniensis</i> alone or by killed <i>C. dubliniensis</i> plus live <i>S. aureus</i><i>S. aureus</i> alone was not protective, and killed <i>S. aureus</i> compromised <i>C. dubliniensis</i>-induced protection. <i>C. dubliniensis</i>/<i>S. aureus</i> also protected against lethal challenge by NAC plus <i>S. aureus</i> and could protect for a long-term duration (60 days between primary challenge and <i>C. albicans/S. aureus</i> rechallenge). Unexpectedly, mice deficient in T and B cells (Rag-1 knockouts [KO]) survived both the initial <i>C. dubliniensis/S. aureus</i> challenge and the <i>C. albicans/S. aureus</i> rechallenge, indicating that adaptive immunity did not play a role. Similarly, mice depleted of macrophages prior to rechallenge were also protected. In contrast, protection was associated with high numbers of Gr-1^hi polymorphonuclear leukocytes (PMNLs) in peritoneal lavage fluid within 4 h of rechallenge, and <i>in vivo</i> depletion of Gr-1⁺ cells prior to rechallenge abrogated protection. These results suggest that <i>Candida</i> species can induce protection against a lethal <i>C. albicans</i>/<i>S. aureus</i> IAI that is mediated by PMNLs and postulated to be a unique form of trained innate immunity.<b>IMPORTANCE</b> Polymicrobial intra-abdominal infections are clinically devastating infections with high mortality rates, particularly those involving fungal pathogens, including <i>Candida</i> species. Even in patients receiving aggressive antimicrobial therapy, mortality rates remain unacceptably high. There are no available vaccines against IAI, which is complicated by the polymicrobial nature of the infection. IAI leads to lethal systemic inflammation (sepsis), which is difficult to target pharmacologically, as components of the inflammatory response are also needed to control the infection. Our studies demonstrate that prior inoculation with low-virulence <i>Candida</i> species provides strong protection against subsequent lethal infection with <i>C. albicans</i> and <i>S. aureus</i> Surprisingly, protection is long-lived but not mediated by adaptive (specific) immunity. Instead, protection is dependent on cells of the innate immune system (nonspecific immunity) and provides protection against other virulent <i>Candida</i> species. This discovery implies that a form of trained innate immunity may be clinically effective against polymicrobial IAI.