Immune Protection against Lethal Fungal-Bacterial Intra-Abdominal Infections
Authors: Lilly Elizabeth A. , Ikeh Melanie , Nash Evelyn E. , Fidel Paul L. , Noverr Mairi C. 
Published: January 2018

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
Candida albicans
or other virulent non-
albicans Candida
(NAC) species plus
Staphylococcus aureus
resulted in 70 to 80% mortality in 48 to 72 h due to robust local and systemic inflammation (sepsis). Surprisingly, inoculation with
Candida dubliniensis
or
Candida glabrata
with
S. aureus
resulted in minimal mortality, and rechallenge of these mice with lethal
C. albicans
/
S. aureus
(i.e., coninfection) resulted in >90% protection. The purpose of this study was to define requirements for
C. dubliniensis
/
S. aureus
-mediated protection and interrogate the mechanism of the protective response. Protection was conferred by
C. dubliniensis
alone or by killed
C. dubliniensis
plus live
S. aureus
.
S. aureus
alone was not protective, and killed
S. aureus
compromised
C. dubliniensis
-induced protection.
C. dubliniensis
/
S. aureus
also protected against lethal challenge by NAC plus
S. aureus
and could protect for a long-term duration (60 days between primary challenge and
C. albicans/S. aureus
rechallenge). Unexpectedly, mice deficient in T and B cells (Rag-1 knockouts [KO]) survived both the initial
C. dubliniensis/S. aureus
challenge and the
C. albicans/S. aureus
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
in vivo
depletion of Gr-1
+
cells prior to rechallenge abrogated protection. These results suggest that
Candida
species can induce protection against a lethal
C. albicans
/
S. aureus
IAI that is mediated by PMNLs and postulated to be a unique form of trained innate immunity.

IMPORTANCE
Polymicrobial intra-abdominal infections are clinically devastating infections with high mortality rates, particularly those involving fungal pathogens, including
Candida
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
Candida
species provides strong protection against subsequent lethal infection with
C. albicans
and
S. aureus
. 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
Candida
species. This discovery implies that a form of trained innate immunity may be clinically effective against polymicrobial IAI.