IL-12 Promotes Myeloid-Derived Suppressor Cell Recruitment and Bacterial Persistence during Staphylococcus aureus Orthopedic Implant Infection

Abstract

Staphylococcus aureus (S. aureus) is a leading cause of human prosthetic joint infections (PJIs) typified by biofilm formation. We recently identified a critical role for myeloid-derived suppressor cells (MDSCs) in S. aureus biofilm persistence. Pro-inflammatory signals induce MDSC recruitment and activation in tumor models; however, the mechanisms responsible for MDSC homing to sites of biofilm infection are unknown. Here we report that several cytokines (IL-12p40, IL-1β, TNF-α, and G-CSF) and chemokines (CXCL2, CCL5) were significantly elevated in a mouse model of S. aureus PJI. This coincided with significantly increased MDSC infiltrates concomitant with reduced monocyte, macrophage, and T cell influx compared with uninfected animals. Of the cytokines detected, IL-12 was of particular interest based on its ability to possess either pro- or anti-inflammatory effects mediated through p35-p40 heterodimers or p40 homodimers, respectively. MDSC recruitment was significantly reduced in both p40 and p35 KO mice, which resulted in enhanced monocyte and neutrophil influx and bacterial clearance. Adoptive transfer of wild type MDSCs into infected p40 KO animals worsened disease outcome, as evidenced by the return of S. aureus burdens to levels typical of wild type mice. Tissues obtained from patients undergoing revision surgery for PJI revealed similar patterns of immune cell influx, with increased MDSC-like cells and significantly fewer T cells compared with aseptic revisions. These findings reveal a critical role for IL-12 in shaping the anti-inflammatory biofilm milieu by promoting MDSC recruitment.

Infectious Dose Dictates the Host Response during Staphylococcus aureus Orthopedic-Implant Biofilm Infection

“…To evaluate the importance of strain on infection outcome and inflammation, we compared three commonly used S. aureus isolates, namely, LAC, MW2, and UAMS-1, in a mouse model of orthopedic-implant biofilm infection that models PJI in humans. As previously reported in our laboratory with USA300 LAC (25)(26)(27), this is a well-established low dose infection model associated with rapid bacterial expansion, with titers most prominent in the soft tissue surrounding the infected knee joint followed by the tendons/ligaments comprising the joint, femur, and implant (Fig. 1).…”

“…As previously described in our S. aureus PJI model (25,26), the innate immune response is predominantly composed of MDSCs (50 to 80% of the CD45 ϩ population), which persist throughout infection ( Fig. 2A).…”

“…Recent studies from our laboratory have focused on characterizing the immune response during S. aureus PJI using a mouse model of orthopedic-implant biofilm infection, which has provided a better understanding of the key leukocyte populations that contribute to chronic biofilm establishment. In particular, myeloid-derived suppressor cells (MDSCs) and anti-inflammatory polarized macrophages have been implicated in promoting S. aureus biofilm persistence (25)(26)(27). In this report, we determined whether biofilm formation or the resultant immune response was strain-dependent using our orthopedic-implant biofilm infection model with LAC, MW2, and UAMS-1.…”

“…To examine the effect of infectious dose on the host immune response and biofilm formation, we compared disease progression following the administration of a standard low-dose S. aureus inoculum (10 3 CFU) that we and others have used to establish chronic biofilm infection (25-27, 37-39) versus a 2-log-higher dose (10 5 CFU). These studies utilized IL-12p40 knockout (KO) mice that were previously shown to have reduced myeloid-derived suppressor cell (MDSC) recruitment concomitant with improved bacterial clearance after lowdose challenge (10 3 CFU) (26) to investigate whether a higher dose (10 5 CFU) would negate this phenotype. Indeed, bacterial burdens and MDSC infiltrates were similar between IL-12p40 KO and wild-type (WT) mice after high-dose (10 5 CFU) LAC challenge, whereas both readouts were significantly reduced in IL-12p40 KO animals in response to 10 3 CFU of bacteria.…”

IL-10 Plays Opposing Roles during Staphylococcus aureus Systemic and Localized Infections

“…Similarly, during a chronic S. aureus orthopedic biofilm infection, Heim et al (22,23) observed an expansion of IL-10-producing MDSCs at the site of infection, which was associated with bacterial persistence. Targeted depletion of these cells resulted in an enhanced proinflammatory response that resulted in increased bacterial clearance (23,24).…”