The rise of antibiotic resistance calls for alternative strategies to treat bacterial infections. One attractive strategy is to directly target bacterial virulence factors with anti-virulence drugs. The expression of virulence traits by pathogens is, however, not constitutive but rather induced by the level of stress encountered within the host. Here we use dual RNA sequencing (RNA-seq) to show that intrinsic variability in the level of host resistance greatly affects the pathogen's transcriptome in vivo. Through analysis of the transcriptional profiles of host and pathogen during Staphylococcus aureus infection of two mouse strains, shown to be susceptible (A/J) or resistant (C57BL/6) to the pathogen, we demonstrate that the expression of virulence factors is dependent on the encountered host resistance. We furthermore provide evidence that this dependence strongly influences the efficacy of anti-virulence strategies, highlighting a potential limitation for the implementation of these strategies.

The complement anaphylatoxin C5a contributes to host defense against Staphylococcus aureus. In this study, we investigated the functional role of the two known C5a receptors, C5aR1 and C5aR2, in the host response to S. aureus. We found that C5aR1(-/)(-) mice exhibited greater susceptibility to S. aureus bloodstream infection than wild type and C5aR2(-/)(-) mice, as demonstrated by the significantly higher bacterial loads in the kidneys and heart at 24 h of infection, and by the higher levels of inflammatory IL-6 in serum. Histological and immunohistochemistry investigation of infected kidneys at 24 h after bacterial inoculation revealed a discrete infiltration of neutrophils in wild type mice but already well-developed abscesses consisting of bacterial clusters surrounded by a large number of neutrophils in both C5aR1(-/)(-) and C5aR2(-/)(-) mice. Furthermore, blood neutrophils from C5aR1(-/)(-) mice were less efficient than those from wild type or C5aR2(-/)(-) mice at killing S. aureus. The requirement of C5aR1 for efficient killing of S. aureus was also demonstrated in human blood after disrupting C5a-C5aR1 signaling using specific inhibitors. These results demonstrated a role for C5aR1 in S. aureus clearance as well as a role for both C5aR1 and C5aR2 in the orchestration of the inflammatory response during infection.

TREM-1 (triggering receptor expressed on myeloid cells) is a surface molecule expressed on neutrophils and macrophages which has been implicated in the amplification of inflammatory responses triggered during infection. In the present study, we have investigated the clinical significance of TREM-1 in Streptococcus pyogenes-induced severe sepsis in both experimentally infected mice as well as in patients with streptococcal toxic shock. We found that S. pyogenes induced a dose-dependent upregulation of TREM-1 in in vitro cultured phagocytic cells and in the organs of S. pyogenes-infected mice. Furthermore, we reported a positive correlation between serum levels of soluble TREM-1 (sTREM-1) and disease severity in infected patients as well as in experimentally infected mice. Hence, sTREM-1 may represent a useful surrogate marker for streptococcal sepsis. We found that modulation of TREM-1 by administration of the TREM-1 decoy receptor rTREM-1/Fc substantially attenuated the synthesis of inflammatory cytokines. More importantly, treatment of S. pyogenes-infected septic mice with rTREM-1/Fc or the synthetically produced conserved extracellular domain LP17 significantly improved disease outcome. In summary, our data suggest that TREM-1 may not only represent a valuable marker for S. pyogenes infection severity but it may also be an attractive target for the treatment of streptococcal sepsis.

We have previously reported that myeloid-derived suppressor cells (MDSC), which are a heterogeneous population of immunosuppressive immature myeloid cells, expanded during chronic Staphylococcus aureus infection and promoted bacterial persistence by inhibiting effector T cells. Two major MDSC subsets including monocytic MDSCs (M-MDSC) and granulocytic MDSCs (G-MDSC) have been described to date. Here, we identified a new subset of MDSC (Eo-MDSC) in S. aureus-infected mice that phenotypically resembles eosinophils. Eo-MDSC exhibit eosinophilic cytoplasmic granules and express CD11b, the eosinophil marker Syglec-F, variable levels of CCR3 and low levels of IL-5R. Furthermore, Eo-MDSC accumulated at the site of infection and exerted a potent immunosuppressive effect on T cell responses that was mediated by nitric oxide-dependent depletion of L-arginine. Increased in the number of Eo-MDSC by adoptive transfer caused a significant exacerbation of infection in S. aureus-infected mice. This study sheds new light on the heterogeneity and complexity of MDSC during chronic infection.