Protective or deleterious role of scavenger receptors SR-A and CD36 on host resistance to Staphylococcus aureus depends on the site of infection.

Staphylococcus aureus is a major human opportunistic pathogen responsible for a broad spectrum of infections ranging from benign skin infection to more severe life threatening disorders (e.g. pneumonia, sepsis), particularly in intensive care patients. Scavenger receptors (SR-A and CD36) are known to be involved in S. aureus recognition by immune cells in addition to MARCO, TLR2, NOD2 and α5ß1 integrin. In the present study, we further deciphered the contribution of SR-A and CD36 scavenger receptors in the control of infection of mice by S. aureus. Using double SR-A/CD36 knockout mice (S/C-KO) and S. aureus strain HG001, a clinically relevant non-mutagenized strain, we showed that the absence of these two scavenger receptors was protective in peritoneal infection. In contrast, the deletion of these two receptors was detrimental in pulmonary infection following intranasal instillation. For pulmonary infection, susceptible mice (S/C-KO) had more colony-forming units (CFU) in their broncho-alveolar lavages fluids, associated with increased recruitment of macrophages and neutrophils. For peritoneal infection, susceptible mice (wild-type) had more CFU in their blood, but recruited less macrophages and neutrophils in the peritoneal cavity than resistant mice. Exacerbated cytokine levels were often observed in the susceptible mice in the infected compartment as well as in the plasma. The exception was the enhanced compartmentalized expression of IL-1ß for the resistant mice (S/C-KO) after peritoneal infection. A similar mirrored susceptibility to S. aureus infection was also observed for MARCO and TLR2. Marco and tlr2 -/- mice were more resistant to peritoneal infection but more susceptible to pulmonary infection than wild type mice. In conclusion, our results show that innate immune receptors can play distinct and opposite roles depending on the site of infection. Their presence is protective for local pulmonary infection, whereas it becomes detrimental in the peritoneal infection.

The WalKR system controls major staphylococcal virulence genes and is involved in triggering the host inflammatory response.

The WalKR two-component system is essential for the viability of Staphylococcus aureus, playing a central role in controlling cell wall metabolism. We produced a constitutively active form of WalR in S. aureus through a phosphomimetic amino acid replacement (WalR(c), D55E). The strain displayed significantly increased biofilm formation and alpha-hemolytic activity. Transcriptome analysis was used to determine the full extent of the WalKR regulon, revealing positive regulation of major virulence genes involved in host matrix interactions (efb, emp, fnbA, and fnbB), cytolysis (hlgACB, hla, and hlb), and innate immune defense evasion (scn, chp, and sbi), through activation of the SaeSR two-component system. The impact on pathogenesis of varying cell envelope dynamics was studied using a murine infection model, showing that strains producing constitutively active WalR(c) are strongly diminished in their virulence due to early triggering of the host inflammatory response associated with higher levels of released peptidoglycan fragments. Indeed, neutrophil recruitment and proinflammatory cytokine production were significantly increased when the constitutively active walR(c) allele was expressed, leading to enhanced bacterial clearance. Taken together, our results indicate that WalKR play an important role in virulence and eliciting the host inflammatory response by controlling autolytic activity.

Early systemic bacterial dissemination and a rapid innate immune response characterize genetic resistance to plague of SEG mice.

BACKGROUND: Although laboratory mice are usually highly susceptible to Yersinia pestis, we recently identified a mouse strain (SEG) that exhibited an exceptional capacity to resist bubonic plague and used it to identify immune mechanisms associated with resistance. METHODS: The kinetics of infection, circulating blood cells, granulopoiesis, lesions, and cellular populations in the spleen, and cytokine production in various tissues were compared in SEG and susceptible C57BL/6J mice after subcutaneous infection with the virulent Y. pestis CO92. RESULTS: Bacterial invasion occurred early (day 2) but was transient in SEG/Pas mice, whereas in C57BL/6J mice it was delayed but continuous until death. The bacterial load in all organs significantly correlated with the production of 5 cytokines (granulocyte colony-stimulating factor, keratinocyte-derived chemokine (KC), macrophage cationic peptide-1 (MCP-1), interleukin 1α, and interleukin 6) involved in monocyte and neutrophil recruitment. Indeed, higher proportions of these 2 cell types in blood and massive recruitment of F4/80(+)CD11b(-) macrophages in the spleen were observed in SEG/Pas mice at an early time point (day 2). Later times after infection (day 4) were characterized in C57BL/6J mice by destructive lesions of the spleen and impaired granulopoiesis. CONCLUSION: A fast and efficient Y. pestis dissemination in SEG mice may be critical for the triggering of an early and effective innate immune response necessary for surviving plague.

Tissue tropism and target cells of NSs-deleted rift valley fever virus in live immunodeficient mice.

BACKGROUND: Rift Valley fever virus (RVFV) causes disease in livestock and humans. It can be transmitted by mosquitoes, inhalation or physical contact with the body fluids of infected animals. Severe clinical cases are characterized by acute hepatitis with hemorrhage, meningoencephalitis and/or retinitis. The dynamics of RVFV infection and the cell types infected in vivo are poorly understood. METHODOLOGY/PRINCIPAL FINDINGS: RVFV strains expressing humanized Renilla luciferase (hRLuc) or green fluorescent protein (GFP) were generated and inoculated to susceptible Ifnar1-deficient mice. We investigated the tissue tropism in these mice and the nature of the target cells in vivo using whole-organ imaging and flow cytometry. After intraperitoneal inoculation, hRLuc signal was observed primarily in the thymus, spleen and liver. Macrophages infiltrating various tissues, in particular the adipose tissue surrounding the pancreas also expressed the virus. The liver rapidly turned into the major luminescent organ and the mice succumbed to severe hepatitis. The brain remained weakly luminescent throughout infection. FACS analysis in RVFV-GFP-infected mice showed that the macrophages, dendritic cells and granulocytes were main target cells for RVFV. The crucial role of cells of the monocyte/macrophage/dendritic lineage during RVFV infection was confirmed by the slower viral dissemination, decrease in RVFV titers in blood, and prolonged survival of macrophage- and dendritic cell-depleted mice following treatment with clodronate liposomes. Upon dermal and nasal inoculations, the viral dissemination was primarily observed in the lymph node draining the injected ear and in the lungs respectively, with a significant increase in survival time. CONCLUSIONS/SIGNIFICANCE: These findings reveal the high levels of phagocytic cells harboring RVFV during viral infection in Ifnar1-deficient mice. They demonstrate that bioluminescent and fluorescent viruses can shed new light into the pathogenesis of RVFV infection.

Cutting edge: crucial role of IL-1 and IL-23 in the innate IL-17 response of peripheral lymph node NK1.1- invariant NKT cells to bacteria.

We have shown previously that peripheral lymph node-resident retinoic acid receptor-related orphan receptor γt(+) NK1.1(-) invariant NKT (iNKT) cells produce IL-17A independently of IL-6. In this study, we show that the concomitant presence of IL-1 and IL-23 is crucial to induce a rapid and sustained IL-17A/F and IL-22 response by these cells that requires TCR-CD1d interaction and partly relies on IL-23-mediated upregulation of IL-23R and IL-1R1 expression. We further show that IL-1 and IL-23 produced by pathogen-associated molecular pattern-stimulated dendritic cells induce this response from NK1.1(-) iNKT cells in vitro, involving mainly TLR2/4-signaling pathways. Finally, we found that IL-17A production by these cells occurs very early and transiently in vivo in response to heat-killed bacteria. Overall, our study indicates that peripheral lymph node NK1.1(-) iNKT cells could be a source of innate Th17-related cytokines during bacterial infections and supports the hypothesis that they are able to provide an efficient first line of defense against bacterial invasion.

Contribution of NOD2 to lung inflammation during Staphylococcus aureus-induced pneumonia.

Staphylococcus aureus is the most commonly found Gram-positive bacterium in patients admitted in intensive-care units, causing septicaemia or pneumonia. In this work, we investigated the role of NOD2 in S. aureus-induced pneumonia. We found that the absence of NOD2 affected weight loss and recovery speed. Nod2-/- mice showed a reduced lung inflammation in comparison to wild-type animals, with lower presence of cytokines in broncho-alveolar lavage fluids and reduced recruitment of neutrophils. Furthermore, histological analysis of the lungs revealed less severe lesions in Nod2-/- mice at day 2 and day 7 post-infection. In conclusion, we demonstrated that NOD2 is not a crucial receptor to fight S. aureus-induced pneumonia, but that it contributes to the inflammatory response in the lungs. Interestingly, the absence of NOD2 led to a lesser inflammation and was finally beneficial for the animal recovery.