Disruption of the sigS gene attenuates the local innate immune response to Staphylococcus aureus in a mouse mastitis model

Staphylococcus aureus SigS Induces Expression of a Regulatory Protein Pair That Modulates Its mRNA Stability

SigS is the sole extracytoplasmic function sigma factor in Staphylococcus aureus and is necessary for virulence, immune evasion, and adaptation to toxic chemicals and environmental stressors. Despite the contribution of SigS to a myriad of critical phenotypes, the downstream effectors of SigS-dependent pathogenesis, immune evasion, and stress adaptation remain elusive. To address this knowledge gap, we analyzed the S. aureus transcriptome following transient overexpression of SigS. We identified a bicistronic transcript, upregulated 1,000-fold, containing two midsized genes, each containing single domains of unknown function (DUFs). We renamed these genes SigS-regulated orfA (sroA) and SigS-regulated orfB (sroB). We demonstrated that SigS regulation of the sroAB operon is direct by using in vitro transcription analysis. Using Northern blot analysis, we also demonstrated that SroA and SroB have opposing autoregulatory functions on the transcriptional architecture of the sigS locus, with SroA stimulating SigS mRNA levels and SroB stimulating s750 (SigS antisense) levels. We hypothesized that these opposing regulatory effects were due to a direct interaction. We subsequently demonstrated a direct interaction between SroA and SroB using an in vivo surrogate genetics approach via bacterial adenylate cyclase-based two-hybrid (BACTH) analysis. We demonstrated that the SroA effect on SigS is at the posttranscriptional level of mRNA stability, highlighting a mechanism likely used by S. aureus to tightly control SigS levels. Finally, we demonstrate that the sroAB locus promotes virulence in a murine pneumonia model of infection. IMPORTANCE SigS is necessary for S. aureus virulence, immune evasion, and adaptation to chemical and environmental stressors. These processes are critically important for the ability of S. aureus to cause disease. However, the SigS-dependent transcriptome has not been identified, hindering our ability to identify downstream effectors of SigS that contribute to these pathogenic and adaptive phenotypes. Here, we identify a regulatory protein pair that is a major direct target of SigS, known as SroA and SroB. SroA also acts to stimulate SigS expression at the posttranscriptional level of RNA turnover, providing insight into intrinsically low levels of SigS. The discovery of SroA and SroB increases our understanding of SigS and the S. aureus pathogenesis process.

Battle royale: Immune response on biofilms – host-pathogen interactions

The research interest of the scientific community in biofilm-forming microorganisms is growing due to the problems caused by their infections affecting humans and animals, mainly because of the difficulty of the host immune system in eradicating these microbial complex communities and the increasing antimicrobial resistance rates worldwide. This review describes the virulence factors and their interaction with the microbial communities of four well-known and highly biofilm-forming pathogens, more exactly, Pseudomonas aeruginosa, Escherichia coli, Staphylococcus spp., and Candida spp. The innate and adaptive immune responses caused by the infection with these microorganisms and their evasion to the host immune system by biofilm formation are discussed in the present work. The relevance of the differences in the expression of certain virulence factors and the immune response in biofilm-associated infections when compared to planktonic infections is usually described as the biofilm architecture protects the pathogen and alters the host immune responses, here we extensively discussed these mechanisms.

Staphylococcus aureus Extracellular Vesicles Elicit an Immunostimulatory Response in vivo on the Murine Mammary Gland

Staphylococcus aureus is a major pathogen responsible for bovine mastitis, the most common and costly disease affecting dairy cattle. S. aureus naturally releases extracellular vesicles (EVs) during its growth. EVs play an important role in the bacteria-bacteria and bacteria-host interactions and are notably considered as nanocarriers that deliver virulence factors to the host tissues. Whether EVs play a role in a mastitis context is still unknown. In this work, we showed that S. aureus Newbould 305 (N305), a bovine mastitis isolate, has the ability to generate EVs in vitro with a designated protein content. Purified S. aureus N305-secreted EVs were not cytotoxic when tested in vitro on MAC-T and PS, two bovine mammary epithelial cell lines. However, they induced the gene expression of inflammatory cytokines at levels similar to those induced by live S. aureus N305. The in vivo immune response to purified S. aureus N305-secreted EVs was tested in a mouse model for bovine mastitis and their immunogenic effect was compared to that of live S. aureus N305, heat-killed S. aureus N305 and to S. aureus lipoteichoic acid (LTA). Clinical and histopathological signs were evaluated and pro-inflammatory and chemotactic cytokine levels were measured in the mammary gland 24 h post-inoculation. Live S. aureus induced a significantly stronger inflammatory response than that of any other condition tested. Nevertheless, S. aureus N305-secreted EVs induced a dose-dependent neutrophil recruitment and the production of a selected set of pro-inflammatory mediators as well as chemokines. This immune response elicited by intramammary S. aureus N305-secreted EVs was comparable to that of heat-killed S. aureus N305 and, partly, by LTA. These results demonstrated that S. aureus N305-secreted EVs induce a mild inflammatory response distinct from the live pathogen after intramammary injection. Overall, our combined in vitro and in vivo data suggest that EVs are worth to be investigated to better understand the S. aureus pathogenesis and are relevant tools to develop strategies against bovine S. aureus mastitis.

Anti-inflammatory signaling by mammary tumor cells mediates prometastatic macrophage polarization in an innovative intraductal mouse model for triple-negative breast cancer

Background

Murine breast cancer models relying on intraductal tumor cell inoculations are attractive because they allow the study of breast cancer from early ductal carcinoma in situ to metastasis. Using a fully immunocompetent 4T1-based intraductal model for triple-negative breast cancer (TNBC) we aimed to investigate the immunological responses that guide such intraductal tumor progression, focusing on the prominent role of macrophages.

Methods

Intraductal inoculations were performed in lactating female mice with luciferase-expressing 4T1 mammary tumor cells either with or without additional RAW264.7 macrophages, mimicking basal versus increased macrophage-tumor cell interactions in the ductal environment. Imaging of 4T1-derived luminescence was used to monitor primary tumor growth and metastases. Tumor proliferation, hypoxia, disruption of the ductal architecture and tumor immune populations were determined immunohistochemically. M1- (pro-inflammatory) and M2-related (anti-inflammatory) cytokine levels were determined by Luminex assays and ELISA to investigate the activation state of the macrophage inoculum. Levels of the metastatic proteins matrix metalloproteinase 9 (MMP-9) and vascular endothelial growth factor (VEGF) as well as of the immune-related disease biomarkers chitinase 3-like 1 (CHI3L1) and lipocalin 2 (LCN2) were measured by ELISA to evaluate disease progression at the protein level.

Results

Mice intraductally co-injected with macrophages showed severe splenomegaly with faster ductal breakthrough of tumor cells and increased metastases in axillary lymph nodes and lungs. These mice showed higher M1-related cytokines in the early disease stages (at 1 to 3 weeks post-inoculation) due to the pro-inflammatory nature of RAW264.7 macrophages with increased Ly6G-positive neutrophils and decreased anti-inflammatory macrophages in the tumor microenvironment. However, upon metastasis (at 5 weeks post-inoculation), a prominent increase in M2-related cytokine levels was detected and established a tumor microenvironment with similar immune populations and cytokine responses as in mice which received only 4T1 tumor cells. The observed tumor-associated immune responses and the increased metastasis were associated with significantly induced local and systemic levels of MMP-9, VEGF, CHI3L1 and LCN2.

Conclusions

The current experimental study with an innovative immunocompetent intraductal model for TNBC pinpoints towards a metastasis-supporting M1 to M2 macrophage polarization in the mammary ducts mediated by 4T1-derived signaling. We propose to explore this process as immunotherapeutic target.

Electronic supplementary material

The online version of this article (10.1186/s13046-018-0860-x) contains supplementary material, which is available to authorized users.

Identification of a unique transcriptional architecture for the sigS operon in Staphylococcus aureus

Staphylococcus aureus possess three alternative σ factors, including a lone extracytoplasmic function σ factor, σS. Our group previously identified and characterized this element, mapping three sigS promoters, demonstrating its inducibility during stress and virulence inducing conditions and demonstrating a role for this factor in disease causation. In the present study, we identify a fourth promoter of the sigS operon, termed P4, located in a unique position internal to the sigS coding region. Transcriptional profiling revealed that expression from P4 is dominant to the three upstream promoters, particularly upon exposure to chemical stressors that elicit DNA damage and disrupt cell wall stability; each of which have previously been shown to stimulate sigS expression. Importantly, expression of this fourth promoter, followed by at least one or more of the upstream promoters, is induced during growth in serum and upon phagocytosis by RAW 264.7 murine macrophage-like cells. Finally, we demonstrate that a downstream gene, SACOL1829, bears a large 3΄ UTR that spans the sigS-SACOL1828 coding region, and may serve to compete with the P4 transcript to inhibit σS production. Collectively, these findings reveal a unique operon architecture for the sigS locus that indicates the potential for novel regulatory mechanisms governing its expression.

Preconditioning with Lipopolysaccharide or Lipoteichoic Acid Protects against Staphylococcus aureus Mammary Infection in Mice

Staphylococcus aureus is one of the most causative agents of mastitis and is associated with chronic udder infections. The persistency of the pathogen is believed to be the result of an insufficient triggering of local inflammatory signaling. In this study, the preclinical mastitis model was used, aiming to evaluate if lipopolysaccharide (LPS) or lipoteichoic acid (LTA) preconditioning could aid the host in more effectively clearing or at least limiting a subsequent S. aureus infection. A prototypic Gram-negative virulence factor, i.e., LPS and Gram-positive virulence factor, i.e., LTA were screened whether they were able to boost the local immune compartment. Compared to S. aureus-induced inflammation, both toxins had a remarkable high potency to efficiently induce two novel selected innate immunity biomarkers i.e., lipocalin 2 (LCN2) and chitinase 3-like 1 (CHI3L1). When combining mammary inoculation of LPS or LTA prior to a local S. aureus infection, we were able to modulate the innate immune response, reduce local bacterial loads, and induce either LCN2 or CHI3L1 at 24 h post-infection. Clodronate depletion of mammary macrophages also identified that macrophages contribute only to a limited extend to the LPS/LTA-induced immunomodulation upon S. aureus infection. Based on histological neutrophil influx evaluation, concomitant local cytokine profiles and LCN2/CHI3L1 patterns, the macrophage-independent signaling plays a major role in the LPS- or LTA-pretreated S. aureus-infected mouse mammary gland. Our results highlight the importance of a vigilant microenvironment during the innate immune response of the mammary gland and offer novel insights for new approaches concerning effective immunomodulation against a local bacterial infection.