Staphylococcal Extracellular Complement-Binding Protein Induces IL-4 Production in Murine Basophils.

The C-type lectin receptor Mincle is functionally expressed by murine bone cells and can mediate inflammatory responses to Staphylococcus aureus 2257334

It is now apparent that osteoblasts (OBs) and osteoclasts (OCs) have immune functions that play a critical role in shaping host responses and the abnormal bone remodeling associated with Staphylococcus aureus (SA) infection. Both express pattern recognition receptors (PRR) to perceive pathogens and initiate the production of mediators that can exacerbate inflammatory bone loss. Macrophage-inducible C-type lectin (Mincle) is a tyrosine activation motif—coupled PRR for microbial glycolipids that has been suggested to play an important role in host responses against Gram positive bacteria in the lungs. However, the ability of Mincle to serve a similar function in bone tissue has not been investigated. RNA sequence analysis was used to assess changes in the transcriptome of murine bone marrow-derived OCs and primary OBs following SA challenge. Additionally, bone cells were either treated with a Mincle-neutralizing antibody, an isotype control antibody, or were left untreated, prior to being stimulated with Mincle-specific ligands or SA infection. Cell supernatants and whole cell protein lysates were subsequently collected to quantify changes in the production of immune mediators by immunoblot analysis and ELISA. RNA Tag-Seq analysis of SA infected OCs and OBs revealed elevated expression of mRNA encoding Mincle and its key downstream signaling components. We found robust levels of Mincle protein in murine OCs and OBs and demonstrated the inducible expression of this molecule in human OBs. The functional nature of Mincle expression by OCs and OBs was confirmed by the ability of Mincle agonists to elicit inflammatory cytokine production. Importantly, we showed that such responses to SA are attenuated following Mincle blockade. These studies show that bone cells functionally express Mincle and indicate that this C-type lectin can mediate, at least in part, the inflammatory responses of OBs to SA challenge. NIAID R01AI170012 Innate Immune Responses and Host Defense: Cellular Mechanisms (INC)

Exploiting classical virulence factor: staphylococcal protein a in immunotherapy and immunodiagnosis

Fibroblasts resist Staphylococcus aureus on the skin surface by responding to interleukin-1 and recruiting neutrophils

Staphylococcus aureus can cause serious infections, yet it can also reside on the skin without causing disease. This apparent paradox implies a strong host defense system, which prevents S. aureus from invading the dermis. In this study, we investigate how the skin detects and responds to superficial S. aureus exposure. Using unbiased transcriptomic, biochemical, and phosphoproteomic analyses, followed by targeted validation in human and mouse models, we found that fibroblast recognition of interleukin-1 is essential for immune response. Deletion or blockade of the interleukin-1 receptor type 1 (IL-1R1) in fibroblasts in vitro abolished keratinocyte-driven changes in gene expression and reduced chemokine production. Furthermore, the skin of mice lacking fibroblast IL-1R1 had fewer neutrophils and higher bacterial load after topical S. aureus application. These findings show that fibroblasts actively participate in innate immunity and highlight an IL-1R1–dependent keratinocyte-fibroblast-neutrophil axis of communication. Understanding this pathway provides insights into mechanisms that initiate neutrophil recruitment to the skin and may help develop new approaches to therapy.

Surface-associated phosphoglycerate kinase (PGK) of Staphylococcus aureus binds human plasminogen in a partially lysine-independent manner.

Findings suggest that PGK may function as a moonlighting protein, playing an essential role in the interaction of S. aureus with its host through plasminogen binding.

ESKAPE bacterial products as modulators of natural killer cell function

Natural killer (NK) cells are innate lymphocytes acting as the key effector cells capable of recognizing and eliminating infected and transformed cells without prior sensitization. Their role in antibacterial defense, inflammatory response regulation and modulation of reproductive processes has been attracting increasing attention. In this regard, of particular interest are opportunistic bacteria of the ESKAPE group (Enterococcus faecium, Staphylococcus aureus, Klebsiella pneumoniae, Acinetobacter baumannii, Pseudomonas aeruginosa, Enterobacter spp.) characterized by high virulence, a prominent potential for developing multiple antibiotic resistance, being a lead cause of severe nosocomial infections, especially in immunocompromised patients. Pathogens of the ESKAPE produce a wide range of cell wall structural components (lipopolysaccharides, peptide glycans, lipoteichoic acids, capsular polysaccharides) as well as secreted virulence factors (pore-forming toxins, proteases, superantigen-like proteins, extracellular membrane vesicles) that can directly modulate immune cell functional activity. This review systematizes current literature data regarding the mechanisms underlying interplay between products of ESKAPE pathogenic bacteria as well as NK cell receptor apparatus and effector functions. Intracellular signaling pathways (NF-κB, MAPK, JAK/STAT) activated upon binding of bacterial ligands to pattern recognition receptors (TLR2, TLR4, TLR9, NOD1/NOD2) and their impact on NK cell cytotoxic potential, cytokine secretion profile (IFNγ, TNFα, IL-1β), and survival are primarily delineated. In addition, the immunomodulatory mechanisms mediated by bacterial metabolites on dendritic cells and macrophages subsequently affecting NK cell activation are discussed as well. Understanding the complex network of molecular crosstalk between ESKAPE pathogen metabolites and NK cells is fundamental for developing novel immunotherapy strategies for drug-resistant infections, designing adjuvant drugs based on bacterial components as well as correcting immune disorders in critical conditions. Altogether, it opens up a new avenue to a personalized treatment approach for patients with severe nosocomial infections.

Beta-2-microglobulin augments neutrophil phagocytosis of bacteria and apoptotic cells.

Polymorphonuclear leukocytes (PMNs), predominantly neutrophil granulocytes, are key components of the innate immune system that eliminate invading pathogens through phagocytosis and clear apoptotic cells through efferocytosis. Beta-2-microglobulin (β2m) is best known as the light chain of major histocompatibility complex class I (MHC I), where it is required for antigen presentation to CD8⁺ T cells. However, emerging evidence suggests that extracellular β2m may also regulate innate immune responses. Here, we show that extracellular β2m enhances neutrophil phagocytosis and efferocytosis. Addition of soluble β2m (50 µg/ml) increased phagocytosis of latex beads by PMNs from 23% to 31%, whereas the proteolytically cleaved variant desLys58-β2m (dK58β2m) had no effect. In contrast, both β2m and dK58β2m enhanced phagocytosis of the Gram-positive bacterium Streptococcus pyogenes and the Gram-negative bacterium Acinetobacter baumannii by more than 3.6-fold. Furthermore, both β2m variants promoted efferocytosis of apoptotic Jurkat cells in a dose-dependent manner, resulting in up to a two-fold increase that was comparable to the effect of GM-CSF. Cytochalasin D abolished β2m-mediated uptake of apoptotic cells. Pre-incubation of latex beads with β2m followed by washing did not enhance phagocytosis, and pre-incubation of PMNs with β2m followed by washing did not enhance subsequent efferocytosis of apoptotic cells. These findings indicate that β2m does not act by coating phagocytic targets or by inducing sustained neutrophil priming. Collectively, these findings identify extracellular β2m as a regulator of neutrophil-mediated phagocytosis and efferocytosis and demonstrate that proteolytic processing differentially influences these activities.