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.