Daptomycin-resistant Staphylococcus aureus clinical isolates are poorly sensed by dendritic cells.

Methicillin-resistant Staphylococcus aureus (MRSA) presents an increasing threat to public health, with antimicrobial resistance on the rise and infections endemic in the hospital setting. Despite a global research effort to understand and combat antimicrobial resistance, less work has focused on understanding the nuances in the immunopathogenesis of clinical strains. In particular, there is a surprising gap of knowledge in the literature pertaining to how clinical strains are recognized by dendritic cells (DCs). Here, we show that the activation of DCs is compromised in response to MRSA strains resistant to the last-line antibiotic daptomycin. We found a significant reduction in the secretion of proinflammatory cytokines including tumor necrosis factor-α, interleukin-6, regulated upon activation, normal T cell expressed, and secreted and macrophage inflammatory protein-1ß, as well as decreased expression of CD80 by DCs responding to daptomycin-resistant MRSA. We further demonstrate that this phenotype is coincident with the acquisition of specific point mutations in the cardiolipin synthase gene cls2, and, partly, in the bifunctional lysylphosphatidylglycerol flippase/synthetase gene mprF, which are genes that are often mutated in clinical daptomycin-resistant strains. Therefore, throughout infection and antibiotic therapy, MRSA has the capacity to not only develop further antibiotic resistance, but also develop resistance to immunological recognition by DCs, because of single amino acid point mutations occurring under the selective pressures of both host immunity and antibiotic therapy. Understanding the diversity of clinical MRSA isolates and the nuances in their immune recognition will have important implications for future therapeutics and the treatment of these infections.

A Simple and Rapid Protocol for the Isolation of Murine Bone Marrow Suitable for the Differentiation of Dendritic Cells.

The generation of bone-marrow-derived dendritic cells is a widely used approach in immunological research to study antigen processing and presentation, as well as T-cell activation responses. However, the initial step of isolating the bone marrow can be time-consuming, especially when larger numbers of precursor cells are required. Here, we assessed whether an accelerated bone marrow isolation method using centrifugation is suitable for the differentiation of FMS-like tyrosine kinase 3 ligand-driven dendritic cells. Compared to the conventional flushing method, the centrifugation-based isolation method resulted in a similar bone marrow cell yield on Day 0, increased cell numbers by Day 8, similar proportions of dendritic cell subsets, and consequently a higher number of type 1 conventional dendritic cells (cDC1) from the culture. Although the primary purpose of this method of optimization was to improve experimental efficiency and increase the output of cDC1s, the protocol is also compatible with the differentiation of other dendritic cell subsets such as cDC2 and plasmacytoid dendritic cells, with an improved output cell count and a consistent phenotype.

Discordance in STING-Induced Activation and Cell Death Between Mouse and Human Dendritic Cell Populations.

Stimulator of Interferon Genes (STING) is a cytosolic sensor of cyclic dinucleotides (CDNs). The activation of dendritic cells (DC) via the STING pathway, and their subsequent production of type I interferon (IFN) is considered central to eradicating tumours in mouse models. However, this contribution of STING in preclinical murine studies has not translated into positive outcomes of STING agonists in phase I & II clinical trials. We therefore questioned whether a difference in human DC responses could be critical to the lack of STING agonist efficacy in human settings. This study sought to directly compare mouse and human plasmacytoid DCs and conventional DC subset responses upon STING activation. We found all mouse and human DC subsets were potently activated by STING stimulation. As expected, Type I IFNs were produced by both mouse and human plasmacytoid DCs. However, mouse and human plasmacytoid and conventional DCs all produced type III IFNs (i.e., IFN-λs) in response to STING activation. Of particular interest, all human DCs produced large amounts of IFN-λ1, not expressed in the mouse genome. Furthermore, we also found differential cell death responses upon STING activation, observing rapid ablation of mouse, but not human, plasmacytoid DCs. STING-induced cell death in murine plasmacytoid DCs occurred in a cell-intrinsic manner and involved intrinsic apoptosis. These data highlight discordance between STING IFN and cell death responses in mouse and human DCs and caution against extrapolating STING-mediated events in mouse models to equivalent human outcomes.