Modulating granulocytic myeloid-derived suppressor cell (G-MDSC) metabolic activity to promote Staphylococcus aureus biofilm clearance

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Staphylococcus aureus (S. aureus) is a leading cause of biofilm-associated prosthetic joint infection (PJI) characterized by antibiotic tolerance and evasion of immune-mediated clearance. Our laboratory has established a critical role for granulocytic myeloid-derived suppressor cells (G-MDSCs), a pathologically activated neutrophil precursor, in attenuating monocyte/macrophage (MФ) proinflammatory properties and neutrophil bactericidal activity that leads to S. aureus biofilm persistence. The metabolic attributes of leukocytes are intimately linked with their inflammatory properties, relationships encompassing the emerging field of immunometabolism. This has been best described for MФs, where biases towards aerobic glycolysis or oxidative phosphorylation (OxPhos) dictate pro- vs. anti-inflammatory activity, respectively. In contrast, little information is available regarding the metabolic tendencies of G-MDSCs during infection and our preliminary studies are the first to demonstrate that G-MDSCs exhibit a glycolytic bias following S. aureus biofilm exposure in vitro and in vivo. Importantly, blocking glycolysis in G-MDSCs attenuated their suppressive activity resulting in decreased biofilm burden in vivo. This provides proof-of-concept that targeting G-MDSC metabolism is a tractable and novel approach to promote biofilm clearance. Recent studies have revealed that mitochondria can directly traffic between cells in vitro and in vivo via tunneling nanotubes (TNTs), where mitochondrial transfer in recipient cells promotes their OxPhos activity. However, most of these reports examined TNT-mediated mitochondrial transfer between mesenchymal stem cells and endothelial cells or cardiomyocytes, whereas this mechanism of intercellular metabolic rewiring has not been explored in the context of MФ-G-MDSC crosstalk. These studies will leverage MФs as a source of mitochondria for reprogramming G-MDSC metabolism, which originated from our prior work showing that MФ adoptive transfer reduced S. aureus biofilm burden in vivo. Indeed, our preliminary data support this innovative concept, where MФs transferred their mitochondria to G-MDSCs in a cell contact-dependent manner, which skewed G-MDSC metabolism towards OxPhos. This R21 revision will investigate the hypothesis that increasing mitochondrial abundance in G-MDSCs will result in metabolic reprogramming from glycolysis to an OxPhos bias coincident with diminished immune suppressive activity, resulting in improved biofilm clearance. Understanding how mitochondrial transfer from MФs can reprogram G- MDSC metabolism will be examined leveraging natural transfer and exogenous mitochondrial treatment paradigms in the following Specific Aims. 1) Identify the functional implications of mitochondrial transfer on G- MDSC anti-inflammatory activity and 2) Determine whether augmenting G-MDSC mitochondrial activity improves S. aureus clearance during PJI. These studies will inform our long-term goal of targeting critical metabolic nodes to reprogram aberrant leukocyte anti-inflammatory responses to eradicate biofilm together with antibiotics.