Staphylococcus aureus induces cell-surface expression of immune stimulatory NKG2D ligands on human monocytes

Staphylococcus aureusis among the leading causes of bacterial infections worldwide. The pathogenicity and establishment ofS. aureusinfections are tightly linked to its ability to modulate host immunity. Persistent infections are often associated with mutant staphylococcal strains that have decreased susceptibility to antibiotics; however, little is known about how these mutations influence bacterial interaction with the host immune system. Here, we discovered that clinicalS. aureusisolates activate human monocytes, leading to cell-surface expression of immune stimulatory natural killer group 2D (NKG2D) ligands on the monocytes. We found that expression of the NKG2D ligand ULBP2 (UL16-binding protein 2) is associated with bacterial degradability and phagolysosomal activity. Moreover,S. aureus-induced ULBP2 expression was linked to altered host cell metabolism, including increased cytoplasmic (iso)citrate levels, reduced glycolytic flux, and functional mitochondrial activity. Interestingly, we found that the ability ofS. aureusto induce ULBP2 and proinflammatory cytokines in human monocytes depends on a functional ClpP protease inS. aureus. These findings indicate thatS. aureusactivates ULBP2 in human monocytes through immunometabolic mechanisms and reveal thatclpPinactivation may function as a potential immune evasion mechanism. Our results provide critical insight into the interplay between the host immune system andS. aureusthat has evolved under the dual selective pressure of host immune responses and antibiotic treatment. Our discovery of an immune stimulatory pathway consisting of human monocyte-based defense againstS. aureussuggests that targeting the NKG2D pathway holds potential for managing persistent staphylococcal infections.