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

Maiken Mellergaard,Rikke Illum Høgh,Astrid Lund,Blanca Irene Aldana,Romain Guérillot,Sofie Hedlund Møller,Ashleigh S Hayes,Nafsika Panagiotopoulou,Zofija Frimand,Stine Dam Jepsen,Camilla Hartmann Friis Hansen,Lars Andresen,Anders Rhod Larsen,Anton Y Peleg,Timothy P Stinear,Benjamin P Howden,Helle S Waagepetersen,Dorte Frees,Søren Skov

doi:10.1074/jbc.ra120.012673

Abstract

Staphylococcus aureus is among the leading causes of bacterial infections worldwide. The pathogenicity and establishment of S. aureus infections 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 clinical S. aureus isolates 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 of S. aureus to induce ULBP2 and proinflammatory cytokines in human monocytes depends on a functional ClpP protease in S. aureus These findings indicate that S. aureus activates ULBP2 in human monocytes through immunometabolic mechanisms and reveal that clpP inactivation may function as a potential immune evasion mechanism. Our results provide critical insight into the interplay between the host immune system and S. aureus that 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 against S. aureus suggests that targeting the NKG2D pathway holds potential for managing persistent staphylococcal infections.

Highlights

Staphylococcus aureus is among the leading causes of bacterial infections worldwide
Upon S. aureus exposure, we found that ULBP2 was induced on the surface of THP1 with expression increasing over time (Fig. 1B)
We show that natural killer group 2D (NKG2D) ligands are induced in human monocytes in response to S. aureus

Summary

Introduction

Staphylococcus aureus is among the leading causes of bacterial infections worldwide. The pathogenicity and establishment of S. aureus infections are tightly linked to its ability to modulate host immunity. We discovered that clinical S. aureus isolates 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. Daptomycin exposure led to mutations in the rpoB and clpP genes, resulting in a thickened bacterial cell wall and increased resistance to lysosomal degradation, and we show that a mutation in the highly conserved S. aureus clpP gene hampered ULBP2-mediated immune activation and secretion of proinflammatory cytokines. We show that S. aureus mediates a metabolic change of monocytes characterized by lowered glycolysis and elevated intracellular level of (iso) citrate that is likely causally involved in NKG2D ligand regulation, because citrate and inhibitors of glycolysis directly induce ULBP2 expression in monocytes. Our study describes induction of NKG2D ligands by S. aureus on human monocytes and connects S. aureus with an altered metabolic phenotype causing surface expression of ULBP2 in human monocytes

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Results

Conclusion