Host mechanisms governing Staphylococcus aureus persistence during implant-associated osteomyelitis

About This Grant

PROJECT SUMMARY Bone implant-associated infections remain a major challenge in orthopaedic surgery. Staphylococcus aureus, a significant human pathogen, continues to be the leading cause of persistent bone implant-associated infections. Reinfections rates following orthopaedic surgery have remained constant over the last half-century, which has led to the orthopaedic paradigm that S. aureus infection of bone is broadly incurable. However, it is also known that some patients can resolve acute infections and live a full life with asymptomatic S. aureus bone infections. Unfortunately, clinical diagnostics to guide conservative vs. aggressive surgical treatment of these patients are anecdotal, time-based, and not evidence-based. To the end of developing functional clinical biomarkers, we performed host studies in a humanized mouse model, and found that implant-associated osteomyelitis is characterized by increases in bacterial load and bone osteolysis, and large numbers of proliferating human T cells adjacent to purulent abscesses in the bone marrow. Subsequent multi-omic investigation of human T cells in an improved humanized NSG-SGM3 BLT mouse model revealed remarkable human T cell heterogeneity in gene expression and numbers, upregulation of immune checkpoint proteins (TIM- 3 and LAG3) in Th1 and Th17 cells, and diminished cytokine production in CD4+ T cells 2-weeks post-infection. Surprisingly, these immune checkpoint proteins were upregulated in the serum of patients with S. aureus osteomyelitis, and a preliminary multiparametric nomogram revealed that TIM-3, LAG3, and PD-1 levels are predictive of adverse outcomes (arthrodesis, reinfection, amputation, and septic death) in these patients. Thus, our results indicate that functional impairment of T cells could occur in chronic osteomyelitis. To unravel this and derive a prognostic to guild life-altering surgical decisions, we hypothesize that persistent S. aureus infections cause impairment of CD4+ Th1/Th17 cells in the form of immune checkpoint expression and functional exhaustion at the bone infection site, and can be leveraged as a functional biomarker of S. aureus implant- associated osteomyelitis disease outcome. To test this, we will characterize the kinetics of human Th1/Th17 dysfunction during S. aureus bone infections in humanized BLT mice as the disease progresses from the acute to the chronic phase (Aim 1), and elucidate the mechanistic link between CD4 T cell exhaustion and disease progression, which could potentially be overcome by immune checkpoint blockade adjuvant therapy (Aim 2). Finally, we will assess if a blood-based multiparametric nomogram examining checkpoint proteins and associated cytokines is predictive of disease outcomes in patients with clinically defined S. aureus “acute” and “chronic” implant-associated bone infections (Aim 3). Collectively, these studies will provide novel insights into mechanisms of musculoskeletal injuries such as S. aureus osteomyelitis and will derive an evidence-based functional biomarker of human disease outcomes that could guide life-changing clinical decisions in orthopaedic surgery.