A novel host-based strategy for detecting low-grade prosthetic joint infection using immune cell pattern in synovial fluid

Reliable biomarkers for the routine diagnostics of low-grade prosthetic joint infection (PJI) remain an unmet clinical need. The cellular and soluble content of synovial fluid (SF) from 108 patients was investigated using flow cytometry and ELISA. The study cohort included 44 patients with total knee/hip arthroplasty (16 with PJI, 8 with low-grade PJI with low α-defensin/CRP levels, 20 with osteolysis/aseptic loosening) and 64 controls with osteoarthritis with/without infection (OA-INF/OA: 28/36). To investigate the interconnectivity between wear particle-associated mechanisms and infection-associated patterns, primary human OA fibroblast-like synoviocytes and OA chondrocytes cultured with/without Ti6Al4V alloy were analysed through RNAseq. A multiparametric patient similarity network (PSN) approach was used to evaluate the results. A multiparametric PSN revealed that the combination of a low proportion of dendritic cells (cDC2s), HLA-DR+ natural killer (NK) cells, a high proportion of NK cells from lymphocytes, a higher percentage of CD88+ cDC1 cells, and neutrophil (NEU) immunophenotypes has demonstrated outstanding performance in detecting PJI, including low-grade PJI, comparing to sterile inflammation induced by wear particles. Moreover, cellular biomarkers on NEUs differed in PJI with culture positivity, low-grade PJI and aseptic loosening. In addition, the gene expression of IL6, CXCL1/3/5/6/8, CCL2/7, and MMP9 of synoviocytes and chondrocytes stimulated by Ti6Al4V alloy particles confirms the interconnectivity between infection- and wear particle-associated mechanisms, which discriminates some traditional soluble infection-associated biomarkers. We propose a novel host-based strategy for detecting low-grade PJI based on composition and immunophenotype of immune cells derived from SF, shifting the emphasis from pathogen detection and ambiguous clinical manifestations to cellular biomarkers. Our findings showed that cellular biomarkers may be beneficial for the detection of PJI, including low-grade PJI, even in the presence of wear particles activating inflammatory responses mimicking infectious conditions. This host-based approach may improve diagnostic strategies for low-grade PJI.