Integrated Single-Cell and Bulk RNA Sequencing Reveals the Diagnostic Value of Synovial Tissue Calprotectin and the MDSC-Associated Immune Microenvironment in Periprosthetic Joint Infection

Background: Periprosthetic joint infection (PJI) lacks a universally accepted gold standard for diagnosis, and the immune microenvironment underlying PJI remains incompletely understood. Calprotectin (S100A8/A9) emerged as a key molecule of interest in PJI, yet its expression profile in synovial tissue has not been fully characterized. Methods: Bulk RNA sequencing was performed on sonication fluid samples from 53 PJI patients and 40 aseptic failure (AF) controls from the GEO database. Differential gene expression, immune cell infiltration, and pathway enrichment analyses were conducted. Single-cell RNA sequencing (scRNA-seq) data from synovial tissue were integrated to characterize the cellular distribution of S100A8 and S100A9 and to investigate immune cell interactions using the CellChat algorithm. Immunohistochemical staining was performed to validate calprotectin expression in synovial tissue. Results: By integrating bulk and scRNA-seq, we characterized the cellular expression profile of S100A8 and S100A9 within the immune microenvironment of PJI synovial tissue. The results showed that S100A8 and S100A9 were markedly upregulated in PJI synovial tissue and predominantly expressed by myeloid-derived suppressor cells (MDSCs). The enrichment of S100A8/A9-expressing MDSCs in PJI synovial tissue was associated with immune features indicative of an immunosuppressive microenvironment. Cell–cell communication analysis revealed extensive interactions between MDSCs and NK cells as well as macrophages in the PJI microenvironment. Furthermore, immunohistochemical analysis demonstrated significantly elevated calprotectin expression in PJI synovial tissue, and receiver operating characteristic (ROC) curve analysis supported the diagnostic value of synovial tissue calprotectin for PJI. Conclusion: This study demonstrates that S100A8 and S100A9 are highly expressed in the synovial tissue of patients with PJI and provides additional evidence supporting the diagnostic value of calprotectin in synovial tissue. In addition, we comprehensively characterize the cellular expression profiles of S100A8 and S100A9 in PJI synovial tissues, revealing a close association with MDSC-related immunosuppression.