Single-cell RNA sequencing unravels chondrocyte heterogeneity and immune cell interactions in knee osteoarthritis...

Osteoarthritis (OA) is a degenerative joint disorder characterized by progressive articular cartilage destruction and chronic inflammation. However, the dynamic changes in immune cell subpopulations during OA progression and their specific regulatory interactions with chondrocyte subsets remain poorly understood.

We aim to comprehensively characterize disease-specific subpopulations of chondrocytes and their distinct molecular signatures, as well as to decode the compositional diversity of infiltrating immune cells by a secondary observational analysis of public GEO database.

We integrated four single-cell RNA sequencing datasets of 20 knee OA patients and 6 healthy donors. Cellular subpopulations were identified using t-SNE-based clustering. Differential gene expression analysis was performed, followed by pathway enrichment analysis to elucidate key biological processes. Chondrocyte differentiation trajectories were reconstructed using pseudotime analysis. Intercellular communication networks were deciphered using CellChat.

Chondrocytes were subdivided into eight distinct subpopulations. GO enrichment analysis highlighted extracellular matrix deposition, mineralization, and chondrocyte differentiation as central biological events. Trajectory analysis indicated that fibrotic chondrocytes occupy terminal positions. Among lymphocytes, effector T cells and NK cells appeared to mediate cytotoxic effects, with complement pathway activation playing a key role in regulating their functions. Macrophages constituted the majority of myeloid cells and were categorized into four subpopulations, among C1Q⁺macrophages were strongly associated with complement activation. CellChat analysis suggested that MIF signaling may promote inflammatory microenvironment formation in OA.

We propose a model wherein chondrocyte-derived MIF recruits immune cells during progression of OA. Attached immune cells collectively sustain a chronic inflammatory milieu. Additionally, C1Q⁺macrophages produce complement components that activate the complement cascade, stimulating C5AR1⁺macrophages and enhancing lymphocyte-mediated cytotoxicity. These findings provide novel insights into the cellular mechanisms and intercellular crosstalk underlying OA progression. Key Points • The fibrotic chondrocytes occupy terminal differentiation stages of chondrocytes in OA. • Complement-activated effector T cells and NK cells mediate cytotoxic effects within the OA microenvironment. • C1Q⁺macrophages were closely linked to complement activation. • MIF signaling may promote inflammatory microenvironment formation in OA.