Spatial-cellular resolution analysis of ferroptosis-associated immune microenvironment heterogeneity in osteoarthritic cartilage.

Osteoarthritis (OA) is a degenerative joint disease characterized by progressive cartilage destruction, in which dysregulated cell death and immune microenvironment remodeling play critical roles. Ferroptosis, an iron-dependent form of regulated cell death, has recently been implicated in OA pathogenesis; however, its role in modulating immune responses remains poorly understood. This study aimed to elucidate the interplay between ferroptosis and the immune microenvironment in osteoarthritic cartilage, with a particular focus on the potential association between ferroptosis and macrophage polarization. Bulk transcriptomic analysis was performed to identify ferroptosis-related genes, followed by least absolute shrinkage and selection operator regression to screen key biomarkers. Integrated single-cell and spatial transcriptomic analyses were used to characterize ferroptotic chondrocytes and their spatial association with immune cells. Gene co-expression and functional enrichment analyses identified a ferroptosis-associated core module enriched in pathways related to ferroptosis, inflammatory responses, and oxidative metabolism. Immune infiltration analysis revealed a significant positive correlation between module hub genes and macrophage infiltration, particularly pro-inflammatory macrophages. Spatial transcriptomics further demonstrated marked colocalization between high-ferroptosis chondrocytes and macrophage-enriched regions. Cell-cell communication analysis highlighted the SPP1 signaling pathway as a potential mediator of communication between ferroptotic chondrocytes and macrophages. Mechanistic and in vitro experiments further suggested that ferroptotic chondrocytes may contribute to macrophage polarization toward a pro-inflammatory phenotype, potentially through the SPP1 signaling axis. Collectively, these findings suggest a potential molecular link between ferroptosis and immune inflammation in OA and identify the SPP1-ferroptosis axis as a promising therapeutic target for OA intervention.