Synovial immune endotypes associate with structural severity across canine and human osteoarthritis

Osteoarthritis (OA) is a heterogeneous disease in which synovial inflammation drives structural progression and pain, yet the molecular diversity of synovitis remains poorly defined. Here, we performed a multimodal characterization of synovial tissues from 48 dogs with naturally occurring OA, integrating histopathology, immunohistochemistry, transcriptomics, MRI, and soluble mediator profiling. This analysis identified three discrete synovial pathotypes, pauci-immune, diffuse myeloid, and lympho-myeloid, defined by distinct immune cell composition and transcriptional programs reflecting biologically distinct endotypes. The lympho-myeloid pathotype, characterized by ectopic lymphoid structures and adaptive immune enrichment, was independently associated with greater structural joint damage on MRI despite comparable clinical scores, demonstrating that tissue stratification captures heterogeneity invisible to clinical assessment alone. Composite soluble mediator signatures correlated with disease severity across pathotypes. Cross-species analyses integrating a large human synovial cohort revealed conserved pathotype architecture and shared gene networks, including tyrosine kinase signaling and B cell receptor activation, validating the translational relevance of the canine model. Together, these findings provide a framework for patient stratification and therapeutic target discovery in OA, with broad implications for precision medicine within a One Medicine perspective.