Cellular crosstalk and signaling networks in the rheumatoid arthritis synovial microenvironment

Rheumatoid arthritis (RA) is a chronic autoimmune disease characterized by synovial inflammation, pannus formation, and progressive cartilage and bone destruction. Within the RA synovial microenvironment, resident synoviocytes, mesenchymal stem cells, fibroblasts, adipocytes, vascular-associated cells, and diverse immune cell populations form a dynamic interaction network through direct contact and paracrine mediators, including cytokines, chemokines, complement components, and extracellular vesicles. This review summarizes how these cellular interactions drive RA along a pathological continuum from early autoimmune initiation, through middle-stage inflammatory amplification and synovial hyperplasia, to late fibrosis, pannus formation, dysregulated bone remodeling, and irreversible structural damage. Particular emphasis is placed on the dynamic balance between pathogenic cellular circuits and immunoregulatory programs within the synovial microenvironment, which helps determine whether the joint remains in an inflammatory-active state, re-enters a remission-associated and relatively rebalanced state, or progresses toward remission failure and structural injury. We further discuss the major signaling pathways that mediate these interactions, especially NF-κB, MAPK, JAK-STAT, TGF-β/Smad, and Wnt/β-catenin signaling, highlighting how pathway crosstalk contributes to inflammatory persistence, loss of tissue plasticity, and progressive remodeling. Importantly, because key cellular subsets and interaction programs may still retain partial plasticity during the early and middle stages of disease, stage-adapted modulation of these pathogenic networks may help restore synovial immune homeostasis, promote remission, delay disease progression, and reduce irreversible tissue damage. A deeper understanding of stage-specific cellular programs and interaction networks may therefore provide a stronger theoretical basis for mechanism-informed precision therapies in RA.