Background: The profound alterations in the structure, cellular composition, and function of synovial tissue in rheumatoid arthritis (RA) underlie the persistent inflammation and cumulative joint destruction that are hallmarks of this disease.

Several biomolecules originating from infiltrating inflammatory cells and activated resident cells in the RA joint, and present in the serum, which remain poorly characterized, likely play a crucial role in the disease’s pathophysiology.

Objectives: To identify synovial tissue secreted proteins that may contribute to the inflammatory serum profile, assess their correlation with clinical manifestations of RA, and evaluate their responsiveness to biologic and targeted synthetic DMARDs.

Methods: Synovial explants obtained from biopsies of 17 RA patients were cultured in vitro for 24h, to characterize the secreted protein profile of this tissue. Tissues from each patient were also treated with etanercept, sarilumab or baricitinib (all 10 micromolar). Protein levels and drug-induced changes were assessed in supernatants by proximity extension assay -PEA- technology (Olink)), analyzing a panel of 92 inflammation-related proteins. Concurrently, that proteomic profile was analyzed in the serum samples from the same subjects. Findings were further validated in the serum of an independent cohort of 185 RA patients.

Results: Levels of ten synovial-secreted proteins correlated with those present in the serum of RA patients (CDCP1, CXCL1, IL10RA, IL10RB, IL10, IL15RA, PDL1, CCL28, IFN gamma, and FGF19). These proteins influence RA pathophysiology, affecting T cells, synovium chemotaxis, immune cells activity, inflammatory cytokines, B cells, autoantibodies, reactive species, antigen presentation, macrophage activation, and cell proliferation.

Clinically, elevated levels of these proteins were noted in RA patients with active disease (DAS28 score > 3.2), increased acute phase reactants (APR), and positivity for ACPAs.

To validate the findings, serum levels of this protein signature was assessed in a separate cohort of 185 RA patients with established disease. Hierarchical unsupervised clustering identified two patient groups, with 46% exhibiting elevated expression of the aforementioned 10 proteins. Clinically, these patients demonstrated increased disease activity, elevated APRs, longer disease duration, and associated comorbidities such as heightened cardiovascular risk, indicated by atheroma plaques, an increased atherogenic index, and arterial hypertension. Intriguingly, these patients also exhibited a more favorable clinical response to bDMARDs after six months, as evaluated by EULAR criteria.

Finally, in vitro studies demonstrated that culturing synovial tissues from patients who exhibited elevated basal levels of the identified 10-protein signature with TNFi, IL6Ri, or JAKinibs for 24 hours resulted in a more significant modulation of inflammatory mediators compared to those with lower protein levels of this signature. Furthermore, distinct responses were observed for each therapeutic approach.