Background/Purpose: To identify synovial tissue (ST) biomarkers differentially expressed in Psoriatic Arthritis (PsA) and seronegative Rheumatoid Arthritis (Ab^neg RA) and test their predictive value of therapeutic response.

Methods: 34 PsA patients [12 DMARDs naive and 22 no-responder to Methotrexate (MTX-IR)] with peripheral joint involvement and 55 Ab^neg RA (27 DMARDs naive and 28 MTX-IR) underwent US-guided ST biopsy and immunohistochemistry (IHC) for CD68⁺, CD3⁺, CD20⁺, CD21⁺, CD117⁺and CD138⁺cells. After study entry, each DMARD naive patient started MTX therapy and was fallowed in an outpatient setting for at least 6 months to define the achievement of Minimal Disease Activity (PsA) and DAS-remission (Ab^neg RA) status respectively. Each IR-MTX patient was treated according to EULAR recommendations.

Results: At study entry, there were no significant differences in terms of demographic and clinical characteristics of enrolled PsA and Ab^neg RA cohorts. IHC analysis revealed that naive PsA patients had lower IHC scores of lining and sublining CD20⁺than Ab^neg RA patients (p=0.002 and p=0.05 respectively), whereas no significant differences were found in terms of lining and sublining CD68⁺, CD21⁺and CD3⁺cells among the two cohorts. Moreover, regardless to the therapeutic scheme, PsA patients showed higher IHC score of lining and sublining CD117⁺cells (p<0.001 for both) compared to Ab^neg RA patients. Conversely, Ab^neg RA patients showed higher IHC score of lining and sublining CD138⁺cells, despite the therapeutic scheme (p=0.04 and p=0.01 for naive and MTX-IR respectively). Analysing the response rate to the therapeutic scheme, naive PsA patients reaching MDA status at 6 months follow-up, showed, at study entry, lower IHC score of sublining CD3⁺cells compared to PsA patients not reaching this outcome (p=0.03), conversely, naive Ab^neg RA patients reaching DAS-remission status at 6 months follow-up, showed, at study entry, lower IHC score of sublinig CD68⁺cells compared to Ab^neg RA patients not reaching this outcome (p<0.001).

Conclusion: CD117⁺and CD138⁺cells are differentially distributed among PsA and Ab^neg RA. Histological analysis of ST may help to solve the clinical overlap between the two diseases and provides prognostic data about the therapy success.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/synovitis-in-psoriatic-arthritis-and-seronegative-rheumatoid-arthritis-differential-histological-features/