Background Psoriatic arthritis (PsA) is an inflammatory joint disease associated with psoriasis. With biologics entering the therapeutic scene, treatment outcome has dramatically improved; nonetheless, pathogenesis of the condiiton is still under investigation. Exploring the biological differences between PsA and RA at tissue level may help in understanding their pathological pathways. Little comparative data is available in early and untreated disease. With the use of a minimally-invasive, ultrasound-guided, needle biopsy technique, synovium from patients with early and untreated disease and small joint involvement may be studied.

Objectives To evaluate clinical, sonographic and histological features of early untreated psoriatic arthritis versus rheumatoid arthritis.

Methods Patients were selected from a large prospective cohort of treatment-naïve patients with inflammatory arthritis of under 12 months' duration. All patients underwent a clinical and sonographic assessment of arthritis activity and an ultrasound-guided synovial biopsy prior to treatment. 32 PsA and 23 gender- and age-matched RA patients were included in this cross-sectional analysis. Synovial inflammatory infiltrate were assessed using immunohistochemical staining for CD3, CD68, CD20 and CD138. Tissue was classified as lymphoid (presence of grade 2/3 CD20+ aggregates); fibroid (CD68 sublining score ≤2 on a semi-quantitative scale of 0 to 4); or myeloid (CD68 sublining score >2, with or without grade 1 aggregates). Gene-expression analysis of 46 pre-selected genes was performed on 16 PsA and 22 RA patients using Fluidigm technology.

Results Demographic and clinical data at the time of biopsy were comparable between PsA and RA; mean symptom duration was 5.5 for PsA and 4.8 months for RA. Mean tender and swollen joint counts were lower in PsA, nonetheless no significant difference could be found in DAS28. US synovial thickness was similar between the two groups, however a significantly higher Power Doppler score was detected in the biopsied joints of RA patients. Histologically, PsA synovitis score was significantly lower compared to RA; accordingly, PsA synovial tissue showed a lower number of B and T lymphocytes, plasma-cells and sub-lining macrophages. Around 50% of RA synovium demonstrated features of the lymphoid pathotype; in contrast, fibroid (40%) and myeloid (37%) pathotypes were prevalent in PsA (chi-square p<0.05). Surprisingly, expression of pro-inflammatory cytokines such as TNFa, IL1b, IL6, IL23, IL17A, IL12A at mRNA level was comparable between RA and PsA.

Conclusions Despite similar clinical features, early treatment-naïve PsA and RA are histologically deeply different; inflammatory infiltrate is lower in PsA compared to RA. However, pro-inflammatory cytokines are similarly expressed, suggesting differential pathways of translation from synovitis to clinical phenotype.

Disclosure of Interest None declared