Background: Ultrasound (US)-guided Synovial Tissue (ST) biopsy has emerged as a safe procedure for translational research into inflammatory arthritis. The histological and molecular analysis of ST heterogeneity have a prospect to unravel the extreme clinical complexity of the disease pathotypes and to inform treatment strategies.

Objectives: To carry out a multi-modal assessment of heterogeneity of ST obtained through US-guided biopsies from patients with Psoriatic Arthritis (PsA), across different disease phases.

Methods: 397 patients fulfilling the CASPAR criteria[1] for PsA underwent US-guided ST biopsy. At baseline, patients were categorized based on their disease phase: n=210 naïve to csDMARDs; n=131 resistant to cs/b-DMARDs and n=56 in sustained clinical and US remission or in low disease activity (Rem/LDA) state based on DAPSA score[2]. All ST specimens were processed for H&E-based semiquantitative synovitis quantification (KSS)[3], implemented with the pathotype assessment[4].Each naive to treatment PsA was treated according to the EULAR/GRAPPA[5,6] recommendations and DAPSA Rem/LDA rate at 6 months was recorded. In addition, ST from n=18 patients, representing all disease phases, were digested and alive cells were sorted using FACS-ARIA III and single-cell RNA sequencing (scRNAseq) conducted using 10x Genomics (Chemistry 3′ v3.1, Illumina HiSeq 4000, with depth of 20,000 reads/cell).

Results: The distribution of KSS was significantly different across the different PsA phases (ANOVA p<0.001). KSS was significantly higher in cs/b-DMARDs resistant patients than naive PsA(p<0.0001) and PsA in remission/LDA (p<0.0001). Regardless of the disease phase, KSS directly correlated with disease activity in terms of DAPSA (r=0.476, p<0.001). Evaluation of cellular composition showed that naive and cs/b-DMARDs resistant PsA had similar rates of diffuse-myeloid (DM) pathotype (47,2% and 45%, p=0.69. However, cs/b-DMARDs resistant PsA exhibited higher rate of lympho-myeloid (LM) pathotype than naïve (p=0,002). Among naïve PsA patients, KSS was significantly higher in those with concomitant dactylitis (p<0.001) and skin (p=0.04) and nail disease (p<0.001). Moreover, those who achieved Rem/LDA at 6 months showed, at baseline, significantly lower KSS compared to those not achieving this clinical outcome (p<0.001). In single cell analysis, we obtained transcriptomic profiles for 174,672 synovial tissue-derived cells from which, 17,932 myeloid cells were computationally isolated based on the expression of CD14, CD163, MARCO, LYZ, CD11b and CD64. We identified 9 different myeloid subclusters: 2 clusters of lining layer synovial tissue macrophages (STMs) based on the expression of MerTK ^pos TREM2 ^pos TIMD4 ^pos and MerTK ^pos TREM2 ^low TIMD4 ^low respectively, 5 clusters of sublining layer STMs (MerTK ^pos FOLR2 ^pos LYVE1 ^pos , MerTK ^pos FOLR2 ^pos LYVE1 ^pos ICAM1 ^pos , MerTK ^neg S100A12 ^pos , MerTK ^neg ISG15 ^pos SLAMF7 ^pos , CLEC10A ^pos ) and 2 clusters of dendritic cells (DCs) (CD1c ^pos CLEC10A ^pos DC2, CD1c ^neg CLEC10A ^pos iDC3). Each of these clusters was present across all disease phases but differed in relative proportions. Specifically, ST of naive PsA with LM pathotype was enriched with sublining MerTK ^neg S100A12 ^pos STMs as compared to naive PsA with DM pathotype. ST of cs/b-DMARDs resistant PsA with DM pathotype showed an expansion of sublining MerTK ^neg S100A12 ^pos and MerTK ^neg ISG15 ^pos SLAMF7 ^pos STMs compared to the same disease state with pauci-immune (PI) pathotype. Finally, ST of naive and cs/b-DMARDs resistant PsA showed an enrichment of CD1c ^neg CLEC10a ^pos /iDC3, both in patients with LM and DM pathotypes, compared to the same disease state with PI pathotype.

Conclusion: KSS is a reliable tool for synovitis assessment in PsA, being related to clinical outcome. Transcriptomic analysis revealed that distinct STM and DC clusters contribute to different pathotypes of naive and resistant to treatment PsA.

REFERENCES: [1] Taylor W, et al. Arthritis Rheum. 2006.

[2] Schoels M, et al. Ann Rheum Dis. 2010.

[3] Krenn V, et al. Histopathology. 2006.

[4] Alivernini S, et al. Arthritis Rheumatol. 2021.

[5] Gossec L, et al. Ann Rheum Dis. 2020.

[6] Coates LC, et al. J Rheumatol. 2022.

Acknowledgements: NIL.

Disclosure of Interests: None declared.