LAUSR

Fibroblasts isolated by traditional explant culture from skin biopsies in systemic sclerosis (SSc) have a profibrotic and “migratory” phenotype. Single cell RNAseq (scRNAseq) studies suggest other fibroblast subpopulations may be important in SSc pathogenesis. We have isolated a second fibroblast population termed “resident” from whole skin in SSc. We used bulk and scRNAseq to further characterise these populations. Forearm skin from dcSSc (n = 3) and HC (n = 3) was biopsied. “Migratory” fibroblasts were isolated by standard explant culture. The remaining biopsy fragments underwent collagenase digestion to yield a second “resident” fibroblast population. Functional characterization included 3-D collagen gel contraction, migratory scratch-wound assays and western blot comparing expression of pro-collagen I, CTGF and αSMA. Significant difference in gene expression was defined by fold change≥1.5, and adjusted p-value<0.05. scRNAseq was performed on 12 SSc whole skin biopsies, and 3 HC, using the 10X Genomics platform. Both SSc fibroblast populations showed a fibrotic phenotype with overexpression of COL1, CTGF and αSMA compared to HC, but with reduced expression αSMA in resident compared to migratory SSc fibroblasts (p < 0.05). RNAseq confirmed 1483 genes being significantly differentially expressed between the SSc fibroblast populations (Table 1). All four populations clustered separately on hierarchical clustering. 10 fibroblast clusters were identified using scRNAseq of whole skin. Of these, cluster 0 and 4 showed greater expression of COL10A1, and STC2 in keeping with migratory fibroblasts. Increased expression of CCL2, and EGR1 were seen in clusters 3 and 6, representing the resident fibroblast population. Gene set enrichment analysis (GSEA) confirmed concordance between KEGG pathway upregulation of the migratory and resident fibroblast clusters and bulk RNAseq analysis. Resident fibroblasts showed upregulation of complement and coagulation genes, and JAK/STAT signalling pathway. There was upregulation of glycolysis gluconeogenesis, and PPAR signalling in migratory fibroblasts. There was agreement in GSEA findings between relevant scRNAseq clusters and bulkRNAseq for resident and migratory subpopulations. We have isolated for the first time two distinct profibrotic fibroblast populations from SSc skin with altered expression of key genes associated with SSc pathogenesis. Future work will define their role in SSc pathogenesis, potentially facilitating a more targeted treatment approach. Disclosure K.E. Clark: None. S. Xu: None. V.H. Ong: None. C.D. Buckley: None. C. Denton: None.