Background/Purpose: Fibrosis in multiple organs is the defining hallmark that accounts for the high mortality associated with systemic sclerosis (SSc). Understanding how SSc patients develop fibrosis is crucial for developing new treatments. Our goal is to discover fibrosis-driving cellular mechanisms that can lead to novel therapeutics for SSc. Here we focus on non-motile primary cilia (PC), dynamic solitary cellular organelles, that serve as antennae for both chemical and mechanical cues.

Methods: We use a comprehensive orthogonal approach encompassing human, cellular, and animal studies.

Results: A robust skin transcriptomic meta-analysis of multiple independent datasets with both microarrays (n=8) and scRNA-seq (n=3), revealed a distinct cilia gene signature associated with SSc. Gene ontology analysis showed a predominant downregulation of ciliary assembly genes and concomitant upregulation of ciliary disassembly genes in SSc biopsies. Pseudotime analysis on PC signature genes indicated that their expression in SSc fibroblasts precedes differentiation into myofibroblasts. Additionally, we observed consistently reduced PC length in SSc skin biopsies (n=5) compared to biopsies from matched healthy donors. Similarly, ex vivo cultured lesional skin fibroblasts from SSc patients (n=4) showed reduced primary cilia length. Interestingly, fibroblasts from skin biopsies of individuals (n=3) with Very Early Diagnosis of SSc (VEDOSS), who develop clinically detectable skin thickening within 1-5 years, also exhibited shorter PC compared to skin fibroblasts from age-matched healthy controls. To determine if PC disruption is sufficient to elicit fibrotic responses, we used the PC inhibitors Ciliobrevin D and HPI-4. Pharmacological disruption (n=3) of PC in mouse skin fibroblasts activated profibrotic TGF-β and Hippo signaling, leading to their myofibroblast transformation. Moreover, genetic disruption (n=6) of the PC-associated gene Spag17 in fibroblasts resulted in shortened PC that was accompanied by ligand-independent activation of TGF-β and Hippo signaling. Targeting PC using LiCl (n=6), a pharmacological compound known to impact PC length, significantly reduced fibrotic features such as ASMA and collagen expression. It also reversed profibrotic signaling and restored PC length in Spag17 knockout fibroblasts.

Conclusion: Differential expression of PC genes (the primary cilia signature) in the skin is associated with SSc, and PC shortening is an early hallmark event in SSc pathogenesis that drives fibrotic reprogramming of dermal fibroblasts. Accordingly, PC may represent a therapeutic target for SSc, suggesting the possibility of ciliotherapy as a treatment.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/unveiling-the-primary-cilia-signature-driving-systemic-sclerosis-pathogenesis/