Background/Purpose: Aminopeptidase N, also known as CD13, is a Zn²⁺-dependent membrane bound ectopeptidase widely expressed in mammalian cells including rheumatoid arthritis (RA) fibroblast-like synoviocytes (FLS), myeloid cells, and endothelial cells. CD13 exists in two forms, the membrane-binding and the MMP14 cleaved-soluble form. We previously showed that CD13 is shed from FLS. In addition, soluble (s)CD13 induces EC angiogenesis, FLS proliferation, and acute inflammatory arthritis in mice. In earlier studies we showed that sCD13 induces immune cell migration by binding to G-protein-coupled receptors. Recently we have identified bradykinin receptor B1 (B1R) as the previously unknown receptor for sCD13. We propose that CD13 is a compelling new target for scleroderma (SSc), as it induces immune cell migration and promotes fibroblast activation.

Methods: Dermal fibroblasts were isolated from skin biopsies from healthy subjects and patients with diffuse cutaneous (dc)SSc. Gene expression was analyzed by qPCR and Western blotting. sCD13 was measured using ELISA. The effects of sCD13 and B1R inhibitor on fibroblast function were analyzed using Western blotting, proliferation, migration, and gel contraction assays. To induce a myofibroblast phenotype, normal fibroblasts were incubated with TGFβ for 72 hours. The bleomycin-induced skin fibrosis mouse model was used to determine the effect of CD13 or B1R inhibition in vivo. To determine the differences between groups, Students t-test, Mann–Whitney U test, and Kruskal–Wallis test were performed. P values of less than 0.05 were considered statistically significant.

Results: CD13 knockout mice are resistant to bleomycin-induced skin fibrosis, as shown by significant reduction in skin thickness and hydroxyproline content after bleomycin injection compared to wild type mice. Dermal fibroblasts from SSc patients showed increased B1R expression and released significantly higher amount of sCD13 compared to healthy controls. In normal fibroblasts, BDKRB1 (gene encoding B1R) and MMP14 were induced by TGFβ, while ANPEP (gene encoding CD13) was downregulated. sCD13 induced pro-fibrotic gene expression, proliferation, migration, and gel contraction in SSc fibroblasts, and these effects were blocked by a B1R antagonist. Inhibition of B1R also prevented bleomycin-induced skin fibrosis in wild type mice.

Conclusion: We’ve established the pro-fibrotic properties of sCD13 in SSc fibroblasts and in an animal model of fibrosis. sCD13 exerts its pro-fibrotic effect by acting on B1R, and this in turn reinforces a myofibroblast phenotype in SSc fibroblasts. The elevated levels of B1R and sCD13 in SSc fibroblasts are mediated by TGFβ, since TGFβ increased BDKRB1 as well as MMP14 expression, which in turn cleaves CD13 into its soluble form. Targeting the sCD13-B1R axis appears to be a promising therapeutic approach for SSc.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/targeting-cd13-aminopeptidase-n-as-a-novel-therapeutic-approach-for-scleroderma-fibrosis/