Background/Purpose: Rheumatoid arthritis (RA) is an inflammatory disorder characterized by synovial hyperplasia, inflammatory cell infiltration and pannus formation. We have shown that soluble CD13 (sCD13) induces angiogenesis, monocyte and lymphocyte chemotaxis and arthritis through G protein-coupled receptors (GPCRs). However, the receptor for sCD13 is not yet defined.

Methods: A GPCR screening assay for potential receptors for sCD13 identified bradykinin receptor B1 (B1R). Binding of sCD13 and B1R was studied by immunoprecipitation and immunoblotting of lysates of FLS that had been treated with a chemical cross-linker, BS3. Immunofluorescence was performed to verify the physical binding of sCD13 to B1R on cell membranes. The expression of B1R on RA synovium and fibroblast-like synoviocytes (FLS) was tested using immunofluorescence and flow cytometry, respectively. We also tested the inducibility of B1R on RA FLS by quantitative PCR and Western blotting. Western blotting was performed to test the role of B1R in sCD13 induced phosphorylation of signaling molecules in RA FLS, including use of the B1R antagonist, R715. Monocyte (MN) chemotaxis assays using a modified Boyden chamber were done to examine the effect of B1R antagonism on sCD13 induced MN migration.  We also performed RA synovial tissue organ culture with a B1R antagonist, SSR240612 (SSR), and cytokines in the culture media were measured by ELISA. In vivo, B1R antagonists including R715 and SSR, or vehicle were injected intraperitoneally in C57Bl/6 mice daily for 9 days during induction of K/BxN serum-transfer arthritis, and ankle circumference was measured every day.

Results: B1R was found to be a potential receptor for sCD13 by GPCR screening. Following cross-linking of sCD13 to FLS, biochemical analysis showed a band that was identified by both anti-B1R and anti-CD13 at an apparent molecular mass consistent with the sum of the masses of sCD13 and B1R. Blockage of RA FLS with recombinant human CD13 decreased the binding of anti-B1R antibody to B1R on the FLS surface, confirming the binding of sCD13 to the same receptor. B1R was present in RA synovium and was highly expressed by RA FLS. Both TNF-α and IL-1β upregulated the expression of B1R in RA FLS, indicating B1R is inducible in inflammation. Blocking B1R with R715 reduced sCD13 induced phosphorylation of signaling molecules such as Erk1/2, and Akt in RA FLS. We also found that R715 inhibited sCD13 induced MN migration (p< 0.01). Synovial tissue organ culture showed that blocking B1R with SSR reduced the secretion of Monocyte Chemoattractant Protein-1 by RA synovium compared to non-treated (p< 0.05). In vivo, R715 or SSR suppressed the development of K/BxN serum-transfer arthritis as shown by significantly reduced ankle swelling compared to vehicle, starting from the 6th day (p< 0.05).

Conclusion: B1R is highly expressed in RA synovium and RA FLS and is inducible in inflammation. B1R is a receptor for sCD13 and mediates CD13 induced chemotaxis and signal transduction. The potent arthritogenic properties of sCD13 indicate that B1R could be an important pro-inflammatory receptor in both RA and an inflammatory arthritis mouse model, and also a compelling novel therapeutic target.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/the-role-of-bradykinin-receptor-b1-and-its-new-ligand-soluble-cd13-in-rheumatoid-arthritis-and-inflammatory-arthritis-mouse-model/