Expression and Function of Aminopeptidase N/CD13 Produced by Fibroblast-like Synoviocytes in Rheumatoid Arthritis Role of CD13 in Chemotaxis of Cytokine-Activated T Cells Independent of Enzymatic Activity

Objective. Aminopeptidase N/CD13 EC 3.4.11.2) is a metalloproteinase expressed by fibroblast-like synoviocytes (FLS). It has been suggested that CD13 can act chemotactically for T cells in rheumatoid arthritis RA). We undertook this study to measure CD13 in vivo and in vitro in RA samples and to determine whether CD13 could play a role in the homing of T cells to the RA joint. Methods. Interleukin-17-treated FLS were used to immunize mice, from which a novel anti-human CD13 monoclonal antibody mAb), 591.1D7.34, was developed. The mAb 591.1D7.34 and a second anti-CD13 mAb, WM15, were used to develop a novel enzyme-linked immunosorbent assay ELISA) for CD13, and CD13 enzymatic activity was measured in parallel. Chemotaxis of cytokine-activated T cells was measured by a chemotaxis-under-agarose assay. Results. We detected substantial amounts of CD13 in synovial fluid SF), sera, FLS lysates, and culture supernatants by ELISA, with a significant increase in CD13 in RA SF when compared to osteoar-thritis SF. CD13 accounted for most but not all of the CD13-like enzymatic activity in SF. Recombinant human CD13 was chemotactic for cytokine-activated T cells through a G protein-coupled receptor and contributed to the chemotactic properties of SF independently of enzymatic activity. Conclusion. CD13 is released from FLS into culture supernatants and is found in SF. CD13 induces chemotaxis of cytokine-activated T cells, a T cell population similar to that found in RA synovium. These data suggest that CD13 could play an important role as a T cell chemoattractant, in a positive feedback loop that contributes to RA synovitis.