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 osteoarthritis 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.