Expression of CD13/Aminopeptidase N by Synovial Fibroblasts: Novel Roles in the Pathogenesis of Rheumatoid Arthritis

Aminopeptidase N/CD13 is a metallopeptidase that is highly expressed by fibroblast-like synoviocytes (FLS) and may play a role in rheumatoid arthritis (RA). The goal of this study was to define CD13 function in the RA joint by monitoring CD13 expression in vivo and in vitro, analyzing regulation by pro-inflammatory cytokines of CD13 expression on FLS, describing the mechanisms by which CD13 is released from FLS, and defining potential roles for CD13 in homing of T cells to the RA joint and in regulating the growth and migration of RA FLS. In overview, we find that CD13 is expressed in synovial fluids, sera, FLS lysates, and culture supernatants as measured by ELISA, with a significant increase of CD13 in RA synovial fluids when compared to osteoarthritis. In FLS, pro-inflammatory cytokines (TNFalpha, IFNgamma, IL-17) are able to upregulate CD13 mRNA. However, as surface expression of CD13 did not correlate with CD13 mRNA, and soluble CD13 is present in RA synovial fluid, potential mechanisms for CD13 release from FLS were examined. CD13 was detected in sera, synovial fluids, and FLS culture supernatants, on extracellular vesicles and in soluble form. The release of soluble CD13 from FLS could be blocked by metalloproteinase inhibitors and siRNA directed against MT1-MMP/MMP14. With regard to biological functions, recombinant human CD13 was chemotactic for cytokine activated T-cells (Tck) through a G-protein-coupled receptor and contributed to the chemotactic properties of synovial fluid independently of enzymatic activity. Furthermore, inhibition of CD13 function in FLS, using enzymatic activity inhibitors or anti-CD13 antibodies, resulted in decreased growth and diminished migration of RAFLS. We conclude that CD13 is upregulated in the RA joint by pro-inflammatory cytokines where it is released on extracellular vesicles and shed as a soluble molecule from the FLS surface by metalloproteinases, including MMP14. Following its release from the FLS surface, CD13 induces chemotaxis of Tck, a T cell population similar to that found in RA synovium. Moreover, CD13 increases FLS growth and migration, thereby contributing to synovial hyperplasia in RA. Our data implicates CD13 in the pathogenesis of RA through enhancement of both T cell infiltration and aggressive FLS outgrowth.