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). The goals of this study were to measure CD13 in vivo and in vitro-in RA samples, and to determine whether CD13 could play a role in homing of T cells to the RA joint. Methods IL-17 treated FLS were used to immunize mice, from which a novel anti-human CD13 monoclonal antibody (591.1D7.34) was developed. 1D7 and a second anti-CD13 monoclonal, WM15, were used to develop a novel ELISA for CD13, and CD13 enzymatic activity was measured in parallel. Chemotaxis of cytokine activated T cells (Tck) was measured by an under-agarose assay. Result We detected substantial amounts of CD13 in synovial fluids, sera, FLS lysates, and culture supernatants by ELISA, with a significant increase in CD13 in RA synovial fluids when compared to osteoarthritis (OA). CD13 accounted for most but not all of the CD13-like enzymatic activity in synovial fluid. Recombinant human CD13 was chemotactic for Tck through a G-protein-coupled-receptor and contributed to the chemotactic properties of synovial fluid independently of enzymatic activity. Conclusion CD13 is released from FLS into culture supernatants and is found in synovial fluid. CD13 induces chemotaxis of Tck, a T cell population similar to that found in RA synovium. This data suggest that CD13 could play an important role as a T cell chemoattractant, in a positive feedback loop that contributes to RA synovitis.

Aminopeptidase N/CD13 is highly expressed by fibroblast like synoviocytes (FLS) and may play a role in rheumatoid arthritis (RA). CD13 was previously detected in human synovial fluid where it was significantly increased in RA compared to osteoarthritis. In this study we found that CD13 in biological fluids (plasma, synovial fluid, FLS culture supernatant) is present as both a soluble molecule and on extracellular vesicles, including exosomes, as assessed by differential ultracentrifugation and density gradient separation. Having determined CD13 could be released as a soluble molecule from FLS, we examined potential mechanisms by which CD13 might be shed from the FLS membrane. The use of protease inhibitors revealed that CD13 is cleaved from the FLS surface by metalloproteinases. siRNA treatment of FLS revealed one of those proteases to be MMP14. We determined that pro-inflammatory cytokines (TNFα, IFNγ, IL-17) upregulated CD13 mRNA in FLS, which may contribute to the increased CD13 in RA synovium and synovial fluid. Inhibition of CD13 function by either inhibitors of enzymatic activity or anti-CD13 antibodies resulted in decreased growth and diminished migration of FLS. This suggests that CD13 may be involved in the pathogenic hyperplasia of RA FLS. This data expands potential roles for CD13 in the pathogenesis of RA.

The use of autografts versus allografts for anterior cruciate ligament (ACL) reconstruction is controversial. The current popular options for ACL reconstruction are patellar tendon or hamstring autografts, yet advances in allograft technologies have made allogeneic grafts a favorable option for repair tissue. Despite this, the mismatched biomechanical properties and risk of osteoarthritis resulting from the current graft technologies have prompted the investigation of new tissue sources for ACL reconstruction. Previous work by our lab has demonstrated that tissue-engineered bone-ligament-bone (BLB) constructs generated from an allogeneic cell source develop structural and functional properties similar to those of native ACL and vascular and neural structures that exceed those of autologous patellar tendon grafts. In this study, we investigated the effectiveness of our tissue-engineered ligament constructs fabricated from autologous versus allogeneic cell sources. Our preliminary results demonstrate that 6 months postimplantation, our tissue-engineered auto- and allogeneic BLB grafts show similar histological and mechanical outcomes indicating that the autologous grafts are a viable option for ACL reconstruction. These data indicate that our tissue-engineered autologous ligament graft could be used in clinical situations where immune rejection and disease transmission may preclude allograft use.

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