Aminopeptidase N (CD13) expression on synovial fluid T lymphocytes from psoriatic arthritis patients

Background: T lymphocyte activation is responsible for articular damage in the course of chronic polyarthritides. The occurrence of T lymphocytes expressing early, intermediate and late activation markers shows a significant increase in synovial fluid (SF)respect to the peripheral blood (PB). Aminopeptidase (CD13) is an activation surface molecule expressed on different cell types except PB T lymphocytes. Objectives: To evaluate the appearance of aminopeptidase N (CD13) on SF T lymphocytes from psoriatic arthritis (PsA) patients, where it has never been investigated, to understand its contribution to the pathogenesis of this disease in relation to other activation molecules. Methods: We obtained SF and PB samples from 13 patients with PsA diagnosed by the presence of psoriasis and seronegative peripheral arthritis [M/F=11/2; mean age=40 yrs (range=24-57); mean disease duration=68 months (range=4-120)] and 11 subjects with rheumatoid arthritis (RA) classified according to Arnett criteria [M/F=3/8; mean age=55 yrs (range=24-70); mean disease duration=90 months (range=4-196)] presenting a knee joint effusion. For all of them we evaluated the main clinical and laboratory parameters including number of painful and swollen joints, erythrocyte sedimentation rate, C-reactive protein, rheumatoid factor and analysis of SF, obtained by therapeutic arthrocentesis. Besides, we performed SF and PB T lymphocyte analysis by direct dual immunofluorescence and flow cytometry using anti-CD3, anti-CD4, anti-CD8, anti-CD19, anti-CD13, anti-CD69, anti-CD25, anti-CD71 and anti-HLA-DR monoclonal antibodies. PB T cell subsets were also examined in 14 healthy donors matched for sex and age. Results: CD13 was expressed by SF T lymphocytes only and its presence did not significantly differ between PsA and RA patients. A significant increase in the number and percentage of CD69+ and HLA-DR+ T cells in SF compared to PB was seen both in PsA and RA. The absolute number of CD13+ T lymphocytes positively correlated with that of HLA-DR+ T cells in PsA (rs 0.839; p<0.0054) and RA (rs 0.781; p<0.019) patients.We did not find any difference in the expression of PB lymphocyte activation markers among PsA, RA and controls.Conclusion: Our results confirm that activated T lymphocytes in PsA and RA SF outnumber those in PB, sustaining the possibility that joint injury may derive from an inflammatory response towards an intraarticular antigen, in which CD13, expressed by SF T lymphocytes only, plays a prominent role.References: 1. Smith MD, Roberts-Thomson PJ. Lymphocytes surface marker expression in rheumatic diseases: evidence for prior activation of lymphocytes in vivo. Ann Rheum Dis 1990; 49: 81-7.2. Riemann D, Schwachula A, Hentschel M, Langner J. Demonstration of CD13/aminopeptidase N on synovial fluid T cells from patients with different forms of joint effusions. Immunobiology 1993; 187: 24-35.