Increased expression of interleukin-8 and aminopeptidase N by cell-cell contact: interleukin-8 is resistant to degradation by aminopeptidase N/CD13

In rheumatic joints, high concentrations of interleukin-8 (IL-8) have been measured in synovial fluid and in pannus tissue. In both locations aminopep tidase N (APN)/CD13, an exopeptidase with reported activity towards IL-8 is also present. The surprising stability of IL-8 in the presence of an alleg ed IL-8-degrading peptidase prompted us to undertake the present study, Coc ultivation of fibroblast-like synoviocytes (SFC) with T cells or with T lym phocytic cell membranes, Or of T cells with SFC cell membranes, ah resulted in increased IL-8 mRNA expression and IL-8 secretion into the medium, and an increase of APN expression:on lymphocytes. IL-8 degradation was monitore d by Western blots and HPLC. IL-8(72), as a partially processed form, was u sed throughout this study since it is abundant in tissues and has increased biological activity in comparison to IL-8(77), Thus its degradation/inacti vation is considered of high biological significance. Whereas trypsin as a positive control rapidly degraded IL-8, we did not see any IL-8 degradation either by a variety of soluble APNs, by leucine. aminopeptidase or by APN expressed on the surface of SFC, or on ECV304 cells transfected with an APN expression vector. The much more sensitive HPLC technique resulted in nega tive results as well.