To assess the serum levels, specific activity and other characteristics of dipeptidylpeptidase IV (DPP IV/CD26), an ectoenzyme that plays a critical role in the modulation and expression of autoimmune and inflammatory diseases, from patients with rheumatoid arthritis (RA), systemic lupus erythematosus (SLE), primary Sjögren syndrome (SS) and normal controls. To study the possible underlying molecular basis if significant differences were found.
Serum DPP IV was purified by ion-exchange and affinity chromatography techniques and its specific activity and sera levels were determined by an enzyme-linked assay (ELISA). The enzyme was further analyzed for its sialic acid content, its adenosine deaminase binding capacity and its electrophoretic mobility. The levels of circulating IgA, IgG, and IgM anti-DPP IV autoantibodies were determined by an ELISA technique.
The median serum levels of DPP IV in RA patients was similar to controls (0.85 microg/ml versus 1.03 microg/ml, p = n.s.); in SLE and SS patients the enzyme serum levels were reduced to nearly one half of controls (p < 0.001). DPP IV specific activity was significantly reduced in serafrom RA patients when compared with those of SLE, SS and normal sera (12.24 versus 16.5, 19.69 and 16.34 mol pNA x 10(-4)/min/mol respectively, p < 0.005). Both RA and SLE enzymes were hypersialylated, but only RA DPP IV augmented its specific activity to close to control values after desialylation with V. cholerae neuraminidase. Sera from all patient groups contained anti-DPP IV autoantibodies, but only those of the IgA isotype were significantly higher than those found in normal subjects.
The specific activity of serum DPP IV was decreased only in RA patients, although its levels were similar to normal controls. While both RA and SLE DPP IV were hypersialylated, desialylation restored the specific activity only of RA DPP IV. This finding suggests that different specific glycosylation sites in the enzyme might be involved as the underlying mechanism of the decreased enzyme specific activity of RA patients. The differences in DPP IV levels observed between RA and SLE patients seem to reflect a different status of T cell activation in both diseases.