CR-1, CD35 IN SYNOVIAL-FLUID FROM PATIENTS WITH INFLAMMATORY JOINT DISEASES

Objective. To investigate synovial fluid (SF) for the presence of CR1 and to study its relationship to SF leukocytes and to serum levels of soluble CR1 (sCR1) in patients with rheumatic diseases. Methods. Synov ial fluids were collected from 35 patients with rheumatoid arthritis ( RA) and 26 patients with other inflammatory joint diseases, Total CR1 in the SF and serum were measured with a sandwich enzyme-linked immuno sorbent assay (ELISA) that recognized both soluble and transmembrane f orms of CR1, The characteristics of CR1 in SF were analyzed by ultrace ntrifugation and by a second ELISA specific for transmembrane CR1. Res ults. CR1 was found in all SF samples tested (range 5-281 ng/ml), SF C R1 was higher in patients with RA (mean +/- SD 81 +/- 66 ng/ml) than i n those with other inflammatory joint diseases (31.8 +/- 23.8 ng/ml) ( P < 0.001), Serum sCR1 was not significantly increased in the patients compared with the normal subjects, There was no correlation between s erum sCR1 and SF CR1, In 14% of the patients, the SF CR1 level was hig her than the serum sCR1 level, A fraction (30-80%) of SF CR1 was pelle ted by ultracentrifugation and, unlike serum sCR1, it reacted in an EL ISA specific for transmembrane CR1, Thus, SF contained 2 forms of CR1: a membrane-associated and a soluble form, which was confirmed by sucr ose density-gradient ultracentrifugation, SF CR1 levels correlated dir ectly with the number of SF total leukocytes and polymorphonuclear leu kocytes (PMN). These 2 forms of CR1 were also found in the supernatant of in vitro-activated PMN from normal subjects, SF CRI exhibited the capacity to act as a cofactor for the factor I degradation of C3b. Con clusion. CR1 is found in the SF of patients with joint inflammation, T he data suggest that SF CR1 originates from the infiltrating leukocyte s, which shed both a soluble and a membrane-associated form, Whether S F CR1 participates in the local regulation of complement activation re mains to be examined.