Demonstration of Fibronectin Associated with Platelets in Synovial Fluid from Patients with Rheumatoid Arthritis

Abstract

Paired samples of peripheral blood and synovial fluid (SF) aspirated from inflamed knee joints from 15 adult patients with classical rheumatoid arthritis, as well as peripheral blood obtained from 15 healthy subjects, were anticoagulated with ACD. Peripheral blood platelets were separated from other plasma constituents by gel filtration of platelet-rich plasma on Sepharose 2B. When using this technique on SF, it was found that platelets could be isolated from other SF constituents, and that each of the SF's gave a high yield of eluted platelets.

Direct immunofluorescent staining for human fibronectin was performed with isolated and suspended platelets obtained from the three different sources. Aliquots of both intact and permeabilized platelets were stained. Intact peripheral platelets from all patients and normal subjects revealed a weakly positive staining, whereas the staining of intact SF platelets from all patients was clear and bright. The fluorescence of intact cells was surface-located and speckled. For permeabilized platelets, the staining had a punctate intracellular pattern, with a varying number of discrete and bright fluorescent foci per cell. Counting of the foci in each platelet specimen revealed that this number, which was also found in peripheral platelets from all patients and normal subjects, was distinctly greater than the number of foci in all the SF platelet specimens. No relationship was found between the various staining results and medical treatment or Waaler-Rose serology of the patients.

The findings indicate that the SF platelets had large amounts of surface-bound and small amounts of intracellular fibronectin, whereas the converse was found in the case of peripheral platelets from the patients and normal subjects. The adhesive properties of fibronectin bound to the surface of SF platelets may, in several ways, contribute to the inflammatory responses in the synovitis of rheumatoid arthritis.