Evidence for activation of platelets in the synovial fluid from patients with rheumatoid arthritis.

Platelets were isolated by gel filtration from paired samples of peripheral blood and synovial fluid (SF) aspirated from inflamed knee joints from 20 adult patients with rheumatoid arthritis (RA) as well as from peripheral blood obtained from 20 healthy subjects. The platelets from the three different sources were investigated for quantitative differences in the number of two distinct types of intracellular storage organelles using immunofluorescence staining for platelet factor 4 (PF4) and labelling with the fluorescent substance mepacrine (MC). The number of PF4-stained organelles per cell was the same in the peripheral normal and RA platelets. This number was distinctly lower in the SF platelets. The peripheral and SF platelets from the RA patients had the same number of MC-labelled organelles. This number was distinctly lower than in the normal cells. The results suggest that the peripheral RA platelets had been activated to liberate serotonin and other substances from one type of organelles, and that the SF platelets had been activated to an additional liberation of PF4 from another such type. Liberated PF4, serotonin, and other substances from SF platelets may, in several ways, contribute to the inflammatory responses of RA.