Identification of the Platelet Activating Activity in Rheumatoid Synovial Fluid as an Intermediate Molecular Weight Complex of IgG

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Abstract
The predominant platelet-activating activity assessed by the release of 14C-serotonin from washed human platelets was purified from the synovial fluid of patients with rheumatoid arthritis. Cation exchange chromatography on carboxymethyl cellulose, gel filtration on Sepharose 6B and preparative isoelectric focusing yielded a single factor with an apparent m.w. of 450,000 and an isoelectric point of 8.36 ± 0.25 (mean ± S.D.). Homogeneity of the purified synovial fluid platelet-activating activity, termed SF-PAA, was confirmed by the complete correspondence between platelet-activating activity and protein during gel filtration and electrophoresis in a glycerol gradient and by the elicitation of a monospecific antiserum. Platelet activation by SF-PAA, although coupled to serotonin secretion, was not associated with the release of the cytoplasmic marker lactic acid dehydrogenase or with platelet aggregation. Ouchterlony analysis of SF-PAA revealed a line of complete identity with human IgG with antiserum to either SF-PAA or γ heavy chains. Kappa and λ light chains were also identified in SF-PAA, whereas µ and α heavy chains, fibrinogen, and the complement components, Clq, C3, properdin, and B, were not present. Thus, SF-PAA appeared to be a complex of IgG of average m.w. 450,000. The inhibition of its action by monomeric IgG confirmed that the complex activated platelets through an IgG receptor.