Characterization of the procoagulant-inducing factor derived from the plasma of BXSB mice

Plasma procoagulant activity inducing factor (PIF) is a spontaneously occurring, potent inducer of macrophage procoagulant activity (PCA) in the male BXSB murine model of systemic lupus erythematosus. The physical characteristics of PCA induction by PIF, aggregated mouse IgG, and lipopolysaccharide (LPS) were compared. Both aggregated IgG and PIF-induced PCA were heat, acid and alkali sensitive. In contrast, LPS-induced PCA was heat resistant and only partially acid and alkali sensitive. Plasma containing PIF was fractionated on Sephacryl S-300. The PIF activity localized to the first protein peak, molecular weight 400,000 to 900,000 daltons. Analysis of peak 1 by an enzyme-linked immunosorbent assay showed the presence of IgM, IgA and IgG. This was confirmed by Western blot analysis using 125I-labelled goat anti-mouse IgM, IgA and IgG probes. The concentration of PIF increased with Sephacryl S-300 chromatography and was reduced by removal of IgG, but not IgA or IgM by affinity chromatography. Peak 1 did not contain DNA as revealed by ethidium bromide staining. Thus, IgG from the plasma of BXSB mice, a strain which develops lupus nephritis, stimulates macrophages to express PCA, accounting for PCA induction in the BXSB model of murine lupus.