Free histones are not detectable in the sera of patients with meningococcal sepsis. (A) The presence of nucleosomes and free histones was analyzed in serum of 2 patients with meningococcal sepsis. As before, total histones, nucleosomes, or free histones were immunoprecipitated and separated on gel, and histone H3 was visualized (A) and quantified (B) using a histone titration on the same blot. (B) Histone levels as determined via densitometric analysis. (C) Nucleosome levels as determined in the sera by ELISA (D). FSAP-AP levels were determined by ELISA to verify FSAP activation. Blots of the patient samples are representative of 2 independent experiments because of limited sample availability. (E) Purified nucleosomes were incubated with plasma-purified FSAP (0, 50, or 200 nM), healthy donor serum (HD), FSAP-deficient serum (F-def), mock-depleted serum (M-depl), or FSAP-depleted serum (F-depl; 50%) and were immunoprecipitated from serum using an anti-DNA antibody coupled to sepharose. The cleavage products and the nucleosomes previously immunoprecipitated from a septic baboon serum (Figure 5D baboon 2, lane 2) were separated by SDS-PAGE, and histones were visualized by instant blue staining (upper panel), or histone H3 and its cleavage product were visualized by an anti–histone H3 antibody on immunoblot (lower panel). Arrows indicate intact histone H3 or a cleavage product of histone H3.