We have previously reported the purification of a 37-kd platelet-agglutinating
protein (PAP p37) from the plasma of a patient with
thrombotic thrombocytopenic purpura (
TTP) that was shown to be present in a subset of
TTP patients. The platelet agglutination induced by PAP p37 has been shown to be inhibited by
IgG from normal human adults and the same
TTP patient after recovery. To elucidate the mechanism of inhibition of
IgG, the interaction between PAP p37 and
IgG was studied. The complex formation was demonstrated by the binding of fluid-phase
IgG from normal adults and the same
TTP patient after recovery to adsorbed PAP by using an
enzyme-linked
immunosorbent assay. The binding was specific, concentration dependent, and saturable.
IgG purified from a 5-month-old baby and the same
TTP patient during active disease did not form complex with PAP p37. The
IgG covalently cross-linked to
Sepharose 4B bound 125I-PAP p37 but not 125I-fibrinogen.
Sucrose density gradient ultracentrifugation of a mixture of 125I-PAP p37 and
IgG also revealed the fluid-phase complex formation with a sedimentation value of 19S. Complexes of molecular weight ranging from 180,000 to over 350,000 daltons were also detected by molecular sieve chromatography. The
IgG that was bound to PAP p37 conjugated to
Sepharose 4B inhibited the agglutination of washed platelets induced by
TTP plasma containing PAP p37, whereas the
IgG that was not bound to PAP p37 did not have a significant inhibitory effect. The complex formation between PAP p37 and specific
IgG is likely to account for the in vitro inhibition of
TTP plasma-induced agglutination and, at least partly, the in vivo successful treatment with specific
IgG-containing normal plasma.