Antiidiotype Antibody against Platelet Anti-Gpiiia Contributes to the Regulation of Thrombocytopenia in HIV-1–Itp Patients

Patients with human immunodeficiency virus 1–associated immunological thrombocytopenia (HIV-1–ITP) have markedly elevated platelet-bound immunoglobulin (Ig)G, IgM, and C3C4, as well as serum circulating immune complexes (CICs) composed of the same. Affinity purification of IgGs from their CICs with fixed platelets reveals high-affinity antibody (Ab) against platelet glycoprotein (GP)IIIa 49–66, which correlates inversely with their platelet count. However, sera from these patients have little to no anti-GPIIIa activity. To investigate this, we assayed serum, purified serum IgG, and CIC-Ig from these patients. This revealed ∼150-fold greater Ab activity in purified serum IgG, and ∼4,000-fold greater reactivity in CIC-IgG. This was shown to be associated with the presence of antiidiotype Ab2 (both IgG and IgM) sequestered in the CIC-IgG. The IgM antiidiotype was predominantly blocking Ab, as demonstrated by specificity for F(ab′)2 fragments of anti–GPIIIa 49–66 of HIV-1–ITP patients and inhibition of reactivity with peptide GPIIIa 49–66, not with a control peptide. The IgM antiidiotype was not polyreactive. Similar measurements were made in nonthrombocytopenic HIV-1–infected patients