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 approximately 150-fold greater Ab activity in purified serum
IgG, and approximately 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. Their serum reactivity was not measurable, but serum Ig and CIC-
IgG against
platelet GPIIIa 49-66 was present, although considerably lower than that found in HIV-1-ITP patients (26- and 35-fold lower, respectively). In addition, their
IgM antiidiotype reactivity was 12-fold greater than that found in HIV-1-ITP patients. The
IgM antiidiotype Ab titer of both cohorts correlated with in vivo platelet count (r = 0.7, P = 0. 0001, n = 32). To test the in vivo effectiveness of the
IgM antiidiotype,
thrombocytopenia was induced in mice with 25 microgram of affinity-purified anti-
GPIIIa 49-66 (mouse
GPIIIa has 83% homology with human
GPIIIa and
Fc receptors for human
IgG1). Maximum effect was obtained at 4-6 h after
intraperitoneal injection into Balb/c mice with a platelet count of approximately 30% baseline value. Preincubation of the anti-
GPIIIa Ab with control
IgM at molar ratios of
IgM/
IgG of 1:7 before
intraperitoneal injection had no effect on the in vivo platelet count, whereas preincubation with patient
IgM antiidiotype improved the platelet count to 50-80% of normal.
Thrombocytopenia could be reversed after addition of
IgM antiidiotype 4 h after induction of
thrombocytopenia. Thus, CICs of HIV-1-infected patients contain
IgM antiidiotype Ab against anti-
GPIIIa, which appears to regulate their serum reactivity in vitro and their level of
thrombocytopenia in vivo.