MECHANISMS OF IMMUNOLOGIC THROMBOCYTOPENIA IN INDIVIDUALS AT RISK FOR AIDS

HIV-seropositive homosexuals, narcotic addicts and hemophiliacs develop a new syndrome of immunologic thrombocytopenic purpura (ITP) which is clinically indistinguishable from classic autoimmune thrombocytopenic purpura (ATP) with respect to increased megakaryocytes in the bone marrow, peripheral destruction of antibody-coated platelets, negative serology for SLE, response to treatment with prednisone and/or splenectomy. However, their platelet immunologic profiles are different.Homosexuals appear to have an immune complex-mediated mechanism: markedly elevated platelet-bound IgG and C3C4 (3.8 and 4.2-fold greater than classic ATP, respectively), elevated circulating immune complexes (3-fold greater than classic ATP), anti-F(ab')2 antibodies and absence of 7S anti-platelet IgG. There is no inverse correlation between platelet count and platelet-bound IgG or platelet-elutable anti-platelet antibody as in classic ATP.Hemophiliacs appear to have an autoimmune 7S IgG-mediated mechanism similar to classic ATP: inverse relationship betweem platelet count and platelet-bound IgG, r = 0.84, p less than 0.001, 26 df, anti-platelet reactive 7S IgG which reacts by its F(ab')2 domain, (reactive at 60-130 ug/ml compared to control IgG), platelet-elutable anti-platelet antibody. However, these patients also have elevated circulating immune complexes (2.4-fold classic ATP level) and markedly elevated platelet-bound IgG and C3C4 (3.4 and 1.2-fold classic ATP level, respectively). Anti-HIV antibody correlated with circulating immune complexes, r = 0.833, p less than 0.001.Narcotic addicts appear to have a mixture of both mechanisms (immune complex as well as autoimmune 7S IgG): markedly elevated platelet-bound IgG and C3C4 (2.6 and 2.4-fold classic ATP level, respectively), elevated circulating immune complexes (7.3-fold classic ATP level), anti-F(ab')2 antibodies, absence of an inverse correlation between platelet count and platelet-bound IgG. However, these patients do have specific 7S IgG anti-platelet antibody, which reacts by its F(ab')2 domain.F(ab')2antibodies were of the IgG class and correlated with circulating immune complex level. They react with autologous, homologous patient and healthy control F(ab')2 fragments. Some anti-F(ab')2 antibodies have broad reactivity, others are more limited. Some immune complexes were shown to contain HIV antibody. It is postulated that the immune complex platelet deposition noted with homosexual and narcotic addict thrombocytopenia may in part be due to HIV antibody complexes, some of which may exist as anti-antibody complexes.