Simon Karpatkin

On the Mechanism of Thrombocytopenic Purpura in Sexually Active Homosexual Men

Thrombocytopenic purpura has recently been noted in sexually active homosexual men. To elucidate the pathogenesis of thrombocytopenic purpura in this population, we compared the disorder in 33 homosexual men with that in 23 patients (15 women and 8 men) thought to have classic autoimmune thrombocytopenic purpura. The homosexual group had 3.8-fold higher levels of platelet-bound IgG and 4.2-fold higher levels of platelet-bound complement than the patients with autoimmune thrombocytopenic purpura. Eluates from the platelets of only 1 of 10 homosexual patients reacted in vitro with normal platelets, as compared with those from the platelets of 12 of 15 patients with autoimmune thrombocytopenic purpura. Twenty-one of 24 homosexual patients (88 per cent) had elevated serum levels of immune complexes that were capable of binding to platelets, whereas none of 5 patients with autoimmune thrombocytopenic purpura had circulating immune complexes. The IgG fraction of positive serum samples from three homosexual patients did not bind to normal platelets, whereas that from the positive serum of two patients with autoimmune thrombocytopenic purpura and one woman in whom isoimmune antiplatelet antibody developed during pregnancy (studied as a positive control) did bind to normal platelets. We conclude that, whereas classic autoimmune thrombocytopenic purpura involves antiplatelet IgG directed against platelet antigenic determinants, the thrombocytopenic purpura that occurs in sexually active homosexual men is usually caused not by antiplatelet IgG but probably by the nonspecific deposition of complement and immune complexes on platelets.

Thrombocytopenia in Homosexual Patients: Prognosis, Response to Therapy, and Prevalence of Antibody to the Retrovirus Associated with the Acquired Immunodeficiency Syndrome

Thirty-three homosexual patients with thrombocytopenia (mean [+/- SE] platelet count, 50 000 +/- 7000/mm3; range, 7 to 135 000/mm3) have been followed for a mean period of 20 +/- 2 months. Six patients have developed the acquired immunodeficiency syndrome 1 to 37 months after the diagnosis of thrombocytopenia. Six patients spontaneously reverted to normal platelet counts 5 to 27 months (median, 10 months) after the diagnosis of thrombocytopenia, in the absence of splenectomy and while not receiving corticosteroids. Sixteen of seventeen patients had a moderate to excellent response while on corticosteroid treatment. Ten of ten patients had an excellent response to splenectomy which has persisted. Fifteen patients did not require treatment for their thrombocytopenia. Thirteen of fourteen patients had antibody against the retrovirus associated with the acquired immunodeficiency syndrome, as did 4 of 12 homosexual controls without thrombocytopenia. Thrombocytopenia in homosexuals is part of the complex related to the acquired immunodeficiency syndrome.

Thrombocytopenic Purpura in Narcotics Addicts

Since November 1982 we have seen an association of thrombocytopenic purpura with chronic narcotic addiction in 70 patients with a mean platelet count of 53000 +/- 4000 (SE); 33 had stopped taking intravenous drugs for an average of 21.2 +/- 4.7 months; 13 of 15 had elevated antibody titers for a virus related to the acquired immunodeficiency syndrome. Platelet-bound IgG, IgM, and complement levels were 16.7-, 5.6-, and 3.1-fold greater than control values, respectively, and 2.6-, 1.9-, and 2.4-fold greater than values in 25 patients with classic autoimmune thrombocytopenic purpura studied at the same time. Thirty-three of thirty-six addicts had elevated circulating immune complexes, whereas 8 patients with autoimmune thrombocytopenia had no elevation. Eleven of eighteen addicts had positive serum platelet-reactive IgG titers, compared to 5 of 19 patients with classic autoimmune thrombocytopenia. The platelet-reactive IgG in sera of addicts was composed of 7S IgG antibody as well as high molecular weight (immune complex) IgG. Thus, chronic addicts appear to have a new immunologic platelet disorder associated with the presence of 7S IgG antiplatelet antibody, like patients with classic autoimmune thrombocytopenic purpura, and immune complex associated nonspecific platelet IgG, like male homosexual patients with thrombocytopenia.

Thrombotic thrombocytopenic purpura associated with human immunodeficiency virus type 1 (HIV-1) infection

The cases of 14 patients with thrombotic thrombocytopenic purpura admitted to one institution after 1980 were reviewed. Three of the fourteen cases occurred in patients with the acquired immunodeficiency syndrome (AIDS)-related complex and one occurred in a patient with probable human immunodeficiency virus (HIV) infection. The diagnosis in all four cases had been made after 1985. The association of thrombotic thrombocytopenic purpura with HIV infection was judged to be statistically significant on the basis of the proportion of patients with AIDS among the general population of patients admitted to the same institution during the same period. The fact that this association is only now being recognized suggests that there may be a long incubation period for thrombotic thrombocytopenic purpura or that the association is a rare one recognized now only because of the increased number of persons with AIDS.

Role of molecular mimicry of hepatitis C virus protein with platelet GPIIIa in hepatitis C-related immunologic thrombocytopenia.

Patients with HIV-1 immune-related thrombocytopenia (HIV-1-ITP) have a unique Ab against platelet GPIIIa49-66 capable of inducing oxidative platelet fragmentation in the absence of complement. HIV-1-seropositive drug abusers are more prone to develop immune thrombocytopenia than non-drug abusers and have a higher coinfection with hepatitis C virus (HCV) than non-drug abusers (90% vs 30%). Molecular mimicry was sought by screening a phage peptide library with anti-GPIIIa49-66 antibody as bait for peptides sharing homology sequences with HCV. Several phage peptide clones had 70% homology with HCV protein. Sera from dually infected thrombocytopenic patients with HCV and HIV-ITP reacted strongly with 4 nonconserved peptides from HCV core envelope 1. Reactivity correlated inversely with platelet count (r(2) = 0.7, P < .01). Ab raised against peptide PHC09 in GPIIIa(-/-) mice induced thrombocytopenia in wild-type mice. Affinity-purified IgG against PHC09 induced oxidative platelet fragmentation in vitro. Drug abusers dually infected with HCV and HIV-1 had a greater incidence and severity of thrombocytopenia as well as titer of anti-GPIIIa49-66/PHC09 Ab. NZB/W F1 mice injected with recombinant core envelope 1 developed Ab versus PHC09 and significantly decreased their platelet count (P < .001). Thus, HCV core envelope 1 can induce thrombocytopenia by molecular mimicry with GPIIIa49-66.

Complement-independent Ab-induced peroxide lysis of platelets requires 12-lipoxygenase and a platelet NADPH oxidase pathway

Antiplatelet GPIIIa49-66 Ab of HIV-related thrombocytopenic patients induces thrombocytopenia and platelet fragmentation by the generation of peroxide and other reactive oxygen species (ROS). Here we report the presence of a functional platelet NADPH oxidase pathway that requires activation by the platelet 12-lipoxygenase (12-LO) pathway to fragment platelets. A new Ab-mediated mechanism is described in which the platelet 12-LO product, 12(S)-HETE activates the NADPH oxidase pathway to generate ROS.

Heavy chain subclass of human anti-platelet antibodies☆

Abstract The IgG subclass distribution of the antiplatelet antibodies present in several clinical conditions was determined. Studies were carried out on antiplatelet antibodies present in patients with autoimmune thrombocytopenic purpura, systemic lupus erythematosis, drug-induced immunologic thrombocytopenic purpura, and after multiple transfusions. The antiplatelet antibodies in autoimmune thrombocytopenic purpura appeared to be restricted to the γG3 subclass in every one of 15 patients studied. In contrast, nine patients with systemic lupus erythematosis who had antiplatelet antibodies were found to have antiplatelet antibodies detectable in three out of four or in all four subclasses. Different patterns of subclass distribution were observed in the other clinical conditions studied. The subclass distribution of antiplatelet antibodies may be of assistance in distinguishing systemic lupus erythematosis from autoimmune thrombocytopenic purpura.

Risk for the Acquired Immunodeficiency Syndrome Among Thrombocytopenic and Nonthrombocytopenic Homosexual Men Seropositive for the Human Immunodeficiency Virus

A group of 44 homosexual patients with immune thrombocytopenia and serologic evidence of infection with the human immunodeficiency virus (HIV) was observed for a total of 844 person-months. The risk of developing the acquired immunodeficiency syndrome (AIDS), an actuarial incidence of 36.5% in 37 months, was no different than that reported for seropositive homosexual men from New York who are nonthrombocytopenic and asymptomatic. A statistical analysis comparing the hazard function with a constant-risk model showed that the hazard of developing AIDS was not constant but increased with duration of seropositivity.

Role of thrombin as a tumor growth factor.

The clinical observation that thrombosis in some patients heralds the onset of malignancy has been recognized for over a century. Thrombin the key terminal enzyme of coagulation also promotes angiogenesis and stimulates tumor-platelet adhesion, adhesion to endothelium, tumor implantation, tumor cell growth and metastasis. The thrombin receptor, a member of the protease-activated receptor family, is expressed on many tumor cell lines and on breast tumor biopsy specimens. In addition to mitogenic effects on fibroblast, smooth muscle cells and endothelial cells, thrombin also exerts direct effects on cancer cells by activation of the cell cycle through down-regulation of p27Kip1and induction of Skp2, and cyclins D and A. MicroRNA 222, which inhibits p27Kip1 , is upregulated by thrombin. In the transgenic TRAMP mouse model of prostate cancer inhibition of endogenous thrombin by hirudin retards spontaneous tumor growth. Inhibition of thrombin may lead to tumor dormancy and could explain inhibition of tumor growth and metastasis by anticoagulants observed in animal models and a beneficial effect on survival observed in some clinical trials of anticoagulants in cancer patients.

Platelet Fragmentation Requires a Specific Structural Conformation of Human Monoclonal Antibody against β3 Integrin

We have described an autoantibody against β3 (GPIIIa49–66), a region of platelet integrin αIIbβ3 that is unique. It induces platelet fragmentation in the absence of complement via antibody activation of platelet NADPH oxidase and 12-lipoxygenase to release reactive oxygen species, which destroy platelets. To study the mechanism of anti-GPIIIa antibody-induced platelet fragmentation, we screened a human single chain Fv antibody library with the GPIIIa49–66 peptide. Nine monoclonal antibodies were identified that were capable of binding to GPIIIa49–66. Surprisingly, binding avidity for GPIIIa49–66 did not correlate with activity of induction of platelet fragmentation. We therefore investigated the requirements for platelet fragmentation. Mutations were introduced into the heavy chain complementary-determining region-3 of clones 11, 43, and 54 by site-directed mutagenesis. The capability of these clones to induce platelet fragmentation or bind to GPIIIa49–66 subsequently changed. Molecular modeling of these clones with their mutants revealed that the ability to induce platelet fragmentation is affected by the side chain orientation of positively charged amino acids in the heavy chain of residues 99–102. Thus, a structural change in the conformation of anti-GPIIIa49–66 antibody contributes to its binding to the β3 integrin and subsequent antibody-induced platelet fragmentation and aggregate dissolution.