Journal of Clinical Immunology, Vol. 16, No. 6, 1996 Antiplatelet Glycoprotein Autoantibodies in Patients with Autoimmune Diseases with and Without Thrombocytopenia I. CORDIANO, 1 F. SALVAN, 1 M. L. RANDI, 1 M. A. RUFFATTL 2 A. STEFFAN, 3 A. GIROLAMI, 1 and F. FABRIS 1.4 Accepted: August 16, 1996 The presence and specificity of antiplatelet autoantibodies in 32 patients with primary and 18 patients with secondary autoim- mune thrombocytopenic purpura (AITP), as well as 11 non- thrombocytopenic patients with systemic autoimmune diseases, were studied. By means of the direct and indirect monoclonal antibody immobilization of platelet antigen (MAIPA) assay, antiplatelet autoantibodies were detected using monoclonal antibodies specific for platelet glycoproteins (GPs) Ib, IIb/IIIa, Ia/IIa, and iV. Serum antiplatelet autoantibodies were found in 18 of 32 primary AITP patients (56%), 6 of 18 secondary AITP patients (33%), and 5 of 11 nonthrombocytopenic patients (45%). Platelet-associated autoantibodies were detected in five of eight patients with primary (62%) and in four of eight patients with secondary AITP (50%) and in two of four patients without thrombocytopenia (50%). Multiple antibody reactivity, mainly against GPs IIb/IlIa and Ib and, in a few patients, against Ia/IIa, was found. Using MAIPA, platelet xylene eluates from 20 patients were also studied. Antiplatelet elutable autoantibodies were related to thrombocytopenia; autoantibod- ies against membrane GPs Ib and IIbfllIa were demonstrable in 84 and 63% of eluates from patients with primary and second- ary AITP, respectively, but not in eluates from nonthrombocy- topenic patients. The presence of antiplatelet antibodies thus appears to be a common feature of many autoimmune diseases apart from the thrombocytopenia, but the (primary or second- ary) etiology of the immune tfirombocytopenia cannot be differentiated on the grounds of their specificity. KEY WORDS: Autoimmune tbrombocytopenia;antiplatelet autoanti- bodies; monoclonal antibody immobilization of platelet antigens; platelet glycoproteins. Institute of Medical Semeiotics, Chairs of Semeiotic and Internal Medicine, University of Padua Medical School, Padua, Italy. 2 Department of Rheumatology, University of Padua Medical School, Padua, Italy. 3 Blood Bank, C.R.O. Aviano Oncology Hospital, Pordenone, italy. 4 To whom correspondence should be addressed at Istituto di Semei- otica Medica, via Ospedale 105, 35100 Padova, Italy. INTRODUCTION Immune-mediated platelet destruction may occur as a primary disease, as in idiopathic thrombocytopenic pur- pura, or may be secondary to other autoimmune disor- ders, as in systemic lupus erythematosus (SLE) (1). Autoimmune thrombocytopenic purpura (AITP) is char- acterized by peripheral platelet destruction caused by autoantibodies directed against platelet surface antigens (2), which can be detected as platelet-associated (PAIg) or serum platelet-bindable (SPBIg) immunoglobulins. PAIg are increased in many patients, whereas SPBIg are found in a smaller percentage of patients with immune thrombocytopenia (3). However, despite the sensitivity of PAlg, their specificity is dubious (4), as elevated levels were reported in patients with primary and sec- ondary AITP, as well as nonimmune thrombocytopenia (5, 6). Several studies demonstrated that autoantibodies from AITP patients are prevalently IgG directed against epitopes located on platelet surface glycoproteins (GP) Ib/IX, llbfliIa, IV, and Ia/iIa (7-11); however, antibodies directed against anionic phospholipids (12) and glyco- sphingolipids (13) were also detected. SLE is the most common cause of secondary AITP. Thrombocytopenia occurs in approximately 20% of SLE patients, and its incidence increases to about 40% in patients with antiphospholipid antibodies (14). The im- mune etiology of thrombocytopenia in SLE is based on the presence of increased PAIg levels (15-18), even though multiple cross-reacting autoantibodies character- ize this disease (19-21). Autoantibodies directed against cytoplasmic platelet components, with an apparent mo- lecular weight ranging from 50 to 120 kDa, were also evidenced in SLE by means of serum immunoblotting (20, 22, 23), but their pathogenetic role is uncertain since similar autoantibodies were detected in the sera of patients with primary AITP as well as normal subjects (24, 25). 340 0271-9142/96/1100-0340509.50/0 9 i996 Plenum Publishing Corporation