Platelet binding properties of monoclonal lupus autoantibodies produced by human hybridomas

Abstract

The platelet binding properties of human monoclonal lupus autoantibodies have been studied. These IgM autoantibodies, produced by human X human hybridomas derived from lymphocytes of patients with systemic lupus erythematosus, are known to bind to single-stranded DNA. Four anti-DNA antibodies that express the dominant 16/6 idiotype--HF2′1/17, HF2′18/2, HF2′1/13b, and HF3′16/6--bound to glutaraldehyde-fixed platelets. In contrast, HF6′21/28, HF9′11/3, and polyclonal IgM bound poorly to platelets. [35S]Methionine was incorporated into HF2′1/17, and the interaction of the intrinsically radiolabeled HF2′1/17 with fixed platelets was evaluated in a solution phase radioimmunoassay. [35S]Methionine HF2′1/17 bound to fixed platelets and could be displaced by equivalent amounts of HF2′1/17, HF2′18/2, HF2′1/13b, and HF3′16/6. HF2′1/17 bound with greater affinity to fresh platelets and to thrombin-activated platelets than to glutaraldehyde-fixed platelets. Single-stranded DNA competed with platelets for the HF2′1/17 combining site. Treatment of fresh platelets with nuclease I, trypsin, chymotrypsin, and neuraminidase did not alter the binding of antibody to the platelet surface. No binding of antibody to phospholipid micelles was observed. Purified IgM autoantibodies did not inhibit platelet aggregation induced with ADP, thrombin, or ristocetin in platelet-rich plasma. These results indicate that the human IgM monoclonal anti-DNA autoantibodies that express the dominant 16/6 idiotype are polyspecific, bind to platelets, and interact with a platelet epitope that does not appear to involve DNA, protein, or sialic acid. These antibodies interact with platelets through the same sites responsible for antibody-DNA binding.

Anti-β2-glycoprotein I (GPI) autoantibodies, annexin V binding and the anti-phospholipid syndrome

“…The precise pathogenic mechanisms which are responsible for the clinical manifestations of the APS are still unknown. The results of in vitro experiments have suggested endothelial cell activation [22–24], platelet activation [25–27] and modulation of coagulation mechanisms such as acquired protein C resistance [28] as potential candidates. Rand et al .…”

“…The precise pathogenic mechanisms underlying the APS are still unknown. A variety of in vitro effects have been attributed to autoimmune aPL antibodies, including endothelial cell activation [22–24], platelet activation [25–27] and modulation of coagulation mechanisms leading to acquired protein C resistance [28]. Recent studies have suggested that inhibition of annexin V binding to procoagulant surfaces may be an additional mechanism through which aPL antibodies mediate their in vivo pathogenic effects [29,30].…”