Regulation of An Acquired Inhibitor to the GPVI Platelet Collagen Receptor in a Patient with An Autoimmune Syndrome and Kidney Disease.

GPVI is a major platelet collagen signaling receptor associated with the ITAM-containing FcRγ in the platelet membrane. Autoantibodies to GPVI result in ADAM10-mediated cleavage and loss of receptor function. We now report a young woman with a history of autoimmune and kidney disease associated with systemic lupus erythematosus, and a strong circulating antibody to GPVI. The patient has been followed over a 3-year period. Particularly studied were (i) the platelet aggregation response to collagen (and convulxin (Cvx)), (ii) platelet GPVI levels (flow cytometry and Western blot (WB)), (iii) plasma soluble GPVI levels (GPVIs, dual antibody ELISA with capture and biotinylated detction MoAbs), and (iv) circulating anti-GPVI antibody titres (ELISA using recombinant GPVIs). When first studied in 2006, the patient had a normal platelet count but a virtual absence of platelet aggregation to collagen and Cvx. Nevertheless, her platelets responded to other agonists including cross-linking FcRγIIA by way of the monoclonal antibody, IV.3, showing that the PLCγ2-dependent signaling pathway shared with GPVI-FcRγ was retained. Western blotting with a series of MoAbs to GPVI (extracellular and intracellular domains) and other platelet receptors showed a major and specific deficit of GPVI with appearance of GPVI dimer and a 47 kDa degradation product. Other platelet receptors known to be sensitive to metalloprotease cleavage, GPIbα, P-selectin, TLT-1 were not reduced. However, FcRγ was moderately decreased in WB. Interestingly, the patient’s plasma was able to activate normal platelets, a phenomenon blocked by recombinant GPVI-Fc protein but not by IV.3 showing that it was independent of FcRγIIA and probably related to GPVI dimerization. Her plasma also specifically blocked the response of control platelets to Cvx in a time-dependent manner. Studies continued in 2007 when the patient additionally experienced neurological and vascular problems. Platelet expression of GPVI continued to be severely reduced by WB, and ELISA confirmed strong anti-GPVI plasma antibody titers. Plasma GPVIs levels were undetectable compared to control donors. Transfusion of donor platelets prior to a renal biopsy was unsuccessful; they were rapidly cleared and failed to correct the platelet defect. The patient was now subjected to increasingly strict immunosuppressive medication including corticoids (prednisolone), imurel (azathioprine) then endoxan (cyclophosphamide). A large reduction in plasma anti-GPVI levels, the appearance of GPVIs in the patient’s plasma (the ELISA results were confirmed by WB of plasma samples), the partial restoration of both platelet GPVI levels and the platelet collagen response accompanied these treatments which was paralleled by an improvement in her nephrotic syndrome. Our results suggest that this patient developed a potent anti-GPVI antibody responsible for the loss of platelet GPVI, with the possible clearance of GPVIs by immune complex formation. The activity of the antibody was much reduced by immunotherapy; further studies are required to know whether this is related to the improvement in her clinical state.