Incidence of the Nak(a)-negative platelet phenotype in African Americans is similar to that of Asians

Background: About 5 to 10 percent of Asians have platelets that lack the major membrane glycoprotein (GP) IV (CD36, GPIIIb) that carries the isoantigen Nak(a). The GPIV-negative platelet phenotype is extremely rare among whites, but its frequency in persons of African ancestry has not yet been determined. Isoimmunization against GPIV can occur in GPIV-negative persons and can lead to platelet transfusion refractoriness. Therefore, the expression of GPIV on platelets from unrelated African Americans was studied. Study Design and Methods: Platelets were obtained from 250 African American and 280 white blood donors. Flow cytometry was used to determine the ability of these platelets to bind a monoclonal antibody that reacted with GPIV. Platelets that failed to react with this probe were tested with other GPIV-specific monoclonal antibodies and with anti-Nak(a), an isoantibody that recognizes an epitope on GPIV. Results: Platelets from 6 of the 250 African American donors (2.4%) lacked GPIV and failed to bind anti-Nak(a), whereas platelets from all of the white donors were GPIV positive (p > 0.05). No platelet-reactive antibodies were identified in the serum of the GPIV-negative donors. Conclusion: The frequency of the GPIV-negative platelet phenotype in African Americans is comparable to that in Asians and much greater than that in whites. Studies are needed to determine the frequency with which African Americans become isoimmunized to GPIV by transfusions and the possible contribution of this isoimmunization to platelet transfusion refractoriness in this population.