Expression of GPIV and Naka antigen on monocytes in Naka-negative subjects whose platelets lack GPIV

Summary. The platelet antigen Nak^a was once considered to be a platelet-specific alloantigen and is carried on platelet membrane glycoprotein (GP) IV. Recent studies suggest that Nak^a-negative subjects lack platelet GPIV. GPIV is an important adhesive receptor and expressed on the surface of monocytes as well as of platelets. In the present study, flow cytometry was used to detect GPIV and Nak^a antigen on the surface of monocytes. Nak^a antigen was expressed on monocytes as well as on platelets in Nak^a-positive subjects ( n = 6) (P-GPIV-positive subjects). To our surprise, monocytes of Nak^a-negative subjects ( n = 7) (P-GPIV-negative subjects) having no anti-Nak^a antibody in their serum expressed GPIV and Nak^a antigen to almost the same degree as did the monocytes of P-GPIV-positive subjects. Competitive experiments using OKM5 (a monoclonal antibody against GPIV) and anti-Nak^a antibody showed that the epitope of anti-Nak^a antibody on monocytes was very close to that of OKM5. In two P-GPIV-negative subjects having anti-Nak^a antibody in their serum, GPIV and Nak^a antigen were not expressed on the surface of either monocytes or platelets. These results indicate that the GPIV molecules and Nak^a antigen are expressed on the surface of monocytes in the majority of P-GPIV-negative subjects, but that in a very few P-GPIV-negative subjects neither GPIV nor Nak^a antigen is expressed on the surface of their monocytes. We hypothesize that P-GPIV-negative subjects who carry neither GPIV nor Nak^a antigen on their monocytes produce anti-Nak^a antibody as a result of transfusion or pregnancy.