SEROLOGICAL CHARACTERIZATION OF THE NAK^a ANTIGEN

Abstract

The Nak^a antigen is an isoantigen on platelet GPIV (CD36) and Nak^a negative platelets are deficient of GPIV. The Nak^a epitope defined by anti-Nak^a serum is also recognized by OKM5, a monoclonal anti-GPIV (CD36). This was shown by blocking experiments, in which the reactivity of OKM5 was completely blocked by the pretreatment of the platelets with anti-Nak^a serum and vice versa. Quantitative studies of the Nak^a-GPIV epitope on platelets and monocytes were performed both with the OKM5 and anti-Nak^a serum. Amounts of the Nak^a-GPIV epitope on monocytes were 4 times as many as those on platelets. Monocytes from Nak^a negative platelets donors could express the epitope, which varied quantitatively from the one comparable to that of Nak^a negative platelets to the other to that of Nak^a positive platelets. Two distinct Nak^a-GPIV positive patterns of Nak^a positive platelets were also observed in monocytes and appeared to be genetically determined. Since mRNA for CD36 is present in Nak^a negative platelets as well as monocytes, the genetic control of Nak^a-GPIV expression must be at post-translation level.