Characterization of HLA Class I antigens on platelets as integral or adsorbed membrane proteins.

To determine whether HLA-A,B antigens on platelets are integral membrane constituents or simply represent adsorbed plasma proteins, the degree to which they are adsorbed, and the relative ease with which they elute off platelet membranes was studied using various treatments known to elute passively adsorbed membrane proteins. In addition, this question was investigated at the RNA level using phytohemagglutinin stimulation to determine whether platelets have the capacity to respond with de-novo synthesis of HLA antigens and by enzymatic amplification of platelet derived mRNA to attempt to demonstrate the presence of nascent message encoding for these antigens. HLA antigen present on platelet membranes did not elute when platelets were incubated in autologous plasma or in plasma from homologous, antigen negative donors. When HLA-A2 negative platelets were incubated in HLA-A2 positive plasma a small amount of HLA-A2 antigen was detectable indicating that platelets possess the ability, to a limited extent, to absorb HLA antigen from the plasma, in vitro. The results indicate that the majority of HLA antigen present on platelet membranes can be selectively eluted without a concomitant loss of known integral membrane proteins such as GPIIIa. These findings argue in favor of the existent hypothesis that HLA antigens are absorbed platelet membrane proteins. (Abstract shortened by UMI.