HLA antigens of platelets. IV. Influence of "private" HLA--B locus specificities on the expression of Bw4 and Bw6 on human platelets.

Quantities of the HLA--B antigens B5, B7, B8, B13, B27, and Bw44 were determined on platelets of subjects heterozygous for those markers using an assay based on inhibition of antibody-mediated 51Cr release from lymphocytes. In confirmation of previous studies, Bw44 was found to vary about 35-fold and B8 about 8-fold. In contrast, B5, B7, B27, and B13 varied only 2--3-fold. Expression of each of the six antigens was proportional to that of its associated broad specificity, Bw4 or Bw6. The antigens Bw4 and Bw6, when measured directly on platelets of 62 normal heterozygous subjects, were found to vary 32-fold and 20-fold, respectively. Bw4 was strongly expressed on platelets carrying B5 or B27 (P less than 0.001), was weakly expressed in the presence of B13 (P less than 0.001) and was variable, possibly bimodal, in the presence of Bw44. Bw6 was strongly expressed on platelets carrying B7, Bw15.1, or Bw35 (P less than 0.01) but was weak on platelets carrying B8 or B14 (P less than 0.01). Little or no variation in Bw4 or Bw6 was observed on lymphocytes of the same donor population. These findings are consistent with the possibility that private HLA--B locus specificities influence the expression of part or all the HLA--B molecule on the platelet surface through an undetermined mechanism. The atypical behavior of Bw44/Bw4 on platelets suggests the possibility that Bw44 consists of two subspecificities not yet detectable with available serologic reagents. Variability of HLA markers in tissues other than platelets could have important immunobiologic implications which seem deserving of further investigation.