HLA Antigens of Platelets. IV. Influence of “Private” HLA-B Locus Specificities on the Expression of Bw4 and Bw6 on Human Platelets

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Abstract
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 on inhibition of antibody-mediated 51Cr release from lymphocytes. In confirmation of previous studies, Bw44 varied about 35-fold and B8 about 8-fold. In contrast, B5, B7, B27 and B13 varied only 2- to 3-fold. Expression of each of the 6 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, varied 32-fold and 20-fold, respectively. Bw4 was strongly expressed on platelets carrying B5 or B27 (P < 0.001), was weakly expressed in the presence of B13 (P < 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 < 0.01) but was weak on platelets carrying B8 or B14 (P < 0.01). Little or no variation in Bw4 or Bw6 was observed on lymphocytes of the same donor population. These findings were consistent with the possibility that private HLA-B locus specificities influence the expression of part or all of 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 2 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.