The class I MHC antigens of erythrocytes: A serologic and biochemical study

We have measured the binding of anti-H-2K monoclonal antibodies to purified erythrocytes of a number of mouse strains. The purified erythrocytes specifically bound between 500 and 2000 molecules of labeled anti-H-2K antibodies per cell, whereas lymphocytes bound approximately 10⁵ molecules of antibody per cell. Not all of the antibodies that bound to lymphocytes bound to erythrocytes. Antibodies to both public and private specificities that reacted with H-2(k) lymphocytes failed to react with H-2(k) erythrocytes. A similar pattern was found when comparing H-2^b erythrocytes with lymphocytes. Failure to bind erythrocytes is not a function of antibody isotype or avidity. Isolated H-2K antigens of erythrocytes are integral membrane proteins identical in apparent m.w. and very similar in charge heterogeneity to the H-2K antigens of lymphocytes. Limited peptide maps of exogenously labeled antigens are also nearly identical. We also find that antibodies against epitopes of all three external domains of H-2K may react with erythrocyte H-2K. The altered reactivity of erythrocyte H-2K antigens is unlikely to be due to domain deletion, or to differences in membrane environments in erythrocytes and lymphocytes. The difference in reactivity could be due to subtle post-transciptional modifications, or to expression of other class I genes in erythroid precursors.