Quantitative expression of epitopes defined by murine monoclonal antibodies on DR molecules from different HLA...

Monoclonal antibodies 50D6 and 21r5, reactive with human class II molecules, were analyzed quantitatively by flow cytometry and cellular radioimmunoassay for their binding to cells of different HLA-DR types. Monoclonal antibody 50D6 bound equally to cells of all DR types tested except DR7, where no reactivity was observed. Monoclonal antibody 21r5 was reactive with all cells. However, the percentage of DR molecules at the cell surface expressing 21r5 epitope varied with the DR type and increased as follows: DR3 = DR7 less than DR2 less than DR5 less than DR4 less than DR1. MAb 50D6 reacted with an epitope spatially related to but distinct from the 21w4 epitope present on all DR molecules. The 50D6 epitope was shown to be present on isolated DR1 molecules.