A CROSS-REACTIVE MOUSE ANTI-I-EK MONOCLONAL-ANTIBODY DETECTS AN HLA-DR POLYMORPHISM LINKED TO HLA-DR1

Two mouse monoclonal anti-I-Ek antibodies (H7-8.26 and H40-242.3) also reacted with human B cells. Cellular radioimmunoassays performed on lymphoblastoid B cell lines suggest that these 2 antibodies recognized broad antigenic determinants on human B cells. In order to determine whether the monoclonal antibody-HLA-DR antigen interaction had the same affinity on cells of different haplotypes, the dissociation rate constants (kds) of radioiodinated Fab fragments from several human lymphoblastoid cell lines were measured. Antibody H7-8.26 bound with the same avidity to all these B cell lines regardless of their origin, suggesting that its target antigenic determinant on HLA-DR molecules was non-polymorphic. Antibody H40-242.3 displayed 2 patterns of binding avidity with either high or low kd values depending on the cell tested. This antibody apparently cross-reacted with different determinants on human B cell lines. High-avidity binding patterns correlated with the classical HLA-DR1 allospecificity and segregated with this antigen in a family study. The amount of HLA-DR antigen immunoprecipitated by H40-242.3 from 125I-radiolabeled NP-40 solubilized membrane extracts of cells from 5 members of the same family depended also on the presence or absence of serologically defined HLA-DR1 on the cells. Avidity measurements apparently can detect some degree of polymorphism of membrane antigens in cases where the cell binding reactions appear to be monomorphic; the immunoprecipitation efficiency of a monoclonal antibody may depend on its avidity and can not be predicted from its cell binding reactivity alone; and an interspecies cross-reactivity can also be related to an allospecificity in the species studied.