Epitope diversity of monoclonal antibodies revealed by cross-species reactivity.

The epitope specificity of two monoclonal antibodies (MAb) which have the same functional activity has been studied. These two independently raised rat IgG2b MAb, NIMP-R10 and M1/70 (Springer et al., 1979), blocked the complement (C) receptor on mouse macrophages. Both MAb showed essentially the same binding pattern with mouse cells, binding to the same extent mouse eosinophils, macrophages, neutrophils, a small proportion of spleen and bone marrow cells, but not thymocytes. That both MAb were apparently recognizing the same epitope was suggested from experiments in which MAb M1/70 inhibited the binding of MAb NIMP-R10. In addition, both MAb showed identity at the molecular level, precipitating the same molecules from the surface of mouse cells. However, NIMP-R10 and M1/70 could be shown to recognize different epitopes when they were tested on human cells. Thus, NIMP-R10 was found to bind to neutrophils and to large granular lymphocytes with natural killer cell activity but not to eosinophils or monocytes, while M1/70 bound to all of these cell types. It is suggested that inter-species testing may have general application in the analysis of antibody specificity.