Characterization of a novel leucocyte surface membrane antigen recognized by the monoclonal antibody WM65

WM65 is a murine mAb whieh recognizes a novel surface membrane antigen present on leukaemic and normal leucocytes. The present study further investigates the nature of this antigen, especially those features which relate to the possible therapeutic applications of the WM65 antibody. There are 1–3 × 10⁴ molecules of this antigen present on normal leucocytes, and the same or greater numbers of antigen molecules are present on a variety of leukaemic cells. In vitro data showed that the WM65 antibody is internalized following interaction with its antigen on normal leucocytes. The affinity of this antibody was calculated using an FLISA method which required neither labelling of the antibody nor purification of the antigen and the affinity constant was found to be 3 × 107±2 × 107 (mol/L)⁻¹. Further data are presented which suggest that this antigen is a differentiation antigen and an integral membrane protein. Despite the relatively low affinity of the WM65 antibody, a number of characteristics of the antigen suggest the antibody may possibly have therapeutic applications. These characteristics include its cellular distribution, the number of antigen molecules expressed on the cell surface and its ability to internalize in vitro.