| Abstract: | Spleen cells from BALBc mice immunized with human epidermoid carcinoma A431 cells were fused with mouse myeloma P3NP cells. One of the isolated hybridoma lines, B4G7, secreted a monoclonal antibody of the IgG class which inhibited the binding of [125I]-EGF to A431 cells and human fibroblasts, but not to mouse 3T3 cells. This inhibition was partial (65-70%) and Scatchard analysis of the EGF binding data suggested that the B4G7 antibody interacts preferentially with a low-affinity class of EGF receptors. This monoclonal antibody specifically precipitated EGF receptors (Mr = 170,000 and 155,000) of A431 cells which were directly crosslinked with [125I]-EGF. It also precipitated EGF receptors from cells whose surface proteins were labeled with 125I, from cells grown in the presence of [35S]-methionine or [32P]-orthophosphate, and from membrane fractions phosphorylated in vitro with [32P]-gamma-ATP. Receptors subjected to EGF-induced phosphorylation, both in vivo and in vitro, were also precipitated. The B4G7 antibody blocked approximately 70% of the EGF receptors in human fibroblasts, but did not stimulate DNA synthesis in these cells. However, in the presence of this antibody, cells showed the full mitogenic response to EGF, presumably through the unblocked receptors that are likely to be of the high-affinity type. |
