| Abstract: | Epstein Barr virus (EBV) infection of human B lymphocytes is initiated by selective binding of the virus to the C3d receptor (EBV/C3d receptor) on the cell surface and results in polyclonal proliferation of infected cells. In these studies we examined the fate of the EBV/C3d receptor during viral infection by using an immunotoxin made from a monoclonal antibody (HB5) reactive with the receptor and the potent toxin, gelonin. Binding of the HB5-gelonin conjugate to the EBV/C3d receptor before EBV infection (at concentrations as low as 10(-11) M) significantly inhibited the subsequent polyclonal proliferation of virus-infected B lymphocytes. HB5 antibody and gelonin alone did not inhibit proliferation. Because internalization of gelonin-antibody conjugates is required to cause cytotoxicity, these results indicate that infection of B lymphocytes with EBV selectively induced endocytosis of the EBV/C3d receptor with concomitant internalization of the immunotoxin. Proliferation of B lymphocytes that were activated by prior infection with EBV, or activated by cross-linking of their surface immunoglobulin molecules, was not inhibited by the antibody-toxin conjugate even at concentrations as high as 10(-7) M. Also, the growth of B lymphoblastoid cell lines cultured in the presence or absence of infectious EBV was not inhibited by HB5-gelonin. Thus, our results suggest that the EBV/C3d receptor is internalized only during the infection of normal B lymphocytes by EBV, with co-internalization of immunotoxin, and indicate that internalization of the EBV/C3d receptor-immunotoxin complex does not occur simply as a consequence of activation and proliferation of B lymphocytes. The use of a ligand to induce endocytosis of its receptor offers a new strategy for the selective delivery of immunotoxins to cells and may be more generally applicable. |
