Fusion between uninfected cells in retrovirus-induced fusion-from-within

We previously examined Moloney murine leukemia virus-induced fusion-from-wi thin (FFWI) and fusion-from-without (FFWO) of SC-1 mouse cells. FFWI and FF WO can be distinguished by their stimulation by ionophores and polycations, respectively. FFWI is caused by infected cells. Normally, fusion between a n infected cell and uninfected cells (heterofusions) is described, but we h ave surprisingly found that the infected cells also caused homofusion betwe en uninfected cells in their vicinity (named neighbor homofusions). It was shown that neighbor homofusions were not induced by free virus particles (b y FFWO). Transfectants expressing envelope proteins only induced heterofusi ons, indicating that virus production is necessary for the formation of nei ghbor homofusions. Both plasma membrane fragments and easily removable mate rial from the surface of infected cells were able to induce fusion with the same stimulation pattern as FFWI and neighbor homofusion. These materials, especially the latter, have properties in common with virions, and it is d iscussed whether immature virions are involved in the formation of the neig hbor homofusions. (C) 1998 Elsevier Science B.V. All rights reserved.