Xenotropism: The elusive viral receptor finally uncovered

Working with knowledge gained from pioneers in the RNA tumor virus field (2), the biologic activity of this NZB virus ultimately was demonstrated in 1970 by cultivating NZB mouse embryo cells with hamster or rat tumor cells containing a nonreplicating sequence of the murine sarcoma virus (3). This procedure, using conventional MLV, leads to the rescue of the sarcoma virus genome in the envelope coat of the replication-competent MLV (2). The newly formed transforming virus pseudotype now has the host range of the “helper” MLV. Supernatants from the NZB cell coculture demonstrated the presence of a replicating virus that could induce foci of transformation in rat cells but not mouse embryo cells (3). The NZB MLV host range thus seemed limited to rat cells. Subsequent work demonstrated that the helper MLV visualized in the NZB mouse cells was infectious but was not infectious at all for mouse cells (i.e., complete block). It was infectious only for cells from heterologous species, including those of avian origin (Table 1). Similar viruses were subsequently found in all laboratory strains of Mus musculus domesticus as …