We have attempted to rescue presumptive human endogenous retrovirus(es) by using a competent animal oncovirus as a helper. Human melanoma cells (line HMB2) were fused, using polyethylene glycol, with mouse NIH-3T3 cells which had been infected and transformed by the Harvey murine leukaemia and sarcoma virus complex (MLV and MSV). The heteropolykaryons obtained were co-cultivated with fresh NIH-3T3 cells; filtered (Millipore 0.22 µm) medium from these was used to infect further NIH-3T3 cells. In these cells after several passages, vesicular stomatitis virus (VSV) pseudotypes could be produced. These were infectious not only for mouse cells (manifesting the helper MLV), but also for human cells (HeLa, HEC human embryo fibroblasts, HMB2); they were not infectious for CCL64 (mink) or for Vero (African green monkey) cells. The presence of such VSV pseudotypes infectious for human cells indicated that a human ecotropic virus [provisionally named rescued human virus (RHV)] had been rescued by the fusion of human melanoma cells with MLV-infected mouse cells. This was supported by the following evidence. (i) The human-specific pseudotype was neutralized by sheep antisera raised to antigens selected by VSV from human tumour cell lines HMB2, T47D and HeLa. (ii) These antisera also aggregated NIH cells infected with MLV and RHV. (iii) Mouse antisera raised to antigens present in NIH cells infected with MLV and RHV, in contrast to sera raised to NIH cells infected with MLV only, immunoprecipitated an 85000 mol. wt. protein band from human cells (HEC, HMB2 and HeLa) surface-labelled with I.


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