Helper specificity for retrieval of defective friend virus.

A Friend virus-induced reticulum-cell sarcoma from BALB/c mice has been cultivated in vitro in our laboratory for more than 11 years and contains no detectable evidence of virus. However, Friend virus could be retrieved readily from the tissue cultures by any of several lymphatic leukemia viruses belonging to the Friend-Moloney-Rauscher (FMR) group, as long as the helper viruses were actively replicating in a culture. Helper activity appeared to be highly specific and limited to the FMR group including the B-tropic Tennant leukemia virus which produced a B-tropic Friend virus pseudotype. The naturally occurring type AKR murine leukemia viruses and the related Gross passage A virus failed both in vivo and in vitro to retrieve the Friend virus genome from the virus-free tumor cells.