Mouse and cat cells were each examined for the mode of restriction of endogenous xenotropic oncornavirus. Murine xenotropic helper virus (MuX) and its pseudo-type of Moloney murine sarcoma virus (MSV(MuX)) were grown in cat cells to high titre. MuX alone did not replicate in any mouse cell tested including normal or transformed outbred Swiss 3T3 cells or SC-1 cells, but did grow in a variety of other mammalian cells. MSV(MuX) was not able to achieve that intracellular state from which it could be rescued by mouse leukaemia virus (MuLV) in any mouse cell tested with the exception of SC-1 cells. Detection of MSV(MuX) foci with appropriate helper virus was as sensitive in SC-1 cells as in the cells of several other species. Sequential passage of MSV(MuX) virus complex in SC-1 cells resulted in a loss of infectious sarcoma and helper viruses, but transformed, MSV rescuable cells were retained.

If cat embryo cells were infected with either the feline endogenous xenotropic virus (FeX) or its MSV pseudotype (MSV(FeX)), two analogous states of restriction were observed. FeX alone did not replicate in cat cells as measured by release of progeny virus or by FeX group-specific antigen induction. Cat cells could be susceptible or insusceptible to the entry of MSV(FeX) as measured by MSV rescue with appropriate ecotropic feline leukaemia virus. The sensitivity of detection of MSV(FeX) foci in some cat cells in the presence of feline ecotropic virus was comparable to that exhibited by cells of other mammalian species. A single strain of cat cells underwent a change in its restrictive capacity for MSV(FeX) on prolonged passage. Late passage cat cells became very insusceptible to MSV(FeX) but not to other pseudotypes of MSV. Infectious FeX or its group-specific antigens were not detected in the insusceptible cells. The major glycoprotein of FeX did appear as a surface antigen of the insusceptible cells. It is apparent that two levels of cellular restriction can be distinguished in each of two mammalian cell systems by the susceptibility to penetration of MSV coated with endogenous xenotropic oncornavirus.


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