The kinetics of intracellular viral antigen production in UC1-B, BALB 3T3 and BL-5 cells (all Fv-1b) and of focus formation in UC1-B cells have been studied following infection with free RadLV*, a naturally occurring, B-tropic murine leukemia virus adapted to continuous propagation in vitro, and with infective centers of BL-5(RadLV) cells chronically infected with RadLV*. Whereas such infective centers induced focus formation in UC1-B cells with high efficiency within a single culture passage, the development of infective center activity in BL-5 cells freshly infected with RadLV* and the direct induction of focus formation in UC1-B cells infected with free RadLV* both required one or more subcultures and several rounds of cell replication. The end-point dilution titer was not detected until four to five subcultures and about 15 rounds of cell cycle replication had been completed. Viral antigen content, measured by indirect immunofluorescence (IF), followed a triphasic pattern with respect to time and viral input concentration. Yet, both the IF assay at 3 days and the focus formation assay at 7 days (one subculture) were characterized by single-hit kinetics, excluding the possibility that mutual complementation of defective RadLV* particles might account for the delayed expression of viral infection. When viral replication was related to the number of rounds of cell replication, rather than to elapsed time, no significant differences among the three cell lines were observed. As in previous studies with NB-tropic viruses, RadLV* failed to rescue a putative sarcoma virus from UC1-B cells. It is concluded that RadLV* is not defective and can replicate to high titer in several Fv-1b cell lines in the absence of a detectable murine sarcoma virus (MSV) genome. However, the kinetics of its replication suggest that certain expressions of infection may be dependent on the attainment of a threshold level of some viral product(s), the synthesis of which is dependent on sustained cell proliferation.
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