Virus production by Abelson murine leukemia virus-transformed lymphoid cells

Cell lines obtained by in vitro transformation of bone marrow with Abelson murine leukemia virus (A-MuLV) can be divided into three classes: producers, releasing reverse transcriptase-containing particles and infectious virus; nonproducers, releasing no viral particles; and defective producers, the most common phenotype, releasing particulate reverse transcriptase in the absence of infectious virus. When such cell lines were analyzed 1 to 2 weeks after their isolation, however, all produced infectious virus. Because these cell lines were carried in culture, many ceased to release infectious virus but produced defective virions. One defective producer, SWR4, has been extensively studied. The particles it produces have the same density as that of virions of Moloney murine leukemia virus (M-MuLV). The particles contain no 35 to 70S RNA, as determined by analysis of [3H]uridine-labeled particles, and exhibit no endogenous reverse transcriptase activity. Although the reverse transcriptase enzyme is of normal size, the major structural protein of the defective virions has a molecular weight of 28,000 (p28), in contrast to the p30 of M-MuLV, and no viral glycoprotein was evident. The defective particles do not appear to arise either from the helper virus or from Abelson virus. An alteration of the protein of the helper virus is an unlikely source of p28 because particles produced by lymphoid cells transformed with another strain of M-MuLV as helper (M-MuLV-TB) contained p28 with an unaltered cleavage pattern, although M-MuLV-TB p30 differs from M-MuLV p30. The A-MuLV genome lacks the capacity to code for the reverse transcriptase virions. Clones of fibroblasts infected with A-MuLV only occasionally produce defective particles. The defective particles therefore probably arose from an endogenous virus that is preferentially expressed in the class of lymphoid cells transformed by A-MuLV. This interpretation implies that the majority of A-MuLV-transformed lymphoid cells completely lose expression of the helper virus genome.