The nef gene product of both human and simian immunodeficiency viruses is critically important for virus replication and disease progression in vivo. However, the precise biological function of Nef remains poorly characterized in vitro, with previous reports suggesting that Nef might be either cytotoxic or cytostatic. As a result of difficulties encountered by several groups in establishing cell lines constitutively expressing Nef, we have developed two inducible systems resulting in stable Nef expression in various mammalian cell lines. Tetracycline-regulated Nef expression was achieved in HeLa cells but could not be established in human T cell lines. Jurkat E6-1 T cell and RAW264.7 murine macrophage cell lines expressing a regulated nef gene were generated using a system in which Nef expression was controlled by a mutated version of the heavy metal-inducible human metallothionein IIA promoter. Induction of high levels of Nef expression in HeLa-Nef and Jurkat-Nef cells resulted in a moderate (2-fold) and a dramatic (10-fold) retardation of cell growth respectively, supporting the contention that Nef may be a cytotoxic or cytostatic factor. This property was also observed at low basal levels of Nef expression in RAW264.7-Nef macrophage clones (5-fold reduction in growth) and was associated with an altered morphological phenotype suggesting that different cell types may be more susceptible to the cytostatic activity of Nef. The regulated Nef-expression systems provide tools for investigating the molecular basis of Nef function, including Nef-mediated cytopathogenicity, CD4 down-regulation and enhancement of virus infectivity.


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