Neural stem cells as engraftable packaging lines can mediate gene deliveryto microglia: Evidence from studying retroviral env-related neurodegeneration

The induction of spongiform myeloencephalopathy by murine leukemia viruses is mediated primarily by infection of central nervous system (CNS) microgli a. In this regard, we have previously shown that CasBrE-induced disease req uires late, rather than early, virus replication events in microglial cells (W.P. Lynch et al., J. Virol. 70:8896-8907, 1996). Furthermore, neurodegen eration requires the presence of unique sequences within the viral env gene . Thus, the neurodegeneration-inducing events could result from microglial expression of retroviral envelope protein alone or from the interaction of envelope protein with other viral structural proteins in the virus assembly and maturation process. To distinguish between these possible mechanisms o f disease induction, we engineered the engraftable neural stem cell line C1 7-2 into packaging/producer cells in order to deliver the neurovirulent Cas BrE env gene to endogenous CNS cells. This strategy resulted in significant CasBrE env expression within CNS microglia without the appearance of repli cation competent virus. CasBrE envelope expression within microglia was acc ompanied by increased expression of activation markers F4/80 and Mac-1 (CD1 1b) but failed to induce spongiform neurodegenerative changes. These result s suggest that envelope expression alone within microglia is not sufficient to induce neurodegeneration. Rather, microglia-mediated disease appears to require neurovirulent Env protein interaction with other viral proteins du ring assembly or maturation. More broadly, the results presented here prove the efficacy of a novel method by which neural stem cell biology may be ha rnessed for genetically manipulating the CNS, not only for studying neurode generation but also as a paradigm for the disseminated distribution of retr oviral vector-transduced genes.