Tissue specific regulation of foamy viruses
Many viruses including retroviruses have evolved elaborate mechanisms of maintaining persistent, non-pathogenic infections. In infected hosts, foamy virus (FV) DNA can be detected in most tissues by nested PCR, but FV RNA remains undetectable by the same method. This indicates that virus is widely dispersed within a host in the absence of robust viral transcription. This phenomenon can be explained by infection of the particular tissues by FV, or by infection of a circulating cell population. By determining the cell type(s) which FV infects in vivo we might better understand how it achieves a lifelong persistent infection. To address this issue, we developed replication competent FV vectors which express the green fluorescent protein (GFP). We hoped the first would indicate in which cells FV replicate, while the constitutive construct would indicate which cell were infected, whether or not replication occurs. In the course of testing these vectors in tissue culture and in vivo in mice we learned that mice are a difficult small animal model for FV infection. Because of the tenability of a murine model and the deletion of GFP in the vectors we developed, we chose to focus our investigation on FV infection of different cell types in tissue culture. Specifically we sought to determine the relative strengths of the two FV promoters, located in the LTR and in the coding region of the genome (the internal promoter or IP), in different cell types. Our studies indicate that there is a complex pattern of promoter regulation which is cell type specific and this regulation can explain differential virus production in vitro.
Using both transient transfection assays, as well as radioimmunoprecipitation assays, were have determined that expression from the LTR and IP varies depending on cell type. In general, lytically infected cells from fibroblast lineages have robust LTR transcription and modest IP transcription. In contrast, hematopoeitic cells, some of which are know to be infected in vivo, have very weak LTR transcription and relatively robust IP transcription. Thus, our data indicate that LTR mediated transcription is a limiting factor in persistently infected cells. Future experiments will look at the roles of the T and B cell response to viral expression in vivo in several different systems, since these could further modulate viral expression.
Meiering, C.M., Comstock, K.E. and Linial, M.L. Human foamy virus multiple integrations in persistently infected human erythroleukemia cells. J. Virol. 74:1718-1726, 2000.