Visna virus cell entry: Studies of an elusive receptor (Immune deficiency)

Visna virus, an ovine pathogen, is a member of the lentivirus family of retroviruses along with the human immunodeficiency virus (HIV). In vivo, visna virus is found to infect cells of the monocyte/macrophage lineage ultimately resulting in multiorgan disease including progressive pneumonia, encephalitis, and arthritis in sheep. The identity of the cellular receptor for visna virus is currently unknown, but the identification and characterization of the molecule(s) involved will advance understanding of disease tropism and pathogenesis. These studies report recent advances in the effort to characterize proteins involved in visna virus binding and entry and to ultimately identify the elusive cellular receptor. First, we have extended studies of the antiserum 2-23 raised against a 45-kDa visna virus binding protein (Crane, S. et al., 1991, J. Virol., 65:6137) using biochemical approaches to protein characterization. The 2-23 antiserum was used to identify membrane proteins of a putative complex consisting of a 30-kDa chondroitin sulfate proteoglycan, as well as the 45-kDa immunogen from visna virus susceptible sheep cells. Inhibition of glycosaminoglycan synthesis reduced visna virus production, suggesting a role for the chondroitin sulfate modification on visna virus entry. Second, we describe here the development of functional MuLV/visna virus pseudotypes as a powerful tool for the study of visna virus entry. Studies on visna virus entry have previously been hindered by the lack of tools to specifically study entry independent of later steps in the virus lifecycle. These pseudotypes demonstrated the ability of visna virus to enter a wide variety of cell types derived from many different species, only murine NIH3T3 cells, chinese hamster CHO cells, and human U937 cells were refractory to visna virus entry. Finally, the use of the MuLV/visna pseudotypes serve as a critical component of the functional genetic screen of a susceptible cell SCP cDNA library for the visna virus receptor. Transfer of the SCP cDNA library to receptor-deficient NIH3T3 cells through retroviral vector delivery and subsequent positive selection with MuLV/visna virus pseudotypes carrying drug resistance markers was done in an effort to identify the receptor cDNA. This initial screening of the SCP cDNA library has provided information necessary for the design of a successful functional screen for the visna virus receptor using the tools described. Collectively, the results of the research that comprises this thesis have advanced the understanding of proteins involved in visna virus entry, host cell tropism, and provided a promising approach for the eventual cloning of the visna virus receptor.