Stable expression of the human immunodeficiency virus type 1 envelope glycoprotein in transfected L cells

An SV40-based expression vector was used to generate CD4-negative murine L cell lines which stably expressed the human immunodeficiency virus envelope glycoprotein (env). Despite the presence of abundant intracellular envelope glycoprotein, the expression of env gp120/41 was not detected on the cell surface. Pulse-chase studies showed that the majority of the gp120 detected at the end of a 20-h chase was in the culture medium. Therefore gp120 was shed and/or secreted from these cells. Transfected L cells (H-2k) served as targets for specific lysis by CTL raised against vaccinia virus-encoded env gp160. The discrepancy in relative levels of intracellular versus surface expression of env was probably due to the highly inefficient processing of newly synthesized gp160, as well as the apparent instability of the gp120/41 complex in the transfected cell lines. Digestion of immunoprecipitated gp120 and gp160 with endoglycosidase H and peptide N-glycosidase F revealed that the envelope glycoprotein in transfected L cells possessed both high mannose and complex N-glycans, analogous to the posttranslational modification of the mature envelope glycoprotein in infected T cells. These studies indicate that the relatively inefficient processing of env gp160 occurs in the absence of CD4, and that the stable surface expression of envelope gp120/41 complex may require additional factors not present in transfected cells.