The double life of a retroviral receptor: TVB(S3) mediates viral endocytosis and apoptosis of bystander cells
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The ability of avian leukosis virus subgroup B (ALV-B) to kill cells in culture has been known for some time, however, the molecular basis of this process has not been established. Here we show that superinfection, which has long been considered the major cause of cell killing by ALV-B, plays no crucial role in cell death progression. Instead, we show that signaling by the ALV-B receptor, TVBS3, a member of the tumor necrosis factor receptor family, is essential for ALV-B mediated cell death and induction of caspase-dependent apoptosis. Strikingly, killing was not a product of direct infection by ALV-B, since most cell death occurred in uninfected cells. Bystander killing by ALV-B was reproduced by coculturing ALV-B envelope expressing cells with receptor-expressing cells. Taken together, our results indicate that this novel bystander killing mechanism is mediated by ALV-B envelope/receptor interactions.;The avian leukosis virus (ALV) entry mechanism is controversial, with evidence for and against a low pH-requirement for viral fusion. To further address this question, we tested the entry of HIV-1 pseudotyped with the envelope protein of ALV-B in the presence of three different lysosomotropic agents. These lysosomotropic agents were able to block the entry of wild type and pseudotyped ALV-B in two different cell lines, strongly suggesting that ALV-B requires a low pH step for entry. ALV-B and pH-dependent Semliki Forest virus (SFV) entered cells with slower uptake kinetics than human immunodeficiency virus type 1 (HIV-1), which is pH-independent. These slow uptake rates support the theory that ALV-B utilizes endocytic pathways to enter cells. Using immunofluorescence and electron microscopy analysis, we visualized the colocalization of viral particles with the endosomal marker transferrin, and demonstrated viral particles in clathrin-coated vesicles and endosome-like structures. Surprisingly, a low pH treatment did not overcome the inhibition of ALV-B entry by lysosomotropic agents. This indicates, that in contrast to SFV, ALV-B is unable to fuse at the cellular surface even at a low pH. Taken together, our findings suggest that endocytosis and a subsequent low pH step are critical for successful ALV-B infection.