Identification and characterization of TNFR-like receptors for subgroups B, D, and E avian leukosis viruses: Two isoforms of a common receptor explain non-reciprocal receptor interference between cytopathic and non-cytopathic avian retroviruses

The cytopathic Avian Leukosis Virus (ALV) subgroups B and D and the non-cytopathic subgroup E ALV were predicted to use related receptors encoded by the tvb locus. Genetic analysis in chickens identified different alleles of this locus: tvbs3 confers susceptibility to ALV-B and D, whereas the tvbs1 allele was predicted to encode a receptor for ALV-B, D and E. In addition, turkeys are susceptible to infection by ALV-E, but not ALV-B, suggesting that the turkey homologue of tvb may encode a subgroup E specific receptor. Receptor interference studies demonstrated that ALV-B/D and ALV-E exhibit a non-reciprocal receptor interference (NRI) pattern. Pre-infection with ALV-B or D leads to interference with superinfection by all three virus subgroups. In contrast, pre-infection by ALV-E, which can be present as an endogenous provirus in the chicken germline, only interferes with ALV-E superinfection.;The questions addressed in this thesis are the differences between ALV-B and ALV-E interactions with the TVB receptors and the molecular basis of the NRI pattern exhibited by these viruses. During the course of this work, I identified two primary binding receptors for ALV-E. The tvb s1 allele from chicken encodes a receptor for ALV-B, D and E (TVBS1) and the tvbt homologue from turkey encodes a receptor specific for ALV-E (TVBT). Sequence comparisons of these receptors identified an important cysteine at residue 62 in the ALV-E receptors that is a serine in the ALV-B/D-specific TVB S3 receptor.;Mutational analysis of TVBS1 indicated that ALV-E entry is dependent on the conformational constraints imposed by a putative disulfide bond between Cys-46 and Cys-59. In contrast, ALV-B infection does not require any of the first four cysteines of TVBS1. Significantly, I reconstituted the NRI pattern exhibited between ALV-B and ALV-E by expressing the cloned TVBS1 receptor with viral Env proteins in transfected human cells. Investigation of the molecular basis of NRI lead to the surprising result that TVBS1 is produced as two protein isoforms from a single cDNA clone. One form serves as a receptor for both ALV-B and ALV-E, and the other is a receptor specific for ALV-B. The formation of both forms of TVBS1 is not due to heterogeneous N-linked glycosylation, and we suggest that these receptor isoforms arose from alternative disulfide bonding. These results provide an explanation for the NRI by these viruses and a rationale for the observed biological differences between ALV-B and ALV-E.