| Full Member, Division of Human Biology, Fred Hutchinson Cancer Research Center Feline leukemia virus (FeLV) subgroups use different receptors, a characteristic reflected by diverse disease outcomes in infected cats. Most FeLV variants enter via a classical pathway involving recognition of a single, small molecule transporter. For example, the transmissible form of the virus, FeLV-A, uses a thiamine transporter. In contrast, the late-stage, immunodeficiency-associated FeLV-T is distinguished by its requirement for both the transporter Pit1 and a soluble cofactor FeLIX that is encoded by endogenous sequences resembling FeLV. The goal of this thesis is to gain a clearer understanding of the determinants for this unusual receptor system by genetically characterizing requirements for FeLV-T entry. Several key findings have resulted from these studies: (1) FeLV envelope determinants that confer the ability to enter via Pitl and FeLIX (non-classical pathway) require a combination of changes at the N-terminus, within a conserved SPHQ motif, and a C-terminal insertion. Importantly, these determinants lie outside of the classical receptor binding domain as defined by the region of envelope that binds to classical receptors. Furthermore, the envelope determinants for FeLV-T are distinct and separable from the determinants of entry via a single transporter (classical pathway). (2) Receptor usage of FeLV-T is not restricted to Pitl and FeLIX, but can be facilitated by some, but not all, other combinations of FeLIX-like, gammaretroviral RBD cofactors with their cognate receptors. (3) Receptor function is influenced by concentrations of both cofactor (soluble gammaretroviral RBDs) and cognate receptors. (4) The FeLV-B envelope (highly related to FeLIX) interacts through multiple receptor contacts, and possibly with different residues of the related Pit1 and Pit2 receptors. Together, the work presented here suggests that specific changes found in FeLV-T are adapted to efficient entry via Pit1 and FeLIX, that and that the stable FeLIX/Pit1 complex rescues FeLV-T entry by fusion activation or facilitating requisite intermolecular interactions. Finally, that FeLV-T uses Pit1/FeLIX, and potentially other cofactor-receptor complexes, in vivo to facilitate infection suggests that the unique features of entry by this pathway may be relevant to the connection between viral receptors and pathogenesis. |
