Selection of a Subgroup A Avian Leukosis Virus [ALV(A)] Envelope Resistant to Soluble ALV(A) Surface Glycoprotein

Abstract

The host developing resistance to retroviral infection is believed to be a major force in the evolution of multiple receptor usage by retroviruses. The avian leukosis-sarcoma virus (ALV) group of retroviruses provides a powerful system for studying the envelope-receptor interactions involved in retrovirus entry; different members of this group of closely related viruses use distinct cellular receptors. Analysis of the ALV envelope subgroups suggests that the different ALVs evolved from a common ancestor by mutations in the env gene. Cells and animals that express subgroup A ALV envelope glycoproteins are highly resistant to ALV(A) infection due to receptor interference. In this study, we tested whether expression of a soluble form of subgroup A surface glycoprotein (SU) would result in receptor interference and whether this interference would select for resistant viruses with altered receptor usage. Chicken cells expressing the secreted ALV(A) SU immunoadhesin SU(A)-rIgG, which contains the subgroup A SU domain fused to the constant region of a rabbit immunoglobulin (IgG) heavy chain, showed significant receptor interference. A variant virus resistant to SU(A)-rIgG receptor interference was obtained. This virus had a six-amino-acid deletion in the subgroup A hr1 that altered receptor usage. This approach may identify regions of SU that play a critical role in receptor specificity.

Evolutionary Pressure of a Receptor Competitor Selects Different Subgroup A Avian Leukosis Virus Escape Variants with Altered Receptor Interactions

“…An earlier study identified another region of ALV(A) hr1 (Fig. 8, residues 155 to 160) deletion of which broadened the receptor usage of the variant but allowed it to retain wild-type binding affinity for the Tva receptors (23). Despite the reports that residues in FIG.…”

“…Our approach to defining these critical determinants utilizes the ability of retroviruses to escape environmental pressure by mutation and/or recombination to select a viable variant. It has been shown that even a modest level of selective pressure on ALV entry results in the generation of viral variants with altered entry properties (e.g., expanded host range or reduced receptor binding affinity) (23,24,34). These genetic approaches do not make assumptions about the location of interaction determinants in SU (in contrast to sitedirected mutagenesis), and viral variants with multiple mutations can be selected to identify functional but noncontiguous determinants.…”

“…There are now four genetically selected ALV(A) variants that display some level of expanded interactions with cellular proteins in addition to Tva: three variants selected with the soluble quail Tva inhibitor (the E149K [24], W141G K261E, and W145R K261E variants) and one variant selected by receptor interference with a soluble form of ALV(A) SU (the ⌬155-160 variant [23]). Compared to wild-type ALV(A), all four variant viruses have an altered receptor interference pattern: an increased entry efficiency in cells previously infected with ALV(A) and a decreased entry efficiency in cells previously infected with ALV(B) or ALV(C).…”

“…However, only a limited number of studies have been conducted to identify residues within the hypervariable regions of ALV SU that are important for receptor interaction. Previous studies have demonstrated that regions and residues in the ALV(A) SU hr1 hypervariable domain that are important for ALV(A) receptor use and virus entry could be identified by using genetic selection strategies that block virus entry based on receptor interference (23) or by using a competitive inhibitor of the Tva receptor (24). Both of these studies identified residues in hr1 that are important for Tva binding affinity and receptor usage.…”

The Spacing between Cysteines Two and Three of the LDL-A Module of Tva Is Important for Subgroup A Avian Sarcoma and Leukosis Virus Entry

“…This sequence divergence may dictate subtle differences in this region and determine specificity in receptor usage. Indeed, it was reported that several ASLV-A variants which were genetically selected for a soluble form of quail Tva could not use quail Tva but could still efficiently use chicken Tva as the receptor in viral entry (14)(15)(16). These results seem to suggest that specific residues in the N terminus of Tva play an important role in specificity in receptor usage.…”