Feline leukemia virus envelope sequences that affect T-cell tropism and syncytium formation are not part of known receptor-binding domains

The envelope protein is a primary pathogenic determinant for T-cell-tropic feline leukemia virus (FeLV) variants, the best studied of which is the imm unodeficiency-inducing virus, 61C, We have previously demonstrated that T-c ell-tropic, cytopathic, and syncytium-inducing viruses evolve in cats infec ted with a relatively avirulent, transmissible form of FeLV, 61E, The envel ope gene of an 81T variant, which encoded scattered single-amino-acid chang es throughout the envelope as well as a 3-amino-acid insertion in the C-ter minal half of the surface unit (SU) of envelope, was sufficient to confer t he T-cell-tropic, cytopathic phenotype (J. L, Rohn, M, S, Moser, S, R, Gwyn n, D. N, Baldwin, and J, Overbaugh, J, Virol, 72:2686-2696, 1998), In the p resent study, we examined the role of the 4-amino-acid insertion in determi ning viral replication and tropism of FeLV-81T, The 4-amino-acid insertion was found to be functionally equivalent to a 6-amino-acid insertion at an i dentical location in the 61C variant. However, viruses expressing a chimeri c 61E/81T SU, containing the insertion together with the N terminus of 61E SU, were found to be replication defective and were impaired in the process ing of the envelope precursor into the functional SU and transmembrane (TM) proteins. In approximately 10% of cultured feline T cells (3201) transfect ed with the 61E/81T envelope chimeras and maintained over time, replication -competent tissue culture-adapted variants were isolated. Compensatory muta tions in the SU of the tissue culture-adapted viruses were identified at po sitions 7 and 375, and each was shown to restore envelope protein processin g when combined with the C-terminal 81T insertion. Unexpectedly, these viru ses displayed different phenotypes in feline T cells: the virus with a chan ge from glutamine to proline at position 7 acquired a T-cell-tropic, cytopa thic phenotype, whereas the virus with a change from valine to leucine at p osition 375 had slower replication kinetics and caused no cytopathic effect s. Given the differences In the replication properties of these viruses, it is noteworthy that the insertion as well as the two single-amino-acid chan ges all occur outside of predicted FeLV receptor-binding domains.