A naturally occurring feline leukaemia virus subgroup A variant exhibits broad mammalian cellular tropism and utilizes canine thiamine transporter 1.

Feline leukaemia virus subgroup A (FeLV-A) has generally been regarded as having a narrow, predominantly feline host range. Here, we characterize a naturally occurring FeLV-A isolate, strain F8701, that exhibits unexpectedly broad mammalian cell tropism. Using an infectious molecular clone of F8701, we show that F8701 infects canine cells efficiently and can utilize the canine orthologue of thiamine transporter 1, the canonical FeLV-A receptor. Compared with reference FeLV-A strains, F8701 displayed markedly higher infectivity in multiple non-feline cell lines, including canine and human cells. Sequence analysis of the envelope glycoprotein revealed that F8701 carries the previously described 83D/91D di-aspartic acid residue combination in variable region A, together with an additional K105N substitution whose functional contribution remains to be determined. Interference assays confirmed that F8701 belongs to FeLV subgroup A, indicating that broad cellular tropism can occur within a naturally occurring FeLV-A background. Although replication in cultured canine cells was observed, whether F8701 can establish productive infection or transmission in dogs in vivo remains unknown. These findings highlight the natural plasticity of the FeLV-A envelope and suggest that receptor compatibility can contribute to the expansion of mammalian cellular tropism, while additional host-level barriers are likely to determine cross-species infection in vivo.