Presence of Endogenous Viral Elements Negatively Correlates with Feline Leukemia Virus Susceptibility in Puma and Domestic Cat Cells

Abstract

While feline leukemia virus (FeLV) has been shown to infect felid species other than the endemic domestic cat host, differences in FeLV susceptibility among species has not been evaluated. Previous reports have noted a negative correlation between endogenous FeLV (enFeLV) copy number and exogenous FeLV (exFeLV) infection outcomes in domestic cats. Since felids outside the genus Felis do not harbor enFeLV genomes, we hypothesized absence of enFeLV results in more severe disease consequences in felid species lacking these genomic elements. We infected primary fibroblasts isolated from domestic cats (Felis catus) and pumas (Puma concolor) with FeLV and quantitated proviral and viral antigen loads. Domestic cat enFeLV env and LTR copy numbers were determined for each individual and compared to FeLV viral outcomes. FeLV proviral and antigen levels were also measured in 6 naturally infected domestic cats and 11 naturally infected Florida panthers (P. concolor coryi). We demonstrated that puma fibroblasts are more permissive to FeLV than domestic cat cells, and domestic cat FeLV restriction was highly related to enFeLV-LTR copy number. Terminal tissues from FeLV-infected Florida panthers and domestic cats had similar exFeLV proviral copy numbers, but Florida panther tissues have higher FeLV antigen loads. Our work indicates enFeLV-LTR elements negatively correlates with exogenous FeLV replication. Further, Puma concolor lacking enFeLV are more permissive to FeLV infection than domestic cats, suggesting endogenization can play a beneficial role in mitigating exogenous retroviral infections. Conversely, presence of endogenous retroelements may relate to new host susceptibility during viral spillover events. IMPORTANCE Feline leukemia virus (FeLV) can infect a variety of felid species. Only the primary domestic cat host and related small cat species harbor a related endogenous virus in their genomes. Previous studies noted a negative association between the endogenous virus copy number and exogenous virus infection in domestic cats. This report shows that puma cells, which lack endogenous FeLV, produce more virus more rapidly than domestic cat fibroblasts following cell culture challenge. We document a strong association between domestic cat cell susceptibility and FeLV long terminal repeat (LTR) copy number, similar to observations in natural FeLV infections. Viral replication does not, however, correlate with FeLV env copy number, suggesting this effect is specific to FeLV-LTR elements. This discovery indicates a protective capacity of the endogenous virus against the exogenous form, either via direct interference or indirectly via gene regulation, and may suggest evolutionary outcomes of retroviral endogenization.

Endogenous feline leukemia virus siRNA transcription may interfere with exogenous FeLV infection

“…LTR transcription is approximately 10 times higher than pol, gag and env (Fig. 4), which may be reflective of the greater number of LTR elements per genome than other segments (Chiu and VandeWoude, 2020). Lymphoid tissue transcription is 1-2 logs greater transcription than other tissues, and one salivary gland transcriptome available for analysis had higher expression than lymphoid (Fig.…”

“…PBMCs were plated at 1×10 6 cells/mL and infected with an MOI of 0.01 FeLV-61E as was previously described (Chiu and VandeWoude, 2020). Briefly, supernatant was sampled at days 0, 1, 3, 5, and 7 and tested for viral antigen using p27 ELISA, as previously described PBMCs were harvested at day 5 to enumerate cell viability and proviral copy number.…”

“…FeLV endogenization has occurred in Felidae of the Felis genus, and has been characterized in the domestic cat ( Felis catus ). Eight to twelve nearly full-length enFeLV genomes are present in each genome, with significantly greater numbers of solo LTR remnants (Chiu and VandeWoude, 2020; Powers et al, 2018; Roca et al, 2005). Full-length enFeLV genomes are 86% similar at the nucleotide level to horizontally transmitted exogenous FeLV (exFeLV) (Chiu et al, 2018).…”

“…This finding was in contrast to cats with lower LTR copy number, which developed progressive infection and accumulated virulent enFeLV-exogenous FeLV recombinants (Powers et al, 2018). We experimentally infected domestic cat fibroblasts with FeLV and likewise demonstrated that primary cells from cats with greater enFeLV-LTR copy number were more resistant to FeLV infection and viral replication (Chiu and VandeWoude, 2020). This relationship was not observed when we examined FeLV infection and replication related to enFeLV- env gene copy number, representing intact full enFeLV genomes, which were found at considerably lower rate of incorporation than enFeLV-LTR (mean of 11 env copies/cell versus 57 LTR copies/cell) (Chiu and VandeWoude, 2020).…”

Bioaccumulation of Pathogen Exposure in Top Predators

“…In contrast to these asymptomatic infections, spillover of feline leukemia virus (FeLV) from domestic cat to puma resulted in fatal outbreaks of disease in the endangered Florida panther (Puma concolor coryi) that significantly impacted the population and hindered the species recovery program [6,36]. Recent evidence suggests FeLV may be able to replicate more competently in puma than the native domestic cat host [37] due to differences in host genomic content. FeLV originating in domestic cats has also been recorded to cause high mortality in Iberian lynx (Lynx pardinus) [7].…”