Immune response of healthy and leukemic cats to the feline oncornavirus-associated cell membrane antigen

Adeno-Associated Vector-Delivered CRISPR/SaCas9 System Reduces Feline Leukemia Virus Production In Vitro

Feline leukemia virus (FeLV) is a retrovirus of cats worldwide. High viral loads are associated with progressive infection and the death of the host, due to FeLV-associated disease. In contrast, low viral loads, an effective immune response, and a better clinical outcome can be observed in cats with regressive infection. We hypothesize that by lowering viral loads in progressively infected cats, using CRISPR/SaCas9-assisted gene therapy, the cat’s immune system may be permitted to direct the infection towards a regressive outcome. In a step towards this goal, the present study evaluates different adeno-associated vectors (AAVs) for their competence in delivering a gene editing system into feline cells, followed by investigations of the CRISPR/SaCas9 targeting efficiency for different sites within the FeLV provirus. Nine natural AAV serotypes, two AAV hybrid strains, and Anc80L65, an in silico predicted AAV ancestor, were tested for their potential to infect different feline cell lines and feline primary cells. AAV-DJ revealed superior infection efficiency and was thus employed in subsequent transduction experiments. The introduction of double-strand breaks, using the CRISPR/SaCas9 system targeting 12 selected FeLV provirus sites, was confirmed by T7 endonuclease 1 (T7E1), as well as Tracking of Indels by Decomposition (TIDE) analysis. The highest percentage (up to 80%) of nonhomologous end-joining (NHEJ) was found in the highly conserved gag and pol regions. Subsequent transduction experiments, using AAV-DJ, confirmed indel formation and showed a significant reduction in FeLV p27 antigen for some targets. The targeting of the FeLV provirus was efficient when using the CRISPR/SaCas9 approach in vitro. Whether the observed extent of provirus targeting will be sufficient to provide progressively FeLV-infected cats with the means to overcome the infection needs to be further investigated in vivo.

Horizontal transmission of feline leukemia virus under natural conditions in a feline leukemia cluster household

Ten post-weanling 4-month-old cats, designated "tracers", were placed in a feline leukemia cluster household to determine the efficiency of horizontal transmission of feline leukemia virus (FeLV). The tracer cats were confirmed as negative for prior exposure to FeLV. Following the placement in the leukemia cluster environment, the tracer cats were serologically monitored at intervals of 3-6 weeks for a total period of 1 year. The tests employed included the detection of FeLV using fixed-cell immunofluorescence and the detection and titration of antibody to : (1) the feline oncornavirus-associated cell membrane antigen (FOCMA), as detected by membrane immunofluorescence; (2) viable FeLV, using serum neutralization; (3) virion core protein p30, using radioimmunoprecipitation; and (4) virion glycoprotein gp70, using radioimmunoprecipitation. All of the tracers had evidence of horizontal infection by FeLV, by several criteria. Seven of the 10 had virus that could be isolated from plasma. All of these 7 developed a terminal illness within 18 months; 3 developed aplastic anemia, 3 infectious peritonitis, and 1 lymphoma. The remaining 3 were negative for FeLV by both virus isolation and fixed-cell immunofluorescence. These 3 did, however, develop high antibody titers by all four criteria and they remained healthy throughout the examination period. These results clearly indicate that unprotected pros-weanling cats brought into a leukemia exposure household environment have a high risk of becoming infected with FeLV. Furthermore, a large proportion of the cats are at risk for development of persistent viremia and FeLV-related diseases.