A vector expressing feline mature IL-18 fused to IL-1beta antagonist protein signal sequence is an effective adjuvant to a DNA vaccine for feline leukaemia virus

Abstract

DNA vaccination using vectors expressing the gag/pol and env genes of feline leukaemia virus (FeLV) and plasmids encoding feline interleukin-12 (IL-12) and IL-18 completely protected cats from viraemia following challenge [Hanlon L, Argyle D, Bain D, Nicolson L, Dunham S, Golder MC, et al. Feline leukaemia virus DNA vaccine efficacy is enhanced by coadministration with interleukin-12 (IL-12) and IL-18 expression vectors. J Virol 2001;75:8424-33]. However, the relative contribution of each cytokine gene towards protection is unknown. This study aimed to resolve this issue. IL-12 and IL-18 constructs were modified to ensure effective expression, and bioactivity was demonstrated using specific assays. Kittens were immunised intramuscularly with FeLV DNA and various cytokine constructs. Together with control kittens, these were challenged oronasally with FeLV and monitored for 15 weeks. All six kittens given FeLV, IL-12 and IL-18 were protected from the establishment of persistent viraemia and four from latent infection. Of six kittens immunised with FeLV DNA and IL-18, all were protected from viraemia and five from latent infection. In contrast, three of five kittens given FeLV DNA and IL-12 became persistently viraemic. Therefore, the adjuvant effect on the FeLV DNA vaccine appears to reside in the expression of IL-18.

Fig. 1

Schedule of the experiment, showing vaccination, viral challenge, blood sampling, virus isolation and antibody detection. Black arrow: immunisation of 100 μg of each DNA construct, intramuscularly to quadriceps femoris; grey arrow: viral challenge of 10⁶ FFU delivered on days 0, 2, 4, and 8 to a total dose of 4 × 10⁶ FFU; white arrow: blood sampling; at position (1) pre-trial, (2) 48 h post-immunisation, (3) pre-challenge; black diamond: virus isolation; black triangle: virus neutralizing antibody detection; black circle: anti-FeLV antibody; black square: bone marrow sample used for virus isolation.

Fig. 2

Expression of feline IL-12. Western blot prepared from a 12% SDS-PAGE gel showing transfection supernatants and cell lysates as follows: pCI-neo supernatant (lane 1), pCI-neo cell lysate (lane 2), feline flexi-IL-12 supernatant 10 × concentrate (lane 3), feline flexi-IL-12 cell lysate (lane 4), feline p40 supernatant (lane 5), feline p40 cell lysate (lane 6).

Fig. 3

Biological activity of flexi-IL-12. Ten-fold dilutions of flexi-IL-12 transfection supernatant were plotted against equine IFNγ production to produce an S-shaped curve. In parallel, flexi-IL-12 supernatant was incubated with mouse anti-human IL-12 p40/p70 feline cross-reactive neutralising antibody and mouse anti-protein A non-neutralising antibody.