Feline leukemia virus immunity induced by whole inactivated virus vaccination

Abstract

A fraction of cats exposed to feline leukemia virus (FeLV) effectively contain virus and resist persistent antigenemia/viremia. Using real-time PCR (qPCR) to quantitate circulating viral DNA levels, previously we detected persistent FeLV DNA in blood cells of non-antigenemic cats considered to have resisted FeLV challenge. In addition, previously we used RNA qPCR to quantitate circulating viral RNA levels and determined that the vast majority of viral DNA is transcriptionally active, even in the absence of antigenemia. A single comparison of all USDA-licensed commercially available FeLV vaccines using these modern sensitive methods has not been reported. To determine whether FeLV vaccination would prevent nucleic acid persistence, we assayed circulating viral DNA, RNA, antigen, infectious virus, and virus neutralizing (VN) antibody in vaccinated and unvaccinated cats challenged with infectious FeLV. We identified challenged vaccinates with undetectable antigenemia and viremia concomitant with persistent FeLV DNA and/or RNA. Moreover, these studies demonstrated that two whole inactivated virus (WIV) adjuvanted FeLV vaccines (Fort Dodge Animal Health's Fel-O-Vax Lv-K^® and Schering-Plough Animal Health's FEVAXYN FeLV^®) provided effective protection against FeLV challenge. In nearly every recipient of these vaccines, neither viral DNA, RNA, antigen, nor infectious virus could be detected in blood after FeLV challenge. Interestingly, this effective viral containment occurred despite a weak to undetectable VN antibody response. The above findings reinforce the precept of FeLV infection as a unique model of effective retroviral immunity elicited by WIV vaccination, and as such holds valuable insights into retroviral immunoprevention and therapy.

Introduction

Feline leukemia virus (FeLV) was identified as a naturally occurring retroviral infection of cats over 40 years ago (Jarrett et al., 1964, Kawakami et al., 1967, Rickard et al., 1969). The primary route of transmission of this gammaretrovirus is horizontally through saliva (Francis et al., 1977, Hardy et al., 1976, Hardy et al., 1973, Hoover et al., 1977a). The pathogenic effects of FeLV infection are both cytoproliferative (e.g. lymphoma, myeloproliferative disorder) and cytosuppressive (e.g. immunodeficiency, myelosuppression) (Hoover and Mullins, 1991).

Historically, FeLV infection has represented a diametric paradigm of effective host response leading to regressive infection vs. ineffective host response leading to progressive infection and disease (Hoover et al., 1981). This model has been based on assays detecting either: (a) viremia by cell culture infectivity (VI) (de Noronha et al., 1977, Fischinger et al., 1974) or (b) intracellular antigenemia in leukocytes by immunofluorescent antibody (IFA) assay (Hardy et al., 1973, Hardy and Zuckerman, 1991a) or (c) extracellular antigenemia in plasma or serum by capture ELISA (Lutz et al., 1983a). Information obtained using these assays was used to estimate that in ∼60% of young adult cats exposed to FeLV, neither p27 capsid antigen nor infectious virus were detectable in the blood after virus challenge (Hardy, 1980, Hardy et al., 1976, Hoover and Mullins, 1991, Rojko et al., 1979). In stark contrast, ∼30% of exposed animals developed persistent antigenemia and viremia. However, subsequent widespread use of the p27 capture ELISA, in combination with the IFA and VI assays, prompted the identification of cats with discordant results (Hardy and Zuckerman, 1991b, Jarrett et al., 1982, Lutz et al., 1980b, Lutz et al., 1983b). In addition, several laboratories demonstrated that it is possible to reactivate FeLV from some cats with regressive infections (Madewell and Jarrett, 1983, Post and Warren, 1980, Rojko et al., 1982). These observations pointed to a more complex, less polar, view of FeLV:host relationships (Hoover and Mullins, 1991) and/or varying limits in assay sensitivity.

We have recently applied quantitative real-time PCR (qPCR) to examine vaccinated and unvaccinated cats challenged oronasally with FeLV-A/61E and found covert FeLV DNA, in both circulation and tissues, in the absence of detectable antigenemia (Torres et al., 2005). Investigators have shown that proviral integration occurs not only in cats with persistent antigenemia, but also in cats without detectable anitgenemia and with lower circulating proviral burdens (Cattori et al., 2006). Additionally, we have reported a near perfect agreement and strong linear correlation between FeLV DNA and RNA in the blood of FeLV-challenged cats, inferring that a substantial fraction of the detected FeLV DNA was indeed integrated into the host cell genome and initiated a transcriptionally active infection (Torres et al., 2008). Consequently, a spectrum of FeLV:host relationships have been identified, including cats with detectable nucleic acids and undetectable antigenemia (latent infections) and cats with both detectable nucleic acids and antigenemia (active infections). These findings, and those of colleagues (Cattori et al., 2006, Flynn et al., 2002, Gomes-Keller et al., 2006a, Gomes-Keller et al., 2006b, Hofmann-Lehmann et al., 2001, Hofmann-Lehmann et al., 2006, Tandon et al., 2005), demonstrated that DNA and RNA qPCR sensitivities are greater than p27 capsid antigen capture ELISA.

A singular feature of FeLV infection has been the development of effective vaccines providing protection against virulent virus challenge. At the time this study was initiated, four FeLV vaccines were commercially available in the USA, each with varying formulations and efficacy [reviewed by Loar, 1993 and Sparkes, 1997]. Despite the accumulation of individual vaccine trials, the differences in experimental designs have made comparisons of vaccine efficacy virtually impossible. Moreover, most of these studies were performed before the advent of qPCR, thereby limiting the ability to detect evidence of viral infection. To our knowledge, a single comparison of every USDA-licensed FeLV vaccine that is commercially available in the USA has not been reported using modern viral nucleic acid detection methods. In addition, our previous work identified several protected vaccinates without any evidence of viral infection despite the use of DNA qPCR. Hofmann-Lehmann et al. however, have not observed this unusual level of protection (Hofmann-Lehmann et al., 2006, Hofmann-Lehmann et al., 2007, Hofmann-Lehmann et al., 2008).

The present study, therefore, had two purposes: (1) to compare all USDA-licensed commercially available FeLV vaccines by determining whether they differed in ability to protect against both active and latent viral infection using contemporary sensitive methods and (2) to determine whether a neutralizing humoral immune response was associated with highly effective viral containment. Accordingly, we examined virulent FeLV challenge outcomes in cohorts of cats vaccinated with one of four commercially available vaccines and have assessed host:virus relationships by criteria of viral DNA, RNA, p27 capsid antigen, infectious virus, and neutralizing antibody.