Elsevier

Vaccine

Volume 24, Issue 8, 20 February 2006, Pages 1087-1094
Vaccine

Reassessment of feline leukaemia virus (FeLV) vaccines with novel sensitive molecular assays

https://doi.org/10.1016/j.vaccine.2005.09.010Get rights and content

Abstract

We previously described antigen negative, provirus positive cats. Subsequently, we hypothesized that efficacious FeLV vaccines cannot prevent minimal viral replication. Thus, we vaccinated cats with either a canarypox-vectored live or a killed virus vaccine and analyzed the challenge outcome with quantitative PCR and a newly established real-time RT-PCR. When judged by conventional parameters (antigenaemia, virus isolation), most of the vaccinated cats were, as expected, protected from persistent viraemia. However, all cats were found to be plasma viral RNA positive. The loads were significantly associated with the infection outcome. Thus, commonly used FeLV vaccines understood to be successful model antiretroviral vaccines protecting against FeLV-related diseases do not confer sterilizing immunity.

Introduction

Feline leukaemia virus (FeLV) [1] is a gamma-retrovirus and a well-known feline pathogen occurring worldwide in domestic cats and some related felids [2], [3]. FeLV infection can lead to fatal neoplasia, degenerative diseases of the haematopoietic system, and immunodeficiency. It is not only of great veterinary but also of human medical interest as it is an important animal model for tumor and AIDS research [4], [5].

In contrast to infections with lentiviruses, such as the feline or human immunodeficiency viruses, the susceptibility of cats to FeLV varies remarkably and different infection outcomes are known [2], [6], [7], [8], [9], [10], [11]. A proportion of cats develop progressive infection with persistent viraemia; they lack FeLV specific humoral and cellular immunity [2], [12], [13], and ultimately develop FeLV-associated diseases [14]. The majority of FeLV exposed cats develop contained infection characterized by transient or undetectable viraemia and an effective immune response [7], [12], [13], [15]. Categorization of cats into these groups is typically accomplished using results of virus isolation and FeLV p27 antigen detection from blood plasma [16]. Latent non-productive infection characterized by absence of viraemia and persistence of virus in the bone marrow can be identified in cats that had a contained course of infection and ostensibly recovered from it; latency is classically detected by cell culture of bone marrow cells in the presence of corticosteroids [17], [18]. In addition, some non-viraemic cats were demonstrated to have localized FeLV infection in selected other tissues, such as spleen, lymph node, small intestine and mammary glands [19], [20].

Recently, we developed a sensitive method for the detection and quantification of FeLV proviral loads [21]. Employing a quantitative polymerase chain reaction (PCR) assay, we demonstrated that not only viraemic cats but unexpectedly all experimentally challenged cats turned provirus positive starting as early as one week after FeLV exposure [21]. Quantifying the proviral burdens, we found that cats with contained infection had lower loads than cats that became persistently viraemic. So far, it is incompletely understood which factors determine the magnitude of the proviral load and with that the infection course. However, we demonstrated that lower initial proviral loads were associated with higher humoral immune responses [21]. Moreover, Flynn et al. demonstrated an important role of cytotoxic T lymphocytes in FeLV specific immunity [12]. The role of plasma viral RNA loads during FeLV infection could up to now not been established as the necessary methods, such as a specific quantitative reverse transcriptase (RT) PCR assay, had not been available.

Excellent FeLV vaccines had been developed several years ago and are now commonly used in veterinary practices [22], [23], [24]. Some are based on formaldehyde-inactivated FeLV proteins produced in cell culture [25], [26], [27], [28], others contain recombinant FeLV surface proteins [29]. More recently, an efficacious recombinant canarypox virus vaccine expressing FeLV genes was introduced that stimulates a strong immune response in the absence of virus amplification in the host [30]. FeLV vaccine evaluation was so far generally conducted assaying conventional parameters including virus isolation and FeLV antigen detection. Since all animals exposed to FeLV, including non-viraemic, protected cats, had previously turned provirus positive [21], we hypothesized that even efficacious commonly used FeLV vaccines would not protect cats from minimal viral replication subsequent to challenge exposure. In the present study, we tested our hypothesis in a prospective experimental FeLV vaccination and challenge study using for the first time a FeLV specific real-time RT-PCR method to assess plasma viral RNA loads in vaccinated cats. We tested FeLV vaccines that represent two different vaccine types, have been reported to be efficacious according to conventional parameters and are widely employed in veterinary practices nowadays.

Section snippets

Animals, experimental design and vaccines

Thirty 11-week-old outbred, specific pathogen-free (SPF) domestic kittens (Charles River Laboratories, Lyon, France) were randomly assigned to three groups of 10 cats each avoiding familial clustering. Each group consisted of seven male castrated and three female spayed cats. Cats were kept under barrier conditions and housed in groups in large rooms as required by the Swiss law under optimal ethological conditions. Group 1 served as unvaccinated control animals. Kittens of group 2 were

FeLV vaccines confer protection against FeLV viraemia as determined by conventional methods

Subsequent to FeLV challenge exposure, 9 out of 10 unvaccinated control kittens became persistently infected as determined by virus isolation and antigen ELISA from plasma (Fig. 1). In contrast, only 2 out of 10 Eurifel vaccinated cats (significant protection; pFisher = 0.0055) and 5 out of 10 Fel-O-Vax vaccinated animals became persistently infected, respectively (Fig. 1). The preventable fraction [36] for Eurifel was 78%; that for Fel-O-Vax was 44%. Similarly, 9 out of 10 controls and 6 out of

Discussion

Judged by conventional diagnostic measures, such as detection of antigenaemia and virus isolation, FeLV exposure of cats leads to different outcomes of the infection: progressive infection defined by persistent viraemia or contained infection characterized by transient or undetectable viraemia [2], [6], [8], [9], [10], [11]. Using a more sensitive molecular real-time PCR method to detect and quantify FeLV provirus in peripheral blood cells of cats, we previously found that all cats

Acknowledgements

We thank Dr. O. Jarrett (University of Glasgow), who kindly provided the FeLV virus stock. We thank Y. Bosshart, C. Brönnimann, C. Brunner, U.M. Dreher, U. Egger, E. Gönczi, E. Grässli, P. Fidler, V. Fornera, M. Huder, B. Lange, M. Rios, E. Rogg, E. Rhiner, J. Wälchli, and B. Wenger for excellent assistance, and Dr. D.D. Addie (University of Glasgow) for helpful discussion. Laboratory work was performed using the logistics of the Center for Clinical Studies at the Vetsuisse Faculty, University

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