Oswald Jarrett

Determinants of the host range of feline leukaemia viruses.

Summary Feline leukaemia viruses of subgroups B and C multiply in human and canine cells, while subgroup A viruses do not. This host range restriction is determined by the virus envelope and operates at the level of virus entry into the cell. Subgroup A virus genomes are expressed and replicated when they are introduced within B subgroup envelopes into human or dog cells. Therefore, since they are phenotypic mixtures of A and B subgroup viruses, the majority of feline leukaemia virus isolates will infect human and canine cells with the subsequent production of FeLV of each subgroup.

Histologic Classification and Immunophenotype of Lymphosarcomas in Cats with Naturally and Experimentally Acquired Feline Immunodeficiency Virus Infections

Lymphosarcoma (malignant lymphoma) is the commonest hematopoietic tumor in the cat. Many cases are associated with feline leukemia virus (FeLV) infection, but epidemiologic and experimental data suggest that feline immunodeficiency virus (FIV) may also have a role in lymphomagenesis. In this paper, we describe the clinical presentation, histologic classification, and immunophenotype of lymphosarcoma in eight domestic cats with natural or experimental FIV infections. The tumors were often of unusual distribution, with the majority of cases conforming to the least common anatomic classification of “miscellaneous.” Histopathologic and immunophenotypic analysis using a panel of anti-cat and cross-reactive anti-human monoclonal and polyclonal antibodies identified seven of these tumors as high-grade B cell lymphomas of the centroblastic or immunoblastic subtypes. The remaining case was a T-cell tumor associated with a concurrent FeLV infection. Our findings, together with the results of an analysis of FIV proviral DNA in these tumors, indicate that the B-cell lymphosarcomas were comparable to those observed in human and simian immunodeficiency virus infections and that the role of FIV in lymphomagenesis is indirect and related to the potential for malignant transformation during polyclonal B cell activation.

Serological responses of cats to feline immunodeficiency virus.

The proteins of feline immunodeficiency virus (FIV) were identified by sodium dodecylsulphate poly-acrylamide gel electrophoresis (SDS-PAGE) and immunoblotting. Purified [35S]methionine/ cysteine-Iabelled virus contained proteins of M r 120, 24, 17, and 10kD, of which the most prominent were p24 and p17, and minor components of 62, 54, 52, 41 and 32kD. Sera from FIV-infected cats precipitated two glycoproteins (gp) of M r 120kD (gp120) and 41kD (gp41) from lysates of [14C]glucosamine-labelled infected cells. Purified virus contained very little or no detectable glycoproteins. The serological response to individual viral proteins was followed in experimentally infected cats by immunoblotting. Since purified virus was a poor source of gp120, a method using FIV-infected cell lysates was developed. Cats produced antibodies to gp120, p55, p24 and p17. (The p55 was presumed to be a precursor of p24 and p17.) Following infection, antibodies developed first to p24 and subsequently to p17, p55 and gp120. Sera from cats infected with three separate isolates of FIV, two from the UK and one from the USA, had cross-reacting antibodies to all of these viral proteins. The criteria for identification of seropositive cats were defined. The minimum requirement for a positive immunoblot was antibody to gp120 or to at least three core proteins (p55, p24 and p17). Comparison of two commercial enzyme-linked immunosorbent assay (ELISA) kits and immunoblotting indicated that false-positive results occurred as a result of non-specific reactions in the ELISA systems.

VALUE OF ALPHA 1-ACID GLYCOPROTEIN IN THE DIAGNOSIS OF FELINE INFECTIOUS PERITONITIS

Feline infectious peritonitis (FIP) is notoriously difficult to differentiate from the many other diseases with similar clinical signs and at present the only conclusive diagnostic test is the histopathological examination of a biopsy. The potential value of raised levels of the acute phase reactants, α1-acid glycoprotein (AGP) and haptoglobin in the diagnosis of the disease was investigated. The concentrations of the two proteins were determined in serum samples from healthy cats and gave reference ranges of 0.1 to 0.48 g/litre and 0.04 to 3.84 g/litre, respectively. Levels of AGP greater than 1.5 g/litre in serum, plasma or effusion samples were found to be of value in distinguishing field cases of FIP from cats with similar clinical signs and differentiated these two groups of cats more effectively than the albumin:globulin ratio. The concentration of haptoglobin was higher in cats with FIP than in the group of healthy cats, but this protein was not of value in the diagnosis of FIP. Serum samples from feline immunodeficiency virusinfected cats were also analysed for these proteins and their concentrations were significantly elevated, illustrating that raised levels of AGP and haptoglobin are not pathognomonic for FIP.

Enhancement after feline immunodeficiency virus vaccination

Cats were vaccinated with one of the three preparations: purified feline immunodeficiency virus (FIV) incorporated into immune stimulating complexes (ISCOMs), recombinant FIV p24 ISCOMs, or a fixed, inactivated cell vaccine in quil A. Cats inoculated with the FIV ISCOMs or the recombinant p24 ISCOMs developed high titres of antibodies against the core protein p24 but had no detectable antibodies against the env protein gp120 or virus neutralising antibodies. In contrast, all of the cats inoculated with the fixed, inactivated cell vaccine developed anti-env antibodies and four of five had detectable levels of neutralising antibody. However, none of the vaccinated cats were protected from infection after intraperitoneal challenge with 20 infectious units of FIV. Indeed there appeared to be enhancement of infection after vaccination as the vaccinated cats become viraemic sooner than the unvaccinated controls, and 100% of the vaccinated cats became viraemic compared with 78% of the controls. The mechanism responsible for this enhancement remains unknown.

Clinical and pathological findings in feline immunodeficiency virus experimental infection

Abstract A study is described of the clinical and pathological findings in 20 specific pathogen free cats infected when 1 year old with feline immunodeficiency virus and monitored over 12 months. Cats were divided into two groups (A and B). The clinical and clinicopathological features were studied in Group A. In Group B, at 1, 2, 4, 9 and 12 months post infection two cats were necropsied. Clinically all cats developed generalised lymphadenopathy, six cats were neutropenic and five cats lymphopenic. Three cats became febrile with conjunctivitis and anterior uveitis and one of these cats ultimately developed jaundice. Postmortem examinations confirmed a generalised lymphadenopathy involving peripheral and visceral lymph nodes with concurrent stimulation of splenic white matter and mucosal lymphoid tissue of the digestive tract and conjunctiva. Within the lymph nodes there was a reactive follicular hyperplasia accompanied by a paracortical hyperplasia with an increased paracortical vascularity. Unusual features were the presence of lymphoid follicles in the bone marrow, thymus and parathyroid tissue. In addition, aggregates of lymphoid cells were found within salivary glands, kidneys, sclera and choroid of the eye. One cat developed a lymphosarcoma affecting the liver and kidneys at 36 weeks post infection. The cat with jaundice had a cholangitis with marked biliary epithelial hyperplasia.

Use of a reverse-transcriptase polymerase chain reaction for monitoring the shedding of feline coronavirus by healthy cats

The pattern of shedding of feline coronavirus (FCoV) was established in 155 naturally infected pet cats from 29 households over periods of up to five years. Viral RNA was detected in faeces by reverse-transcriptase PCR (RT-PCR), and plasma antiviral antibodies by immunofluorescence. The cats rarely shed FCoV in their saliva. Three patterns of FCoV shedding were observed. Eighteen of the cats shed virus continuously, so were persistent, and possibly lifelong, carriers; none of them developed feline infectious peritonitis. Fifty-six cats ceased shedding virus, although they were susceptible to reinfection, and 44 shed intermittently or were being continuously reinfected. Four of the cats were resistant to infection. Seventy-three per cent of the virus shedding episodes lasted up to three months and 95 per cent up to nine months. There was a correlation between shedding and antibody titre but the cats could remain seropositive for some time after they had ceased shedding virus. One-off testing for FcoV by RT-PCR iS inappropriate. Identification of longterm carriers requires that a positive result be obtained by RT-PCR on faecal samples for at least eight consecutive months. A cat should be shown to be negative over five months, or to have become seronegative, to ensure that it has ceased shedding virus.

Feline leukemia virus DNA vaccine efficacy is enhanced by coadministration with interleukin-12 (IL-12) and IL-18 expression vectors

ABSTRACT The expectation that cell-mediated immunity is important in the control of feline leukemia virus (FeLV) infection led us to test a DNA vaccine administered alone or with cytokines that favored the development of a Th1 immune response. The vaccine consisted of two plasmids, one expressing the gag/pol genes and the other expressing the env gene of FeLV-A/Glasgow-1. The genetic adjuvants were plasmids encoding the feline cytokines interleukin-12 (IL-12), IL-18, or gamma interferon (IFN-γ). Kittens were immunized by three intramuscular inoculations of the FeLV DNA vaccine alone or in combination with plasmids expressing IFN-γ, IL-12, or both IL-12 and IL-18. Control kittens were inoculated with empty plasmid. Following immunization, anti-FeLV antibodies were not detected in any kitten. Three weeks after the final immunization, the kittens were challenged by the intraperitoneal inoculation of FeLV-A/Glasgow-1 and were then monitored for a further 15 weeks for the presence of virus in plasma and, at the end of the trial, for latent virus in bone marrow. The vaccine consisting of FeLV DNA with the IL-12 and IL-18 genes conferred significant immunity, protecting completely against transient and persistent viremia, and in five of six kittens protecting against latent infection. None of the other vaccines provided significant protection.

Productive infection of T-helper lymphocytes with feline immunodeficiency virus is accompanied by reduced expression of CD4.

An antigen-specific feline T-lymphocyte cell line (Q201) was generated and infected in vitro with the feline immunodeficiency virus (FIV). Syncytium formation and the release of the viral core protein p24 into culture fluid were accompanied by a reduction in expression of the CD4 surface antigen. The reduction in CD4 expression was transient, the resulting persistently infected population of cells expressing levels of CD4 comparable to those observed prior to infection. Persistently infected cells gradually lost expression of major histocompatibility antigen (MHC) class II while maintaining pre-infection levels of expression of CD4, MHC class I, CD18 or CD29.

Partial dissociation of subgroup C phenotype and in vivo behaviour in feline leukaemia viruses with chimeric envelope genes

Feline leukaemia viruses (FeLVs) are classified into subgroups A, B and C by their use of different host cell receptors on feline cells, a phenotype which is determined by the viral envelope. FeLV-A is the ubiquitous, highly infectious form of FeLV, and FeLV-C isolates are rare variants which are invariably isolated along with FeLV-A. The FeLV-C isolates share the capacity to induce acute non-regenerative anaemia and the prototype, FeLV-C/Sarma, has strongly age-restricted infectivity for cats. The FeLV-C/Sarma env sequence is closely related to that of common, weakly pathogenic FeLV-A isolates. We now show by construction of chimeric viruses that the receptor specificity of FeLV-A/Glasgow-1 virus can be converted to that of FeLV-C by exchange of a single env variable domain, Vr1, which differs by a three codon deletion and nine adjacent substitutions. Attempts to dissect this region further by directed mutagenesis resulted in disabled proviruses. Sequence analysis of independent natural FeLV-C isolates showed that they have unique Vr1 sequences which are distinct from the conserved FeLV-A pattern. The chimeric viruses which acquired the host range and subgroup properties of FeLV-C retained certain FeLV-A-like properties in that they were non-cytopathogenic in 3201B feline T cells and readily induced viraemia in weanling animals. They also induced a profound anaemia in neonates which had a more prolonged course than that induced by FeLV-C/Sarma and which was macrocytic rather than non-regenerative in nature. Although receptor specificity and a major determinant of pathogenicity segregate with Vr1, it appears that sequences elsewhere in the genome influence infectivity and pathogenicity independently of the subgroup phenotype.