Multiple recombination events between endogenous retroviral elements and feline leukemia virus

Feline leukemia virus (FeLV) is an exogenous retrovirus that causes malignant hematopoietic disorders in domestic cats, and its virulence may be closely associated with viral sequences. FeLV is classified into several subgroups, including A, B, C, D, E, and T, based on viral receptor interference properties or receptor usage. However, the transmission manner and disease specificity of the recombinant viruses FeLV-D and FeLV-B remain unclear. The aim of this study was to understand recombination events between exogenous and endogenous retroviruses within a host and elucidate the emergence and transmission of recombinant viruses. We observed multiple recombination events involving endogenous retroviruses (ERVs) in FeLV from a family of domestic cats kept in one house; two of these cats (ON-T and ON-C) presented with lymphoma and leukemia, respectively. Clonal integration of FeLV-D was observed in the ON-T case, suggesting an association with FeLV-D pathogenesis. Notably, the receptor usage of FeLV-B observed in ON-T was mediated by feline Pit1 and feline Pit2, whereas only feline Pit1 was used in ON-C. Furthermore, XR-FeLV, a recombinant FeLV containing an unrelated sequence referred to the X-region, which is homologous to a portion of the 5'-leader sequence of Felis catus endogenous gammaretrovirus 4 (FcERV-gamma4), was isolated. Genetic analysis suggested that most recombinant viruses occurred de novo; however, the possibility of FeLV-B transmission was also recognized in the family. This study demonstrated the occurrence of multiple recombination events between exogenous and endogenous retroviruses in domestic cats, highlighting the contribution of ERVs to pathogenic recombinant viruses.IMPORTANCEFeline leukemia virus subgroup A (FeLV-A) is primarily transmitted among cats. During viral transmission, genetic changes in the viral genome lead to the emergence of novel FeLV subgroups or variants with altered virulence. We isolated three FeLV subgroups (A, B, and D) and XR-FeLV from two cats and identified multiple recombination events in feline endogenous retroviruses (ERVs), such as enFeLV, ERV-DC, and FcERV-gamma4, which are present in the cat genome. This study highlights the pathogenic contribution of ERVs in the emergence of FeLV-B, FeLV-D, and XR-FeLV in a feline population.

Feline Leukemia Virus p27 Antigen Concentration and Proviral DNA Load Are Associated with Survival in Naturally Infected Cats

Longitudinal studies of cats naturally infected with feline leukemia virus (FeLV) are important for understanding disease outcomes. Levels of p27 antigen and copy numbers of proviral DNA have been associated with FeLV-infection courses. The purpose of this prospective study was to establish cutoff values for p27 antigen concentration and proviral DNA load that distinguished high positive from low positive groups of cats and to evaluate an association with survival. At enrollment, 254 cats were tested by point-of-care and microtiter plate enzyme-linked immunosorbent assays (ELISAs) for p27 antigen and real-time polymerase chain reaction (PCR) for proviral DNA. The 127 positive cats were retested monthly for six months and monitored for survival over the four-year study. A receiver operating characteristic-based analysis of samples with concordant or discordant qualitative results for p27 antigen and proviral DNA was used to establish cutoff values, and when applied to test results at enrollment for classifying cats as high positive or low positive, a significant difference in survival was observed. High positive cats had a median survival of 1.37 years (95% CI 0.83-2.02) from time of enrollment, while most low positive cats were still alive (93.1% survival). Quantitative results for p27 antigen concentration and proviral DNA load were highly correlated with survival times in FeLV-infected cats.

Co-infection with feline retrovirus is related to changes in immunological parameters of cats with sporotrichosis

Feline sporotrichosis due to Sporothrix brasiliensis is frequently severe and often correlated to zoonotic transmission. Feline Immunodeficiency Virus (FIV) and Feline Leukemia Virus (FeLV) cause immunodeficiency in cats; no association has been identified with critical cases of sporotrichosis. Moreover, the cytokine profile in Sporothrix-infected cats and a potential impact of retrovirus co-infections on their immunity is unknown. This study assessed immunological parameters in cats with sporotrichosis with and without FIV or FeLV co-infection. FeLV infection was detected by antigen ELISA and by provirus PCR. FIV infection was investigated through ELISA and Western blot. Cytokine transcription (IFN-γ, IL-4, IL-5, IL-6, IL-10, IL-12, TNF-α) was quantified using RT-qPCR and lymphocyte subpopulations (CD4, CD8, CD5 and CD21) were assessed by flow cytometry. Thirty cats with sporotrichosis were recruited to the study, including three FIV-positive and five FeLV-positive (progressive infection) cats. One cat with regressive FeLV infection was excluded from statistics. In comparison to retrovirus-negative cats, FIV-positive cats and FeLV-positive cats had higher IL-10 levels, FeLV-positive cats had lower IL-4 levels and FIV-positive cats had lower IL-12 levels and a lower CD4+/CD8+ ratio. Remarkably, all cats with poor general condition were FeLV (progressive infection) or FIV-positive, but the retrovirus status was not associated with the sporotrichosis treatment length or outcome. The immunological changes and the more severe clinical presentation observed in cats with retrovirus co-infections encourage future prospective studies that address the impact of these changes on prognostic determinants of feline sporotrichosis and the development of new therapy strategies that control disease spread.

A Retrospective Examination of Feline Leukemia Subgroup Characterization: Viral Interference Assays to Deep Sequencing

Feline leukemia virus (FeLV) was the first feline retrovirus discovered, and is associated with multiple fatal disease syndromes in cats, including lymphoma. The original research conducted on FeLV employed classical virological techniques. As methods have evolved to allow FeLV genetic characterization, investigators have continued to unravel the molecular pathology associated with this fascinating agent. In this review, we discuss how FeLV classification, transmission, and disease-inducing potential have been defined sequentially by viral interference assays, Sanger sequencing, PCR, and next-generation sequencing. In particular, we highlight the influences of endogenous FeLV and host genetics that represent FeLV research opportunities on the near horizon.

[Efficacy of siRNA on feline leukemia virus replication in vitro]

Feline leukemia virus (FeLV) can lead to severe clinical signs in cats. Until now, there is no effective therapy for FeLV-infected cats. RNA interference-based antiviral therapy is a new concept. Specific small interfering RNA (siRNA) are designed complementary to the mRNA of a target region, and thus inhibit replication. Several studies have proven efficacy of siRNAs in inhibiting virus replication. The aim of this study was to evaluate the inhibitory potential of siRNAs against FeLV replication in vitro. siRNAs against the FeLV env gene and the host cell surface receptor (feTHTR1) which is used by FeLV-A for entry as well as siRNA that were not complementary to the FeLV or cat genome, were tested. Crandell feline kidney cells (CrFK cells) were transfected with FeLV-A/Glasgow-1. On day 13, infected cells were transfected with siRNAs. As control, cells were mock-transfected or treated with azidothymidine (AZT) (5 μg/ml). Culture supernatants were analyzed for FeLV RNA using quantitative real-time RT-PCR and for FeLV p27 by ELISA every 24 hours for five days. All siRNAs significantly reduced viral RNA and p27 production, starting after 48 hours. The fact that non-complementary siRNAs also inhibited virus replication may lead to the conclusion that unspecific mechanisms rather than specific binding lead to inhibition.

Clinical aspects of feline retroviruses: a review

Feline leukemia virus (FeLV) and feline immunodeficiency virus (FIV) are retroviruses with global impact on the health of domestic cats. The two viruses differ in their potential to cause disease. FeLV is more pathogenic, and was long considered to be responsible for more clinical syndromes than any other agent in cats. FeLV can cause tumors (mainly lymphoma), bone marrow suppression syndromes (mainly anemia), and lead to secondary infectious diseases caused by suppressive effects of the virus on bone marrow and the immune system. Today, FeLV is less commonly diagnosed than in the previous 20 years; prevalence has been decreasing in most countries. However, FeLV importance may be underestimated as it has been shown that regressively infected cats (that are negative in routinely used FeLV tests) also can develop clinical signs. FIV can cause an acquired immunodeficiency syndrome that increases the risk of opportunistic infections, neurological diseases, and tumors. In most naturally infected cats, however, FIV itself does not cause severe clinical signs, and FIV-infected cats may live many years without any health problems. This article provides a review of clinical syndromes in progressively and regressively FeLV-infected cats as well as in FIV-infected cats.

Identification of novel subgroup A variants with enhanced receptor binding and replicative capacity in primary isolates of anaemogenic strains of feline leukaemia virus

BACKGROUND

The development of anaemia in feline leukaemia virus (FeLV)-infected cats is associated with the emergence of a novel viral subgroup, FeLV-C. FeLV-C arises from the subgroup that is transmitted, FeLV-A, through alterations in the amino acid sequence of the receptor binding domain (RBD) of the envelope glycoprotein that result in a shift in the receptor usage and the cell tropism of the virus. The factors that influence the transition from subgroup A to subgroup C remain unclear, one possibility is that a selective pressure in the host drives the acquisition of mutations in the RBD, creating A/C intermediates with enhanced abilities to interact with the FeLV-C receptor, FLVCR. In order to understand further the emergence of FeLV-C in the infected cat, we examined primary isolates of FeLV-C for evidence of FeLV-A variants that bore mutations consistent with a gradual evolution from FeLV-A to FeLV-C.

RESULTS

Within each isolate of FeLV-C, we identified variants that were ostensibly subgroup A by nucleic acid sequence comparisons, but which bore mutations in the RBD. One such mutation, N91D, was present in multiple isolates and when engineered into a molecular clone of the prototypic FeLV-A (Glasgow-1), enhanced replication was noted in feline cells. Expression of the N91D Env on murine leukaemia virus (MLV) pseudotypes enhanced viral entry mediated by the FeLV-A receptor THTR1 while soluble FeLV-A Env bearing the N91D mutation bound more efficiently to mouse or guinea pig cells bearing the FeLV-A and -C receptors. Long-term in vitro culture of variants bearing the N91D substitution in the presence of anti-FeLV gp70 antibodies did not result in the emergence of FeLV-C variants, suggesting that additional selective pressures in the infected cat may drive the subsequent evolution from subgroup A to subgroup C.

CONCLUSIONS

Our data support a model in which variants of FeLV-A, bearing subtle differences in the RBD of Env, may be predisposed towards enhanced replication in vivo and subsequent conversion to FeLV-C. The selection pressures in vivo that drive the emergence of FeLV-C in a proportion of infected cats remain to be established.

Identification of a novel common proviral integration site, flit-1, in feline leukemia virus induced thymic lymphoma

A new proviral integration site for feline leukemia virus (FeLV), termed flit-1, was identified from feline thymic lymphoma. Among 35 FeLV-related tumors examined, 5 of 25 thymic lymphomas demonstrated proviral insertion within flit-1 locus whereas none of four alimentary and five multicentric lymphomas and one T-lymphoid leukemia examined had rearrangement in this region. Extensive sequence analysis has shown that flit-1, which is noncoding, is conserved on human chromosome 12 and mouse chromosome 15. The human and murine homologs of flit-1 are positioned approximately 30-kb upstream to activin-A receptor type II-like 1 (ACVRL1/ALK1) gene. Expression of ACVRL1 mRNA was examined in two of five lymphomas with flit-1 rearrangement and detected in both of the two whereas normal thymuses and seven lymphoid tumors without flit-1 rearrangement had no detectable expression. Therefore, flit-1 appears to represent a novel FeLV proviral common integration domain that may influence lymphomagenesis as insertional mutagenesis.

Feline leukemia virus T entry is dependent on both expression levels and specific interactions between cofactor and receptor

Feline leukemia virus (FeLV) subgroup T uses both a multiple membrane-spanning receptor, FePit1, and a soluble cofactor, FeLIX, to enter feline cells. FeLIX is expressed from endogenous FeLV-related sequence and resembles the receptor binding domain (RBD) of the viral envelope protein. It remains unclear whether FeLV-T receptor activity requires specific residues within FePit1 and FeLIX and/or a threshold level of receptor/cofactor expression. To address this, we examined FeLV-T infection of cells expressing variable levels of FePit1 and other gammaretroviral receptors in the presence of variable amounts of soluble cofactor, either RBD or the envelope surface subunit (SU). Cofactor-receptor pairs fall into three groups with regard to mediating FeLV-T infection: those that are efficient at all concentrations tested, such as FePit1 and FeLIX; those requiring high expression of both cofactor and receptor; and those that are non-functional as receptors even at high expression. This suggests that both expression levels and specific interactions with receptor and cofactor are critical for mediating entry of FeLV-T.

Host cell range of T-lymphotropic feline leukemia virus in vitro

We compared the host cell range of T-lymphotropic feline leukemia virus (FeLV-T) with that of FeLV subgroup B (FeLV-B) by pseudotype assay in the presence of FeLIX, a truncated envelope glycoprotein of endogenous FeLV. Although both viruses use Pit1 as a receptor and FeLIX does not hamper FeLV-B infection by receptor interference, the host ranges of FeLV-T and -B were not exactly the same, suggesting a different Pit1 usage at the post-binding level. A comparison of Pit1 sequences of various mammalian species indicated that extracellular loop 1 in a topology model deduced with the PHD PredictProtein algorism may be one of the regions responsible for efficient infection by FeLV-T.

A putative thiamine transport protein is a receptor for feline leukemia virus subgroup A

Feline leukemia virus (FeLV) is a horizontally transmitted virus that causes a variety of proliferative and immunosuppressive diseases in cats. There are four subgroups of FeLV, A, B, C, and T, each of which has a distinct receptor requirement. The receptors for all but the FeLV-A subgroup have been defined previously. Here, we report the identification of the cellular receptor for FeLV-A, which is the most transmissible form of FeLV. The receptor cDNA was isolated using a gene transfer approach, which involved introducing sequences from a feline cell line permissive to FeLV-A into a murine cell line that was not permissive. The feline cDNA identified by this method was approximately 3.5 kb, and included an open reading frame predicted to encode a protein of 490 amino acids. This feline cDNA conferred susceptibility to FeLV-A when reintroduced into nonpermissive cells, but it did not render these cells permissive to any other FeLV subgroup. Moreover, these cells specifically bound FeLV-A-pseudotyped virus particles, indicating that the cDNA encodes a binding receptor for FeLV-A. The feline cDNA shares approximately 93% amino acid sequence identity with the human thiamine transport protein 1 (THTR1). The human THTR1 receptor was also functional as a receptor for FeLV-A, albeit with reduced efficiency compared to the feline orthologue. On the basis of these data, which strongly suggest the feline protein is the orthologue of human THTR1, we have named the feline receptor feTHTR1. Identification of this receptor will allow more detailed studies of the early events in FeLV transmission and may provide insights into FeLV pathogenesis.

Envelope determinants for dual-receptor specificity in feline leukemia virus subgroup A and T variants

Gammaretroviruses, including the subgroups A, B, and C of feline leukemia virus (FeLV), use a multiple-membrane-spanning transport protein as a receptor. In some cases, such as FeLV-T, a nonclassical receptor that includes both a transport protein (Pit1) and a soluble cofactor (FeLIX) is required for entry. To define which regions confer specificity to classical versus nonclassical receptor pathways, we engineered mutations found in either FeLV-A/T or FeLV-T, individually and in combination, into the backbone of the transmissible form of the virus, FeLV-A. The receptor specificities of these viruses were tested by measuring infection and binding to cells expressing the FeLV-A receptor or the FeLV-T receptors. FeLV-A receptor specificity was maintained when changes at amino acid position 6, 7, or 8 of the mature envelope glycoprotein were introduced, although differences in infection efficiency were observed. When these N-terminal mutations were introduced together with a C-terminal 4-amino-acid insertion and an adjacent amino acid change, the resulting viruses acquired FeLV-T receptor specificity. Additionally, a W-->L change at amino acid position 378, although not required, enhanced infectivity for some viruses. Thus, we have found that determinants in the N and C termini of the envelope surface unit can direct entry via the nonclassical FeLV-T receptor pathway. The region that has been defined as the receptor binding domain of gammaretroviral envelope proteins determined entry via the FeLV-A receptor independently of the presence of the N- and C-terminal FeLV-T receptor determinants.

Comprehensive mapping of receptor-functioning domains in feline leukemia virus subgroup C receptor FLVCR1

Infection of cells by the highly anemogenic feline leukemia virus subgroup C (FeLV-C) is mediated by the heme exporter FLVCR1, a cell surface protein containing 12 potential transmembrane segments with six presumptive extracellular loops (ECLs). To identify FLVCR1 residues critical for mediating FeLV-C infection, we first independently isolated a human cDNA encoding the FLVCR2 protein that shares 52% identity to human FLVCR1, and we show that FLVCR2 does not function as a receptor for FeLV-C. Then, by generating specific hybrids between FLVCR1 and FLVCR2 and testing susceptibility of mouse cells expressing these hybrids to beta-galactosidase encoding FeLV-C, we identify FLVCR1 ECLs 1 and 6 as critical for mediating FeLV-C infection. Mouse cells expressing a hybrid protein containing FLVCR2 backbone with the ECL6 sequence from FLVCR1 were highly susceptible to FeLV-C infection. Using site-directed mutagenesis, we show that a single mutation of Asn463 in FLVCR2 ECL6 to an acidic Asp residue (a residue present in the corresponding position 487 in FLVCR1 ECL6) is sufficient to render FLVCR2 functional as an FeLV-C receptor. However, an Asp487Asn mutation in FLVCR1 ECL6 or substitution of the entire FLVCR1 ECL6 sequence for FLVCR2 ECL6 sequence does not disrupt receptor function. Subsequent substitutions show that residues within FLVCR1 ECL1 also contribute to mediating FeLV-C infection. Furthermore, our results suggest that FLVCR1 regions that mediate FeLV-C surface unit binding are distinct from ECL1 and ECL6. Our results are consistent with previous conclusions that infection of cells by gammaretroviruses involves interaction of virus with multiple receptor regions.

In vivo expression of GFP transgene delivered via a replicating feline leukemia virus

We previously described replication-competent feline leukemia virus (FeLV) vectors with high-level and stable expression of a green fluorescent protein (GFP) or a suicide transgene in cell cultures in vitro. Considering that FeLV might potentially be used to deliver therapeutic genes in vivo, we first evaluated the expression of the GFP gene introduced in cats by the FeLV, Rickard subgroup A (FRA) construct. Eight newborn kittens were either inoculated with pFRA-GFP plasmid DNA intradermally, or challenged intraperitoneally with FRA-GFP-infected feline fibroblasts. During a 12-week observation period, five cats were shown to be progressively viremic. Quantitative PCR and RT-PCR analyses of plasma and tissue samples from these cats showed that GFP was retained in FeLV DNA or RNA to a variable degree, ranging from 0.002 to 27.890%. Tissue DNA samples were analyzed by PCR for the status of GFP and the env-transgene complex. While the proviruses carrying the GFP transgene were shown to be minor species, all tissues, however, retained the full-length GFP transgene. Despite the occurrence of predominant species with various deletions in the viral genome, approximately 1-3% of the total cell population was GFP-positive in the lymphoid tissues as visualized by laser confocal microscopy. Co-localization of immunofluorescent cells indicated that CD3-positive T cells, dendritic cells and macrophages were the major targets for GFP expression. These findings on the detectable in vivo expression of GFP for as long as a period of 3 months could be viewed positively for contemplating a therapeutic strategy for control of FeLV infection in the cats.

Insertional polymorphisms of endogenous feline leukemia viruses

The number, chromosomal distribution, and insertional polymorphisms of endogenous feline leukemia viruses (enFeLVs) were determined in four domestic cats (Burmese, Egyptian Mau, Persian, and nonbreed) using fluorescent in situ hybridization and radiation hybrid mapping. Twenty-nine distinct enFeLV loci were detected across 12 of the 18 autosomes. Each cat carried enFeLV at only 9 to 16 of the loci, and many loci were heterozygous for presence of the provirus. Thus, an average of 19 autosomal copies of enFeLV were present per cat diploid genome. Only five of the autosomal enFeLV sites were present in all four cats, and at only one autosomal locus, B4q15, was enFeLV present in both homologues of all four cats. A single enFeLV occurred in the X chromosome of the Burmese cat, while three to five enFeLV proviruses occurred in each Y chromosome. The X chromosome and nine autosomal enFeLV loci were telomeric, suggesting that ectopic recombination between nonhomologous subtelomeres may contribute to enFeLV distribution. Since endogenous FeLVs may affect the infectiousness or pathogenicity of exogenous FeLVs, genomic variation in enFeLVs represents a candidate for genetic influences on FeLV leukemogenesis in cats.

Characterization of a newly established nonproducer lymphoma cell line for feline leukemia virus

A feline lymphoblastoid cell line (KO-1) was established from a 5-year-old neutered female cat with naturally occurring thymic lymphoma. KO-1 cells had a rearrangement of T-cell receptor beta-chain gene and a germ-line configuration of immunoglobulin heavy chain gene, however, they were devoid of T-cell-specific surface phenotype. Cytogenetically, KO-1 cells showed a hyperploidy (2n = 41) due to the trisomy of B2, F2 and X chromosomes. Although KO-1 cells were shown to be clonally expanded cells integrated with feline leukemia virus (FeLV) proviruses and expressed its structural proteins in their cytoplasm, they did not produce virus particles as shown by transmission electron microscopy and the absence of the viral protein and reverse transcriptase activity in the culture supernatant. The present study showed that the KO-1 cell line established here was a feline T-cell lymphoma cell line having a unique characteristic as an FeLV nonproducer.

Regulation of FeLV-945 by c-Myb binding and CBP recruitment to the LTR

Background

Feline leukemia virus (FeLV) induces degenerative, proliferative and malignant hematologic disorders in its natural host, the domestic cat. FeLV-945 is a viral variant identified as predominant in a cohort of naturally infected animals. FeLV-945 contains a unique sequence motif in the long terminal repeat (LTR) comprised of a single copy of transcriptional enhancer followed by a 21-bp sequence triplicated in tandem. The LTR is precisely conserved among independent cases of multicentric lymphoma, myeloproliferative disease and anemia in animals from the cohort. The 21-bp triplication was previously shown to act as a transcriptional enhancer preferentially in hematopoietic cells and to confer a replicative advantage. The objective of the present study was to examine the molecular mechanism by which the 21-bp triplication exerts its influence and the selective advantage responsible for its precise conservation.

Results

Potential binding sites for the transcription factor, c-Myb, were identified across the repeat junctions of the 21-bp triplication. Such sites would not occur in the absence of the repeat; thus, a requirement for c-Myb binding to the repeat junctions of the triplication would exert a selective pressure to conserve its sequence precisely. Electrophoretic mobility shift assays demonstrated specific binding of c-Myb to the 21-bp triplication. Reporter gene assays showed that the triplication-containing LTR is responsive to c-Myb, and that responsiveness requires the presence of both c-Myb binding sites. Results further indicated that c-Myb in complex with the 21-bp triplication recruits the transcriptional co-activator, CBP, a regulator of normal hematopoiesis. FeLV-945 replication was shown to be positively regulated by CBP in a manner dependent on the presence of the 21-bp triplication.

Conclusion

Binding sites for c-Myb across the repeat junctions of the 21-bp triplication may account for its precise conservation in the FeLV-945 LTR. c-Myb binding and CBP recruitment to the LTR positively regulated virus production, and thus may be responsible for the replicative advantage conferred by the 21-bp triplication. Considering that CBP is present in hematopoietic cells in limiting amounts, we hypothesize that FeLV-945 replication in bone marrow may influence CBP availability and thereby alter the regulation of CBP-responsive genes, thus contributing to altered hematopoiesis and consequent hematologic disease.

Reevaluation of host ranges of feline leukemia virus subgroups

We reevaluated the host ranges of feline leukemia virus (FeLV) subgroups A, B and C using pseudotype assays based on recombinant NB-tropic murine leukemia virus, which is not usually blocked after viral entry in mammalian cells. Pseudotype viruses of FeLV-B and -C infected a variety of cell lines from many mammalian species. Unexpectedly, FeLV-A pseudotype viruses of two independent isolates from the UK and US also infected a variety of non-feline cell lines including cells from humans, rabbits, pigs and minks. Moreover, both isolates of FeLV-A productively infected human embryonic kidney 293 and mink Mv-1-Lu cells. We conclude that FeLV-A is not strictly ecotropic.

Disease propagation in connected host populations with density-dependent dynamics: the case of the Feline Leukemia Virus

Spatial heterogeneity is a strong determinant of host-parasite relationships, however local-scale mechanisms are often not elucidated. Generally speaking, in many circumstances dispersal is expected to increase disease persistence. We consider the case when host populations show density-dependent dynamics and are connected through the dispersal of individuals. Taking the domestic cats (Felis catus)--Feline Leukemia Virus (FeLV) as a toy model of host-microparasite system, we predict the disease dynamics when two host populations with distinct or similar structures are connected together and to the surrounding environment by dispersal. Our model brings qualitatively different predictions from one-population models. First, as expected, biologically realistic rates of dispersal may allow FeLV to persist in sets of populations where the virus would have gone extinct otherwise, but a reverse outcome is also possible: eradication of FeLV from a small population by connexion to a larger population where it is not persistent. Second, overall prevalence as well as depression of host population size due to infection are both enhanced by dispersal, even at low dispersal rates when disease persistence is not achieved in the two populations. This unexpected prediction is probably due to the combination of dispersal with density-dependent population dynamics. Third, the dispersal of non-infectious cats has more influence on virus prevalence than the dispersal of infectious. Finally, prevalence and depression of host population size are both related to the rate of dispersion, to the health status of individuals dispersing and to the dynamics of host populations.

Mutations that abrogate transactivational activity of the feline leukemia virus long terminal repeat do not affect virus replication

The U3 region of the LTR of oncogenic Moloney murine leukemia virus (Mo-MuLV) and feline leukemia viruses (FeLV) have been previously reported to activate expression of specific cellular genes in trans, such as MHC class I, collagenase IV, and MCP-1, in an integration-independent manner. It has been suggested that transactivation of these specific cellular genes by leukemia virus U3-LTR may contribute to the multistage process of leukemogenesis. The U3-LTR region, necessary for gene transactivational activity, also contains multiple transcription factor-binding sites that are essential for normal virus replication. To dissect the promoter activity and the gene transactivational activity of the U3-LTR, we conducted mutational analysis of the U3-LTR region of FeLV-A molecular clone 61E. We identified minimal nucleotide substitution mutants on the U3 LTR that did not disturb transcription factor-binding sites but abrogated its ability to transactivate the collagenase gene promoter. To determine if these mutations actually have altered any uncharacterized important transcription factor-binding site, we introduced these U3-LTR mutations into the full-length infectious molecular clone 61E. We demonstrate that the mutant virus was replication competent but could not transactivate cellular gene expression. These results thus suggest that the gene transactivational activity is a distinct property of the LTR and possibly not related to its promoter activity. The cellular gene transactivational activity-deficient mutant FeLV generated in this study may also serve as a valuable reagent for testing the biological significance of LTR-mediated cellular gene activation in the tumorigenesis caused by leukemia viruses.

A potential therapeutic strategy to combat leukemia virus infection

To test the concept that a replication-competent retrovirus carrying a suicide gene could have potential utility in the control of the natural virus infection in mammalian species, we constructed derivatives of a feline leukemia virus (FeLV) that is commonly associated with leukemia-lymphomas in this species. The FeLV, Rickard strain, subgroup A (FRA) genome contained at the 3' end of the envgene, an insert of an internal ribosomal entry site (IRES) linked to cDNA sequence of either herpes simplex virus thymidine kinase (HSV-TK) or a truncated HSV-TK (HSV-ATK) or yeast cytosine deaminase (CD). These constructs were transfected into feline fibroblast cells (H927). The viruses produced were determined to be replication-competent. The stable propagation of the full-length transgene was, however, dependent on the size of the insert, IRES-CD being the smallest in size (1031 bp) exhibiting maximal stability for at least up to six months. The protein products of the transgenes could be detected, despite the appearance of deleted proviruses at late passages. The transduced cells were susceptible to cytotoxic killing when the appropriate prodrug, ganciclovir (GCV), acyclovir (ACV) or 5-fluorocytosine (5-FC) was added to the culture medium. H927 cells, infected with another subgroup of FeLV, namely, FeLV-B or FeLV-C, could be superinfected by the FRA-suicide gene viruses and thus, subjected to killing. Interestingly, at an early stage of infection by the parental FRA, H927 cells could also be reinfected by the same subgroup FRA constructs to induce the suicide effect. Among the three constructs, the vector with the CD gene was determined to be superior to others in terms of stability, therapeutic index and bystander effect in the cell culture test system. While the in vivo correlates of the therapeutic effect in the feline model remain to be determined, our results do encourage investigation of the same concept in the control of HTLV and, perhaps even, HIV infection in humans.

Host range and receptor binding properties of vectors bearing feline leukemia virus subgroup B envelopes can be modulated by envelope sequences outside of the receptor binding domain

To evaluate host range differences between two different strains of feline leukemia virus subgroup B (FeLV-B), we compared the binding and infectivity patterns of retrovirus vectors bearing either FeLV-B-90Z or FeLV-B-GA envelopes. We report here that the ability of these envelopes to utilize different Pit1 orthologs is mediated primarily by the receptor binding domain; however, in the case of FeLV-B-90Z, the C terminus also contributes to the recognition of certain Pit1 orthologs.

Specificity in receptor usage by T-cell-tropic feline leukemia viruses: implications for the in vivo tropism of immunodeficiency-inducing variants

Cytopathic, T-cell-tropic feline leukemia viruses (FeLV-T) evolve from FeLV-A in infected animals and demonstrate host cell specificities that are distinct from those of their parent viruses. We recently identified two cellular proteins, FeLIX and Pit1, required for productive infection by these immunodeficiency-inducing FeLV-T variants (M. M. Anderson, A. S. Lauring, C. C. Burns, and J. Overbaugh, Science 287:1828-1830, 2000). FeLV-T is the first example of a naturally occurring type C retrovirus that requires two proteins to gain entry into target cells. FeLIX is an endogenous protein that is highly related to the N-terminal portion of the FeLV envelope protein, which includes the receptor-binding domain. Pit1 is a multiple-transmembrane phosphate transport protein that also functions as a receptor for FeLV-B. The FeLV-B envelope gene is derived by recombination with endogenous FeLV-like sequences, and its product can functionally substitute for FeLIX in facilitating entry through the Pit1 receptor. In the present study, we tested other retrovirus envelope surface units (SUs) with their cognate receptors to determine whether they also could mediate infection by FeLV-T. Cells were engineered to coexpress the transmembrane form of the envelope proteins and their cognate receptors, or SU protein was added as a soluble protein to cells expressing the receptor. Of the FeLV, murine leukemia virus, and gibbon ape leukemia virus envelopes tested, we found that only those with receptor-binding domains derived from endogenous FeLV could render cells permissive for FeLV-T. We also found that there is a strong preference for Pit1 as the transmembrane receptor. Specifically, FeLV-B SUs could efficiently mediate infection of cells expressing the Pit1 receptor but could only inefficiently mediate infection of cells expressing the Pit2 receptor, even though these SUs are able to bind to Pit2. Expression analysis of feline Pit1 and FeLIX suggests that FeLIX is likely the primary determinant of FeLV-T tropism. These results are discussed in terms of current models for retrovirus entry and the interrelationship among FeLV variants that evolve in vivo.

A replication-competent feline leukemia virus, subgroup A (FeLV-A), tagged with green fluorescent protein reporter exhibits in vitro biological properties similar to those of the parental FeLV-A

We previously established that lymphoid tumors could be induced in cats by intradermal injection of ecotropic feline leukemia virus (FeLV), subgroup A, plasmid DNA. In preparation for in vivo experiments to study the cell-to-cell pathway for the spread of the virus from the site of inoculation, the green fluorescent protein (GFP) transgene fused to an internal ribosome entry site (IRES) was inserted after the last nucleotide of the env gene in the ecotropic FeLV-A Rickard (FRA) provirus. The engineered plasmid was transfected into feline fibroblast cells for production of viruses and determination of GFP expression. The virions produced were highly infectious, and the infected cells could continue to mediate strong expression of GFP after long-term propagation in culture. Similar to parental virus, the transgene-containing ecotropic virus demonstrated recombinogenic activity with endogenous FeLV sequences in feline cells to produce polytropic recombinant FeLV subgroup B-like viruses which also contained the IRES-GFP transgene in the majority of recombinants. To date, the engineered virus has been propagated in cell culture for up to 8 months without diminished GFP expression. This is the first report of a replication-competent FeLV vector with high-level and stable expression of a transgene.

Selection forces and constraints on retroviral sequence variation

All retroviruses possess a highly error-prone reverse transcriptase, but the extent of the consequent sequence diversity and the rate of evolution differ greatly among retroviruses. Because of the high mutability of retroviruses, it is not the generation of new viral variants that limits the extent of diversity and the rate of evolution of retroviruses, but rather the selection forces that act on these variants. Here, we suggest that two selection forces--the immune response and the limited availability of appropriate target cells during transmission and persistence--are chiefly responsible for the observed sequence diversity in untreated retroviral infections. We illustrate these aspects of positive selection by reference to specific lentiviruses [human and simian immunodeficiency viruses (HIV and SIV)] and oncoviruses [feline leukemia virus (FeLV) and human T cell leukemia virus (HTLV)] that differ in their extent of variation and in disease outcomes.

Cellular and species resistance to murine amphotropic, gibbon ape, and feline subgroup C leukemia viruses is strongly influenced by receptor expression levels and by receptor masking mechanisms

Chinese hamster ovary (CHO) cells are resistant to infections by gibbon ape leukemia virus (GALV) and amphotropic murine leukemia virus (A-MLV) unless they are pretreated with tunicamycin, an inhibitor of N-linked glycosylation. These viruses use the related sodium-phosphate symporters Pit1 and Pit2, respectively, as receptors in nonhamster cells, and evidence has suggested that the corresponding transporters of CHO cells may be masked by tunicamycin-sensitive secreted inhibitors. Although the E36 line of Chinese hamster cells was reported to secrete the putative Pit2 inhibitor and to be sensitive to the inhibitory CHO factors, E36 cells are highly susceptible to both GALV and A-MLV in the absence of tunicamycin. Moreover, expression of E36 Pit2 in CHO cells conferred tunicamycin-independent susceptibilities to both viruses. Based on the latter results, it was suggested that E36 Pit2 must functionally differ from the endogenous Pit2 of CHO cells. To test these ideas, we analyzed the receptor properties of CHO Pit1 and Pit2 in CHO cells. Surprisingly, and counterintuitively, transfection of a CHO Pit2 expression vector into CHO cells conferred strong susceptibility to both GALV and A-MLV, and similar overexpression of CHO Pit1 conferred susceptibility to GALV. Thus, CHO Pit2 is a promiscuous functional receptor for both viruses, and CHO Pit1 is a functional receptor for GALV. Similarly, we found that the natural resistance of Mus dunni tail fibroblasts to subgroup C feline leukemia viruses (FeLV-C) was eliminated simply by overexpression of the endogenous FeLV-C receptor homologue. These results demonstrate a novel and simple method to unmask latent retroviral receptor activities that occur in some cells. Specifically, resistances to retroviruses that are caused by subthreshold levels of receptor expression or by stoichiometrically limited masking or interference mechanisms can be efficiently overcome simply by overexpressing the endogenous receptors in the same cells.

Feline leukemia virus envelope sequences that affect T-cell tropism and syncytium formation are not part of known receptor-binding domains

The envelope protein is a primary pathogenic determinant for T-cell-tropic feline leukemia virus (FeLV) variants, the best studied of which is the immunodeficiency-inducing virus, 61C. We have previously demonstrated that T-cell-tropic, cytopathic, and syncytium-inducing viruses evolve in cats infected with a relatively avirulent, transmissible form of FeLV, 61E. The envelope gene of an 81T variant, which encoded scattered single-amino-acid changes throughout the envelope as well as a 4-amino-acid insertion in the C-terminal half of the surface unit (SU) of envelope, was sufficient to confer the T-cell-tropic, cytopathic phenotype (J. L. Rohn, M. S. Moser, S. R. Gwynn, D. N. Baldwin, and J. Overbaugh, J. Virol. 72:2686-2696, 1998). In the present study, we examined the role of the 4-amino-acid insertion in determining viral replication and tropism of FeLV-81T. The 4-amino-acid insertion was found to be functionally equivalent to a 6-amino-acid insertion at an identical location in the 61C variant. However, viruses expressing a chimeric 61E/81T SU, containing the insertion together with the N terminus of 61E SU, were found to be replication defective and were impaired in the processing of the envelope precursor into the functional SU and transmembrane (TM) proteins. In approximately 10% of cultured feline T cells (3201) transfected with the 61E/81T envelope chimeras and maintained over time, replication-competent tissue culture-adapted variants were isolated. Compensatory mutations in the SU of the tissue culture-adapted viruses were identified at positions 7 and 375, and each was shown to restore envelope protein processing when combined with the C-terminal 81T insertion. Unexpectedly, these viruses displayed different phenotypes in feline T cells: the virus with a change from glutamine to proline at position 7 acquired a T-cell-tropic, cytopathic phenotype, whereas the virus with a change from valine to leucine at position 375 had slower replication kinetics and caused no cytopathic effects. Given the differences in the replication properties of these viruses, it is noteworthy that the insertion as well as the two single-amino-acid changes all occur outside of predicted FeLV receptor-binding domains.

Identification of a cellular cofactor required for infection by feline leukemia virus

Retroviral infection involves continued genetic variation, leading to phenotypic and immunological selection for more fit virus variants in the host. For retroviruses that cause immunodeficiency, pathogenesis is linked to the emergence of T cell-tropic, cytopathic viruses. Here we show that an immunodeficiency-inducing, T cell-tropic feline leukemia virus (FeLV) has evolved such that it cannot infect cells unless both a classic multiple membrane-spanning receptor molecule (Pit1) and a second coreceptor or entry factor are present. This second receptor component, which we call FeLIX, was identified as an endogenously expressed protein that is similar to a portion of the FeLV envelope protein. This cellular protein can function either as a transmembrane protein or as a soluble component to facilitate infection.

A comprehensive approach to mapping the interacting surfaces of murine amphotropic and feline subgroup B leukemia viruses with their cell surface receptors

Because mutations in envelope glycoproteins of retroviruses or in their cell surface receptors can eliminate function by multiple mechanisms, it has been difficult to unambiguously identify sites for their interactions by site-directed mutagenesis. Recently, we developed a gain-of-function approach to overcome this problem. Our strategy relies on the fact that feline leukemia virus subgroup B (FeLV-B) and amphotropic murine leukemia virus (A-MLV) have closely related gp70 surface envelope glycoproteins and use related Na(+)-dependent phosphate symporters, Pit1 and Pit2, respectively, as their receptors. We previously observed that FeLV-B/A-MLV envelope glycoprotein chimeras spliced between the variable regions VRA and VRB were unable to use Pit1 or Pit2 as a receptor but could efficiently use specific Pit1/Pit2 chimeras. The latter study suggested that the VRA of A-MLV and FeLV-B functionally interact with the presumptive extracellular loops 4 and 5 (ECL4 and -5) of their respective receptors, whereas VRB interacts with ECL2. We also found that FeLV-B gp70 residues F60 and P61 and A-MLV residues Y60 and V61 in the first disulfide-bonded loop of VRA were important for functional interaction with the receptor's ECL4 or -5. We have now extended this approach to identify additional VRA and VRB residues that are involved in receptor recognition. Our studies imply that FeLV-B VRA residues F60 and P61 interact with the Pit1 ECL5 region, whereas VRA residues 66 to 78 interact with Pit1 ECL4. Correspondingly, A-MLV VRA residues Y60 and V61 interact with the Pit2 ECL5 region, whereas residues 66 to 78 interact with Pit2 ECL4. Similar studies that focused on the gp70 VRB implicated residues 129 to 139 as contributing to specific interactions with the receptor ECL2. These results identify three regions of gp70 that interact in a specific manner with distinct portions of their receptors, thereby providing a map of the functionally interacting surfaces.

Modulation of lipid polymorphism by the feline leukemia virus fusion peptide: implications for the fusion mechanism

The structural effects of the fusion peptide of feline leukemia virus (FeLV) on lipid polymorphism were studied, using differential scanning calorimetry (DSC), 31P nuclear magnetic resonance (NMR), and time-resolved X-ray diffraction. This peptide lowers the bilayer to inverted hexagonal phase transition temperature, TH, of dipalmitoleoylphosphatidylethanolamine (DiPoPE) at peptide mole fractions of up to 1.5 x 10(-3) at pH 5.0 and at pH 7.4. The temperature at which isotropic 31P NMR signals for monomethyldioleoylphosphatidylethanolamine (MeDOPE) first occurred is lowered by the FeLV peptide. The amount of isotropic signal seen at 40 degrees C is directly correlated to the peptide:lipid molar ratio. In the peptide-containing samples, more lipid remains in the isotropic state over the whole recorded temperature range. Isotropic 31P NMR signals were observed for DiPoPE in the presence of the FeLV peptide for the entire recorded temperature range of 35-50 degrees C, while pure DiPoPE showed no significant amount of isotropic signal. X-ray studies of DiPoPE show the formation of a new lipid phase with peptide, which is not seen in the pure lipid samples. Disordering of the Lalpha phase is evidenced by broadening of the diffraction peaks, and the hexagonal cell parameter is decreased with peptide present. Our results suggest that the FeLV peptide is increasing the negative curvature of the lipid system, which is thought to be crucial to the formation of highly bent, high-energy structural fusion intermediates, such as the "stalk" model. Fusion activity for this putative fusogenic peptide was also demonstrated, using a resonance energy transfer (RET) lipid mixing assay. To our knowledge, this work provides the first published experimental evidence of both fusogenic activity and effects on lipid polymorphism for the FeLV fusion peptide.

In vivo evolution of a novel, syncytium-inducing and cytopathic feline leukemia virus variant

Studies of feline leukemia virus (FeLV) have illustrated the importance of the genotype of the infecting virus in determining disease outcome. In FeLV infections, as in other retroviral infections, it is less clear how virus variants that evolve from the transmitted virus affect pathogenesis. We previously reported an analysis of the genotypic changes that occur in the viral envelope gene (env) in cats infected with a prototype transmissible FeLV clone, 61E (J. Rohn, M. Linenberger, E. Hoover, and J. Overbaugh, J. Virol. 68:2458-2467, 1994). In one cat, each variant (81T) had evolved, in addition to scattered amino acid changes, a four-amino-acid insertion with respect to 61E. This insertion was located at the same site in the extracellular envelope glycoprotein where the immunodeficiency-inducing molecular clone 61C possesses a six-amino-acid insertion critical for its pathogenic phenotype, although the sequences of the insertions were distinct. To determine whether acquisition of the four-amino-acid insertion was associated with a change in the replication or cytopathic properties of the virus, we constructed chimeras encoding 81T env genes in a 61E background. One representative chimeric virus, EET(TE)-109, was highly cytopathic despite the fact that it replicated with delayed kinetics in the feline T-cell line 3201 compared to the parental 61E virus. The phenotype of this virus was also novel compared to other FeLVs, including both the parental virus 61E and the immunodeficiency-inducing variant 61C, because infection of T cells was associated with syncytium formation. Moreover, in single-cycle infection assays, the 81T-109 envelope demonstrated receptor usage properties distinct from those of both 61E and 61C envelope. Thus, these studies demonstrate the evolution of a novel T-cell cytopathic and syncytium-inducing FeLV in the host. The 81T virus will be valuable for dissecting the mechanism of T-cell killing by cytopathic variants in the FeLV model.

The receptor binding site of feline leukemia virus surface glycoprotein is distinct from the site involved in virus neutralization

The external surface glycoprotein (SU) of feline leukemia virus (FeLV) contains sites which define the viral subgroup and induce virus-neutralizing antibodies. The subgroup phenotypic determinants have been located to a small variable region, VR1, towards the amino terminus of SU. The sites which function as neutralizing epitopes in vivo are unknown. Recombinant SU proteins were produced by using baculoviruses that contained sequences encoding the SUs of FeLV subgroup A (FeLV-A), FeLV-C, and two chimeric FeLVs (FeLV-215 and FeLV-VC) in which the VR1 domain of FeLV-A had been replaced by the corresponding regions of FeLV-C isolates. The recombinant glycoproteins, designated Bgp70-A, -C, -215, and -VC, respectively, were similar to their wild-type counterparts in several immunoblots and inhibited infection of susceptible cell lines in a subgroup-specific manner. Thus, Bgp70-A interfered with infection by FeLV-A, whereas Bgp70-C, -VC, and -215 did not. Conversely, Bgp70-C, -VC, and -215 blocked infection with FeLV-C, while Bgp70-A had no effect. These results indicate that the site on SU which binds to the FeLV cell surface receptor was preserved in the recombinant glycoproteins. It was also found that the recombinant proteins were able to bind naturally occurring neutralizing antibodies. Bgp70-A, -VC, and -215 interfered with the action of anti-FeLV-A neutralizing antibodies, whereas Bgp70-C did not. Furthermore, Bgp70-C interfered with the action of anti-FeLV-C neutralizing antibodies, while the other proteins did not. These results indicate that the neutralizing epitope(s) of FeLV SU lies outside the subgroup-determining VR1 domain.

Retroviruses and sexual size dimorphism in domestic cats (Felis catus L.)

Hochberg and co-workers have predicted that an increase in host adult mortality due to parasites is balanced by an earlier age at first reproduction. In polygynous species we hypothesize that such a pattern would lead to diverging selection pressure on body size between sexes and increased sexual size dimorphism. In polygynous mammals, male body size is considered to be an important factor for reproductive success. Thus, under the pressure of a virulent infection, males should be selected for rapid growth and/or higher body size to be able to compete successfully as soon as possible with opponents. In contrast, under the same selection pressure, females should be selected for lighter adult body size or rapid growth to reach sexual maturity earlier. We investigated this hypothesis in the domestic cat Felis catus. Orange cats have greater body size dimorphism than non-orange cats. Orange females are lighter than non-orange females, and orange males are heavier than non-orange males. Here, we report the extent to which orange and non-orange individuals differ in infection prevelance for two retroviruses, feline immunodeficiency virus (FIV) and feline leukaemia virus (FeLV). FIV is thought to be transmitted almost exclusively through aggressive contacts between individuals, whereas FeLV transmission occurs mainly through social contacts. The pattern of infection of both diseases is consistent with the higher aggressiveness of orange cats. In both sexes, orange cats are significantly more infected by FIV, and tend to be less infected by FeLV than other cats. The pattern of infection is also consistent with an earlier age at first reproduction in orange than in non-orange cats, at least for females. These results suggest that microparasitism may have played an important role in the evolution of sexual size dimorphism of domestic cats.

Dynamics of two feline retroviruses (FIV and FeLV) within one population of cats

We present a deterministic model of the dynamics of two microparasites simultaneously infecting a single host population. Both microparasites are feline retroviruses, namely Feline Immunodeficiency Virus (FIV) and Feline Leukaemia Virus (FeLV). The host is the domestic cat Felis catus. The model has been tested with data generated by a long-term study of several natural cat populations. Stability analysis and simulations show that, once introduced in a population, FIV spreads and is maintained, while FeLV can either disappear or persist. Moreover, introduction of both viruses into the population induces an equilibrium state for individuals of each different pathological class. The viruses never induce the extinction of the population. Furthermore, whatever the outcome for the host population (persistence of FIV only, or of both viruses), the global population size at the equilibrium state is only slightly lower than it would have been in the absence of the infections (i.e. at the carrying capacity), indicating a low impact of the viruses on the population. Finally, the impact of the diseases examined simultaneously is higher than the sum of the impact of the two diseases examined separately. This seems to be due to a higher mortality rate when both viruses infect a single individual.

Retrospective study of 60 cases of feline lymphosarcoma

OBJECTIVE

To characterise epidemiological and clinical findings, and diagnostic procedures undertaken, in cats with lymphosarcoma at a veterinary teaching hospital.

DESIGN

Retrospective case study.

PROCEDURE

Hospital records were reviewed for 7159 cats, sick or healthy, examined during a 10-year period (1984 to 1994). Sixty cats with lymphosarcoma were identified and classified by anatomical location of the tumor. Data on breed, age, sex, clinical signs and diagnostic procedures were collated.

RESULTS

The prevalence of feline lymphosarcoma in the hospital population was 0.84%. Siamese cats appeared predisposed to lymphosarcoma but other purebreds were not. Males were somewhat overrepresented amongst affected cats. Similar numbers of cases (12 to 18) were seen in each of the four anatomic categories (multicentric, mediastinal, alimentary and extranodal). Cats with mediastinal lymphosarcoma were mostly young and Siamese. Clinical signs in affected cats were varied, usually multiple and often nonspecific. Two of 22 cases tested positive for feline leukaemia virus antigen in blood and 6 of 13 were positive for feline immunodeficiency virus antibody.

CONCLUSIONS

Extranodal lymphosarcoma seemed more prevalent in this study than reported elsewhere. Siamese cats in the study population may have had a genetic predisposition to lymphosarcoma. Limited evidence suggested feline leukaemia virus may be less important, and feline immunodeficiency virus more important, in the local population than indicated in overseas reports. Additional studies are needed to investigate breed predisposition and feline leukaemia virus and feline immunodeficiency virus status in Australian cats with lymphosarcoma.

The role of interferon-gamma in antiretroviral activity of methionine enkephalin and AZT in a murine cell culture

The ability of spleen cells treated with methionine enkephalin (Met-ENK) in the presence of 3'-azido-3'-deoxythymidine (AZT) to produce cytokines and inhibit Friend leukemia virus (FLV) replication in Mus dunni cell cultures was investigated. In the presence of murine spleen cells, combination treatments using AZT plus Met-ENK or concanavalin A reduced FLV replication by 63% and 84%, respectively, as compared with 47% for AZT alone. When interleukin (IL)-2, IL-4 and interferon (IFN gamma) levels were measured in FLV-infected cell cultures, both AZT and Met-ENK treatments induced a higher production of IFN gamma and a slight increase in IL-2 and IL-4, as compared with either treatment alone. Subsequent treatment of FLV-infected cells with concanavalin A-stimulated cell supernatants, containing approximately 10 U/ml each of IFN gamma and IL-2, resulted in inhibition of viral replication. Thus, in the absence of spleen cells, IFN gamma was added to cell cultures to determine whether this cytokine contributed to combination antiviral effects. Results show that addition of IFN gamma alone results in a slight suppression of FLV expression, whereas treatment with both AZT and IFN gamma inhibits FLV replication significantly. Subsequently, addition of anti-IFN gamma antibody to cell cultures treated with Met-ENK blocked antiviral effects due to this neuropeptide. Thus anti-FLV effects of spleen cells treated with Met-ENK in combination with AZT are mediated to a large degree by IFN gamma.

Identification and deletion of sequences required for feline leukemia virus RNA packaging and construction of a high-titer feline leukemia virus packaging cell line

Sequences required for specific encapsidation of feline leukemia virus (FeLV) genomic RNA have not yet been defined. Deletion of 107 nucleotides between the splice donor (SD) and the gag coding region of a prototypic subgroup A FeLV, 61E, resulted in an approximately 200-fold reduction of packaged viral RNA. Virus particle production was not disrupted by the deletion, although viral infectivity was dramatically reduced. These data indicate that the 107-nucleotide sequence comprises a portion of the FeLV packaging signal. FeLV particles expressed from the deleted genome were able to efficiently package murine leukemia virus vectors, resulting in high-titer G418R virus production. This system can be easily adapted to produce FeLV particles that contain envelope proteins from other feline leukemia virus subgroups and will be broadly useful for studies of FeLV envelope/receptor interactions.

Replication kinetics and cell tropism of an immunosuppressive feline leukaemia virus

To elucidate in vivo cell tropism and infection kinetics of an immunodeficiency-inducing isolate of feline leukaemia virus (FeLV-FAIDS), we quantified the two major genotypes comprising FeLV-FAIDS [the replication-competent common form (clone 61E) and the replication-defective variant (clone 61C)] in lymphocyte and leukocyte populations from infected cats. Micromagnetic separation of cell subsets, virus genome-specific PCR and flow cytometry were used to demonstrate the following sequence of events in infected animals: (i) very early replication of both 61E and 61C in CD4 T cells (provirus burden 0.2 to 1 copy/cell at 2-4 weeks post-infection); (ii) lower magnitude replication of both viruses in CD8 T cells and B cells during this initial phase of infection; (iii) plateauing of CD4 cell virus burden accompanied by escalation in CD8 and B cell provirus burdens after 4 weeks; (iv) extensive infection of haemopoietic and circulating myeloid cells. FeLV-FAIDS 61E and 61C replication kinetics and lymphocyte tropisms were similar in blood and lymph nodes, where provirus burdens ranged from 0.15 to 1.0 copy/cell. Moreover, virus infection was productive; 8-48 percent of blood lymphocytes, 35-81 percent of node lymphocytes and 53-98 percent of bone marrow cells expressed FeLV capsid antigen (p27 Gag). These findings suggest that the immunosuppressive potency of FeLV-FAIDS reflects the unique cytopathicity rather than unique cytotropism of its 61C (versus 61E) component.

Generation and role of defective proviruses in cytopathic feline leukemia virus (FeLV-FAIDS) infections

Cytopathic feline leukemia virus (FeLV) infections of feline T-cell line (FeT-cell) cultures led to the accumulation and maintenance of threefold more proviruses with deletions within the polymerase gene (pol) than minimally cytopathic FeLV infections. Over 60% of the viral DNA from cytopathic infections bore deletions in pol. Characterization of DNA sequences adjoining the deletions revealed that the junctions were most often flanked by RNA splice donor and acceptor consensus motifs. A thymidine-to-cytidine mutation introduced at the +2 position of one RNA splice donor-like motif inhibited formation of the two most prevalent viral DNA species with deletions, confirming the origin of many proviruses with deletions from reverse transcription of aberrantly spliced viral RNA species. An example of deletion by misalignment was also characterized. Viral inocula obtained from cells recovered after cytopathic infections were attenuated in their ability to cause cytopathic effects (CPE) and were able to confer superinfection resistance to naïve FeT-cells, despite maintaining envelope gene (env) sequences with full cytopathic potential. This suggested that viral genomes with deletions, rather than being required for cytopathicity, play a role in protecting cells from CPE. Indeed, expression of a molecularly cloned provirus bearing one of the characterized deletions attenuated CPE in FeT-cells caused by superinfecting cytopathic virus.

Molecular analysis of tumours from feline immunodeficiency virus (FIV)-infected cats: an indirect role for FIV?

Five tumours, which arose in cats naturally or experimentally infected with feline immunodeficiency virus (FIV), were examined with molecular probes to establish tumour cell lineage and to screen for integrated viral sequences. Three of the tumours were classed as B-cell lymphomas on the basis of morphology, immunocytochemistry, rearrangement of immunoglobulin heavy chain genes and lack of rearrangement of T-cell receptor (TCR) beta-chain genes. Two of these B-cell tumours arose in specific pathogen-free (SPF) cats experimentally infected with FIV. One case of multi-centric lymphosarcoma came from a cat naturally infected with both FIV and feline leukaemia virus (FeLV). This tumour contained integrated FeLV proviral sequences and was judged to be of T-cell origin on the basis of TCR gene rearrangement. The fifth case was a mast cell tumour. Rearrangement of the c-myc locus was not found in any of the FIV-associated tumours but was shown to be present in a rare immunoblastic B-cell lymphoma which arose in an uninfected SPF cat. None of the FIV-associated tumours showed evidence of integrated FIV sequences by Southern blot hybridisation, despite isolation of infectious virus from in vitro cultures of tumour cells in I case. These results confirm that FIV-associated tumours can occur in the absence of FeLV and suggest that the role of FIV in lymphomagenesis is generally indirect.

Haematological disorders associated with feline retrovirus infections

Feline oncornavirus and lentivirus infections have provided useful models to characterize the virus and host cell factors involved in a variety of marrow suppressive disorders and haematological malignancies. Exciting recent progress has been made in the characterization of the viral genotypic features involved in FeLV-associated diseases. Molecular studies have clearly defined the causal role of variant FeLV env gene determinants in two disorders: the T-lymphocyte cytopathicity and the clinical acute immunosuppression induced by the FeLV-FAIDS variant and the pure red cell aplasia induced by FeLV-C/Sarma. Variant or enFeLV env sequences also appear to play a role in FeLV-associated lymphomas. Additional studies are required to determine the host cell processes that are perturbed by these variant env gene products. In the case of the FeLV-FAIDS variant, the aberrant env gene products appear to impair superinfection interference, resulting in accumulation of unintegrated viral DNA and cell death. In other cases it is likely that the viral env proteins interact with host products that are important in cell viability and/or proliferation. Understanding of these mechanisms will therefore provide insights to factors involved in normal lymphohaematopoiesis. Similarly, studies of FeLV-induced haematological neoplasms should reveal recombination or rearrangement events involving as yet unidentified host gene sequences that encode products involved in normal cell growth regulation. These sequences may include novel protoncogenes or sequences homologous to genes implicated in human haematological malignancies. The haematological consequences of FIV are quite similar to those associated with HIV. As with HIV, FIV does not appear to directly infect myeloid or erythroid precursors, and the mechanisms of marrow suppression likely involve virus, viral antigen, and/or infected accessory cells in the marrow microenvironment. Studies using in vitro experimental models are required to define the effects of each of these microenvironmental elements on haematopoietic progenitors. As little is known about the molecular mechanisms of FIV pathogenesis, additional studies of disease-inducing FIV strains are needed to identify the genotypic features that correlate with virulent phenotypic features. Finally, experimental FIV infection in cats provides the opportunity to correlate in vivo virological and haematological changes with in vitro observations in a large animal model that closely mimics HIV infection in man.

Conversion of human fibroblasts to tissue macrophages by the Snyder-Theilen feline sarcoma virus (ST:FeSV(FeLV)): productive infection by Leishmania major

The decisive role of macrophages in T-cell differentiation is best exemplified by cutaneous leishmaniasis. During infection, Leishmania attach to macrophages, the only site of replication for the parasite. We have recently demonstrated the conversion of human fibroblasts to tissue macrophages (TM) by transduction with the Snyder-Theilen feline sarcoma virus (ST:FeSV-(FeLV)). Since Leishmania have tropism only for macrophages, we have used the parasite to ascertain the functional phenotype of the ST:FeSV-induced TM. Here, we have demonstrated the productive infection of the ST:FeSV-induced TM by L. major. These results point to the utility of ST:FeSV-induced TM in studies that concern the role of human macrophages in T-cell differentiation during the course of infection by Leishmania.

Evaluation of in vivo and in vitro interactions of feline immunodeficiency virus and feline leukemia virus

OBJECTIVE

To determine the potential mechanisms for disease potentiation where feline immunodeficiency virus (FIV) infection of persistently feline leukemia virus (FeLV)-infected cats results in more severe FIV disease and increased mortality than FIV infection of specific pathogen-free cats.

DESIGN AND METHODS

To determine whether pseudotype formation resulting in expanded cell tropism may be an important mechanism, cellular targets and tissue distribution of FIV and FeLV were determined by in situ hybridization and/or immunohistochemistry. To determine whether FeLV can transactivate the FIV long terminal repeat (LTR) resulting in increased FIV expression, in vitro transient expression assays were performed. To examine whether persistent FeLV infection can cause the deletion of a suppressive T-lymphocyte population, peripheral blood mononuclear cell (PBMC) cultures from persistently FeLV-infected cats were infected with FIV and monitored for FIV antigen levels.

RESULTS

Macrophages were the predominant target of FIV infection and were disseminated in a similar pattern in lymphoid and nonlymphoid tissues of both FIV-infected and FeLV/FIV-coinfected cats. FeLV-infected cells expressing FIV RNA were not present. Significant transactivation of the FIV LTR in FeLV-infected cells was not demonstrated. FIV antigen production was similar upon in vitro infection of PBMC from FeLV-infected and uninfected cats.

CONCLUSIONS

Neither direct virus/virus interactions, such as FeLV/FIV pseudotype formation or transactivation of the FIV LTR in FeLV-infected cells, nor deletion of a regulatory cell subset from the blood of FeLV-infected cats, was found to be the mechanism of disease potentiation.

Abnormal processing of a recombinant feline leukemia virus envelope polyprotein and its interference with subgroup C virus infection

Processing of the env polyprotein of a noninfectious feline leukemia virus (FeLV) recombinant, named r6gp, was examined in human-transfected cells. The r6gp provirus was previously generated in the frame of FeLV, subgroup B, GA clone with substitution of all but 40 C-terminal amino acid sequences of the surface glycoprotein (SU) from an endogenous FeLV provirus element (CFE-6). Although r6gp produced a normal size (85 kDa) env glycoprotein precursor, the product, unlike the precursor of the parental virus, was neither additionally glycosylated nor further processed into mature env proteins. Biochemical observations were consistent with the idea that the chimeric env polyprotein was trapped in the endoplasmic reticulum (ER) and were directly supported by immunofluorescence microscopy analyses. Interestingly, the residence of the chimeric protein in the ER specifically interfered with FeLV, subgroup C (Sarma) virus infection but not the parental FeLV-B virus infection. Since FeLV-C provirus sequences could be readily detected in the infected cells, it appeared that r6gp env expression did not block entry of the challenge virus. While FeLV-B and CFE-6 env genes share an extensive overall sequence homology, a variable region (region VI) of CFE-6 near the C-terminus of SU, which was retained in the r6gp construct, exhibits a considerably higher degree of homology to FeLV-C than FeLV-B. Thus, we propose that region VI is involved in conferring specificity for the env polyprotein oligomerization in the ER, and that co-oligomerization of the trapped r6gp env with FeLV-C is the reason for specific interference with FeLV-C infection. The results also demonstrate for the first time a functional abnormality of a recombinant FeLV env gene which is structurally similar to those commonly detected in FeLV-induced feline lymphosarcomas.

Induction of feline acquired immune deficiency syndrome by feline leukemia virus: immuno- and neuroendocrine dysfunctions

Young cats, when chronically infected with feline leukemia virus (FeLV), developed feline acquired immune deficiency syndrome (FAIDS). The syndrome was associated with a sequence of dysfunctions in the hypothalamic-pituitary-gonadal (HPG) and the immune system, manifested in the reduction of luteinizing hormone-releasing hormone (LHRH), follicle stimulating hormone (FSH), luteinizing hormone (LH), and testosterone in blood plasma. The average FSH and LH (in plasma or lymphocyte), testosterone, and LHRH concentrations in the 20 FeLV-infected cats were measured by radioimmunoassay. The results were compared with those of the 12 control cats that were not FeLV-infected. Four weeks after infection, the plasma LHRH concentration in the infected cats showed a 43% reduction. Five to six weeks after infection, the content of FSH and LH in lymphocyte was reduced by 50% and 28%, respectively, whereas, the plasma FSH and LH was reduced by 52% and 42%, respectively. A significant reduction in testosterone content was detected at Week 11 of infection. The onset of the immuno- and neuroendocrine dysfunctions in FAIDs cats followed this sequence: hypothalamus, lymphocyte, pituitary, adrenal gland, and gonads. Indirect immunofluorescence assay showed the presence of FeLV cytoplasmic antigens in the fibers of the hypothalamic preoptic region and the Leydig cells. The possible causal relationship between the dysfunction of the lymphocyte and HPG systems and the presence of FeLV was discussed.