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Host genetic resistance to viral infection controls the pathogenicity and epidemic dynamics of infectious diseases. Refrex-1 is a restriction factor against feline leukemia virus subgroup D (FeLV-D) and an endogenous retrovirus (ERV) in domestic cats (ERV-DC). Refrex-1 is encoded by a subset of ERV-DC loci with truncated envelope genes and secreted from cells as a soluble protein. Here, we identified the copper transporter CTR1 as the entry receptor for FeLV-D and genotype I ERV-DCs. We also identified CTR1 as a receptor for primate ERVs from crab-eating macaques and rhesus macaques, which were found in a search of intact envelope genes capable of forming infectious viruses. Refrex-1 counteracted infection by FeLV-D and ERV-DCs via competition for the entry receptor CTR1; the antiviral effects extended to primate ERVs with CTR1-dependent entry. Furthermore, truncated ERV envelope genes found in chimpanzee, bonobo, gorilla, crab-eating macaque, and rhesus macaque genomes could also block infection by feline and primate retroviruses. Genetic analyses showed that these ERV envelope genes were acquired in a species- or genus-specific manner during host evolution. These results indicated that soluble envelope proteins could suppress retroviral infection across species boundaries, suggesting that they function to control retroviral spread. Our findings revealed that several mammalian species acquired antiviral machinery from various ancient retroviruses, leading to convergent evolution for host defense.

Feline lymphomas are associated with the transduction and activation of cellular proto-oncogenes, such as c-myc, by feline leukemia virus (FeLV). We describe a polymerase chain reaction assay for detection of myc transduction usable in clinical diagnosis. The assay targets c-myc exons 2 and 3, which together result in a FeLV-specific fusion gene following c-myc transduction. When this assay was conducted on FeLV-infected feline tissues submitted for clinical diagnosis of tumors, myc transduction was detected in 14% of T-cell lymphoma/leukemias. This newly established system could become a useful diagnostic tool in veterinary medicine.

SUMMARYCats are known to be the main reservoir forBartonella henselaeandBartonella clarridgeiae, which are the agents of ‘cat-scratch disease’ in humans. In the present study, we investigated the prevalence of the twoBartonellaspecies on 1754 cat bloods collected from all prefectures in Japan during 2007–2008 by a nested-polymerase chain reaction (PCR) targeting the 16S–23S rRNA internal transcribed spacer region. Overall,BartonellaDNA was detected in 4·6% (80/1754) of the cats examined. The nested-PCR showed that 48·8% (39/80) of the positive cats were infected withB. henselaemono-infection, 33·8% (27/80) withB. clarridgeiaemono-infection and 17·5% (14/80) were infected with both species. The prevalence (5·9%; 65/1103) ofBartonellainfection in the western part of Japan was significantly higher than that (2·3%; 15/651) of eastern Japan (P< 0·001). Statistical analysis of the cats examined suggested a significant association betweenBartonellainfection and FeLV infection (OR = 1·9; 95% CI = 1·1–3·4), but not with FIV infection (OR = 1·6; 95% CI = 1·0–2·6).

A 4-y-old cat exhibited neurologic signs such as wobbling, right head tilt, and intention tremor, and MRI revealed a mass in the cerebellum. The cat died 5 mo after initial presentation, and no neoplastic lesions, other than the cerebellar mass, were observed at autopsy. Histologically, large atypical cells resembling Hodgkin cells, with single large inclusion-like nucleoli, and those resembling Reed–Sternberg cells, with symmetrically arranged nuclei, had infiltrated the left side of the cerebellum and were admixed with small lymphocytes. These atypical cells were positive for feline leukemia virus (FeLV), CD20, BLA36, vimentin, p16, p53, and Pax5, and negative for CD3, CD79a, and Iba1 by immunohistochemistry. Multiplex PCR for immunoglobulin heavy-chain gene rearrangement revealed monoclonal proliferation of B-lymphocytes. We describe this feline primary cerebellar B-cell lymphoma that displayed Hodgkin lymphoma–like tumor cells with FeLV protein expression.

Endogenous retroviruses (ERVs) of domestic cats (ERV-DCs) are one of the youngest feline ERV groups in domestic cats (Felis silvestris catus); some members are replication competent (ERV-DC10, ERV-DC18, and ERV-DC14), produce the antiretroviral soluble factor Refrex-1 (ERV-DC7 and ERV-DC16), or can generate recombinant feline leukemia virus (FeLV). Here, we investigated ERV-DC in European wildcats (Felis silvestris silvestris) and detected four loci: ERV-DC6, ERV-DC7, ERV-DC14, and ERV-DC16. ERV-DC14 was detected at a high frequency in European wildcats; however, it was replication defective due to a single G → A nucleotide substitution, resulting in an E148K substitution in the ERV-DC14 envelope (Env). This mutation results in a cleavage-defective Env that is not incorporated into viral particles. Introduction of the same mutation into feline and murine infectious gammaretroviruses resulted in a similar Env dysfunction. Interestingly, the same mutation was found in an FeLV isolate from naturally occurring thymic lymphoma and a mouse ERV, suggesting a common mechanism of virus inactivation. Refrex-1 was present in European wildcats; however, ERV-DC16, but not ERV-DC7, was unfixed in European wildcats. Thus, Refrex-1 has had an antiviral role throughout the evolution of the genus Felis, predating cat exposure to feline retroviruses. ERV-DC sequence diversity was present across wild and domestic cats but was locus dependent. In conclusion, ERVs have evolved species-specific phenotypes through the interplay between ERVs and their hosts. The mechanism of viral inactivation may be similar irrespective of the evolutionary history of retroviruses. The tracking of ancestral retroviruses can shed light on their roles in pathogenesis and host-virus evolution. IMPORTANCE Domestic cats (Felis silvestris catus) were domesticated from wildcats approximately 9,000 years ago via close interaction between humans and cats. During cat evolution, various exogenous retroviruses infected different cat lineages and generated numerous ERVs in the host genome, some of which remain replication competent. Here, we detected several ERV-DC loci in Felis silvestris silvestris. Notably, a species-specific single nucleotide polymorphism in the ERV-DC14 env gene, which results in a replication-defective product, is highly prevalent in European wildcats, unlike the replication-competent ERV-DC14 that is commonly present in domestic cats. The presence of the same lethal mutation in the env genes of both FeLV and murine ERV provides a common mechanism shared by endogenous and exogenous retroviruses by which ERVs can be inactivated after endogenization. The antiviral role of Refrex-1 predates cat exposure to feline retroviruses. The existence of two ERV-DC14 phenotypes provides a unique model for understanding both ERV fate and cat domestication.

Feline leukemia virus (FeLV) is horizontally transmitted among cats and causes a variety of hematopoietic disorders. Five subgroups of FeLV, A to D and T, each with distinct receptor usages, have been described. Recently, we identified a new FeLV Env (TG35-2) gene from a pseudotyped virus that does not belong to any known subgroup. FeLV-A is the primary virus from which other subgroups have emerged via mutation or recombination of the subgroup A env gene. Retrovirus entry into cells is mediated by the interaction of envelope protein (Env) with specific cell surface receptors. Here, phenotypic screening of a human/hamster radiation hybrid panel identified SLC19A1, a feline reduced folate carrier (RFC) and potential receptor for TG35-2-phenotypic virus. RFC is a multipass transmembrane protein. Feline and human RFC cDNAs conferred susceptibility to TG35-2-pseudotyped virus when introduced into nonpermissive cells but did not render these cells permissive to other FeLV subgroups or feline endogenous retrovirus. Moreover, human cells with genomic deletion of RFC were nonpermissive for TG35-2-pseudotyped virus infection, but the introduction of feline and human cDNAs rendered them permissive. Mutation analysis of FeLV Env demonstrated that amino acid substitutions within variable region A altered the specificity of the Env-receptor interaction. We isolated and reconstructed the full-length infectious TG35-2-phenotypic provirus from a naturally FeLV-infected cat, from which the FeLV Env (TG35-2) gene was previously isolated, and compared the replication of the virus in hematopoietic cell lines with that of FeLV-A 61E by measuring the viral RNA copy numbers. These results provide a tool for further investigation of FeLV infectious disease. IMPORTANCE Feline leukemia virus (FeLV) is a member of the genus Gammaretrovirus, which causes malignant diseases in cats. The most prevalent FeLV among cats is FeLV subgroup A (FeLV-A), and specific binding of FeLV-A Env to its viral receptor, thiamine transporter feTHTR1, is the first step of infection. In infected cats, novel variants of FeLV with altered receptor specificity for viral entry have emerged by mutation or recombination of the env gene. A novel FeLV variant arose from a subtle mutation of FeLV-A Env, which altered the specific interaction of the virus with its receptor. RFC, a folate transporter, is a potential receptor for the novel FeLV variant. The perturbation of specific retrovirus-receptor interactions under selective pressure by the host results in the emergence of novel viruses.

Oncogene-containing retroviruses are generated by recombination events between viral and cellular sequences, a phenomenon called "oncogene capture". The captured cellular genes, referred to as "v-onc" genes, then acquire new oncogenic properties. We report a novel feline leukemia virus (FeLV), designated "FeLV-AKT", that has captured feline c-AKT1 in feline lymphoma. FeLV-AKT contains a gag-AKT fusion gene that encodes the myristoylated Gag matrix protein and the kinase domain of feline c-AKT1, but not its pleckstrin homology domain. Therefore, it differs structurally from the v-Akt gene of murine retrovirus AKT8. AKT may be involved in the mechanisms underlying malignant diseases in cats.

Feline leukemia virus (FeLV) subgroups have emerged in infected cats via the mutation or recombination of the env gene of subgroup A FeLV (FeLV-A), the primary virus. We report the isolation and characterization of a novel env gene, TG35-2, and report that the TG35-2 pseudotype can be categorized as a novel FeLV subgroup. The TG35-2 envelope protein displays strong sequence identity to FeLV-A Env, suggesting that selection pressure in cats causes novel FeLV subgroups to emerge.

Endogenous retroviruses (ERVs) are remnants of ancestral retroviral infections of germ cells. Retroviral endogenization is an adaptation process for the host genome, and ERVs are gradually attenuated or inactivated by mutation. However, some ERVs that have been “domesticated” by their hosts eventually gain physiological functions, such as placentation or viral resistance. We previously reported the discovery of Refrex-1, a soluble antiretroviral factor in domestic cats that specifically inhibits infection by feline leukemia virus subgroup D (FeLV-D), a chimeric virus of FeLV, and a feline ERV, ERV-DC. Refrex-1 is a truncated envelope protein (Env) encoded by both ERV-DC7 and ERV-DC16 proviral loci. Here, we reconstituted ancestral and functional Env from ERV-DC7 and ERV-DC16 envelope genes ( env ) by inducing reverse mutations. Unexpectedly, ERV-DC7 and ERV-DC16 full-length Env (ERV-DC7 fl and ERV-DC16 fl), reconstructed by removing stop codons, did not produce infectious viral particles. ERV-DC7 fl and ERV-DC16 fl were highly expressed in cells but were not cleaved into surface subunits (SU) and transmembrane subunits, nor were they incorporated into virions. G407R/N427I-A429T and Y431D substitutions within the SU C-terminal domain of ERV-DC7 fl and ERV-DC16 fl, respectively, caused these dysfunctions. The residues glycine 407 and tyrosine 431 are relatively conserved among infectious gammaretroviruses, and their substitution causes the same dysfunctions as the tested retroviruses. Our results reveal that specific mutations within the SU C-terminal domain suppressed Env cleavage and incorporation into virions and indicate that these mutations contributed to the domestication of Refrex-1 through multistep events that occurred in the postintegration period. IMPORTANCE Domestic cats are colonized with various exogenous retroviruses (exRVs), such as feline leukemia virus (FeLV), and their genomes contain numerous ERVs, some of which are replication-competent proviruses. The feline hosts, exRVs, and ERVs have complicated genetic interactions and provide an interesting field model for triangular relationships: recombination between FeLV and ERV-DC, which is a feline ERV, generated FeLV-D, a chimeric virus, and FeLV-D is restricted by Refrex-1, an antiretroviral factor corresponding to truncated Env of ERV-DC7 and ERV-DC16. Here, we reconstructed ancestral, functional Env from ERV-DC7 and ERV-DC16 env by inducing reverse mutations to elucidate how Refrex-1 was generated from its ancestor. Our results reveal that they were repeatedly inactivated by mutations preventing Env maturation. Our results provide insights into how ERVs were “domesticated” by their hosts and identify the mutations that mediated these evolutions. Notably, experiments that restore inactivated ERVs might uncover previously unrecognized features or properties of retroviruses.

Felis catus gammaherpesvirus 1 (FcaGHV1) is a widely endemic infection of domestic cats. Current epidemiological data identify domestic cats as the sole natural host for FcaGHV1. The Tsushima leopard cat (TLC; Prionailurus bengalensis euptilurus) is a critically endangered species that lives only on Tsushima Island, Nagasaki, Japan. Nested PCR was used to test the blood or spleen of 89 TLCs for FcaGHV1 DNA; three (3.37%; 95% CI, 0.70–9.54) were positive. For TLC management purposes, we also screened domestic cats and the virus was detected in 13.02% (95% CI, 8.83–18.27) of 215 cats. Regarding phylogeny, the partial sequences of FcaGHV1 from domestic cats and TLCs formed one cluster, indicating that similar strains circulate in both populations. In domestic cats, we found no significant difference in FcaGHV1 detection in feline immunodeficiency virus-infected (p = 0.080) or feline leukemia virus-infected (p = 0.163) cats, but males were significantly more likely to be FcaGHV1 positive (odds ratio, 5.86; 95% CI, 2.27–15.14) than females. The higher frequency of FcaGHV1 detection in domestic cats than TLCs, and the location of the viral DNA sequences from both cats within the same genetic cluster suggests that virus transmission from domestic cats to TLCs is likely.

Gammaherpesviruses (GHVs) are members of an emerging subfamily of the family Herpesviridae. A recent study identified a novel GHV in domestic cats (Felis catus GHV1, FcaGHV1), and epidemiological surveys have found that FcaGHV1 is distributed worldwide. In this study, we investigated the prevalence of GHVs in domestic cats in Japan with a molecular epidemiological survey. Blood samples were collected from 1,738 domestic cats and GHV-derived DNA was detected with PCR in 1.3% (23/1,738) of the Japanese domestic cats. The FcaGHV1 detected in this study was very similar to FcaGHV1 detected in a domestic cat in North America. Older age (>5 years old) and Feline immunodeficiency virus infection were identified as risk factors for GHV infection.

The Tsushima leopard cat (TLC) Prionailurus bengalensis euptilurus, a subspecies of P. bengalensis, is designated a National Natural Monument of Japan, and lives only on Tsushima Island, Nagasaki Prefecture, Japan. TLCs are threatened by various infectious diseases. Feline leukemia virus (FeLV) causes a serious infectious disease with a poor prognosis in cats. Therefore, the transmission of FeLV from Tsushima domestic cats (TDCs) to TLCs may threaten the TLC population. We investigated the FeLV infection status of both TDCs and TLCs on Tsushima Island by screening blood samples for FeLV p27 antigen and using PCR to amplify the full-length FeLV env gene. The prevalence of FeLV was 6.4% in TDCs and 0% in TLCs. We also demonstrated that the virus can replicate in the cells of TLCs, suggesting its potential cross-species transmission. The viruses in TDCs were classified as genotype I/clade 3, which is prevalent on a nearby island, based on previous studies of FeLV genotypes and FeLV epidemiology. The FeLV viruses identified on Tsushima Island can be further divided into 2 lineages within genotype I/clade 3, which are geographically separated in Kamijima and Shimojima, indicating that FeLV may have been transmitted to Tsushima Island at least twice. Monitoring FeLV infection in the TDC and TLC populations is highly recommended as part of the TLC surveillance and management strategy.

Recombination events induce significant genetic changes, and this process can result in virus genetic diversity or in the generation of novel pathogenicity. We discovered a new recombinant feline leukemia virus (FeLV) gag gene harboring an unrelated insertion, termed the X region, which was derived from Felis catus endogenous gammaretrovirus 4 (FcERV-gamma4). The identified FcERV-gamma4 proviruses have lost their coding capabilities, but some can express their viral RNA in feline tissues. Although the X-region-carrying recombinant FeLVs appeared to be replication-defective viruses, they were detected in 6.4% of tested FeLV-infected cats. All isolated recombinant FeLV clones commonly incorporated a middle part of the FcERV-gamma4 5′-leader region as an X region. Surprisingly, a sequence corresponding to the portion contained in all X regions is also present in at least 13 endogenous retroviruses (ERVs) observed in the cat, human, primate, and pig genomes. We termed this shared genetic feature the commonly shared (CS) sequence. Despite our phylogenetic analysis indicating that all CS-sequence-carrying ERVs are classified as gammaretroviruses, no obvious closeness was revealed among these ERVs. However, the Shannon entropy in the CS sequence was lower than that in other parts of the provirus genome. Notably, the CS sequence of human endogenous retrovirus T had 73.8% similarity with that of FcERV-gamma4, and specific signals were detected in the human genome by Southern blot analysis using a probe for the FcERV-gamma4 CS sequence. Our results provide an interesting evolutionary history for CS-sequence circulation among several distinct ancestral viruses and a novel recombined virus over a prolonged period. IMPORTANCE Recombination among ERVs or modern viral genomes causes a rapid evolution of retroviruses, and this phenomenon can result in the serious situation of viral disease reemergence. We identified a novel recombinant FeLV gag gene that contains an unrelated sequence, termed the X region. This region originated from the 5′ leader of FcERV-gamma4, a replication-incompetent feline ERV. Surprisingly, a sequence corresponding to the X region is also present in the 5′ portion of other ERVs, including human endogenous retroviruses. Scattered copies of the ERVs carrying the unique genetic feature, here named the commonly shared (CS) sequence, were found in each host genome, suggesting that ancestral viruses may have captured and maintained the CS sequence. More recently, a novel recombinant FeLV hijacked the CS sequence from inactivated FcERV-gamma4 as the X region. Therefore, tracing the CS sequences can provide unique models for not only the modern reservoir of new recombinant viruses but also the genetic features shared among ancient retroviruses.

The host defense against viral infection is acquired during the coevolution or symbiosis of the host and pathogen. Several cellular factors that restrict retroviral infection have been identified in the hosts. Feline leukemia virus (FeLV) is a gammaretrovirus that is classified into several receptor interference groups, including a novel FeLV-subgroup D (FeLV-D) that we recently identified. FeLV-D is generated by transduction of the env gene of feline endogenous gammaretrovirus of the domestic cat (ERV-DCs) into FeLV. Some ERV-DCs are replication competent viruses which are present and hereditary in cats. We report here the determination of new viral receptor interference groups and the discovery of a soluble antiretroviral factor, termed Refrex-1. Detailed analysis of FeLV-D strains and ERV-DCs showed two receptor interference groups that are distinct from other FeLV subgroups, and Refrex-1 specifically inhibited one of them. Refrex-1 is characterized as a truncated envelope protein of ERV-DC and includes the N-terminal region of surface unit, which is a putative receptor-binding domain, but lacks the transmembrane region. Refrex-1 is efficiently secreted from the cells and appears to cause receptor interference extracellularly. Two variants of Refrex-1 encoded by provirus loci, ERV-DC7 and DC16, are expressed in a broad range of feline tissues. The host retains Refrex-1 as an antiretroviral factor, which may potentially prevent reemergence of the ERVs and the emergence of novel ERV-related viruses in cats. Refrex-1 may have been acquired during endogenization of ERV-DCs and may play an important role in retroviral restriction and antiviral defense in cats.

Endogenous retroviruses (ERVs) are the remnants of ancient retroviral infections of germ cells. Previous work identified one of the youngest feline ERV groups, ERV-DC, and reported that two ERV-DC loci, ERV-DC10 and ERV-DC18 (ERV-DC10/DC18), can replicate in cultured cells. Here, we identified another replication-competent provirus, ERV-DC14, on chromosome C1q32. ERV-DC14 differs from ERV-DC10/DC18 in its phylogeny, receptor usage, and, most notably, transcriptional activities; although ERV-DC14 can replicate in cultured cells, it cannot establish a persistent infection owing to its low transcriptional activity. Furthermore, we examined ERV-DC transcription and its regulation in feline tissues. Quantitative reverse transcription-PCR (RT-PCR) detected extremely low ERV-DC10 expression levels in feline tissues, and bisulfite sequencing showed that 5′ long terminal repeats (LTRs) of ERV-DC10/DC18 are significantly hypermethylated in feline blood cells. Reporter assays found that the 5′-LTR promoter activities of ERV-DC10/DC18 are high, whereas that of ERV-DC14 is low. This difference in promoter activity is due to a single substitution from A to T in the LTR, and reverse mutation at this nucleotide in ERV-DC14 enhanced its replication and enabled it to persistently infect cultured cells. Therefore, ERV-DC LTRs can be divided into two types based on this nucleotide, the A type or T type, which have strong or attenuated promoter activity, respectively. Notably, ERV-DCs with T-type LTRs, such as ERV-DC14, have expanded in the cat genome significantly more than A-type ERV-DCs, despite their low promoter activities. Our results provide insights into how the host controls potentially infectious ERVs and, conversely, how ERVs adapt to and invade the host genome. IMPORTANCE The domestic cat genome contains many endogenous retroviruses, including ERV-DCs. These ERV-DCs have been acquired through germ cell infections with exogenous retroviruses. Some of these ERV-DCs are still capable of producing infectious virions. Hosts must tightly control these ERVs because replication-competent viruses in the genome pose a risk to the host. Here, we investigated how ERV-DCs are adapted by their hosts. Replication-competent viruses with strong promoter activity, such as ERV-DC10 and ERV-DC18, were suppressed by promoter methylation in LTRs. On the other hand, replication-competent viruses with weak promoter activity, such as ERV-DC14, seemed to escape strict control via promoter methylation by the host. Interestingly, ERV-DCs with weak promoter activity, such as ERV-DC14, have expanded in the cat genome significantly more than ERV-DCs with strong promoter activity. Our results improve the understanding of the host-virus conflict and how ERVs adapt in their hosts over time.

The prevalence of Rickettsia , Ehrlichia , and Anaplasma in 1764 DNA samples extracted from feline peripheral blood from all 47 prefectures in Japan was evaluated by screening real-time PCR, genus-specific PCR, and DNA nucleotide sequencing. The survey revealed that all cats were negative for Rickettsia infection. Two cats were positive for Ehrlichia or Anaplasma based on the screening PCR assay. Nucleotide sequence analysis of the partial 16S rRNA including the divergent region near the 3′-end revealed that the 2 positives were most similar to Anaplasma bovis with percent identities of 99.8% and 99.2%. This was the first detection of A. bovis DNA fragments in cats. Although these 2 cats showed stomatitis, both were also infected with feline immunodeficiency virus. The relationship between A. bovis carriage and clinical disease is not yet understood.

Infection of erythroid cells by Friend spleen focus-forming virus (SFFV) leads to acute erythroid hyperplasia in mice, due to expression of its unique envelope glycoprotein, gp55. Erythroid cells expressing SFFV gp55 proliferate in the absence of their normal regulator, erythropoietin, because of the interaction among the viral envelope protein, the erythropoietin receptor, and a short form of the receptor tyrosine kinase Stk (sf-Stk). This leads to constitutive activation of several signal transduction pathways. Our previous studies showed that sf-Stk interacts with SFFV gp55, forming disulfide-linked complexes. This covalent interaction, along with other noncovalent interactions with SFFV-gp55, results in constitutive tyrosine phosphorylation of sf-Stk and rodent fibroblast transformation. Here, we determined the precise amino acid region within sf-Stk that contributes to fibroblast transformation by the polycythemia-inducing (SFFV-P) and the anemia-inducing (SFFV-A) strains of SFFV. Sf-Stk deletion mutants showed different transforming abilities in fibroblasts infected with SFFV-P and SFFV-A, although the N-terminal extracellular domain of sf-Stk was essential for fibroblast transformation by both viruses. Point mutations of sf-Stk indicated that cysteine 19 was critical for fibroblast transformation by SFFV-P, although all four cysteines (8, 19, 37 and 42) appeared to be important for fibroblast transformation by both SFFV-P and SFFV-A. Mutation of sf-Stk cysteine 19 abolished its ability to form dimers with SFFV-P and SFFV-A gp55. These results suggest that the interaction between sf-Stk and the envelope proteins of the polycythemia- and anemia-inducing variants of SFFV is architecturally different.

We established a novel feline B-cell line, MS4, from the neoplastic pleural effusion of a cat with cutaneous B-cell lymphoma. Immunophenotype staining of the MS4 cells was positive for CD20, CD79α, and IgA and negative for CD3, CD4, CD5, CD8α, CD18, CD21, CD22, IgM, IgG, Ig light chain, and MHC class II. PCR analysis for immunoglobulin heavy chain gene rearrangements revealed a monoclonal rearrangement, whereas no clonal rearrangement of the T-cell receptor γ gene was detected. Southern blotting with an exogenous feline leukemia virus (FeLV) U3 probe revealed no integration of exogenous FeLV provirus. The MS4 cell line is the first FeLV-negative feline B-cell lymphoma cell line, and may be used to investigate the pathogenesis of spontaneously occurring feline lymphoma and the development of new therapies.

An endogenous retrovirus (ERV) is a remnant of an ancient retroviral infection in the host genome. Although most ERVs have lost their viral productivity, a few ERVs retain their replication capacity. In addition, partially inactivated ERVs can present a potential risk to the host via their encoded virulence factors or the generation of novel viruses by viral recombination. ERVs can also eventually acquire a biological function, and this ability has been a driving force of host evolution. Therefore, the presence of an ERV can be harmful or beneficial to the host. Various reports about paleovirology have revealed each event in ERV evolution, but the continuous processes of ERV evolution over millions of years are mainly unknown. A unique ERV family, ERV-DC, is present in the domestic cat (Felis silvestris catus) genome. ERV-DC proviruses are phylogenetically classified into three genotypes, and the specific characteristics of each genotype have been clarified: their capacity to produce infectious viruses; their recombination with other retroviruses, such as feline leukemia virus or RD-114; and their biological functions as host antiviral factors. In this review, we describe ERV-DC-related phenomena and discuss the continuous changes in the evolution of this ERV in the domestic cat.

Severe acute respiratory syndrome coronavirus (SARS-CoV) causes a lung disease with high mortality. In addition, osteonecrosis and bone abnormalities with reduced bone density have been observed in patients following recovery from SARS, which were partly but not entirely explained by the short-term use of steroids. Here, we demonstrate that human monocytes, potential precursors of osteoclasts, partly express angiotensin converting enzyme 2 (ACE2), a cellular receptor of SARS-CoV, and that expression of an accessory protein of SARS-CoV, 3a/X1, in murine macrophage cell line RAW264.7 cells, enhanced NF-kappaB activity and differentiation into osteoclast-like cells in the presence of receptor activator of NF-kappaB ligand (RANKL). Furthermore, human epithelial A549 cells expressed ACE2, and expression of 3a/X1 in these cells up-regulated TNF-alpha, which is known to accelerate osteoclastogenesis. 3a/X1 also enhanced RANKL expression in mouse stromal ST2 cells. These findings indicate that SARS-CoV 3a/X1 might promote osteoclastogenesis by direct and indirect mechanisms.

Feline leukemia virus (FeLV) clone33 was obtained from a domestic cat with acute myeloid leukemia (AML). The long terminal repeat (LTR) of this virus, like the LTRs present in FeLV from other cats with AML, differs from the LTRs of other known FeLV in that it has 3 tandem direct 47-bp repeats in the upstream region of the enhancer (URE). Here, we injected cats with FeLV clone33 and found 41% developed myelodysplastic syndromes (MDS) characterized by peripheral blood cytopenias and dysplastic changes in the bone marrow. Some of the cats with MDS eventually developed AML. The bone marrow of the majority of cats with FeLV clone33 induced MDS produced fewer erythroid and myeloid colonies upon being cultured with erythropoietin or granulocyte-macrophage colony-stimulating factor (GM-SCF) than bone marrow from normal control cats. Furthermore, the bone marrow of some of the cats expressed high-levels of the apoptosis-related genes TNF-alpha and survivin. Analysis of the proviral sequences obtained from 13 cats with naturally occurring MDS reveal they also bear the characteristic URE repeats seen in the LTR of FeLV clone33 and other proviruses from cats with AML. Deletions and mutations within the enhancer elements are frequently observed in naturally occurring MDS as well as AML. These results suggest that FeLV variants that bear URE repeats in their LTR strongly associate with the induction of both MDS and AML in cats.

Feline leukemia virus (FeLV) belongs to the Gammaretrovirus genus and is horizontally transmitted among cats. FeLV is known to undergo recombination with endogenous retroviruses already present in the host during FeLV-subgroup A infection. Such recombinant FeLVs, designated FeLV-subgroup B or FeLV-subgroup D, can be generated by transduced endogenous retroviral env sequences encoding the viral envelope. These recombinant viruses have biologically distinct properties and may mediate different disease outcomes. The generation of such recombinant viruses resulted in structural diversity of the FeLV particle and genetic diversity of the virus itself. FeLV env diversity through mutation and recombination has been studied, while gag diversity and its possible effects are less well understood. In this study, we investigated recombination events in the gag genes of FeLVs isolated from naturally infected cats and reference isolates. Recombination and phylogenetic analyses indicated that the gag genes often contain endogenous FeLV sequences and were occasionally replaced by entire endogenous FeLV gag genes. Phylogenetic reconstructions of FeLV gag sequences allowed for classification into three distinct clusters, similar to those previously established for the env gene. Analysis of the recombination junctions in FeLV gag indicated that these variants have similar recombination patterns within the same genotypes, indicating that the recombinant viruses were horizontally transmitted among cats. It remains to be investigated whether the recombinant sequences affect the molecular mechanism of FeLV transmission. These findings extend our understanding of gammaretrovirus evolutionary patterns in the field.

Molecularly cloned feline leukemia virus (FeLV)-clone 33 (C-33), derived from a cat with acute myelocytic leukemia (AML), was examined to assess its relation to the pathogenesis of AML and myelodysplastic syndrome (MDS). To evaluate in vitro pathogenicity of FeLV C-33, bone marrow colony-forming assay was performed on marrow cells infected with FeLV C-33 or an FeLV subgroup A strain (61E, a molecularly cloned strain with minimal pathogenicity). The myeloid colony-forming activity of feline bone marrow mononuclear cells infected with FeLV C-33 was significantly lower than that of cells infected with 61E. This suggests that FeLV C-33 has myeloid lineage-specific pathogenicity for cats, and that FeLV C-33 infection is useful as an experimental model for investigating pathogenesis of MDS and AML.

The erythroleukemia-inducing Friend spleen focus-forming virus (SFFV) encodes a unique envelope protein, gp55, which interacts with the erythropoietin (Epo) receptor complex, causing proliferation and differentiation of erythroid cells in the absence of Epo. Susceptibility to SFFV-induced erythroleukemia is conferred by the Fv-2 gene, which encodes a short form of the receptor tyrosine kinase Stk/Ron (sf-Stk) only in susceptible strains of mice. We recently demonstrated that sf-Stk becomes activated by forming a strong interaction with SFFV gp55. To examine the biological consequences of activated sf-Stk on erythroid cell growth, we prepared retroviral vectors which express sf-Stk, either in conjunction with gp55 or alone in a constitutively activated mutant form, and tested them for their ability to induce Epo-independent erythroid colonies ex vivo and disease in mice. Our data indicate that both gp55-activated sf-Stk and the constitutively activated mutant of sf-Stk induce erythroid cells from Fv-2 -susceptible and Fv-2 -resistant (sf-Stk null) mice to form Epo-independent colonies. Mutational analysis of sf-Stk indicated that a functional kinase domain and 8 of its 12 tyrosine residues are required for the induction of Epo-independent colonies. Further studies demonstrated that coexpression of SFFV gp55 with sf-Stk significantly extends the half-life of the kinase. When injected into Fv-2 -resistant mice, neither the gp55-activated sf-Stk nor the constitutively activated mutant caused erythroleukemia. Surprisingly, both Fv-2 -susceptible and -resistant mice injected with the gp55-sf-Stk vector developed clinical signs not previously associated with SFFV-induced disease. We conclude that sf-Stk, activated by either point mutation or interaction with SFFV gp55, is sufficient to induce Epo-independent erythroid colonies from both Fv-2 -susceptible and -resistant mice but is unable to cause erythroleukemia in Fv-2 -resistant mice.

A novel protein kinase, polyploidy-associated protein kinase (PAPK), was isolated using a subtraction cDNA library approach from a mouse erythroleukemia cell line that had been induced to polyploidy after serum withdrawal. PAPK shares homology with members of the Ste20/germinal center kinase family of protein kinases and is ubiquitously expressed as two spliced forms, PAPK-A and PAPK-B, that encode for proteins of 418 and 189 amino acids, respectively. The expression of endogenous PAPK-A protein increased after growth factor withdrawal in murine hematopoietic and fibroblast cells. When tested in an in vitro kinase assay, PAPK-A was activated in response to the stress-inducing agent hydrogen peroxide and slightly by fetal calf serum. Biochemical characterization of the PAPK-A-initiated pathway revealed that this novel kinase does not affect MAP kinase activity but can stimulate both c-Jun N-terminal kinase 1 (JNK1) and ERK6/p38 gamma. The kinase activity of PAPK appears to be required for the activation of ERK6/p38 gamma but not JNK1. When an inducible construct of PAPK-A was expressed in stably transfected NIH3T3 cells, the cells exhibited distinct cytoskeletal changes and became resistant to apoptotic cell death induced by serum withdrawal, effects of PAPK that require its kinase activity. These data suggest that PAPK is a new member of the Ste20/germinal center kinase family that modulates cytoskeletal organization and cell survival.

The clonality analysis of the bone marrow cells was carried out by detecting the integrated proviruses of feline leukemia virus (FeLV) to understand the pathogenesis of FeLV-associated hematopoietic disorders in cats. Bone marrow cells from 4 cases with acute myeloid leukemia (AML), 9 cases with myelodysplastic syndromes (MDS), 2 cases with pure red cell aplasia (PRCA) and 3 healthy carriers infected with FeLV were subjected to Southern blot analyses using an exogenous FeLV probe. Clonal hematopoiesis was found in all the cases with AML and in 6 of the 9 cases with MDS, but not in the cases with both PRCA and healthy carriers infected with FeLV. In the 2 cases with MDS, it was thought that the same clones of the hematopoietic cells might proliferate before and after the progression of the disease irrespective of the changes of the hematological diagnoses by cytological examination. This study indicates that MDS in cats is a disease manifestation as a result of clonal proliferation of hematopoietic cells and can be recognized as a pre-leukemic state of AML.

Long terminal repeats of feline leukemia viruses cloned from feline acute myeloid leukemias frequently contained direct repeats of 40 to 74 bp in the upstream region of the enhancer (URE). The repetitive URE conferred an enhancer function upon gene expression in myeloid cells, suggesting its association with tumorigenic potential in myeloid cells.

For investigation of the relation of cell cycle regulation with tumorigenesis in cats, we carried out molecular cloning of feline p21WAF1 and p27Kip1 cDNAs and chromosomal mapping of these genes on the cat genome. The feline p21WAF1 cDNA clone obtained in this study encoded 164 amino acids (aa) showing 83.5% and 76.8% sequence similarity with those of the human and mouse counterparts, respectively. The cat p27Kip1 cDNA clone isolated here encoded 198 aa, showing sequence similarities of 93.4% and 90.4% with its human and mouse counterparts, respectively. Using a panel of feline×rodent somatic cell hybrids, the feline CDKN1A (p21WAF1) and CDKN1B (p27Kip1) loci were assigned to feline chromosomes B2 and B4, respectively. Southern-blot analyses of 17 feline spontaneous leukemia and lymphoma cases using these cDNAs as probes did not reveal any rearrangements in either the p21WAF1 or the p27Kip1 gene. RT-PCR/SSCP (single strand conformation polymorphism) analysis of p27Kip1 cDNA did not uncover any amino acid substitutions in the 10 feline leukemia and lymphoma cases that were examined.

Feline leukemia virus (FeLV) belongs to the genus Gammaretrovirus, and causes a variety of neoplastic and non-neoplastic diseases in cats. Alteration of viral env sequences is thought to be associated with disease specificity, but the way in which genetic diversity of FeLV contributes to the generation of such variants in nature is poorly understood. We isolated FeLV env genes from naturally infected cats in Japan and analyzed the evolutionary dynamics of these genes. Phylogenetic reconstructions separated our FeLV samples into three distinct genetic clusters, termed Genotypes I, II, and III. Genotype I is a major genetic cluster and can be further classified into Clades 1-7 in Japan. Genotypes were correlated with geographical distribution; Genotypes I and II were distributed within Japan, whilst FeLV samples from outside Japan belonged to Genotype III. These results may be due to geographical isolation of FeLVs in Japan. The observed structural diversity of the FeLV env gene appears to be caused primarily by mutation, deletion, insertion and recombination, and these variants may be generated de novo in individual cats. FeLV interference assay revealed that FeLV genotypes did not correlate with known FeLV receptor subgroups. We have identified the genotypes which we consider to be reliable for evaluating phylogenetic relationships of FeLV, which embrace the high structural diversity observed in our sample. Overall, these findings extend our understanding of Gammaretrovirus evolutionary patterns in the field, and may provide a useful basis for assessing the emergence of novel strains and understanding the molecular mechanisms of FeLV transmission in cats.

Endogenous retroviruses (ERVs) comprise a significant percentage of the mammalian genome, and it is poorly understood whether they will remain as inactive genomes or emerge as infectious retroviruses. Although several types of ERVs are present in domestic cats, infectious ERVs have not been demonstrated. Here, we report a previously uncharacterized class of endogenous gammaretroviruses, termed ERV-DCs, that is present and hereditary in the domestic cat genome. We have characterized a subset of ERV-DC proviral clones, which are numbered according to their genomic insertions. One of these, ERV-DC10, located in the q12-q21 region on chromosome C1, is an infectious gammaretrovirus capable of infecting a broad range of cells, including human. Our studies indicate that ERV-DC10 entered the genome of domestic cats in the recent past and appeared to translocate to or reintegrate at a distinct locus as infectious ERV-DC18. Insertional polymorphism analysis revealed that 92 of 244 domestic cats had ERV-DC10 on a homozygous or heterozygous locus. ERV-DC-like sequences were found in primate and rodent genomes, suggesting that these ERVs, and recombinant viruses such as RD-114 and BaEV, originated from an ancestor of ERV-DC. We also found that a novel recombinant virus, feline leukemia virus subgroup D (FeLV-D), was generated by ERV-DC env transduction into feline leukemia virus in domestic cats. Our results indicate that ERV-DCs behave as donors and/or acceptors in the generation of infectious, recombinant viruses. The presence of such infectious endogenous retroviruses, which could be harmful or beneficial to the host, may affect veterinary medicine and public health.

Infection of erythroid cells by Friend spleen focus-forming virus (SFFV) leads to acute erythroid hyperplasia in mice due to expression of its unique envelope glycoprotein, gp55. Erythroid cells expressing SFFV gp55 proliferate in the absence of their normal regulator, erythropoietin (Epo), because of interaction of the viral envelope protein with the erythropoietin receptor and a short form of the receptor tyrosine kinase Stk (sf-Stk), leading to constitutive activation of several signal transduction pathways. Our previous in vitro studies showed that phosphatidylinositol 3-kinase (PI3-kinase) is activated in SFFV-infected cells and is important in mediating the biological effects of the virus. To determine the role of PI3-kinase in SFFV-induced disease, mice deficient in the p85α regulatory subunit of class IA PI3-kinase were inoculated with different strains of SFFV. We observed that p85α status determined the extent of erythroid hyperplasia induced by the sf-Stk-dependent viruses SFFV-P (polycythemia-inducing strain of SFFV) and SFFV-A (anemia-inducing strain of SFFV) but not by the sf-Stk-independent SFFV variant BB6. Our data also indicate that p85α status determines the response of mice to stress erythropoiesis, consistent with a previous report showing that SFFV uses a stress erythropoiesis pathway to induce erythroleukemia. We further showed that sf-Stk interacts with p85α and that this interaction depends upon sf-Stk kinase activity and tyrosine 436 in the multifunctional docking site. Pharmacological inhibition of PI3-kinase blocked proliferation of primary erythroleukemia cells from SFFV-infected mice and the erythroleukemia cell lines derived from them. These results indicate that p85α may regulate sf-Stk-dependent erythroid proliferation induced by SFFV as well as stress-induced erythroid hyperplasia.

Hemoplasma (hemotropic mycoplasma) often causes hemolytic anemia in infected cats, especially those with immune suppression. An updated nationwide epidemiological survey of feline hemoplasmosis was conducted in Japan. Blood samples were collected from 1,770 outdoor-accessing cats from March to October 2008. The infections were molecularly detected by PCR analyses, which are able to distinguish Mycoplasma haemofelis (Mhf), `Candidatus M. haemominutum' (CMhm), and `Candidatus M. turicensis' (CMt) infections. Of the 1,770 cats, 468 cases (26.4%) revealed a single- or co-infection of feline hemoplasmas [Mhf alone, 42 cases (2.4%); CMhm alone, 280 cases (15.8%); CMt alone, 48 cases (2.7%); Mhf+CMhm, 28 cases (1.6%); Mhf+CMt, 6 cases (0.3%); CMhm+CMt, 50 cases (2.8%); Mhf+CMhm+CMt, 14 cases (0.8%)]. In addition, male gender, middle to old age, history of fight wounds, and feline immunodeficiency virus infection were shown to be risk factors for hemoplasma infection. Close attention must be paid to the acute onset of disease in feline practice because a prevalence of hemoplasma infection was detected even in clinically healthy cats.

Infection of erythroid progenitor cells by Friend spleen focus-forming virus (SFFV) leads to acute erythroid hyperplasia and eventually to erythroleukemia in susceptible strains of mice. The viral envelope protein, SFFV gp55, forms a complex with the erythropoietin receptor (EpoR) and a short form of the receptor tyrosine kinase Stk (sf-Stk), activating both and inducing Epo-independent proliferation. Recently, we discovered that coexpression of SFFV gp55 and sf-Stk is sufficient to transform NIH 3T3 and primary fibroblasts. In the current study, we demonstrate that sf-Stk and its downstream effectors are critical to this transformation. Unlike SFFV-derived erythroleukemia cells, which depend on PU.1 expression for maintenance of the transformed phenotype, SFFV gp55-sf-Stk-transformed fibroblasts are negative for PU.1. Underscoring the importance of sf-Stk to fibroblast transformation, knockdown of sf-Stk abolished the ability of these cells to form anchorage-independent colonies. Like SFFV-infected erythroid cells, SFFV gp55-sf-Stk-transformed fibroblasts express high levels of phosphorylated MEK, ERK, phosphatidylinositol 3-kinase (PI3K), Gab1/2, Akt, Jun kinase (JNK), and STAT3, but unlike virus-infected erythroid cells they fail to express phosphorylated STATs 1 and 5, which may require involvement of the EpoR. In addition, the p38 mitogen-activated protein kinase (MAPK) stress response is suppressed in the transformed fibroblasts. Inhibition of either JNK or the PI3K pathway decreases both monolayer proliferation and anchorage-independent growth of the transformed fibroblasts as does the putative kinase inhibitor luteolin, but inhibition of p38 MAPK has no effect. Our results indicate that sf-Stk is a molecular endpoint of transformation that could be targeted directly or with agents against its downstream effectors.

Infection of mice with Friend spleen focus-forming virus (SFFV) results in a multistage erythroleukemia. In the first stage, the SFFV envelope glycoprotein interacts with the erythropoietin receptor and a short form of the receptor tyrosine kinase sf-Stk, resulting in constitutive activation of signal transducing molecules and the development of erythropoietin (Epo)-independent erythroid hyperplasia and polycythemia. The second stage results from the outgrowth of a rare virus-infected erythroid cell that expresses nonphysiological levels of the myeloid transcription factor PU.1. These cells exhibit a differentiation block and can be grown as murine erythroleukemia (MEL) cell lines. In this study, we examined SFFV MEL cells to determine whether their transformed phenotype was associated with a block in the activation of any Epo signal-transducing molecules. Our studies indicate that Epo- or SFFV-induced activation of STAT1/3 DNA binding activity is blocked in SFFV MEL cells. The block is at the level of tyrosine phosphorylation of STAT1, although Jak2 phosphorylation is not blocked in these cells. In contrast to Epo, alpha interferon can induce STAT1 phosphorylation and DNA binding in SFFV MEL cells. The SFFV-transformed cells were shown to express elevated levels of the hematopoietic phosphatase SHP-1, and treatment of the cells with a phosphatase inhibitor restored STAT1 tyrosine phosphorylation. MEL cells derived from Friend murine leukemia virus (MuLV) or ME26 MuLV-infected mice, which do not express PU.1, express lower levels of SHP-1 and are not blocked in STAT1/3 DNA-binding activity. Our studies suggest that SFFV-infected erythroid cells become transformed when differentiation signals activated by STAT1/3 are blocked due to high SHP-1 levels induced by inappropriate expression of the PU.1 protein.

Friend spleen focus-forming virus (SFFV) causes rapid erythroleukemia in mice due to expression of its unique envelope glycoprotein, gp55. Erythroid cells expressing SFFV gp55 proliferate in the absence of their normal regulator erythropoietin (Epo) because of constitutive activation of Epo signal transduction pathways. Although SFFV infects many cell types, deregulation of cell growth occurs only when SFFV infects erythroid cells, suggesting that these cells express unique proteins that the virus requires to mediate its biological effects. Not only do erythroid cells express the Epo receptor (EpoR), but those from mice susceptible to SFFV-induced erythroleukemia also express a short form of the receptor tyrosine kinase Stk (sf-Stk). In erythroid cells, SFFV gp55 interacts with the EpoR complex and sf-Stk, leading to activation of the kinase and constitutive activation of signal transducing molecules. In this study, we demonstrate that SFFV gp55 can also deregulate the growth of nonerythroid cells when it is coexpressed with sf-Stk. Expression of SFFV gp55 in rodent fibroblasts engineered to express sf-Stk induced their transformation, as demonstrated by focus formation and anchorage-independent growth in vitro . This transformation by SFFV gp55 requires the kinase activity of sf-Stk and the presence of its extracellular domain but not expression of the EpoR or the tyrosine kinase Jak2, which is required for activation of signal transduction pathways through the EpoR. Thus, expression of SFFV gp55 in nonerythroid cells coexpressing sf-Stk results in their uncontrolled growth, demonstrating a previously unrecognized mechanism for retrovirus transformation of rodent fibroblasts and providing insight into SFFV-induced disease.

Members of the mitogen-activated protein kinase (MAPK) family, including Jun amino-terminal kinase (JNK) and extracellular signal-related kinase (ERK), play an important role in the proliferation of erythroid cells in response to erythropoietin (Epo). Erythroid cells infected with the Friend spleen focus-forming virus (SFFV) proliferate in the absence of Epo and show constitutive activation of Epo signal transduction pathways. We previously demonstrated that the ERK pathway was constitutively activated in Friend SFFV-infected erythroid cells, and in this study JNK is also shown to be constitutively activated. Pharmacological inhibitors of both the ERK and JNK pathways stopped the proliferation of primary erythroleukemic cells from Friend SFFV-infected mice, with little induction of apoptosis, and furthermore blocked their ability to form Epo-independent colonies. However, only the JNK inhibitor blocked the proliferation of erythroleukemia cell lines derived from these mice. The JNK inhibitor caused significant apoptosis in these cell lines as well as an increase in the fraction of cells in G 2 /M and undergoing endoreduplication. In contrast, the growth of erythroleukemia cell lines derived from Friend murine leukemia virus (MuLV)-infected mice was inhibited by both the MEK and JNK inhibitors. JNK is important for AP1 activity, and we found that JNK inhibitor treatment reduced AP1 DNA-binding activity in primary erythroleukemic splenocytes from Friend SFFV-infected mice and in erythroleukemia cell lines from Friend MuLV-infected mice but did not alter AP1 DNA binding in erythroleukemia cell lines from Friend SFFV-infected mice. These data suggest that JNK plays an important role in cell proliferation and/or the survival of erythroleukemia cells.

We have molecularly cloned a feline leukemia virus (FeLV) (clone 33) from a domestic cat with acute myeloid leukemia (AML). The long terminal repeat (LTR) of this virus, like the LTRs present in FeLV proviruses from other cats with AML, contains an unusual structure in its U3 region upstream of the enhancer (URE) consisting of three tandem direct repeats of 47 bp. To test the disease potential and specificity of this unique FeLV LTR, we replaced the U3 region of the LTR of the erythroleukemia-inducing Friend murine leukemia virus (F-MuLV) with that of FeLV clone 33. When the resulting virus, F33V, was injected into newborn mice, almost all of the mice eventually developed hematopoietic malignancies, with a significant percentage being in the myeloid lineage. This is in contrast to mice injected with an F-MuLV recombinant containing the U3 region of another FeLV that lacks repetitive URE sequences, none of which developed myeloid malignancies. Examination of tumor proviruses from F33V-infected mice failed to detect any changes in FeLV U3 sequences other than that in the URE. Like F-MuLV-infected mice, those infected with the F-MuLV/FeLV recombinants were able to generate and replicate mink cell focus-inducing viruses. Our studies are consistent with the idea that the presence of repetitive sequences upstream of the enhancer in the LTR of FeLV may favor the activation of this promoter in myeloid cells and contribute to the development of malignancies in this hematopoietic lineage.

The Friend spleen focus-forming virus (SFFV) causes an acute erythroleukemia in susceptible strains of mice (for a review, see reference 37). SFFV encodes a unique envelope glycoprotein, gp55, that associates specifically with the erythropoietin receptor (EpoR) at the cell surface (4, 10, 21, 45), allowing erythroid cells to proliferate in the absence of erythropoietin (Epo), the normal regulator of erythropoiesis. Epo stimulation of the EpoR activates a number of signal transduction pathways, including the Jak-Stat, the Ras/Raf-1/mitogen-activated protein kinase (MAPK), and the phosphatidylinositol 3-kinase (PI3-kinase) pathways (for a review, see reference 46). Using the Epo-dependent erythroleukemia cell line HCD-57, we have previously demonstrated that infection with SFFV, which abrogates the Epo dependence of these cells (36), constitutively activates Stat DNA-binding activity (30); Ras (27); Raf-1, MEK, and MAPK (26); PI 3-kinase and Akt kinase (29); and protein kinase C (27). Although interaction of the SFFV envelope glycoprotein with the EpoR complex is essential for inducing the biological effects of the virus, other factors must also be involved, since not all strains of mice are susceptible to SFFV-induced erythroleukemia. A number of polymorphic mouse genes have been identified that affect susceptibility to Friend SFFV-induced erythroleukemia. Most of these host genes interfere with viral entry or integration, or they control immune responses against the virus-infected cells (for a review, see reference 7). However, the Fv-2 gene (22), which is located on mouse chromosome 9, acts at the level of the erythroid target cell for the virus (2, 3, 9, 13, 39). Mice carrying at least one copy of the Fv-2s allele are susceptible to SFFV-induced erythroleukemia, and their erythroid cells will form Epo-independent erythroid bursts when infected in vitro with SFFV. In contrast, mice homozygous for the Fv-2r allele fail to develop SFFV-induced erythroleukemia, and SFFV infection of erythroid cells from these mice does not result in the formation of Epo-independent erythroid bursts. Thus, erythroid cells from Fv-2-resistant mice appear to lack a component necessary for mediating the biological effects of SFFV infection. Although earlier studies suggested that there were differences between Fv-2-susceptible and -resistant erythroid cells in cell cycling (42), it was unclear how this affected the ability of SFFV to alter the growth of these cells. Recently, however, it was shown that the Fv-2 gene is identical to the gene encoding the Met-related tyrosine kinase Stk/Ron (33). Mice that carry the Fv-2s allele express both a full-length Stk and a truncated form of the kinase, sf-Stk, which is expressed from an alternate promoter. sf-Stk lacks almost the entire extracellular domain but retains the transmembrane and tyrosine kinase domains. In contrast to Fv-2s mice, mice that are homozygous for the Fv-2r allele express only full-length Stk due to a 3-nucleotide deletion in the alternate promoter. Transfer of bone marrow cells expressing sf-Stk to Fv-2rr mice confers susceptibility to SFFV-induced erythroleukemia, while targeted disruption of the Stk gene in Fv-2s mice results in resistance to disease (33). Thus, expression of sf-Stk in erythroid cells appears to play a critical role in determining the biological consequences of SFFV infection in the mouse. We, therefore, carried out studies to determine if SFFV gp55 interacts with sf-Stk and whether this interaction results in the activation of this truncated kinase and the downstream activation of signal transduction pathways.

Clinical and immunological characteristics were investigated in six cases of feline granular lymphocyte (GL) tumor. The ages of the affected cats were relatively old, ranging from 4 to 13 years of age. Gastrointestinal signs were commonly observed in these cases. Only one of the six GL tumor cases was positive for feline leukemia virus (FeLV) antigen. Phenotypic analysis revealed that the GL tumor cells from all of the six cases lacked the T- or B-cell markers. These GL tumor cells were examined by Southern blot analysis using feline immunoglobulin (Ig) and T-cell receptor (TCR) gene probes. GL tumor cells obtained from two cases were identified as cells of T-cell lineage by the presence of a rearranged TCR beta gene, whereas those from the other four cases were considered to be derived from non-T- non-B-cell lineage because of the absence of rearrangement of these genes. These findings indicated that feline GL tumors can be considered as a specific disease entity in feline lymphomas because the cases examined in this study showed onset at an older age, a low incidence of FeLV infection and frequent involvement of gastrointestinal lesions, which are not found in typical FeLV-associated lymphomas. Although no specific phenotypes was observed by phenotypic analysis, the feline GL tumor cells were divided into two consistent genotypes of T-cell or non-T- non-B-cell lineages.

Neoplastic diseases, especially malignant lymphomas, are often observed in cats infected with feline immunodeficiency virus (FIV). In order to clarify the characteristics of lymphoma cells and to investigate the pathogenesis in FIV-infected cats, we examined the lymphoma tissues developed in five cats naturally infected with FIV by Southern blot analyses using feline immunoglobulin (Ig), T-cell receptors (TCR) and FIV probes. All of the five cases were serologically positive for anti-FIV antibody and negative for feline leukemia virus antigen. Of these five lymphoma samples, two displayed rearrangement of the Ig heavy chain gene and deletion of the Ig light (κ) chain gene, indicating that the tumor cells were committed to B-cell development. One tumor sample was identified as a T-cell lymphoma because of the presence of a rearranged TCR β-chain gene. The other two cases were considered to be non-T non-B cell lymphoma because they did not show any rearrangement of the Ig and TCR genes. Therefore, no consistent tumor type was found in lymphoma cases infected with FIV. Clonal integration of FIV provirus was not detected in any of the five lymphoma samples obtained from FIV-infected cats using Southern blot analysis, although FIV proviral genome was detected in the genomic DNA of all the lymphoma samples by using a polymerase chain reaction (PCR). These results indicated that FIV might not play a direct role in tumorigenesis of lymphoma in cats.

Feline leukemia viruses carrying transduced v-myc genes (myc-FeLV) induce tumors of clonal origin, suggesting that activated myc alone is not sufficient for tumorigenesis. To investigate the hypothesis that insertional mutagenesis plays a role by activating genes which collaborate with v-myc, we looked for evidence of common proviral integration sites in these tumors. By inverse polymerase chain reaction we identified a 6-kb domain, designated fit-1, in which FeLV proviruses were inserted in four of nine (44%) T-cell tumors induced by myc-FeLV. The fit-1 locus was mapped to feline chromosome B2 and appears to be distinct from known oncogenes located on this chromosome. Fit-1 represents a novel common proviral integration region which may harbor a cellular gene which acts in concert with the myc gene in T-cell tumorigenesis.

An updated nation-wide epidemiological survey of feline immunodeficiency virus (FIV) infection was conducted in Japan. Blood samples were collected from 1,770 outdoor accessing cats from March to October 2008. Serologically, 410 cats (23.2%) were positive for anti-FIV antibody. Proviral DNA of the FIV env V3-V5 region isolated from 348 cases could be phylogenetically analyzed. The present study disclosed a geographic distribution of four subtypes (A, B, C and D) of FIV in Japan. Even though an FIV vaccine was introduced in Japan, we do not currently know whether this vaccine is effective against all strains of FIV in Japan or not. Therefore, close attention still has to be paid to epidemic and genotypic trends of FIV.