In vitro transformation of cells from human neoplasms

The possible involvement of RNA tumor virus genomes in human cell transformation was investigated. Forty-nine cell cultures from neoplastic, normal, or embryo tissues were examined for transformation, following inoculation of murine leukemia virus (MuLV), feline leukemia virus (FeLV) grown in human cells, or bone marrow aspirates from leukemia patients. Five cultures exhibited transformation (1 after inoculation of MuLV grown in human cells; 4 after inoculation of human leukemic bone marrow), and 4 were established as cell lines. They were derived from giant cell tumor and fibrosarcomas. The established transformed cells formed colonies in soft agar, grew progressively in immunosuppressed mice, and carried antigens common to FeLV and MuLV. Although virus particles were not seen in these cultures, 68S RNA was detected in their media. Medium from nontransformed parent cultures also contained 68S RNA but in amounts about 15 times less than in transformed cultures. Transformed human cells passaged in mice produced both type C virus particles and 68S RNA. Antigens common to MuLV and FeLV were found in these particles. However, the results of biological and serological studies indicate their difference from conventional MuLV and FeLV. The relationship of this virus and 68S RNA found in transformed cultures remains to be determined.

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.

Feline leukemia virus-associated myelopathy in cats

Feline leukemia virus (FeLV) infection is associated with distinct neoplastic, hematologic, and immunosuppressive diseases. Here we report on a novel neurologic syndrome in 16 cats infected with FeLV for more than 2 years. Clinical signs consisted of abnormal vocalization, hyperesthesia, and paresis progressing to paralysis. The clinical course of affected cats involved gradually progressive neurologic dysfunction invariably resulting in euthanasia. Microscopically, white-matter degeneration with dilation of myelin sheaths and swollen axons was identified in the spinal cord and brain stem of affected animals. Neither neoplastic nor hematologic diseases commonly associated with FeLV infection were present. Fungal and protozoal infection in one animal was suggestive of impaired immune competence. Immunohistochemical staining of affected tissues revealed consistent expression of FeLV p27 antigens in neurons, endothelial cells, and glial cells. Furthermore, proviral DNA was amplified from multiple sections of spinal cord as well as intestine, spleen, and lymph nodes. These findings suggest that in a proportion of chronically FeLV-infected cats, a virus evolved with cytopathic potential for cells in the central nervous system.

The FeLV-945 LTR confers a replicative advantage dependent on the presence of a tandem triplication

Feline leukemia virus (FeLV), like other naturally occurring retroviruses, is characterized by a high degree of genetic diversity. FeLV-945 is a natural isolate derived from non-B-cell non-T-cell lymphomas classified anatomically as multicentric. FeLV-945 exhibits a unique structural motif in the LTR composed of a 21-bp tandem triplication downstream of a single copy of enhancer. The unique FeLV-945 LTR is precisely conserved among eight independent multicentric lymphomas collected in a geographic cluster. Previous studies using reporter gene constructs predict that the FeLV-945 LTR would confer a replicative advantage on the virus that contains it, particularly in primitive hematopoietic cells. Such an advantage may account for the precise conservation of the unique LTR sequence. To test that prediction, a set of recombinant, infectious FeLVs was developed that are isogenic other than the presence of the FeLV-945 LTR or mutations of it. Replication assays show that the FeLV-945 LTR confers a distinct growth advantage in K-562, FEA, and 3201 cells and implicate the 21-bp triplication in that function. Replacement of two copies of the triplicated element with random sequence greatly diminished the replicative capacity, thus implicating the triplicated sequence itself in LTR function. The 21-bp triplication was shown to contain specific nuclear protein binding sites, which may account for the selective pressure to conserve the sequence.

Retrovirus receptor PiT-1 of the Felis catus

We isolated a cDNA encoding a feline homolog of human PiT-1, a sodium-dependent phosphate symporter which is utilized by gibbon ape leukemia virus (GALV) as a receptor for entry into host cells. The overall homology between the human and feline receptors is 92 and 93% at the nucleotide and deduced amino acid levels, respectively. Hydropathy analyses implied ten potential membrane spanning regions and, in analogy to human and murine homologs, five extracellular and four intracellular loops. Strikingly, the amino acid sequence of the fourth extracellular loop, which is critical for GALV surface glycoprotein binding, has complete identity between the human and feline PiT-1s, while the mouse PiT-1, non-functional for GALV entry, is quite divergent. Ectopic expression of the feline PiT-1 in guinea pig cells, which are non-permissive to feline leukemia virus (FeLV), subgroup B virus, conferred susceptibility to FeLV-B infection confirming the functional ability of the cloned product to serve as a receptor for a natural retrovirus of the homologous species.

The host range and interference properties of two closely related feline leukemia variants suggest that they use distinct receptors

The proviral clones 61E and 61C represent two closely related variants of feline leukemia virus (FeLV) that exhibit significant differences in their biological and pathogenic properties. The major pathogenic determinant has been mapped to the extracellular envelope glycoprotein (Env-SU), but the mechanism by which envelope differences influence pathogenesis is not well understood. Moreover, it is unclear whether these viruses infect the same target cells and/or enter cells using the same receptor. In the present study, we exploited a recently developed single cycle infection assay to examine the host range and interference properties of 61E and 61C FeLVs and found that these two FeLV variants differ significantly in their host ranges and receptor usages. FeLV-61C was found to be an ecotropic virus; the entry of viruses bearing a 61C envelope protein (Env-SU) into cell lines was limited to feline T-cells and feline fibroblasts. In contrast, the host range of 61E includes, in addition to all feline cells examined, some canine, murine, and human cell lines. Feline fibroblast and feline T-cells that expressed 61E envelope were resistant to infection with a virus bearing a 61E Env-SU, whereas these same cells were susceptible to infection by an otherwise similar virus pseudotyped with the 61C Env-SU. This pattern of interference was observed in cells expressing 61E envelope alone, in the absence of other FeLV gene products, demonstrating that interference was mediated specifically by Env-SU. Fibroblast cells chronically infected with a 61C virus were partially resistant to infection with a virus having a 61C Env-SU, but were not resistant to infection by a virus having a 61E Env-SU. On the basis of the current understanding of virus-receptor interactions, the lack of interference between 61E and 61C under conditions where there is significant homologous interference, combined with the differences in their host cell range, leads us to conclude that 61E and 61C use two distinct primary cellular receptors for entry.

Isolation of a novel subgroup B feline leukemia virus from a cat infected with FeLV-A

Proviruses were cloned directly from a cat that developed neurological disorders approximately 28 months after inoculation with a molecularly cloned, minimally pathogenic subgroup A feline leukemia virus (FeLV-A). In addition to FeLV-A proviruses that were nearly identical to the inoculated virus, we detected a subgroup B-like variant in brain, bone marrow, and lymph node that apparently had acquired the major portion of its extracellular envelope gene (gp70) from endogenous FeLV-related sequences. A similar recombinant was also detected, by PCR, at low levels in bone marrow from an early time postinfection (2.5 months). A full-length proviral variant with this recombinant structure cloned from brain tissue encoded a replication-defective virus. A chimera encoding the 5' gag-pol portion of FeLV-A and the 3' env-LTR portion of the defective brain-derived clone was replication-competent and had the extended host range properties of FeLV-B.

Mutation of amino acids within the gibbon ape leukemia virus (GALV) receptor differentially affects feline leukemia virus subgroup B, simian sarcoma-associated virus, and GALV infections

The three type C retroviruses, gibbon ape leukemia virus (GALV), simian sarcoma-associated virus (SSAV), and feline leukemia virus subgroup B (FeLV-B), infect human cells by interacting with the same cell surface receptor, GLVR1. Using LacZ retroviral pseudotypes and murine cells transfected with mutant GLVR1 expression vectors, we show that the same 9-amino-acid region of human GLVR1 is critical for infection by the three viruses. Rat cells were not susceptible to infection by LacZ (FeLV-B) pseudotypes because of a block at the receptor level. We found multiple amino acid differences from human GLVR1 in the 9-amino-acid critical region of rat GLVR1. Expression of a human-rat chimeric GLVR1 in murine cells demonstrated that rat GLVR1 could function as a receptor for GALV and SSAV but not for FeLV-B. Substitution of human GLVR1 amino acids in the critical region of rat GLVR1 identified three amino acids as responsible for resistance to FeLV-B infection; two of these affect SSAV infection, but none affects GALV infection.

Interference with superinfection and with cell killing and determination of host range and growth kinetics mediated by feline leukemia virus surface glycoproteins

The functions of the surface glycoproteins (SU) of feline leukemia viruses (FeLVs) are of interest since these proteins mediate virus infection and interference and are critical determinants of disease specificity. In this study, we examined the biochemical and genetic determinants of SU important to virus entry and cell killing. In particular, we developed and used vesicular stomatitis virus (VSV)/FeLV pseudotype virus interference assays to determine interference subgroupings and assess mechanisms of host cell restriction. We also assessed roles of SU in virus growth kinetics and in the inhibition of cell killing caused by superinfection with cytopathic virus. Subgroup classification by VSV/FeLV pseudotype assay was in agreement with that defined previously by focus interference assay and was found to be determined by changes near the N terminus of SU for FeLV subgroups A (FeLV-A) and C. Virus host range restriction was found to be mediated at the level of virus entry in most cases, although postentry events mediated restriction in the failure of a subgroup A-like, T-cell cytopathic and immunodeficiency-inducing clone (FeLV-FAIDS-EECC) to replicate in feline fibroblasts. FeLV-FAIDS-EECC-induced cell killing was also inhibited by prior infection with one of two FeLV-A isolates. This inhibition could be conveyed by as few as four amino acid changes near the N terminus of the FeLV-A SU and also appeared to be mediated at a postentry level. Lastly, the SU-coding sequence was also found to determine differences in growth kinetics of viruses within the same subgroup. These studies demonstrate that subtle alterations in the FeLV SU, particularly in the N-terminal region, impart multiple significant functional differences which distinguish virus variants.

Protection of cats against feline leukemia virus by vaccination with a canarypox virus recombinant, ALVAC-FL

Two ALVAC (canarypox virus)-based recombinant viruses expressing the feline leukemia virus (FeLV) subgroup A env and gag genes were assessed for their protective efficacy in cats. Both recombinant viruses contained the entire gag gene. ALVAC-FL also expressed the entire envelope glycoprotein, while ALVAC-FL(dl IS) expressed an env-specific gene product deleted of the putative immunosuppressive region. Although only 50% of the cats vaccinated with ALVAC-FL(dl IS) were protected against persistent viremia after oronasal exposure to a homologous FeLV isolate, all cats administered ALVAC-FL resisted the challenge exposure. Significantly, protection was afforded in the absence of detectable FeLV-neutralizing antibodies. These results represent the first effective vaccination of cats against FeLV with a poxvirus-based recombinant vector and have implications that are relevant not only to FeLV vaccine development but also to developing vaccines against other retroviruses, including human immunodeficiency virus.

Pathogenic and host range determinants of the feline aplastic anemia retrovirus

Feline leukemia virus (FeLV) C-Sarma (or FSC) is a prototype of subgroup C FeLVs, which induce fatal aplastic anemia in outbred specific-pathogen-free (SPF) cats. FeLV C isolates also possess an extended host range in vitro, including an ability, unique among FeLVs, to replicate in guinea pig cells. To identify the viral determinants responsible for the pathogenicity and host range of FSC we constructed a series of proviral DNAs by exchanging gene fragments between FSC and FeLV-61E (or F6A), the latter of which is minimally pathogenic and whose host range in vitro is restricted to feline cells. Transfer of an 886-base-pair (bp) fragment of FSC, encompassing the codons for 73 amino acids at the 3' end of pol (the integrase/endonuclease gene) and the codons for 241 amino acids of the N-terminal portion of env [the extracellular glycoprotein (gp70) gene], into the F6A genome was sufficient to confer onto chimeric viruses the ability to induce fatal aplastic anemia in SPF cats. In contrast, no chimera lacking this sequence induced disease. When assayed in vitro, all chimeric viruses containing the 886-bp fragment of FSC acquired the ability to replicate in heterologous cells, including dog and guinea pig cells. Thus, the pathogenic and the host range determinants of the feline aplastic anemia retrovirus colocalize to a 3' pol-5' env region of the FSC genome and likely reside within a region encoding 241 amino acid residues of the N terminus of the extracellular glycoprotein.

Experimental transmission and pathogenesis of immunodeficiency syndrome in cats

We describe the identification, experimental transmission, and pathogenesis of a naturally occurring powerfully immunosuppressive isolate of feline leukemia virus (designated here as FeLV-FAIDS) which induces fatal acquired immunodeficiency syndrome (AIDS) in 100% (25 of 25) of persistently viremic experimentally infected specific pathogen-free (SPF) cats after predictable survival periods ranging from less than 3 months (acute immunodeficiency syndrome) to greater than one year (chronic immunodeficiency syndrome), depending on the age of the cat at time of virus exposure. The pathogenesis of FeLV-FAIDS-induced feline immunodeficiency disease is characterized by: a prodromal period of largely asymptomatic viremia; progressive weight loss, lymphoid hyperplasia associated with viral replication in lymphoid follicles, lymphoid depletion associated with extinction of viral replication in lymphoid follicles, intractable diarrhea associated with necrosis of intestinal crypt epithelium, lymphopenia, suppressed lymphocyte blastogenesis, impaired cutaneous allograft rejection, hypogammaglobulinemia, and opportunistic infections such as bacterial respiratory disease and necrotizing stomatitis. The clinical onset of immunodeficiency syndrome correlates with the replication of a specific FeLV-FAIDS viral variant, detected principally as unintegrated viral DNA, in bone marrow, lymphoid tissues, and intestine. Two of seven cats with chronic immunodeficiency disease that survived greater than 1 year after inoculation developed lymphoma affecting the marrow, intestine, spleen, and mesenteric nodes. Experimentally induced feline immunodeficiency syndrome, therefore, is a rapid and consistent in vivo model for prospective studies of the viral genetic determinants, pathogenesis, prevention, and therapy of retrovirus-induced immunodeficiency disease.

Feline leukaemia vaccine protection against viral latency

Seventeen cats, which were previously vaccinated with a subunit, feline leukaemia vaccine (Leukocell) and subsequently challenged with virulent feline leukaemia virus (FeLV), were tested at 2 to 4 years postchallenge for reactivation of latent FeLV infections. Administration of weekly doses of methylprednisolone induced significant decreases in lymphocyte numbers, but did not reactivate virus in bone marrow cultures from 15 cats in vivo or in vitro. These cats were observed to be neither persistently or latently viraemic prior to corticosteroid administration. The results of this study indicate that the vaccine is effective in affording significant protection against latent FeLV infections, even after severe immunosuppression. This finding, coupled with previously published results indicating protection against persistent viraemia and tumour formation, makes this vaccine highly effective in protecting against FeLV infections and associated disease.

Characterization of a newly established feline lymphoma-derived cell line (BKD) lacking T and B cell surface markers

A new feline lymphoma-derived cell line, designated BKD, was isolated from an anterior mediastinal tumor. Cells of this line were characterized as lymphoid based on morphology, the lack of intracellular esterase and peroxidase activity, and absence of phagocytic function. In contrast with other established feline lymphoma-derived cell lines, cells of the BKD line lack characteristics of both feline T-cells and B-cells in that they neither form rosettes with guinea pig erythrocytes nor have demonstrable surface or cytoplasmic immunoglobulin. Approximately one third of BKD cells form EAC rosettes, a significant number of rosette forming cells (p = 0.0001) when compared to background sheep E-rosetting activity. In addition, a consistently titratable level of interleukin-2-like activity was produced when BKD cells were coincubated with concanavalin A and phorbol-12-myristate-13-acetate. Chromosome analysis showed that a majority of BKD cells are diploid. This new cell line has been continuously replicating in culture for over one year and produces feline leukemia virus as demonstrated by several analyses.

Remission of leukemia and loss of feline leukemia virus in cats injected with Staphylococcus protein A: association with increased circulating interferon and complement-dependent cytotoxic antibody

We have injected purified Staphylococcus aureus protein A intraperitoneally into leukemic cats infected with feline leukemia virus, into cats persistently infected with feline leukemia virus but without hematologic or cytologic abnormalities, and into healthy cats without feline leukemia virus infection. Pre- and post-treatment serum samples were evaluated sequentially for interferon activity and for complement-dependent cytotoxic antibody. Our results indicate that serum interferon increased dramatically (less than 3 to 324 units/ml) during treatment only in cats that responded to staphylococcal protein A therapy. Increase of interferon preceded or was closely associated with increasing levels of cytotoxic antibody, loss of viremia, and correction of cytological and hematological abnormalities of three leukemic cats. The cytotoxic antibody was shown to be specific for envelope glycoprotein gp70 of the feline leukemia virus. One persistently feline leukemia virus-infected cat without leukemia that became nonviremic also developed high levels of interferon and specific cytotoxic antibody. By contrast, interferon levels of cats not responding to treatment had levels of less than 3 to 27 units/ml. Normal healthy cats injected with staphylococcal protein A showed moderate transient increases of interferon but no detectable cytotoxic antibodies to FL-74 cells. These data suggest that interferon and cytotoxic antibody may play important, possibly complementary roles in inducing remission of leukemia and loss of viremia in cats treated with staphylococcal protein A.

Reactivation of latent feline leukaemia virus infection

In most cats exposed to the contagious feline leukemia virus (FeLV), viral replication is contained in target haematopoietic tissues and elicits humoral immunity to FeLV and to the feline oncornavirus-associated cell membrane antigen (FOCMA). Recently, we and others have considered that these ostensibly self-limiting infections might be persistent nonproductive (latent) infections in certain haematopoietic cells. This hypothesis could account for the relapsing viraemias, protracted incubation periods, persistently high titres of antiviral and anti-FOCMA antibodies, appearance of FeLV p27 antigen in serum of otherwise non-viraemic animals and occurrence of FeLV-negative but FOCMA-positive leukaemias in naturally infected pet cats. Here we describe the reactivation of latent FeLV from myelomonocytic and lymphoid cells of cats immune to FeLV, cats bearing FeLV-negative tumours, and kittens congenitally exposed to FeLV. Furthermore, the reappearance of FeLV infection is suppressed by the Host's immune system but his can be altered by adrenal corticosteroid hormones in vivo and in vitro.

Selective host range restriction of goat cells for recombinant murine leukemia virus and feline leukemia virus type A

We isolated a strain of normal goat fibroblasts which was uniquely selective in that it allowed the replication of xenotropic murine leukemia virus but not polytropic recombinant murine leukemia virus. In addition, feline leukemia virus type A replication was severely diminished in these goat cells, whereas feline leukemia virus type B and feline endogenous RD114-CCC viruses replicated efficiently. No other known cells exhibit this pattern of virus growth restriction. These goat cells allow the study of xenotropic murine leukemia virus in mixtures which also contain recombinant murine leukemia virus and may be helpful in eliminating feline leukemia virus type which often coexists in feline sarcoma or leukemia virus mixtures with other feline leukemia virus types.

Feline leukemia virus: survival under home and laboratory conditions

Feline leukemia virus maintained its infectiousness in cell culture medium at 37 degrees C or lower for at least 48 h. However, virus in saliva or medium was inactivated within a few hours if allowed to dry.

Differential growth and transmission in cats of feline leukaemia viruses of subgroups A and B

The outcome of infecting cats with FeLV of sub-groups A (FeLV-A) or B (FeLV-B) was different. After FeLV-A infection, virus was quickly recovered from the blood and oropharynx of most animals. Following infection with FeLV-B, only a small proportion of cats developed a viraemia, and this after a long interval. There was no evidence that FeLV-B was transmitted by contact. The result of infecting cats with mixtures of FeLV-A and B (FeLV-AB) was that to a large extent each virus operated independently. FeLV-A was recovered from the plasma first, in a high proportion of the cats, and FeLV-B appeared later but not in all cats. There was evidence of interaction, however, in that the proportion of cats which were viraemic with FeLV-B was greater following FeLV-AB infection than after infection with FeLV-B alone; also FeLV-B was transmitted by contact from cats excreting FeLV-AB. These results help to explain the apparent dependence of FeLV-B on FeLV-A under natural conditions and the frequency of occurrence of sugroups A and B in FeLV isolates.

Deposition of retrovirus associated antigens (p30 and gp70) on cell membranes of feline and murine leukaemia virus infected cells

A quantitative estimation of retrovirus associated cell membrane antigens of murine and feline cells infected with their respective type C leukosis virus is presented. Using a radio-immune assay with three broadly reactive antisera, the minimum estimated number of retrovirus associated antigenic determinants on YAC [Moloney leukaemia virus (MuLV) infected murine] and FL-74 [feline leukaemia virus (FeLV) infected feline] cells was 1.3 x 10(6) and 1.6 x10(6) determinants per cell respectively. The virus structural proteins p27-30 and gp70 were detected by three component specific antisera on murine and feline cell surfaces in amounts which varied between cell isolates. MuLV infected cells produced as many as 1.9 x 10(5) p30 antigenic determinants and 7.5 x 10(5) gp70 determinants on infected cells. FeLV infected cells (FL-74) expressed 5.6 x 10(5) p27 and 7.5 x 10(5) gp70 antigenic determinants per single cell surface. The major core protein (p27-30) and the major envelope glycoprotein (gp70) antigens are sufficiently physically separated on cell surfaces so that binding of either of the membrane antigens with component specific antibodies does not interfere with binding of antibodies specific for the other. Despite the expression of interspecies determinants for p30, gp70, and other retrovirus associated antigens detected by antibody procedures, interspecies determinants of cell mediated immunity could not be demonstrated in immune mice bearing Moloney sarcoma virus (MSV) induced tumours. Furthermore, xenogeneic immunization of mice with FL-74 cells failed to protect mice against the growth of MSV induced lymphoma or sarcoma.

Effect of helper virus on the number of murine sarcoma virus DNA copies in infected mammalian cells

Cell lines of four mammalian species were each examined for the number of Moloney murine sarcoma virus (M-MSV) DNA copies in total cellular DNA after M-MSV transformation. Sarcoma-positive, leukemia-negative (S+L-) M-MSV-transformed cells were compared to M-MSV-transformed cells infected with a replicating leukemia virus. Both unfractionated M-MSV complementary DNA and complementary DNA representing the MSV-specific and the MSV-murine leukemia virus-common regions of the M-MSV genome were hybridized to total cellular DNA of various species. DNAs of mouse, cat, dog, and human S+L-cells contained from less than one to a few proviral M-MSV DNA copies per haploid genome. In contrast, helper virus-coinfected, M-MSV-producing cells of each species showed a 3- to 10-fold increase in M-MSV proviral DNA over that found in corresponding S+L- cells. MSV-specific and MSV-murine leukemia virus-common nucleotide sequences were each increased to a similar degree. A corresponding examination of cellular DNA of leukemia virus-infected normal or S+L- mammalian cells was performed to establish the resulting number of leukemia proviral DNA copies. The infection of normal or S+L- mammalian cells with several leukemia-type viruses that did not have nucleotide sequences closely related to the cell before infection resulted in the appearance of one to three corresponding leukemia proviral DNA copies.

Feline oncornavirus-associated cell membrane antigen: expression in transformed nonproducer mink cells

The feline oncornavirus-associated cell membrane antigen (FOCMA) is a target for naturally occurring immunity that protects the cat against development of fibrosarcoma and leukemia. Feline sarcoma virus-transformed "nonproducer" mink cells express high levels of FOCMA, but not the major viral structural proteins. Transformation of the same cells by murine sarcoma virus, or infection with feline leukemia virus, which is nontransforming for epithelial or fibroblastic cells, did not induce FOCMA. Thus, FOCMA expression in mind lung cells is specifically associated with transformation by feline sarcoma virus.

Isolation of feline leukemia virus from clinical specimens

Specimens obtained from feline leukemia virus (FeLV)-positive cats were examined for infectious FeLV. Feline leukemia virus was detected by a focus-forming assay and confirmed by florescent antibody. Techniques of sample processing were evaluated and adjusted for optimum detection of FeLV. Low levels of FeLV were detected in 2 of 10 oral samples; however, the majority of these samples (17 of 27 tested) produced cytopathic effects in tissue culture which prevented Fe LV detection. Three of 24 urine samples and 1 of 20 rectal specimens were positive for FeLV. One milk sample contained high levels of FeLV.

Bvr-1, a restriction locus of a type C RNA virus in the feline cellular genome: identification, location, and phenotypic characterization in cat X mouse somatic cell hybrids

Somatic cell hybrids were constructed between BALB/c-RAG mouse cells and feline lymphoma cells by the hypoxanthine-aminopterin-thymidine selection scheme. RAG cells spontaneously produce an endogenous B-tropic type C virus. Cat-mouse hybrids preferentially segregate feline chromosomes and retain murine chromosomes-demonstrable by karyotypic and isozyme analyses. Despite the presence of the complete mouse genome, including the viral genome, virus production was diminished to 1-5% of the levels observed in RAG parents based upon particle-associated RNA-dependent DNA polymerase (reverse transcriptase) activity in the culture fluid. Thirty-seven hybrids made on four different occasions had suppressed virus levels, and no hybrids expressed parental virus levels. Reverse selection experiments on 6-thioguanine demonstrated that a restriction gene, tentatively named Bvr-1, was linked to the feline structural genes for hypoxanthine phosphoribosyltransferase (IMP:pyrophosphate phosphoribosyltransferase; EC 2.4.4.8) and glucose-6-phosphate dehydrogenase (D-glucose-6-phosphate: NADP+ 1-oxidoreductase; EC 1.1.1.49) in cats, probably on the X-chromosome. The genetic mode of action of Bvr-1 is trans dominant in restriction of murine leukemia virus. The restriction locus results in a block late in virus maturation but prior to release, since expression of antigens for viral structural proteins and matrue budding particles is apparent on surfaces of restriced hybrid cells but not in high-speed pellets from culture fluid of restricted cells.

An improved assay for feline leukaemia virus pseudotypes of murine sarcoma virus

An assay is described for feline leukaemia virus pseudotypes of murine sarcoma virus which increased the virus titre by about 100-fold over conventional assays. The titre is independent of dilution and no secondary focus formation occurs. The assay may be used to study virus neutralization and to detect and type feline leukaemia virus in feline embryo cells by interference.

Influence of culture conditions of growth of FL-74 cells and feline oncornavirus cell membrane associated antigen production

The FL-74 cell, a feline lymphoblastoid cell line derived from a tumor induced by leukemia virus, grows equally well in static suspension culture (plastic T-flask or silicone treated glass bottles) or in spinner culture. No growth was observed in unsiliconized glass bottles. Although feline leukemia virus production was nearly the same in FL-74 grown in each of the above types of vessel, the expression of the feline oncornavirus membrane associated antigen (FOCMA), as determined by membrane immunofluorescence, was more intense and more complete on cells grown in static suspension. Moreover, higher fluorescent antibody titer endpoints were observed with cells from static suspension cultures than with cells from spinner cultures, FL-74 cells grown in spinner culture, when subjected to partial synchrony by cold block or by deprivation of essential amino acids (arginine and/or isoleucine) for 12 hr, achieved a membrane fluorescent pattern for FOCMA similar to cells grown in static suspension. It is proposed that the expression of FOCMA on the cell membrane surface is cell-cycle dependent, and that the rate at which a cell passes through the cell cycle determines the pattern and intensity of the fluorescence of the cell membrane.

Establishment of two canine sarcoma cell lines: productive infection with feline leukemia virus

Two canine sarcoma cell lines (11028, 11031) were established in vitro and have been transferred 213 and 306 times, respectively, since 1970. These cell lines had a chromosome pattern consistent with their canine origin. Both cultures were infected with feline leukemia virus (FeLV), which caused a morphologic and karyotypic changes. The cells became rounded after infection with FeLV and both cultures showed the presence of a single, large, acrocentric chromosome considered to be a marker chromosome. All tumors were transplanted into newborn beagle pups, but only the 11028-FeLV formed metastatic tumors. No helper or focus-forming activity or virus particles were found in the uninfected cultures. Helper activity and virus were demonstrated in both sarcoma lines after infection with FeLV, though no focus-forming activity was noted. Helper activity of progeny virus could be assayed on either cat or dog embryo cells.

An unusual case of feline leukemia and an associated syncytium-forming virus

A 3-year-old female Siamese cat was admitted to a local animal hospital with a history of recent extreme lethargy and anorexia. A hemorrhagic tumor was removed from an area of oral buccal skin and diagnosed histopathologically as lymphosarcoma. Rapid physical deterioration occurred, and the cat became moribund 2 weeks after surgical operation. Necropsy revealed at least 200 spherical hemorrhagic neoplastic nodules attached to the omentum, mesentery, and peritoneum. Examination of histopathologic sections confirmed the striking characteristics of an extremely vascular and highly invasive malignant lymphoma, which was designated feline tumor No. 01 (FeT-01). There was no evidence of peripheral blood leukemia. Electron microscopic examination of tumor tissue revealed numerous viral particles having characteristics common to both feline leukemia virus (FeLV) and feline syncytium-forming virus (FeSFV). Primary cells and cultures propagated from tumor tissue were found to be negative or weakly positive for group-specific (gs) antigen by radioimmunoassay but strongly positive when assayed by indirect immunofluorescence. Co-cultivation of cells from tumor tissue, with normal prescreened feline embryo cells, revealed the presence of numerous FeSFV-like viral particles in the absence of C-type virus. A FeSFV was isolated from these passaged cells, with characteristics similar to FeSFV isolates previously described in the literature. The apparent presence of FeSFV in lymphosarcomatous tissue and the apparent absence of FeLV C-type particles in passaged cells indicate the need to make a more intensive study of the FeSFV group of viruses and the possible etiologic relationship to feline malignancies.

Feline leukaemia virus and its clinical effects in cats

Feline leukemia virus (FeLV) infection is common among cats where contact is high. The virus can be transmitted readily between cats. It causes a variety of haemopoietic and lymphoid neoplasms; the most common types are alimentary, multicentric and thymic lymphosarcoma and lymphatic leukaemia. The virus is involved in the aetiology of certain other diseases including anaemia, glomerulonephritis and an immunosuppressive syndrome which predisposes cats to intercurrent infections. Many infected cats mount an immune response and do not suffer from any of these. The immune status is shown by serum antibody levels to feline leukaemia virus associated cell membrane antigens. Cats with a titre of 32 or more are most unlikely to suffer any ill effects and may eliminate the virus infection. The outcome of infection in an individual cat depends on the immunological competence of the cat, the dose of virus received and its ability to induce immunosuppression. FeLV infection can be detected by examination of tissues by electron microscopy, and by culture of virus from plasma and other tissues. In the United States, a method is now in use for the detection of leukaemia virus antigen in peripheral blood leukocytes; this is carried out on ordinary blood films. Successful prototype vaccines have been developed against FeLV. This paper describes the natural history of the virus, the diseases in which it is implicated and discusses recently developed diagnostic methods.

RD 114 virus-specific sequences in feline cellular RNA: detection and characterization

RNA extracted from cat cells contains sequences homologous to RD-114 viral RNA. The sequences are measured by molecular hybridization with a single-stranded DNA probe synthesized by the virion polymerase using the endogenous viral RNA as template. Viral-specific RNA has been detected in all cells of cat origin tested thus far, but not in cells of other animals, except for the virus-producing human rhabdomyosarcoma cell, RD-114. The extent of hybridization of the DNA probe to cellular RNA was equivalent to that obtained with viral 70S RNA indicating that an equal extent of viral specific sequences is present in all cat cells as well as in RD-114 cells. The amounts of this viral RNA reach approximately 100 copies per cell in cat cells, while virus-producing RD-114 cells contain about 1,000 copies per cell. The viral RNA is present in cat cells in two distinct sizes of about 35S and 18S, whereas in RD-114 cells virus-specific RNA is quite heterogeneous in size.