Establishment of a feline T-lymphoblastoid cell line highly sensitive for replication of feline immunodeficiency virus

Inhibitory effect of stromal cell derived factor-1 on the replication of divergent strains of feline immunodeficiency virus in a feline T-lymphoid cell line

The effect of a CXC-chemokine, stromal cell derived factor-1 (SDF-1), on the replication of divergent strains of feline immunodeficiency virus (FIV) was examined in order to identify the mechanism of cell entry of FIV. A chemotaxis assay, using a modified Boyden chamber method, confirmed the biological activity of recombinant human (rh) SDF-1 for a feline T-lymphoid cell line (Kumi-1). The viral replication of FIV, as measured by the reverse transcriptase (RT) activity in the culture supernatant, was significantly suppressed by addition of rhSDF-1 in a dose-dependent manner in Kumi-1 cells. Furthermore, PCR analysis of the FIV proviral genome indicated that the inhibitory effect of rhSDF-1 on the replication of FIV in Kumi-1 cells was due to the inhibitory effect in the early event of replication. The inhibitory effect on viral replication by exogenous rhSDF-1 was shown for four divergent FIV isolates of subtypes A, B, and D in Kumi-1 cells.

Development of restriction fragment-length polymorphism method to differentiate five subtypes of feline immunodeficiency virus

Feline immunodeficiency virus (FIV) isolates have been classified into five subtypes (A to E) based on the sequences of the env variable V3 to V5 region. In this study, we sequenced a partial gag region of 4 and 3 isolates belonging to subtypes C and E, respectively. Phylogenetic analysis revealed that the branching pattern based on the region was similar to that based on the env V3 to V5 region. Here, we propose a protocol to differentiate five subtypes by polymerase chain reaction amplifying 329 bp within the region followed by restriction fragment-length polymorphism analysis using four restriction enzymes.

Isolation of feline parvovirus from peripheral blood mononuclear cells of cats in northern Vietnam

Feline parvovirus (FPV) was isolated rather frequently from the peripheral blood mononuclear cells (PBMCs) of cats in northern Vietnam by coculturing with MYA-1 cells (an interleukin-2-dependent feline T lymphoblastoid cell line) or Crandell feline kidney (CRFK) cells (a feline renal cell line). Efficiency of virus isolation was higher in MYA-1 cells than in CRFK cells. Interestingly, among the 17 cats from which FPV was isolated, 9 cats were positive for virus neutralizing (VN) antibody against FPV, indicating that FPV infected PBMCs and was not eliminated from PBMCs even in the presence of VN antibodies in the cats.

Feline immunodeficiency virus subtype C is prevalent in northern part of Taiwan

The seroepidemiological survey of cats conducted in northern part of Taiwan in 1998 revealed that the positive rate of feline immunodeficiency virus (FIV)-infection was 21.9% (7/32) and the rate was much higher than those of previous reports. We succeeded in isolation of three strains of FIV from peripheral blood mononuclear cells of the blood samples. Nucleotide sequences of the env variable V3 to V5 region of the strains revealed that the isolates from distinct areas belong to subtype C. These data together with our previous report (Inada et al. 1997. Arch. Virol., 142: 1459-1467) indicate that FIV subtype C is prevalent in northern part of Taiwan.

Suppression of feline immunodeficiency virus replication in vitro by a soluble factor secreted by CD8+ T lymphocytes

Mitogen-activated lymphoblasts isolated from the blood and lymph nodes, but not the spleen, of domestic cats acutely infected with the Petaluma or Glasgow8 isolates of feline immunodeficiency virus (FIV), suppressed the replication of FIV in the MYA-1 T-cell line in a dose-dependent manner. This effect was not limited to the homologous isolate of FIV. The suppressor activity declined with progression to chronic infection, with lower levels of activity detectable only in the lymph nodes. Immunization of domestic cats with whole inactivated FIV vaccine elicited profound suppressor activity in both the blood and lymph nodes. The suppressor activity was associated with the CD8+ T-cell subpopulation, the effect did not appear to be major histocompatibility complex-restricted, and was mediated by a soluble factor(s). This activity may be associated with the control of virus replication during both the asymptomatic stages of FIV infection, and in the protective immunity observed in cats immunized with whole inactivated virus vaccines.

Molecular cloning and expression of feline CD3epsilon

The cDNA of feline CD3epsilon, one of the T-cell receptor components, was cloned from a feline T-lymphoblastoid cell line (MYA-1 cells) and peripheral blood mononuclear cells and thymocytes of cats by polymerase chain reaction. Sequencing analysis revealed that the open reading frame of feline CD3epsilon consists of 606 base pairs encoding a predicted molecular mass of 25 kDa transmembrane protein which lacks N-glycosylation site. Comparison of the predicted amino acid sequence of feline CD3epsilon with those of other mammalians' homologues revealed that a relatively low homology was present in the extracellular domain. However, the cytoplasmic domain contained several characteristic motifs highly conserved across the species. These motifs were known to be important for signal transduction upon T-cell activation or endoplasmic reticulum retention. In addition, the feline CD3epsilon protein was expressed in an insect cell line (Sf9) by a baculovirus expression system. The expression was confirmed by indirect immunofluorescence assay and immunoblotting analysis using an anti-human CD3epsilon polyclonal antibody. These results will provide additional information for understanding the feline immune system.

DNA vaccination affords significant protection against feline immunodeficiency virus infection without inducing detectable antiviral antibodies

To test the potential of a multigene DNA vaccine against lentivirus infection, we generated a defective mutant provirus of feline immunodeficiency virus (FIV) with an in-frame deletion in pol (FIVDeltaRT). In a first experiment, FIVDeltaRT DNA was administered intramuscularly to 10 animals, half of which also received feline gamma interferon (IFN-gamma) DNA. The DNA was administered in four 100-microg doses at 0, 10, and 23 weeks. Immunization with FIVDeltaRT elicited cytotoxic T-cell (CTL) responses to FIV Gag and Env in the absence of a serological response. After challenge with homologous virus at week 26, all 10 of the control animals became seropositive and viremic but 4 of the 10 vaccinates remained seronegative and virus free. Furthermore, quantitative virus isolation and quantitative PCR analysis of viral DNA in peripheral blood mononuclear cells revealed significantly lower virus loads in the FIVDeltaRT vaccinates than in the controls. Immunization with FIVDeltaRT in conjunction with IFN-gamma gave the highest proportion of protected cats, with only two of five vaccinates showing evidence of infection following challenge. In a second experiment involving two groups (FIVDeltaRT plus IFN-gamma and IFN-gamma alone), the immunization schedule was reduced to 0, 4, and 8 weeks. Once again, CTL responses were seen prior to challenge in the absence of detectable antibodies. Two of five cats receiving the proviral DNA vaccine were protected against infection, with an overall reduction in virus load compared to the five infected controls. These findings demonstrate that DNA vaccination can elicit protection against lentivirus infection in the absence of a serological response and suggest the need to reconsider efficacy criteria for lentivirus vaccines.

Apoptosis in feline panleukopenia virus-infected lymphocytes

Feline panleukopenia virus (FPLV) was shown to induce apoptosis to feline lymphoid cells and to reduce the expression of interleukin-2 receptor alpha on the cells. FPLV-induced apoptosis might be a key element in the pathophysiology of atrophy of lymphoid tissues associated with feline panleukopenia caused by FPLV.

Modulation of feline immunodeficiency virus infection by stromal cell-derived factor

The alpha-chemokine receptor CXCR4 has recently been shown to support syncytium formation mediated by strains of feline immunodeficiency virus (FIV) that have been selected for growth in the Crandell feline kidney cell line (CrFK-tropic virus). Given that both human and feline CXCR4 support syncytium formation mediated by FIV, we investigated whether human stromal cell-derived factor (SDF-1) would inhibit infection with FIV. Human SDF-1alpha and SDF-1beta bound with a high affinity (K(D)s of 12.0 and 10.4 nM, respectively) to human cells stably expressing feline CXCR4, and treatment of CrFK cells with human SDF-1alpha resulted in a dose-dependent inhibition of infection by FIV(PET). No inhibitory activity was detected when the interleukin-2 (IL-2)-dependent feline T-cell line Mya-1 was used in place of CrFK cells, suggesting the existence of a CXCR4-independent mechanism of infection. Furthermore, neither the human beta-chemokines RANTES, MIP-1alpha, MIP-1beta, and MCP-1 nor the alpha-chemokine IL-8 had an effect on infection of either CrFK or Mya-1 cells with CrFK-tropic virus. Envelope glycoprotein purified from CrFK-tropic virus competed specifically for binding of SDF-1alpha to feline CXCR4 and CXCR4 expression was reduced in FIV-infected cells, suggesting that the inhibitory activity of SDF-1alpha in CrFK cells may be the result of steric hindrance of the virus-receptor interaction following the interaction between SDF and CXCR4. Prolonged incubation of CrFK cells with SDF-1alpha led to an enhancement rather than an inhibition of infection. Flow cytometric analysis revealed that this effect may be due largely to up-regulation of CXCR4 expression by SDF-1alpha on CrFK cells, an effect mimicked by treatment of the cells with phorbol myristate acetate. The data suggest that infection of feline cells with FIV can be mediated by CXCR4 and that, depending on the assay conditions, infection can be either inhibited or enhanced by SDF-1alpha. Infection with FIV may therefore prove a valuable model in which to study the development of novel therapeutic interventions for the treatment of AIDS.

Molecular and immunophenotypical characterization of a feline immunodeficiency virus (FIV)-associated lymphoma: a direct role for FIV in B-lymphocyte transformation?

We describe the histopathological, immunohistochemical, and molecular characterization of a lymphoma arising in a 7-year-old cat following experimental infection with feline immunodeficiency virus (FIV). The tumor was high grade and of B-cell lineage. The transformed cell had an immature phenotype (CD79a+, CD79b-, CD21-, immunoglobulin heavy and light chain negative), confirmed by antigen receptor gene analysis, which showed germ line configuration. Single-copy, clonally integrated FIV provirus was detected in tumor genomic DNA. FIV p24 antigen was not detected in tumor cells by immunostaining. This study provides the first evidence that the feline lentivirus may play a direct role in cell transformation under certain circumstances.

Immunological and histological disorders in cats experimentally infected with feline immunodeficiency virus subtype B (TM2 stain)

Three conventional cats were experimentally infected with subtype B (TM2 strain) of FIV, and two conventional cats served as controls. The infected cats were examined immunologically 99-176 weeks post FIV inoculation (wpi) and histologically at 130 wpi. Two of the three infected cats exhibited lower CD4/CD8 T cell ratios and hypergammaglobulinemia compared with two control cats. Further, all the infected cats showed morphological changes in popliteal lymph nodes such as lymphoid depletion, atrophy and plasma cell hyperplasia. In addition, apoptosis was induced in peripheral blood mononuclear cells (PBMC) from the FIV-infected cats after in vitro culture for one or two days, but not in PBMC from uninfected cats. These observations indicate that FIV subtype B has the potential to induce some immunological and histological disorders in cats.

Vaccination with a feline immunodeficiency virus multiepitopic peptide induces cell-mediated and humoral immune responses in cats, but does not confer protection

Cats were immunized with a 46-residue multiepitopic synthetic peptide of feline immunodeficiency virus (FIV) comprising immunodominant epitopes present in the third variable domain of the envelope glycoprotein, transmembrane glycoprotein (TM), and p24 Gag core protein, using Quil A as an adjuvant. All vaccinated cats developed a humoral response which recognized the synthetic peptide immunogen and the intact viral core and envelope proteins. A FIV Gag- and Env-specific effector cytotoxic T-lymphocyte response was also detected in the peripheral blood of vaccinated cats, which peaked at week 30. This response appeared to be major histocompatibility complex restricted. Epitope mapping studies revealed that both the cellular and humoral immune responses were directed principally to a peptide within the TM glycoprotein, CNQNQFFCK. However, vaccination did not confer protection when cats were challenged with the Petaluma isolate of FIV at week 35.

The effects of treatment with chemical agents or infection with feline viruses on protein-binding properties of the feline immunodeficiency virus long terminal repeat

The effects of treatment with chemical agents or infection with feline viruses on protein-binding properties of the feline immunodeficiency virus (FIV) long terminal repeat (LTR) were examined by gel-mobility-shift assays using oligonucleotides designed to represent putative AP-1 or ATF motif from the FIV LTR. Infection with FIV led to less nuclear proteins binding to the AP-1 and ATF sites, suggesting that proteins binding to the sites were consumed or suppressed by FIV-replication in FIV-infected cells. Nuclear proteins that bind to the AP-1 or ATF site were examined by using extracts from Crandell feline kidney (CRFK) cells treated with TPA (a phorbol ester; a strong activator of protein kinase C) or forskolin (an inducer of cyclic-AMP), or infection with feline herpesvirus type 1 (FHV-1). Although TPA or forskolin treatment moderately increased the level of both proteins that bound to AP-1 and ATF sites, FHV-1 infection markedly changed the protein-binding patterns of the sites. Furthermore, FHV-1-induced proteins that bind adjacent to the transcriptional initiation site of FIV promoter were also observed in FHV-1-infected CRFK cells, suggesting that the FHV-1-induced-proteins affects the transcription of FIV through the AP-1, ATF and leader sequences.

Shared usage of the chemokine receptor CXCR4 by the feline and human immunodeficiency viruses

Feline immunodeficiency virus (FIV) induces a disease state in the domestic cat that is similar to AIDS in human immunodeficiency virus (HIV)-infected individuals. As with HIV, FIV can be divided into primary and cell culture-adapted isolates. Adaptation of FIV to replicate and form syncytia in the Crandell feline kidney (CrFK) cell line is accompanied by an increase in the net charge of the V3 loop of the envelope glycoprotein, mirroring the changes observed in the V3 loop of HIV gp120 with the switch from a non-syncytium-inducing phenotype to a syncytium-inducing phenotype. These data suggest a common mechanism of infection with FIV and HIV. In this study, we demonstrate that cell culture-adapted strains of FIV are able to use the alpha-chemokine receptor CXCR4 for cell fusion. Following ectopic expression of human CXCR4 on nonpermissive human cells, the cells are able to fuse with FIV-infected feline cells. Moreover, fusion between FIV-infected feline cells and CXCR4-transfected human cells is inhibited by both anti-CXCR4 and anti-FIV antibodies. cDNAs encoding the feline CXCR4 homolog were cloned from both T-lymphoblastoid and kidney cell lines. Feline CXCR4 displayed 94.9% amino acid sequence identity with human CXCR4 and was found to be expressed widely on cell lines susceptible to infection with cell culture-adapted strains FIV. Ectopic expression of feline CXCR4 on human cells rendered the cells susceptible to FIV-dependent fusion. Moreover, feline CXCR4 was found to be as efficient as human CXCR4 in supporting cell fusion between CD4-expressing murine fibroblast cells and either HIV type 1 (HIV-1) or HIV-2 Env-expressing human cells. Previous studies have demonstrated that feline cells expressing human CD4 are not susceptible to infection with HIV-1; therefore, further restrictions to HIV-1 Env-dependent fusion may exist in feline cells. As feline and human CXCR4 support both FIV- and HIV-dependent cell fusion, these results suggest a close evolutionary link between FIV and HIV and a common mechanism of infection involving an interaction between the virus and a member of the seven-transmembrane domain chemokine receptor family of molecules.

Comparative efficiency of feline immunodeficiency virus infection by DNA inoculation

Direct inoculation of genetic material in DNA form is a novel approach to vaccination that has proved efficacious for a number of viral agents. We are interested in the potential of this approach for the delivery of vaccines based on attenuated or replication-defective retroviruses. Toward this goal, we tested the effect of intramuscular inoculation of a plasmid containing the entire genome of feline immunodeficiency virus (FIV-Petaluma, F14 clone). DNA delivery was compared with intramuscular or intraperitoneal inoculation of virus reconstituted from the same molecular clone. The outcome was monitored by serological analysis and quantitative virus load determination over a 31-week period. DNA inoculation was found to be a reliable means of infection, although seroconversion and the rise in PBMC virus load were delayed relative to intramuscular or intraperitoneal inoculation of virus. At 31 weeks, similar levels of proviral DNA were detected in central lymphoid tissue of all infected animals. In conclusion, DNA inoculation of proviral DNA will be of use as a novel method of cell-free virus challenge and may have further potential for the delivery of lentiviral vaccines.

Replication of feline syncytial virus in feline T-lymphoblastoid cells and induction of apoptosis in the cells

Feline syncytial virus (FSV) was isolated from feline peripheral blood mononuclear cells of FSV-seropositive cats. When the susceptibility of feline T-lymphocytes to FSV was examined using three strains of FSV, FSV antigens were detected in the FSV-infected T-lymphoblastoid cells. Further, a diversity of biological properties, including replication kinetics and syncytia formation, was noted among the strains, and condensation of chromatin and the fragmentation of cellular DNA were observed in the infected cells. From these data, we conclude that FSV is lymphotropic and can induce apoptosis in the lymphocytes.

Roles of the auxiliary genes and AP-1 binding site in the long terminal repeat of feline immunodeficiency virus in the early stage of infection in cats

To examine the roles of auxiliary genes and the AP-1 binding site in the long terminal repeat of feline immunodeficiency virus (FIV) in vivo, three mutant viruses, which are defective in the vif gene ([delta]vif), ORF-A gene (deltaORF-A), and AP-1 binding site (deltaAP-1), and wild-type virus as a positive control were separately inoculated into three specific-pathogen-free cats. These cats were assessed by measuring the number of proviral DNA copies in peripheral blood mononuclear cells (PBMCs), the CD4/CD8 ratio and antibody responses to FIV for 16 weeks and then examining histological changes at necropsy. Although viral DNAs were detected in PBMCs from all 12 cats to various degrees until 16 weeks postinoculation, no virus was recovered from PBMCs of cats infected with (delta)vif virus during the observation period. However, a very weak antibody response was induced in one cat infected with the (delta)vif virus. In contrast, despite the successful recovery of virus from both groups of cats infected with deltaORF-A and deltaAP-1 virus, antibody responses and decrease in the CD4/CD8 ratio in the groups were milder than those in cats infected with wild-type virus. Furthermore, the numbers of proviral DNA copies in PBMCs from the two groups were not able to reach the level in cats infected with wild-type virus during the observation period. From these results, we conclude that these mutant viruses are still infectious for cats but failed in efficient viral replication and suggest that these auxiliary genes and enhancer element are important or essential to full viral replication kinetics and presumably to full pathogenicity during the early stage of infection in vivo.

Feline immunodeficiency virus vaccination: characterization of the immune correlates of protection

Whole inactivated virus (WIV) vaccines derived from the FL4 cell line protect cats against challenge with feline immunodeficiency virus (FIV). To investigate the correlates of protective immunity induced by WIV, we established an immunization regimen which protected a proportion of the vaccinates against challenge. A strong correlation was observed between high virus neutralizing antibody titers and protection following challenge. To investigate further the immune mechanisms responsible for immunity, all of the vaccinates were rechallenged 35 weeks following the initial challenge. Results of virus isolation from peripheral blood mononuclear cells indicated that 9 of 10 vaccinates were protected from viremia following the second challenge, suggesting that vaccine-induced immunity to FIV persisted for at least 8 months. However, more stringent analysis for evidence of infection revealed that 5 of 10 vaccinates harbored virus in lymphoid tissues. Unlike the protection observed immediately following vaccination, which correlated positively with virus neutralizing antibody titer, the ability to resist a second challenge with FIV was more closely correlated with the induction of Env-specific cytotoxic T-cell activity. The results indicate that both virus-specific humoral immunity and cellular immunity play a role in the protection induced in cats by WIV immunization but their relative importance may be dependent on the interval between vaccination and exposure to virus.

A longitudinal study of feline immunodeficiency virus-specific cytotoxic T lymphocytes in experimentally infected cats, using antigen-specific induction

The evolution of the virus-specific cytotoxic T-lymphocyte response in two cats experimentally infected with feline immunodeficiency virus (FIV) was monitored. Effector cells were derived from peripheral blood lymphocytes during the acute and chronic phases of infection (0 to 21 and 62 to 127 weeks, respectively) and from the spleen and lymph nodes at 127 weeks after infection. Lymphocytes were restimulated in vitro with paraformaldehyde-fixed, autologous lymphoblasts which had been infected with recombinant vaccinia viruses expressing FIV GAG or ENV proteins. Unstimulated lymphocytes were also used as effectors in some assays. 51Cr-labelled autologous skin fibroblasts infected with recombinant vaccinia viruses were used as targets. FIV GAG-specific cytotoxic precursors were detected in restimulated circulating lymphocytes during acute infection in both cats. The onset of this activity was as early as 2 weeks postinfection (p.i.) in one cat. From 62 weeks p.i. neither FIV GAG- nor ENV-specific precursors could be detected in the peripheral blood. However, at 127 weeks p.i., GAG- and ENV-specific cytotoxic precursors were detected in lymphocytes isolated from lymph nodes. The FIV-specific cytotoxic cells were predominantly major histocompatibility complex class I restricted. No cytotoxic activity was detected from unstimulated lymphocytes. These studies demonstrate the use of an assay system for dissecting the FIV-specific cytotoxic cell response and show that precursor cells appear in the circulation very early after infection and prior to a detectable antibody response. Our results also suggest that the persistent high-level circulating antiviral cytotoxic T-lymphocyte responses seen in human immunodeficiency virus-infected humans may not be a feature of FIV infections in cats.

Detection of feline immunodeficiency virus transcripts by quantitative reverse transcription-polymerase chain reaction

We developed a quantitative reverse transcription-polymerase chain reaction (RT-PCR) procedure to estimate replication status of feline immunodeficiency virus (FIV) in peripheral blood mononuclear cells (PBMCs) of cats. Primers used for the RT-PCR were designed to detect only multiple spliced transcripts of FIV and allowed us to detect the specific transcripts with high specificity. By using limiting-cell-dilution RT-PCR, we demonstrated that the specific transcripts were quantitatively detected in a single infected cell in a background of 1 x 10(6) uninfected cells without Southern blot hybridization. Furthermore, the transcripts were observed efficiently in all PBMCs of the chronically FIV-infected cats when examined by this RT-PCR technique. There results demonstrated that this RT-PCR method is applicable for specific detection of the FIV-specific transcripts in the PBMCs and for estimation of the viral replication status in vivo.

Development of a simple, rapid and accurate in vitro whole blood technique for the detection and semi-quantification of FIV cellular viremia

A new, simple, rapid and accurate culture technique is described for a semi-quantitative analysis of cellular viremia in FIV-infected cats. This assay can be carried out with small amounts of whole blood, and is based on the detection of FIV core gag antigen, which is released in culture supernatants. The amount of core antigen produced is measured with an enzyme-linked immunoassay using specific monoclonal antibodies. This whole blood technique (WB method) was compared with a culture method using isolated peripheral blood mononuclear cells (PBMC method). FIV could be detected in whole blood of all experimentally infected cats, but not from uninfected cats. This assay offers a number of advantages (small blood samples required, no leukocyte separation and lymphocyte purification procedures) and its reproducibility is very good. It provides a convenient in vitro cellular assay for viral semi-quantitation, well adapted for monitoring efficacy of prototype FIV vaccines or experimental antiviral drugs. Also, it could facilitate the study of the pathogenesis of FIV-related progressive immunodepression. Finally, it offers an alternative to serological techniques for diagnostic purposes in several circumstances: early viremia, maternal antibodies.

Poly A-linked non-isotopic microtiter plate reverse transcriptase assay for sensitive detection of clinical human immunodeficiency virus isolates

A colorimetric reverse transcriptase assay (cRT assay) was developed for quantitative detection of HIV-1. In this format, reverse transcriptase incorporates biotin-labeled dUTP onto oligo-dT primers hybridized to poly A templates. The templates are covalently bound to the surface of microtiter wells. The amount of incorporated biotin-labeled dUTP is measured by binding horseradish peroxidase conjugated streptavidin, washing away unbound peroxidase, adding colorimetric substrate and then reading with a standard colorimetric reader. The sensitivity of the assay is very good. As little as 3 x 10(5) molecules of recombinant HIV-RT can be detected after 20 h of reaction time. Direct comparison using 3 cultured clinical isolates indicates that this level of detection is equivalent to the commercially available p24 antigen capture assay and the HIV-RNA assay based on branched DNA signal amplification. Other retroviruses, such as HIV-2 and feline immunodeficiency virus (FIV), can also be detected in this format. This non-isotopic assay is easy to perform and could provide a convenient and quantitative method for HIV study by monitoring reverse transcriptase, an essential activity in the infection process.

Feline immunodeficiency virus replicates in salivary gland ductular epithelium during the initial phase of infection

Feline immunodeficiency virus (FIV) antigen was detected by immunochemistry in salivary glands of cats experimentally inoculated with West Australian isolate T91. Six cats were inoculated subcutaneously with 1.0 ml of tissue culture supernatant fluid from a feline T-lymphoblastoid cell line (MYA-1) infected with T91. FIV antigens were detected in the interlobular ducts of the salivary gland of cats infected with FIV 2, 4 and 6 weeks previously. FIV antigen was not detected in the salivary glands of three FIV negative cats and one naturally infected cat. Further, FIV antigen was located only in interlobular duct epithelial cells. The distribution of FIV in the interlobular ducts confirms the important role of salivary glands as a major reservoir of FIV in the early phase of infection and strengthens suggestions that the salivary route is an important mode of transmission of FIV.

The feline lymphoid cell line MBM and its use for feline immunodeficiency virus isolation and quantitation

We report on the development of a feline T lymphoblastoid cell line obtained from the peripheral blood mononuclear cells (PBMC) of a specific pathogen free cat and designated MBM. The cells are pan-T+, CD4- and CD8- and remained interleukin-2-dependent and concanavalin A-dependent throughout the period of observation. MBM cells have proved at least as sensitive as fresh blasts to infection with cell-free stocks of three feline immunodeficiency virus (FIV) isolates. Upon infection, they exhibit a lytic cytopathic effect. Repeated attempts to establish a chronic infection have failed. Using a limiting cell dilution method, it has been shown that MBM cells may be more sensitive than fresh blasts as substrate for isolating FIV from the PBMC of infected cats. These studies have also shown that considerable individual variations exist in the virus loads present in the PBMC of naturally infected cats, and that load size does not appear to correlate with cat age, clinical status, CD4/CD8 ratio and titer of serum neutralizing antibody.

Phylogenetic analysis of the long terminal repeat of feline immunodeficiency viruses from Japan, Argentina and Australia

The nucleotide sequences of the long terminal repeat of five Japanese, five Argentine and three Australian isolates of feline immunodeficiency virus (FIV) were determined and compared with those of isolates previously described. The results revealed that the Japanese isolates were found to cluster with nucleotide sequence similarity of 95.6%-99.4%. The Australian isolates also clustered with nucleotide sequence similarity of 97.2%-99.4%. The Argentine isolates formed two groups; the LP9 isolate is closely related to the Japanese isolates, whereas the LP1, LP3, LP20 and LP24 isolates are distant from both the Japanese and Australian isolates. From these results, FIV can be divided into three groups, namely: (I) the Californian, Australian and British isolates; (II) the Japanese isolates and one Argentine LP9 isolate; (III) the other Argentine isolates.

Carrier-state infection of feline T-lymphoblastoid cells with feline calicivirus

The susceptibility of feline T lymphocytes to feline calicivirus (FCV) in vitro was investigated using feline T-lymphoblastoid cell lines, namely MYA-1 and FL74 cells. The virus titers of supernatants in FCV-infected MYA-1 and FL74 cell cultures increased rapidly, and FCV antigens were also detected in the FCV-infected cells. There were slight differences in the molecular weights of capsid proteins expressed in FCV-infected MYA-1, FL74 and Crandell feline kidney cells. MYA-1 and FL74 cells were productively and persistently infected with FCV, and FCV antigens were observed in the FCV-infected cells for more than one month. At 3 months post infection, FCV-infected FL74 cells that stopped producing infectious FCV could be reinfected with FCV. However, no cytopathic effects were observed.

Induction of apoptosis in a T lymphoblastoid cell line infected with feline immunodeficiency virus

The mechanism of cell death induced by feline immunodeficiency virus (FIV) infection was investigated in an interleukin 2(IL-2)-dependent T-lymphoblastoid cell line (MYA-1). DNA extracted from FIV-infected MYA-1 cells showed a ladder of nucleosomal DNA, indicating that the cytopathic effect (CPE) observed in these cells was due to apoptosis. Infection of MYA-1 cells with FIV was associated with suppression of the proliferative response of the cells to exogenous IL-2 prior to DNA fragmentation. These findings suggest that FIV-induced CPE in these T-lymphoblastoid cells is associated with apoptosis possibly due to a defect in the IL-2 signal transduction pathway.

Effects of insertion of multiple AP-1 binding sites into the U3 region of the long terminal repeat of feline immunodeficiency virus

An oligonucleotide containing multiple AP-1 binding sites was introduced into the regulatory sequence in the long terminal repeat (LTR) of feline immunodeficiency virus (FIV). Chloramphenicol acetyltransferase assay revealed that basal promoter activity of the mutated LTR was higher than that of the wild-type LTR in Crandell feline kidney (CRFK) cells. The mutated LTR was introduced into an infectious molecular clone of FIV and the clone was transfected into CRFK cells. The virus production of the mutant in the cells was as high as that of the wild-type when determined by the reverse transcriptase activity assay. The growth of the mutant virus obtained from the transfected CRFK cells was examined in feline T lymphoblastoid cell lines (MYA-1 and FeL-039 cells) and primary feline peripheral blood mononuclear cells (fPBMCs). The growth was delayed when compared with that of the wild-type virus in all the cells used. Upon examination by polymerase chain reaction, the length of the LTR of the mutant virus was shortened in both MYA-1 cells and fPBMCs. Sequence analysis revealed that the insertion was completely deleted 39 days after infection in the MYA-1 cells.

The detection and quantification of feline immunodeficiency provirus in peripheral blood mononuclear cells using the polymerase chain reaction

The polymerase chain reaction method (PCR) was used to detect feline immunodeficiency virus proviral DNA in peripheral blood mononuclear cells (PBMC) of a group of 8 experimentally infected cats. The proportion of PBMC containing provirus was determined from 6 to 32 weeks post inoculation (p.i.) by performing PCR on serially diluted samples of PBMC. Primers from the p15 and p24 regions of the gag gene were used and Southern hybridization using an end-labelled probe was required to confirm primer-specific products. Provirus was detected in 5 of 8 cats by 6 weeks p.i. in 50000 PBMC, and in all 8 infected cats by 8 weeks p.i. Provirus was not detected in PBMC from any of 3 FIV negative cats. The proportion of PBMC containing provirus in individual cats ranged from 1 in 70 to 1 in 99600 PBMC. There was no significant decline over time in the proportion of PBMC containing provirus. Sequencing of a segment (287 bases) of the gag region of a West Australian FIV isolate (T90) revealed only slight nucleotide divergence from the North American Petaluma and PPR isolates and wider divergence from the Japanese TM2 clone.

The feline immunodeficiency virus ORF-A gene facilitates efficient viral replication in established T-cell lines and peripheral blood lymphocytes

Frameshift mutants corresponding to all of the identified open reading frames of feline immunodeficiency virus, including the ORF-A gene, which has an unknown function, were constructed in vitro. Upon transfection into cells, no significant difference between the phenotypes of ORF-A mutant clones and those of wild-type clones was demonstrated. Although only ORF-A mutant virus among all mutant viruses from transfected cells showed infectivity in established T-cell lines, the replication and propagation of the ORF-A mutant virus were efficiently reduced compared with those of the wild-type virus. Moreover, the loss of the function of the ORF-A gene resulted in a severe defect in productive infection in primary peripheral blood lymphocytes both in the amount of reverse transcriptase activity produced and in core protein expression. These findings demonstrate that the ORF-A gene of feline immunodeficiency virus is required for efficient viral replication and suggest that the ORF-A gene is likely to be important for natural infection.

Nucleotide sequences of Australian isolates of the feline immunodeficiency virus: comparison with other feline lentiviruses

Proviral DNA from four Australian isolates of feline immunodeficiency virus (FIV) was amplified by PCR and the nucleotide sequence determined for two conserved regions within gag (p15/p24) and pol (RT) genes. Comparison with the nucleotide and deduced amino acid sequence of two previously described U.S. isolates from California (Petaluma and PPR), and a third from Maryland (MD) as well as the Japanese isolate TM2, revealed a close similarity between the Australian and Californian isolates with 95-97% nucleotide and 96-99% amino acid homologies. By contrast, the Maryland and Japanese isolates were more distantly related with only 84-87% nucleotide and 90-94% amino acid homology with either the Australian or Californian isolates. The relationship of the Australian FIV isolates to other domestic isolates as well as eight lentiviral isolates from wild felidae (panthers) published previously, was investigated further by constructing a phylogenetic tree based on the pol sequence. This revealed two subgroups of FIV, an Australian/Californian group and a less tightly clustered Maryland/Japanese group. These results suggest that the genomic variability of FIV is reflected by more than simply geographic distance. Furthermore, the relative genetic homogeneity found between Australian isolates suggest a shorter period of evolution of the virus in Australia than in North America.

Localization of the viral antigen of feline immunodeficiency virus in the lymph nodes of cats at the early stage of infection

Immunohistochemical examinations of localization of feline immunodeficiency virus (FIV) Gag protein were performed on lymph nodes of cats experimentally inoculated with three different strains of FIV (infectious molecular clone of TM 1, Petaluma, and KYO-1 strains), using rabbit anti-FIV Gag serum. The FIV Gag antigens were observed in many follicular dendritic cells (FDCs) and sparsely in small lymphocytes of paracortical area in the lymph nodes of cats inoculated with Petaluma and KYO-1 strains. However, the antigens were present only in small lymphocytes, and not in FDCs of a cat inoculated with infectious molecular clone of the TM1 strain. The cell type differences in expression of the viral antigen in vivo might reflect on the cell tropisms of the FIV strains in vitro. By double immunohistochemical staining with rabbit anti-FIV Gag serum and monoclonal antibodies which recognize feline CD4, feline CD8 or feline pan-T molecules, the FIV Gag-positive lymphocytes were characterized as feline CD4-positive T cells. Since the distributions of FIV Gag antigens were mainly in the FDCs, the FDCs may play an important role as a major reservoir and may be a primary target of FIV at early stages of infection.

Establishment of a feline T-lymphoblastoid cell line susceptible to feline immunodeficiency viruses

A feline T-lymphoblastoid cell line susceptible to feline immunodeficiency viruses (FIV) was established and designated as Yu-1 cells. Yu-1 cells continued to grow over one year with more than 100 successive passages in the presence of human recombinant interleukin-2. Surface antigens of Yu-1 cells were feline Pan-T+, CD4+, and CD8-. Susceptibility of Yu-1 cells to FIV strains were higher than that of the primary culture of the feline peripheral blood mononuclear cells, indicating that this cell line should be useful for isolation, propagation, and neutralization test of FIV.

Extensive sequence variation of feline immunodeficiency virus env genes in isolates from naturally infected cats

In an investigation of the evolution of feline immunodeficiency virus (FIV) in vivo, sequential isolates from a persistently infected cat were examined by direct sequencing following amplification of selected subgenomic regions by polymerase chain reaction (PCR). Three isolates, T90, T91, and T92, obtained over a three-year period revealed no changes to regions known to be conserved within gag and pol genes. Additionally, no change occurred within gag and pol in an isolate recovered from a second cat which was experimentally infected with T90. Changes were detected within an N-terminal region of the envelope glycoprotein gp 120 (env). These consisted of point mutations, some of which would result in amino acid substitutions and the predicted amino acid changes tended to cluster within variable domains. Inoculation of T90 into a second cat resulted in a different pattern of mutations than that observed for the three isolates from the first cat. In all cases, virus isolates derived from the same cat were much more highly related to each other (extent of env variation was 0.5-1.5%) than to isolates from other cats (10-12% env variation). The rate of change of FIV was estimated to be 3.4 x 10(-3) nucleotide substitutions per site per year for the env gene and less than 10(-4) nucleotide substitutions per site per year for the gag and pol genes, values concordant with that found for human immunodeficiency virus 1. Both nucleotide and amino acid changes in the gp 120 region were found to be directional, suggesting that selective pressures influence FIV envelope gene sequences.

Feline CD4 molecules expressed on feline non-lymphoid cell lines are not enough for productive infection of highly lymphotropic feline immunodeficiency virus isolates

To investigate whether the feline CD 4 (fCD4) molecules are involved in infections of highly lymphotropic feline immunodeficiency virus (FIV) isolates, we expressed fCD4 stably on Crandell feline kidney cells and Felis catus whole foetus 4 cells by transfection of a cDNA encoding the fCD4 glycoprotein, and then infected them with TM 1 and TM 2 strains of FIV, which are unable to infect these cells productively. In spite of fCD 4 being expressed on these cells, no virus production was observed. This result indicates that fCD 4 expression alone cannot induce a productive infection of the FIV TM 1 and TM 2 strains.

Detection of feline immunodeficiency virus infection in bone marrow of cats

Natural or experimental feline immunodeficiency virus (FIV) infection in cats is often associated with hematologic abnormalities which are similar to those observed in human immunodeficiency virus (HIV) infected patients. To determine if cells in bone marrow are infected with FIV and whether severity of hematopoietic disorder is correlated with the level of viral infection, bone marrow tissues from ten experimentally and two naturally FIV infected cats were examined by in situ hybridization for presence of FIV RNA. Seven of the 12 FIV infected cats were also naturally or experimentally coinfected with feline leukemia virus (FeLV). FIV RNA was detected mainly in megakaryocytes and unidentified mononuclear cells in the bone marrow of cats that were sick and had marrow hypercellularity and immaturity. These included all cats in the acute phase of FIV infection and two of seven long term FIV infected cats. One long term FIV infected cat with lymphosarcoma was also positive for FIV RNA in bone marrow cells. The other four long term FIV infected cats were relatively healthy, with normal bone marrow morphology, and were negative for FIV infected cells. Bone marrow from three non-infected and two cats infected with FeLV alone were also negative for FIV RNA by in situ hybridization. We concluded that megakaryocytes and mononuclear cells were targets of the viral infection and that the presence of FIV RNA in cells of the bone marrow correlated with marrow hypercellularity and immaturity, and severity of illness.

Identification of a feline immunodeficiency virus gene which is essential for cell-free virus infectivity

Feline immunodeficiency virus (FIV) contains at least three small open reading frames (ORFs) in the genome, in addition to the three structural genes. Two of these ORFs (putative vif and ORF-A) have unknown functions. Northern (RNA) blot analysis of mRNAs from an FIV-infected cell line showed that the putative-vif-specific mRNA was expressed as a 5.2-kb species. To examine the function of the putative vif gene, we constructed mutants carrying a deletion in either the vif-like gene or the rev gene from an infectious molecular clone of FIV. Although the vif mutant produced virion-associated reverse transcriptase at a normal level upon transfection, cell-free virus prepared from the transfected cells could not infect feline CD4+ cells. The infectivity of the vif mutant, however, was demonstrated in a coculture of the transfected cells and feline CD4+ cells. We conclude that FIV contains the vif gene, which is structurally and functionally similar to that of the primate lentiviruses.

Detection of anti-gag antibodies of feline immunodeficiency virus in cat sera by enzyme-linked immunosorbent assay

Using gag protein of feline immunodeficiency virus (FIV) expressed in Escherichia coli, an enzyme-linked immunosorbent assay (ELISA) system was developed for detection of antibodies to FIV gag protein in cat sera. With serum samples from cats experimentally infected with several strains and an infectious molecular clone of FIV, increases of the antibody titers to FIV gag protein were observed in all cases by the ELISA at early stage of infection. When we examined a total of 415 field cat sera which were previously tested by an indirect immunofluorescence assay (IFA), 9 (12.9%) out of 70 IFA positive sera were judged as negative by the ELISA. However, all 3 serum samples tested among the 9 IFA positive sera had antibodies to gp130 but not to p26 by a radioimmunoprecipitation assay. The results indicated that some IFA positive sera did not have antibodies to the p26 though they have antibodies to other proteins specific for FIV.

Anti-human immunodeficiency virus (HIV) agents are also potent and selective inhibitors of feline immunodeficiency virus (FIV)-induced cytopathic effect: development of a new method for screening of anti-FIV substances in vitro

The ability of several known anti-HIV substances to inhibit feline immunodeficiency virus (FIV) was tested. The results showed that FIV infection of feline T-cells was almost completely blocked in the presence of all of the agents tested. However, FIV-induced syncytium formation between a human T-cell line (MT-2 cells) and a FIV-infected feline lymphocyte cell line (3201/FIV) was inhibited only by dextran sulfate and pradimicin A. The assay used to measure syncytium inhibition was rapid and did not use potentially hazardous human immunodeficiency virus (HIV)-infected cells. The efficacy results coincided with those of HIV studies.

Altered cell tropism and cytopathicity of feline immunodeficiency viruses in two different feline CD4-positive, CD8-negative cell lines

Two interleukin-2-dependent feline CD4-positive and CD8-negative cell lines, MYA-1 and the newly established FeL-039, were used as host cells for infection with feline immunodeficiency virus (FIV). All FIV strains used, the Petaluma strain and several new isolates, were highly cytopathic to MYA-1. In contrast, the kinetics of FIV replication in FeL-039 differed greatly depending on the strain tested, i.e., noninfectious strain, highly cytopathic strain, and less cytopathic strain producing a persistent state for a long period. It appears, therefore, that cell tropism for FIV differed with each FIV strain tested even in T-cell lines showing similar cell surface phenotypes. Cytopathicity of FIV is evidently due to both the FIV strain and the host T cell.

Replicative difference in early-passage feline brain cells among feline immunodeficiency virus isolates

The susceptibility of early-passage feline brain cells and Crandell feline kidney (CRFK) cells to infection with three isolates of feline immunodeficiency virus (FIV) was investigated. The Petaluma strain of FIV could well infect both the feline brain cells and CRFK cells. The KYO-1 strain could well infect the feline brain cells but the replication in CRFK cells was demonstrated only by coculturing fresh feline T-lymphoblastoid cells with the infected cells. On the other hand, the TM1 strain could infect the feline brain cells but not CRFK cells. Moreover, the replicative ability of the TM1 strain in the feline brain cells was much less than the KYO-1 and Petaluma strains. These results indicate that biological differences can be detected among the FIV isolates.

Feline immunodeficiency virus: quantification in peripheral blood mononuclear cells and isolation from plasma of infected cats

The titer of feline immunodeficiency virus in peripheral blood mononuclear cells (PBMC) and the presence of infectious virus in plasma was investigated over 20 week period in 8 experimentally infected cats, 3 uninfected cats and 2 naturally infected cats by end point dilution cultures using a feline T-lymphoblastoid cell line (MYA-1). FIV was isolated from PBMC of all infected cats, but not from the uninfected cats. FIV was also isolated consistently from 100 microliters plasma from most of the experimentally infected cats, but not from the 2 naturally infected cats. The virus titer in PBMCs in both experimentally and naturally infected cats was comparatively high, ranging from 10 TCID/10(6) PBMC to 14,286 TCID/10(6) PBMC. The titers in PBMC of individual cats remained unchanged or varied only slightly over the 20 week period. In contrast, the titers varied substantially between cats, with significantly higher titers in the youngest litter (4 cats) than in the oldest litter (3 cats). This suggests that there is an age-related factor influencing the level of PBMC virus titers in experimental infection with FIV. A similar age-related susceptibility has been shown with feline leukemia virus. More importantly, the sustained titers in the experimentally infected cats bear close resemblance to infection of children with human immunodeficiency virus. These data reinforce suggestions that age and immune maturity have a fundamental influence on PBMC and plasma titers in lentivirus infections.

Molecular characterization and heterogeneity of feline immunodeficiency virus isolates

We have molecularly cloned the complete genomic DNA of TM2 strain of feline immunodeficiency virus (FIV) isolated in Japan and compared its nucleotide and the deduced amino acid sequence with those of previously described U.S. isolates, FIV Petaluma and FIV PPR. The infectious molecular clone of FIV TM2 is different from FIV Petaluma in host cell range; the clone can not infect Crandell feline kidney cells which were permissive for FIV Petaluma. The amino acid sequence homologies, in gag, pol, and env genes between FIV TM2 and Petaluma were 90%, 87%, and 81%, respectively. On the other hand, comparative analysis of each gene between FIV Petaluma and PPR showed 96,95, and 85%, respectively. These results suggested that the genomic diversity was present among FIV strains isolated from geographically distant areas. Interestingly, tat- and rev-like short open reading frames contained inframe stop codons in the FIV Petaluma but not in the FIV TM2.

Major core proteins, p24s, of human, simian, and feline immunodeficiency viruses are partly expressed on the surface of the virus-infected cells

We have previously shown the expression of human immunodeficiency virus type 1 (HIV-1) major gag protein, p24, on the surface of persistently HIV-1-infected cells by using murine monoclonal antibodies (mAb). We now report that the cell surface gag p24 antigen expression is a universal phenomenon among HIV-1, simian immunodeficiency virus (SIV), and feline immunodeficiency virus (FIV). The mAbs prepared by immunization with purified HIV-1 particles were used as antibodies cross-reactive to HIV-1 and SIVagmp24 antigens. The mAbs to FIV p24 were raised against the gag precursor 50 kDa protein of FIV, which was expressed by Baculovirus vector. The p24 antigen expression on the cell surface was detectable in certain combinations of virus-host cell systems in all of these viruses. Since these p24 regions of the animal viruses seem to play as important a role in cell-mediated immunity as that of HIV-1, the p24 applicability as a candidate epitope for vaccine development could be evaluated in those animals.

Activation of feline immunodeficiency virus long terminal repeat by feline herpesvirus type 1

By transfection of a recombinant plasmid containing the feline immunodeficiency virus (FIV) long terminal repeat (LTR) linked to the chloramphenicol acetyltransferase (CAT) gene followed by infection of feline herpesvirus type 1 (FHV-1) into Crandell feline kidney cells and Felis catus whole fetus 4 cells, enhancement of CAT activity was demonstrated. Furthermore, individual feline T-lymphocytes were productively co-infected with both FIV and FHV-1 in vitro as determined by two-color immunofluorescence and electron microscopy analyses. These results revealed the transactivation of the FIV LTR by FHV-1 and the dual infection of T-lymphocytes with both viruses. The possibility that FHV-1 might be a cofactor which plays a role in the pathogenesis of FIV infection is discussed.

Identification of feline immunodeficiency virus rev gene activity

We constructed 16 deletion mutants from an infectious molecular clone of feline immunodeficiency virus (FIV) and a reporter plasmid carrying the bacterial chloramphenicol acetyltransferase (CAT) gene to identify the rev transactivator activity of the virus. Cotransfections of various mutants and the rev reporter clone bearing a portion of FIV env in addition to the CAT gene revealed that the sequence important for the augmentation of CAT production was located in three separate parts of the virus genome. This enhancement was FIV specific in that the human retrovirus rev and rex gene products did not activate the reporter. The phenotypic properties of an FIV proviral mutant containing a small deletion in the genome were similar to those of rev mutants derived from primate immunodeficiency viruses. These results indicate that FIV, like the other lentiviruses, contains the rev gene in its genome.

Feline immunodeficiency virus infects both CD4+ and CD8+ T lymphocytes

Monoclonal populations of feline T cells, derived from a specific-pathogen-free cat and expressing either the CD4 or CD8 surface antigen, were infected in vitro with two geographically distinct isolates of feline immunodeficiency virus (FIV). Both infected T-cell subsets exhibited decreased cell viability, expressed FIV-encoded proteins, and generated reverse transcriptase activity. All clones examined retained their original surface phenotype after infection. It appears, therefore, that both CD4+ and CD8+ T cells may be productively infected by FIV in vivo.

Molecular cloning of a novel isolate of feline immunodeficiency virus biologically and genetically different from the original U.S. isolate

The Japanese isolate (TM1 strain) of feline immunodeficiency virus (FIV) which replicates in a feline CD4 (fCD4)-positive lymphoblastoid cell line (MYA-1 cells) was molecularly cloned from extrachromosomal closed circular DNA. The restriction map of the clone, termed pFTM 191 complete genome (CG), showed a considerable difference from that of the U.S. isolate (Petaluma strain) of FIV. The sequence homology in the long terminal repeat between the TM1 and Petaluma strain was 82%. The pFTM 191 CG was biologically active after transfection into Crandell feline kidney cells which were permissive for replication of FIV Petaluma. However, the progeny virions could not reinfect fCD4-negative Crandell feline kidney cells but could infect fCD4-positive MYA-1 cells. When a specific-pathogen-free cat was inoculated with the virus derived from the pFTM 191 CG, the cat seroconverted within 8 weeks postinoculation and FIV was reisolated at 4, 8, and 20 weeks postinoculation. These results indicate the infectivity of the pFTM 191 CG in vivo.