Identifying Age Cohorts Responsible for Peste Des Petits Ruminants Virus Transmission among Sheep, Goats, and Cattle in Northern Tanzania.

Peste des petits ruminants virus (PPRV) causes a contagious disease of high morbidity and mortality in global sheep and goat populations. To better control this disease and inform eradication strategies, an improved understanding of how PPRV transmission risk varies by age is needed. Our study used a piece-wise catalytic model to estimate the age-specific force of infection (FOI, per capita infection rate of susceptible hosts) among sheep, goats, and cattle from a cross-sectional serosurvey dataset collected in 2016 in Tanzania. Apparent seroprevalence increased with age, reaching 53.6%, 46.8%, and 11.6% (true seroprevalence: 52.7%, 52.8%, 39.2%) for sheep, goats, and cattle, respectively. Seroprevalence was significantly higher among pastoral animals than agropastoral animals across all ages, with pastoral sheep and goat seroprevalence approaching 70% and 80%, respectively, suggesting pastoral endemicity. The best fitting piece-wise catalytic models merged age groups: two for sheep, three for goats, and four for cattle. The signal of these age heterogeneities were weak, except for a significant FOI peak among 2.5-3.5-year-old pastoral cattle. The subtle age-specific heterogeneities identified in this study suggest that targeting control efforts by age may not be as effective as targeting by other risk factors, such as production system type. Further research should investigate how specific husbandry practices affect PPRV transmission.

Pastoral production is associated with increased peste des petits ruminants seroprevalence in northern Tanzania across sheep, goats and cattle.

Peste des petits ruminants virus (PPRV) causes a contagious disease of high morbidity and mortality in small ruminant populations globally. Using cross-sectional serosurvey data collected in 2016, our study investigated PPRV seroprevalence and risk factors among sheep, goats and cattle in 20 agropastoral (AP) and pastoral (P) villages in northern Tanzania. Overall observed seroprevalence was 21.1% (95% exact confidence interval (CI) 20.1-22.0) with 5.8% seroprevalence among agropastoral (95% CI 5.0-6.7) and 30.7% among pastoral villages (95% CI 29.3-32.0). Seropositivity varied significantly by management (production) system. Our study applied the catalytic framework to estimate the force of infection. The associated reproductive numbers (R0) were estimated at 1.36 (95% CI 1.32-1.39), 1.40 (95% CI 1.37-1.44) and 1.13 (95% CI 1.11-1.14) for sheep, goats and cattle, respectively. For sheep and goats, these R0 values are likely underestimates due to infection-associated mortality. Spatial heterogeneity in risk among pairs of species across 20 villages was significantly positively correlated (R2: 0.59-0.69), suggesting either cross-species transmission or common, external risk factors affecting all species. The non-negligible seroconversion in cattle may represent spillover or cattle-to-cattle transmission and must be investigated further to understand the role of cattle in PPRV transmission ahead of upcoming eradication efforts.

Complete genome sequences of two feline leukemia virus subgroup B isolates with novel recombination sites.

It is generally accepted that all primary isolates of feline leukemia virus (FeLV) contain a subgroup A virus (FeLV-A) that is essential for transmission. In contrast, FeLV-B is thought to arise de novo in the infected animal through RNA recombination events with endogenous FeLV transcripts, presumably through copackaging of RNA from endogenous FeLV and exogenous FeLV-A. Here, we report the complete genome sequences of two novel strains of FeLV-B (FeLV-2518 and FeLV-4314) that were isolated in the absence of FeLV-A. The env genes of these isolates have been characterized previously, and the 3' recombination sites have been identified. We describe herein the 5' recombination breakpoints of each virus. These breakpoints were found to be within the signal peptide of the env gene and the reverse transcriptase-coding region, respectively. This is the first report of a recombination site within the pol gene of an FeLV-B genome and the first genetic characterization of multiple independently arising FeLV-B isolates that have been identified without a functional FeLV-A ancestral virus.

Are endogenous feline leukemia viruses really endogenous?

Full length endogenous feline leukemia virus (FeLV) proviruses exist within the genomes of many breeds of domestic cat raising the possibility that they may also exist in a transmissible exogenous form. Such viruses would share receptor usage with the recombinant FeLV-B subgroup, a viral subgroup that arises in vivo by recombination between exogenous subgroup A virus (FeLV-A) and endogenous FeLV. Accordingly, all isolates of FeLV-B made to date have contained a "helper" FeLV-A, consistent with their recombinatorial origin. In order to assess whether endogenous viruses are transmitted between cats, we examined primary isolates of FeLV for which the viral subgroup had been determined for the presence of a subgroup B virus that lacked an FeLV-A. Here we describe the identification of two primary field isolates of FeLV (2518 and 4314) that appeared to contain subgroup B virus only by classical interference assays, raising the possibility of between-host transmission of endogenous FeLV. Sequencing of the env gene and U3 region of the 3' long terminal repeat (LTR) confirmed that both viral genomes contained endogenous viral env genes. However the viral 3' LTRs appeared exogenous in origin with a putative 3' recombination breakpoint residing at the 3' end of the env gene. Further, the FeLV-2518 virions also co-packaged a truncated FeLV-A genome containing a defective env gene, termed FeLV-2518(A) whilst no helper subgroup A viral genome was detected in virions of FeLV-4314. The acquisition of an exogenous LTR by the endogenous FeLV in 4314 may have allowed a recombinant FeLV variant to outgrow an exogenous FeLV-A virus that was presumably present during first infection. Given time, a similar evolution may also occur within the 2518 isolate. The data suggest that endogenous FeLVs may be mobilised by acquisition of exogenous LTRs yielding novel viruses that type biologically as FeLV-B.

Isolation and partial characterization of Brazilian samples of feline immunodeficiency virus.

Feline immunodeficiency virus (FIV) causes a slow progressive degeneration of the immune system which eventually leads to a disease comparable to acquired immune deficiency syndrome (AIDS) in humans. FIV has extensive sequence variation, a typical feature of lentiviruses. Sequence analysis showed that diversity was not evenly distributed throughout the genome, but was greatest in the envelope gene, env. The virus enters host cells via a sequential interaction, initiated by the envelope glycoprotein (env) binding the primary receptor molecule CD134 and followed by a subsequent interaction with chemokine co-receptor CXCR4. The purpose of this study was to isolate and characterize isolates of FIV from an open shelter in São Paulo, Brazil. The separated PBMC from 11 positive cats were co-cultured with MYA-1 cells. Full-length viral env glycoprotein genes were amplified and determined. Chimeric feline × human CD134 receptors were used to investigate the receptor utilization of 17 clones from Brazilian isolates of FIV. Analyses of the sequence present of molecular clones showed that all clones grouped within subtype B. In contrast to the virulent primary isolate FIV-GL8, expression of the first cysteine-rich domain (CRD1) of feline CD134 in the context of human CD134 was sufficient for optimal receptor function for all Brazilian FIV isolates tested.

Phylogenetic characterisation of naturally occurring feline immunodeficiency virus in the United Kingdom.

Feline immunodeficiency virus (FIV) is a significant pathogen of domestic and non-domestic felids worldwide. In domestic cats, FIV is classified into five distinct subtypes (A-E) with subtypes A and B distributed most widely. However, little is known about the degree of intrasubtype viral diversity and this may prove critical in determining whether monovalent vaccines are likely to protect against FIV strains within a single subtype. Here, we characterise novel env sequences from 47 FIV strains recovered from infected cats in the United Kingdom and its environs. Phylogenetic analyses revealed that all bar one sequence belonged to subtype A, the predominant subtype in Western Europe. A single sequence was identified as a likely subtype A/C recombinant, intriguing given that subtype C does not appear to exist in either the UK or North Western Europe and suggestive of a recombination event predating its introduction into the UK. Subtype A strains from the UK were not significantly differentiated from representative subtype A isolates found elsewhere suggesting multiple introductions of FIV into the country. Divergence among isolates was comparable to that observed for subtype A isolates worldwide, indicating that FIV in the UK covers the full spectrum of subtype A diversity seen globally. This study demonstrates that while subtype A is predominant in the UK, novel introductions may result in the emergence of novel subtypes or intersubtype recombinants, potentially circumventing vaccine strategies. However, the dominance of subtype A suggests that the development of a regional or subtype-specific protective vaccine for the UK could be achievable.

Lymphocyte migration through the blood-brain barrier (BBB) in feline immunodeficiency virus infection is significantly influenced by the pre-existence of virus and tumour necrosis factor (TNF)-alpha within the central nervous system (CNS): studies using an in vitro feline BBB model.

AIMS: In human immunodeficiency virus infection, macrophage-tropic and lymphotropic viruses exist in the host. Central nervous system (CNS) infection is an early and ongoing event, important to understand when developing strategies to treat infection. Some knowledge exists on macrophage-tropic virus interactions with the blood-brain barrier (BBB), and the aim of this study was to investigate lymphotropic lentivirus interactions with the BBB. METHODS: Interactions of the lymphotropic feline immunodeficiency virus (FIV) with an in vitro model of the feline BBB were evaluated in scenarios to mimic in vivo infections. RESULTS: Cell-free FIV crossed the BBB in very low quantities, and in the presence of tumour necrosis factor (TNF)-alpha, BBB integrity was unaffected. However, cell-associated FIV readily crossed the BBB, but BBB integrity was not significantly altered. Transmigration of uninfected and infected lymphocytes increased in response to TNF-alpha, accompanied by a moderate disruption of barrier integrity and an upregulation of vascular cell adhesion molecule-1 rather than intercellular adhesion molecule-1. Significant enhancement of migration and disruption of BBB tight junctions occurred when infected cells and TNF-alpha were added to the brain side of the BBB and this enhancement was not mediated through additional TNF-alpha production. CONCLUSIONS: Small quantities of virus in the brain together with TNF-alpha have the potential to stimulate greater cell and viral entry into the CNS and this is likely to involve important factors other than further TNF-alpha production. Lymphotropic lentivirus entry to the CNS is governed by many factors similar to macrophage-tropic strains.

Diagnosis and management of B cell chronic lymphocytic leukaemia in a cat.

A four-year-old, female neutered domestic shorthair cat had a history of chronic intermittent vomiting and lymphocytosis. B cell chronic lymphocytic leukaemia was diagnosed by flow cytometry, which revealed abnormally large numbers of mature B lymphocytes in the peripheral blood. The cat was treated conservatively with antiemetic drugs and remained stable without chemotherapy for over a year.

Up-regulation by feline interleukin-4 and down-regulation by feline interferon-gamma of major histocompatibility complex class II on cat B-lymphocytes.

Interleukin-4 (IL-4) exhibits numerous biological and immunoregulatory functions on B- and T-lymphocytes, monocytes, and dendritic cells in both mice and humans. In the present study, we show that IL-4 also has a regulatory function in the cat species. Cells transfected with IL-4 DNA expressed a biologically active protein as demonstrated by the up-regulation of MHC class II molecules on B-lymphocytes (CD21(+)) in a flow cytometric assay. Increased levels of MHC class II expression on CD21(+) cells were seen in 11 out of 12 cats (p<0.05). In addition, 12 out of 12 cats showed up-regulation of MHC class II on CD21(-) cells, mainly consisting of T-lymphocytes (p<0.05). In contrast, concanavalin A (ConA)-induced culture supernatant from peripheral blood mononuclear cells (PBMCs) containing high levels of interferon-gamma (IFN-gamma) transcripts induced down-regulation of MHC class II molecules on CD21(+) cells of all samples (p<0.05). Variable results were observed for CD21(-) cells incubated with ConA-conditioned medium (p=0.71). The nature of the cytokine(s) responsible for these effects remains to be determined. However, the fact that down-regulation of MHC class II molecules on B cells occurred in all cats tested suggests that IFN-gamma may be involved. These data provide further insight into the mechanism by which MHC class II expression is regulated in feline lymphocytes, and suggest that the Th1/Th2 paradigm is also present in the cat.

Vaccination with inactivated virus but not viral DNA reduces virus load following challenge with a heterologous and virulent isolate of feline immunodeficiency virus.

It has been shown that cats can be protected against infection with the prototypic Petaluma strain of feline immunodeficiency virus (FIV(PET)) using vaccines based on either inactivated virus particles or replication-defective proviral DNA. However, the utility of such vaccines in the field is uncertain, given the absence of consistent protection against antigenically distinct strains and the concern that the Petaluma strain may be an unrepresentative, attenuated isolate. Since reduction of viral pathogenicity and dissemination may be useful outcomes of vaccination, even in the absence of complete protection, we tested whether either of these vaccine strategies ameliorates the early course of infection following challenge with heterologous and more virulent isolates. We now report that an inactivated virus vaccine, which generates high levels of virus neutralizing antibodies, confers reduced virus loads following challenge with two heterologous isolates, FIV(AM6) and FIV(GL8). This vaccine also prevented the marked early decline in CD4/CD8 ratio seen in FIV(GL8)-infected cats. In contrast, DNA vaccines based on either FIV(PET) or FIV(GL8), which induce cell-mediated responses but no detectable antiviral antibodies, protected a fraction of cats against infection with FIV(PET) but had no measurable effect on virus load when the infecting virus was FIV(GL8). These results indicate that the more virulent FIV(GL8) is intrinsically more resistant to vaccinal immunity than the FIV(PET) strain and that a broad spectrum of responses which includes virus neutralizing antibodies is a desirable goal for lentivirus vaccine development.

A putative cell surface receptor for anemia-inducing feline leukemia virus subgroup C is a member of a transporter superfamily.

Domestic cats infected with the horizontally transmitted feline leukemia virus subgroup A (FeLV-A) often produce mutants (termed FeLV-C) that bind to a distinct cell surface receptor and cause severe aplastic anemia in vivo and erythroblast destruction in bone marrow cultures. The major determinant for FeLV-C-induced anemia has been mapped to a small region of the surface envelope glycoprotein that is responsible for its receptor binding specificity. Thus, erythroblast destruction may directly or indirectly result from FeLV-C binding to its receptor. To address these issues, we functionally cloned a putative cell surface receptor for FeLV-C (FLVCR) by using a human T-lymphocyte cDNA library in a retroviral vector. Expression of the 2.0-kbp FLVCR cDNA in naturally resistant Swiss mouse fibroblasts and Chinese hamster ovary cells caused substantial susceptibility to FeLV-C but no change in susceptibilities to FeLV-B and other retroviruses. The predicted FLVCR protein contains 555 amino acids and 12 hydrophobic potential membrane-spanning sequences. Database searches indicated that FLVCR is a member of the major-facilitator superfamily of transporters and implied that it may transport an organic anion. RNA blot analyses showed that FLVCR mRNA is expressed in multiple hematopoietic lineages rather than specifically in erythroblasts. These results suggest that the targeted destruction of erythroblasts by FeLV-C may derive from their greater sensitivity to this virus rather than from a preferential susceptibility to infection.

The role of the chemokine receptor CXCR4 in infection with feline immunodeficiency virus.

Infection with feline immunodeficiency virus (FIV) leads to the development of a disease state similar to AIDS in man. Recent studies have identified the chemokine receptor CXCR4 as the major receptor for cell culture-adapted strains of FIV, suggesting that FIV and human immunodeficiency virus (HIV) share a common mechanism of infection involving an interaction between the virus and a member of the seven transmembrane domain superfamily of molecules. This article reviews the evidence for the involvement of chemokine receptors in FIV infection and contrasts these findings with similar studies on the primate lentiviruses HIV and SIV (simian immunodeficiency virus).

Effect of mutations in the second extracellular loop of CXCR4 on its utilization by human and feline immunodeficiency viruses.

CCR5 and CXCR4 are the principal CD4-associated coreceptors used by human immunodeficiency virus type 1 (HIV-1). CXCR4 is also a receptor for the feline immunodeficiency virus (FIV). The rat CXCR4 cannot mediate infection by HIV-1NDK or by FIVPET (both cell line-adapted strains) because of sequence differences with human CXCR4 in the second extracellular loop (ECL2). Here we made similar observations for HIV-189.6 (a strain also using CCR5) and for a primary HIV-1 isolate. It showed the role of ECL2 in the coreceptor activity of CXCR4 for different types of HIV-1 strains. By exchanging ECL2 residues between human and rat CXCR4, we found that several amino acid differences contributed to the inactivity of the rat CXCR4 toward HIV-189.6. In contrast, its inactivity toward HIV-1NDK seemed principally due to a serine at position 193 instead of to an aspartic acid (Asp193) in human CXCR4. Likewise, a mutation of Asp187 prevented usage of CXCR4 by FIVPET. Different mutations of Asp193, including its replacement by a glutamic acid, markedly reduced or suppressed the activity of CXCR4 for HIV-1NDK infection, indicating that the negative charge was not the only requirement. Mutations of Asp193 and of arginine residues (Arg183 and Arg188) of CXCR4 reduced the efficiency of HIV-1 infection for all HIV-1 strains tested. Other ECL2 mutations tested had strain-specific effects or no apparent effect on HIV-1 infection. The ECL2 mutants allowed us to identify residues contributing to the epitope of the 12G5 monoclonal antibody. Overall, residues with different charges and interspersed in ECL2 seem to participate in the coreceptor activity of CXCR4. This suggests that a conformational rather than linear epitope of ECL2 contributes to the HIV-1 binding site. However, certain HIV-1 and FIV strains seem to require the presence of a particular ECL2 residue.

The role of in vitro-induced lymphocyte apoptosis in feline immunodeficiency virus infection: correlation with different markers of disease progression.

Human immunodeficiency virus infection is characterized by a progressive decline in the number of peripheral blood CD4(+) T lymphocytes, which finally leads to AIDS. This T-cell decline correlates with the degree of in vitro-induced lymphocyte apoptosis. However, such a correlation has not yet been described in feline AIDS, caused by feline immunodeficiency virus (FIV) infection. We therefore investigated the intensity of in vitro-induced apoptosis in peripheral blood lymphocytes from cats experimentally infected with a Swiss isolate of FIV for 1 year and for 6 years and from a number of long-term FIV-infected cats which were coinfected with feline leukemia virus. Purified peripheral blood lymphocytes were either cultured overnight under nonstimulating conditions or stimulated with phytohemagglutinin and interleukin-2 for 60 h. Under stimulating conditions, the isolates from the infected cats showed significantly higher relative counts of apoptotic cells than did those from noninfected controls (1-year-infected cats, P = 0.01; 6-year-infected cats, P = 0.006). The frequency of in vitro-induced apoptosis was inversely correlated with the CD4(+) cell count (P = 0. 002), bright CD8(+) cell count (P = 0.009), and CD4/CD8 ratio (P = 0. 01) and directly correlated with the percentage of bright major histocompatibility complex class II-positive peripheral blood lymphocytes (P = 0.004). However, we found no correlation between in vitro-induced apoptosis and the viral load in serum samples. Coinfection with feline leukemia virus enhanced the degree of in vitro-induced apoptosis compared with that in FIV monoinfected cats. We concluded that the degree of in vitro-induced apoptosis was closely related to FIV-mediated T-cell depletion and lymphocyte activation and could be used as an additional marker for disease progression in FIV infection.

A comparison of DNA amplification, isolation and serology for the detection of Chlamydia psittaci infection in cats.

Chlamydia psittaci is a significant cause of conjunctivitis in cats, but can be difficult to diagnose owing to the small number of organisms in conjunctival swabs. In the United Kingdom laboratory diagnosis is based on three techniques: isolation of the infectious organism, amplification of chlamydial DNA by the polymerase chain reaction (PCR) or the detection of anti-chlamydial antibodies by immunofluorescence assay. To determine the most sensitive method these techniques were compared in the field. The PCR based on previously published protocols was less sensitive than isolation, but by modifying the protocol its sensitivity was increased by a factor of 25 to 1250 and it was then more sensitive than isolation. The modified PCR detected chlamydia in samples containing non-infectious organisms. Serology was of limited use in predicting which cats shed C psittaci although seronegative cats were negative by PCR and isolation. The modified PCR was the most sensitive and robust method for confirming C psittaci infection in cases of conjunctivitis in pet cats.

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.

The second extracellular loop of CXCR4 determines its function as a receptor for feline immunodeficiency virus.

The feline homolog of the alpha-chemokine receptor CXCR4 has recently been shown to support cell-cell fusion mediated by CXCR4-dependent strains of human immunodeficiency virus (HIV) and strains of feline immunodeficiency virus (FIV) that have been selected for growth in the Crandell feline kidney (CrFK) cell line. In this report we demonstrate that expression of CXCR4 alone is sufficient to render cells from diverse species permissive for fusion with FIV-infected cells, suggesting that CXCR4 is the sole receptor for CrFK-tropic strains of FIV, analogous to CD4-independent strains of HIV-2. To identify the regions of CXCR4 involved in fusion mediated by FIV, we screened panels of chimeric CXCR4 molecules for the ability to support fusion with FIV-infected cells. Human CXCR4 supported fusion more efficiently than feline CXCR4 and feline/human CXCR4 chimeras, suggesting that the second and third extracellular loops of human CXCR4 contain a critical determinant for receptor function. Rat/human CXCR4 chimeras suggested that the second extracellular loop contained the principal determinant for receptor function; however, chimeras constructed between human CXCR2 and CXCR4 revealed that the first and third loops of CXCR4 contribute to the FIV Env binding site, as replacement of these domains with the corresponding domains of CXCR2 rendered the molecule nonfunctional in fusion assays. Mutation of the DRY motif and the C-terminal cytoplasmic tail of CXCR4 did not affect the ability of the molecule to support fusion, suggesting that neither signalling via G proteins nor receptor internalization was required for fusion mediated by FIV; similarly, truncation of the N terminus of CXCR4 did not affect the function of the molecule as a receptor for FIV. CXCR4-transfected feline cells were rendered permissive for infection with both the CrFK-tropic PET isolate of FIV and the CXCR4-dependent RF strain of HIV-1, and susceptibility to infection correlated well with ability to support fusion. The data suggest that the second extracellular loop of CXCR4 is the major determinant of CXCR4 usage by FIV.

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.

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.