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

Expression of APOBEC3 Lentiviral Restriction Factors in Cats

Feline immunodeficiency virus (FIV) is a naturally occurring T-cell tropic lentiviral disease of felids with many similarities to HIV/AIDS in humans. Similar to primate lentiviral-host interactions, feline APOBEC3 (A3) has been shown to inhibit FIV infection in a host-specific manner and feline A3 degradation is mediated by FIV Vif. Further, infection of felids with non-native FIV strains results in restricted viral replication in both experimental and naturally occurring infections. However, the link between molecular A3-Vif interactions and A3 biological activity during FIV infection has not been well characterized. We thus examined expression of the feline A3 genes A3Z2, A3Z3 and A3Z2-Z3 during experimental infection of domestic cats with host-adapted domestic cat FIV (referred to as FIV) and non-adapted Puma concolor FIV (referred to as puma lentivirus, PLV). We determined A3 expression in different tissues and blood cells from uninfected, FIV-infected, PLV-infected and FIV/PLV co-infected cats; and in purified blood cell subpopulations from FIV-infected and uninfected cats. Additionally, we evaluated regulation of A3 expression by cytokines, mitogens, and FIV infection in cultured cells. In all feline cells and tissues studied, there was a striking difference in expression between the A3 genes which encode FIV inhibitors, with A3Z3 mRNA abundance exceeding that of A3Z2-Z3 by 300-fold or more. Interferon-alpha treatment of cat T cells resulted in upregulation of A3 expression, while treatment with interferon-gamma enhanced expression in cat cell lines. In cats, secondary lymphoid organs and peripheral blood mononuclear cells (PBMC) had the highest basal A3 expression levels and A3 genes were differentially expressed among blood T cells, B cells, and monocytes. Acute FIV and PLV infection of cats, and FIV infection of primary PBMC resulted in no detectable change in A3 expression with the exception of significantly elevated A3 expression in the thymus, the site of highest FIV replication. We conclude that cat A3 expression is regulated by cytokine treatment but, by and large, lentiviral infection did not appear to alter expression. Differences in A3 expression in different blood cell subsets did not appear to impact FIV viral replication kinetics within these cells. Furthermore, the relative abundance of A3Z3 mRNA compared to A3Z2-Z3 suggests that A3Z3 may be the major active anti-lentiviral APOBEC3 gene product in domestic cats.

Low-level laser therapy ameliorates disease progression in a mouse model of Alzheimer's disease

Low-level laser therapy (LLLT) has been used to treat inflammation, tissue healing, and repair processes. We recently reported that LLLT to the bone marrow (BM) led to proliferation of mesenchymal stem cells (MSCs) and their homing in the ischemic heart suggesting its role in regenerative medicine. The aim of the present study was to investigate the ability of LLLT to stimulate MSCs of autologous BM in order to affect neurological behavior and β-amyloid burden in progressive stages of Alzheimer's disease (AD) mouse model. MSCs from wild-type mice stimulated with LLLT showed to increase their ability to maturate towards a monocyte lineage and to increase phagocytosis activity towards soluble amyloid beta (Aβ). Furthermore, weekly LLLT to BM of AD mice for 2 months, starting at 4 months of age (progressive stage of AD), improved cognitive capacity and spatial learning, as compared to sham-treated AD mice. Histology revealed a significant reduction in Aβ brain burden. Our results suggest the use of LLLT as a therapeutic application in progressive stages of AD and imply its role in mediating MSC therapy in brain amyloidogenic diseases.

Accessory genes confer a high replication rate to virulent feline immunodeficiency virus

Feline immunodeficiency virus (FIV) is a lentivirus that causes AIDS in domestic cats, similar to human immunodeficiency virus (HIV)/AIDS in humans. The FIV accessory protein Vif abrogates the inhibition of infection by cat APOBEC3 restriction factors. FIV also encodes a multifunctional OrfA accessory protein that has characteristics similar to HIV Tat, Vpu, Vpr, and Nef. To examine the role of vif and orfA accessory genes in FIV replication and pathogenicity, we generated chimeras between two FIV molecular clones with divergent disease potentials: a highly pathogenic isolate that replicates rapidly in vitro and is associated with significant immunopathology in vivo, FIV-C36 (referred to here as high-virulence FIV [HV-FIV]), and a less-pathogenic strain, FIV-PPR (referred to here as low-virulence FIV [LV-FIV]). Using PCR-driven overlap extension, we produced viruses in which vif, orfA, or both genes from virulent HV-FIV replaced equivalent genes in LV-FIV. The generation of these chimeras is more straightforward in FIV than in primate lentiviruses, since FIV accessory gene open reading frames have very little overlap with other genes. All three chimeric viruses exhibited increased replication kinetics in vitro compared to the replication kinetics of LV-FIV. Chimeras containing HV-Vif or Vif/OrfA had replication rates equivalent to those of the virulent HV-FIV parental virus. Furthermore, small interfering RNA knockdown of feline APOBEC3 genes resulted in equalization of replication rates between LV-FIV and LV-FIV encoding HV-FIV Vif. These findings demonstrate that Vif-APOBEC interactions play a key role in controlling the replication and pathogenicity of this immunodeficiency-inducing virus in its native host species and that accessory genes act as mediators of lentiviral strain-specific virulence.

Selective expansion of viral variants following experimental transmission of a reconstituted feline immunodeficiency virus quasispecies

Following long-term infection with virus derived from the pathogenic GL8 molecular clone of feline immunodeficiency virus (FIV), a range of viral variants emerged with distinct modes of interaction with the viral receptors CD134 and CXCR4, and sensitivities to neutralizing antibodies. In order to assess whether this viral diversity would be maintained following subsequent transmission, a synthetic quasispecies was reconstituted comprising molecular clones bearing envs from six viral variants and its replicative capacity compared in vivo with a clonal preparation of the parent virus. Infection with either clonal (Group 1) or diverse (Group 2) challenge viruses, resulted in a reduction in CD4+ lymphocytes and an increase in CD8+ lymphocytes. Proviral loads were similar in both study groups, peaking by 10 weeks post-infection, a higher plateau (set-point) being achieved and maintained in study Group 1. Marked differences in the ability of individual viral variants to replicate were noted in Group 2; those most similar to GL8 achieved higher viral loads while variants such as the chimaeras bearing the B14 and B28 Envs grew less well. The defective replication of these variants was not due to suppression by the humoral immune response as virus neutralising antibodies were not elicited within the study period. Similarly, although potent cellular immune responses were detected against determinants in Env, no qualitative differences were revealed between animals infected with either the clonal or the diverse inocula. However, in vitro studies indicated that the reduced replicative capacity of variants B14 and B28 in vivo was associated with altered interactions between the viruses and the viral receptor and co-receptor. The data suggest that viral variants with GL8-like characteristics have an early, replicative advantage and should provide the focus for future vaccine development.

Disruption of thiamine uptake and growth of cells by feline leukemia virus subgroup A

Feline leukemia virus (FeLV) is still a major cause of morbidity and mortality in domestic cats and some wild cats despite the availability of relatively effective vaccines against the virus. FeLV subgroup A (FeLV-A) is transmitted in natural infections, and FeLV subgroups B, C, and T can evolve directly from FeLV-A by mutation and/or recombination with endogenous retroviruses in domestic cats, resulting in a variety of pathogenic outcomes. The cell surface entry receptor for FeLV-A is a putative thiamine transporter (THTR1). Here, we have addressed whether FeLV-A infection might disrupt thiamine uptake into cells and, because thiamine is an essential nutrient, whether this disruption might have pathological consequences. First, we cloned the cat ortholog of the other of the two known thiamine transporters in mammals, THTR2, and we show that feline THTR1 (feTHTR1) and feTHTR2 both mediate thiamine uptake, but feTHTR2 does not function as a receptor for FeLV-A. We found that feTHTR1 is widely expressed in cat tissues and in cell lines, while expression of feTHTR2 is restricted. Thiamine uptake mediated by feTHTR1 was indeed blocked by FeLV-A infection, and in feline fibroblasts that naturally express feTHTR1 and not feTHTR2, this blockade resulted in a growth arrest at physiological concentrations of extracellular thiamine. The growth arrest was reversed at high extracellular concentrations of thiamine. Our results show that FeLV-A infection can indeed disrupt thiamine uptake with pathological consequences. A prediction of these experiments is that raising the plasma levels of thiamine in FeLV-infected cats may ameliorate the pathogenic effects of infection.

Prostratin exhibits both replication enhancing and inhibiting effects on FIV infection of feline CD4+ T-cells

The phorbol ester Prostratin may either stimulate or inhibit human immunodeficiency virus-1 (HIV-1) replication. Here we report that Prostratin also exhibits a similar dual action upon feline immunodeficiency virus (FIV) replication in an IL-2-dependent feline CD4⁺ T-cell line (MYA-1). While withdrawal of IL-2 halted FIV spread, Prostratin rescued virus production and cell viability, mimicking the functions of the cytokine. Conversely, FIV grew rapidly in the presence of IL-2 and this was inhibited by Prostratin. In contrast to HIV-1, Prostratin mediated inhibition of FIV through means other than blocking virus entry. Co-application of the protein kinase C (PKC) inhibitor Gö6850 with Prostratin reversed both the inhibitory and stimulatory effects, suggesting that PKC is crucial for FIV replication.

Feline immunodeficiency virus (FIV) neutralization: a review

One of the major obstacles that must be overcome in the design of effective lentiviral vaccines is the ability of lentiviruses to evolve in order to escape from neutralizing antibodies. The primary target for neutralizing antibodies is the highly variable viral envelope glycoprotein (Env), a glycoprotein that is essential for viral entry and comprises both variable and conserved regions. As a result of the complex trimeric nature of Env, there is steric hindrance of conserved epitopes required for receptor binding so that these are not accessible to antibodies. Instead, the humoral response is targeted towards decoy immunodominant epitopes on variable domains such as the third hypervariable loop (V3) of Env. For feline immunodeficiency virus (FIV), as well as the related human immunodeficiency virus-1 (HIV-1), little is known about the factors that lead to the development of broadly neutralizing antibodies. In cats infected with FIV and patients infected with HIV-1, only rarely are plasma samples found that contain antibodies capable of neutralizing isolates from other clades. In this review we examine the neutralizing response to FIV, comparing and contrasting with the response to HIV. We ask whether broadly neutralizing antibodies are induced by FIV infection and discuss the comparative value of studies of neutralizing antibodies in FIV infection for the development of more effective vaccine strategies against lentiviral infections in general, including HIV-1.

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.

Feline tetherin efficiently restricts release of feline immunodeficiency virus but not spreading of infection

Domestic cats endure infections by all three subfamilies of the retroviridae: lentiviruses (feline immunodeficiency virus [FIV]), gammaretroviruses (feline leukemia virus [FeLV]), and spumaretroviruses (feline foamy virus [FFV]). Thus, cats present an insight into the evolution of the host-retrovirus relationship and the development of intrinsic/innate immune mechanisms. Tetherin (BST-2) is an interferon-inducible transmembrane protein that inhibits the release of enveloped viruses from infected cells. Here, we characterize the feline homologue of tetherin and assess its effects on the replication of FIV. Tetherin was expressed in many feline cell lines, and expression was induced by interferons, including alpha interferon (IFN-α), IFN-ω, and IFN-γ. Like human tetherin, feline tetherin displayed potent inhibition of FIV and HIV-1 particle release; however, this activity resisted antagonism by either HIV-1 Vpu or the FIV Env and "OrfA" proteins. Further, as overexpression of complete FIV genomes in trans could not overcome feline tetherin, these data suggest that FIV lacks a functional tetherin antagonist. However, when expressed stably in feline cell lines, tetherin did not abrogate the replication of FIV; indeed, syncytium formation was significantly enhanced in tetherin-expressing cells infected with cell culture-adapted (CD134-independent) strains of FIV (FIV Fca-F14 and FIV Pco-CoLV). Thus, while tetherin may prevent the release of nascent viral particles, cell-to-cell spread remains efficient in the presence of abundant viral receptors and tetherin upregulation may enhance syncytium formation. Accordingly, tetherin expression in vivo may promote the selective expansion of viral variants capable of more efficient cell-to-cell spread.

Modulation of the virus-receptor interaction by mutations in the V5 loop of feline immunodeficiency virus (FIV) following in vivo escape from neutralising antibody

Background

In the acute phase of infection with feline immunodeficiency virus (FIV), the virus targets activated CD4+ T cells by utilising CD134 (OX40) as a primary attachment receptor and CXCR4 as a co-receptor. The nature of the virus-receptor interaction varies between isolates; strains such as GL8 and CPGammer recognise a "complex" determinant on CD134 formed by cysteine-rich domains (CRDs) 1 and 2 of the molecule while strains such as PPR and B2542 require a more "simple" determinant comprising CRD1 only for infection. These differences in receptor recognition manifest as variations in sensitivity to receptor antagonists. In this study, we ask whether the nature of the virus-receptor interaction evolves in vivo.

Results

Following infection with a homogeneous viral population derived from a pathogenic molecular clone, a quasispecies emerged comprising variants with distinct sensitivities to neutralising antibody and displaying evidence of conversion from a "complex" to a "simple" interaction with CD134. Escape from neutralising antibody was mediated primarily by length and sequence polymorphisms in the V5 region of Env, and these alterations in V5 modulated the virus-receptor interaction as indicated by altered sensitivities to antagonism by both anti-CD134 antibody and soluble CD134.

Conclusions

The FIV-receptor interaction evolves under the selective pressure of the host humoral immune response, and the V5 loop contributes to the virus-receptor interaction. Our data are consistent with a model whereby viruses with distinct biological properties are present in early versus late infection and with a shift from a "complex" to a "simple" interaction with CD134 with time post-infection.

[Receptors for animal retroviruses]

Diseases caused by animal retroviruses have been recognized since 19th century in veterinary field. Most livestock and companion animals have own retroviruses. To disclose the receptors for these retroviruses will be useful for understanding retroviral pathogenesis, developments of anti-retroviral drugs and vectors for human and animal gene therapies. Of retroviruses in veterinary field, receptors for the following viruses have been identified; equine infectious anemia virus, feline immunodeficiency virus, feline leukemia virus subgroups A, B, C, and T, Jaagsiekte sheep retrovirus, enzootic nasal tumor virus, avian leukosis virus subgroups A, B, C, D, E, and J, reticuloendotheliosis virus, RD-114 virus (a feline endogenous retrovirus), and porcine endogenous retrovirus subgroup A. Primate lentiviruses require two molecules (CD4 and chemokine receptors such as CXCR4) as receptors. Likewise, feline immunodeficiency virus also requires two molecules, i.e., CD134 (an activation marker of CD4 T cells) and CXCR4 in infection. Gammaretroviruses utilize multi-spanning transmembrane proteins, most of which are transporters of amino acids, vitamins and inorganic ions. Betaretroviruses and alpharetroviruses utilize transmembrane and/or GPI-anchored proteins as receptors. In this review, I overviewed receptors for animal retroviruses in veterinary field.

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.

Enforced covalent trimerisation of soluble feline CD134 (OX40)-ligand generates a functional antagonist of feline immunodeficiency virus

The feline immunodeficiency virus (FIV) targets activated CD4-positive helper T cells preferentially, inducing an AIDS-like immunodeficiency in its natural host species, the domestic cat. The primary receptor for FIV is CD134, a member of the tumour necrosis factor receptor superfamily (TNFRSF) and all primary viral strains tested to date use CD134 for infection. To investigate the effect of the natural ligand for CD134 on FIV infection, feline CD134L was cloned and expressed in soluble forms. However, in contrast to murine or human CD134L, soluble feline CD134L (sCD134L) did not bind to CD134. Receptor-binding activity was restored by enforced covalent trimerisation following the introduction of a synthetic trimerisation domain from tenascin (TNC). Feline and human TNC-CD134Ls retained the species-specificity of the membrane-bound forms of the ligand while murine TNC-CD134L displayed promiscuous binding to feline, human or murine CD134. Feline and murine TNC-CD134Ls were antagonists of FIV infection; however, potency was both strain-specific and substrate-dependent, indicating that the modulatory effects of endogenous sCD134L, or exogenous CD134Lbased therapeutics, may vary depending on the viral strain.

Antiviral activity of membrane fusion inhibitors that target gp40 of the feline immunodeficiency virus envelope protein

For the entry of lentivirus into target cells, fusion between its viral membrane and cellular membrane is essential. The present study was conducted to examine the inhibitory effect of modified peptides corresponding to heptad repeats (HR) 1 and 2 of feline immunodeficiency virus (FIV) envelope gp40 on the fusion between the viral and cellular membranes. FIV-N36 and FIV-C35 were synthesized as authentic peptides of the N-terminal HR1 domain and C-terminal HR2 domain of FIV gp40, respectively. FIV-C35EK1, FIV-C35EK2, and FIV-C35EK3 were peptides synthesized by modifying FIV-C35 as the X-EE-XX-KK concept to increase their solubility in water and the stability of their alpha-helicity. FIV-C35 and FIV-C35EK1 inhibited the cell membrane fusion mediated by FIV-infected cells and the replication of FIV. FIV-N36, FIV-C35EK2, and FIV-C35EK3 did not show any apparent inhibitory effect. These results indicated that the newly developed membrane fusion inhibitors could facilitate the development of novel anti-lentiviral chemotherapies.

A single site for N-linked glycosylation in the envelope glycoprotein of feline immunodeficiency virus modulates the virus-receptor interaction

Feline immunodeficiency virus (FIV) targets helper T cells by attachment of the envelope glycoprotein (Env) to CD134, a subsequent interaction with CXCR4 then facilitating the process of viral entry. As the CXCR4 binding site is not exposed until CD134-binding has occurred then the virus is protected from neutralising antibodies targeting the CXCR4-binding site on Env. Prototypic FIV vaccines based on the FL4 strain of FIV contain a cell culture-adapted strain of FIV Petaluma, a CD134-independent strain of FIV that interacts directly with CXCR4. In addition to a characteristic increase in charge in the V3 loop homologue of FIV_FL4, we identified two mutations in potential sites for N-linked glycosylation in the region of FIV Env analogous to the V1–V2 region of HIV and SIV Env, T271I and N342Y. When these mutations were introduced into the primary GL8 and CPG41 strains of FIV, the T271I mutation was found to alter the nature of the virus-CD134 interaction; primary viruses carrying the T271I mutation no longer required determinants in cysteine-rich domain (CRD) 2 of CD134 for viral entry. The T271I mutation did not confer CD134-independent infection upon GL8 or CPG41, nor did it increase the affinity of the CXCR4 interaction, suggesting that the principal effect was targeted at reducing the complexity of the Env-CD134 interaction.

Genetically divergent strains of feline immunodeficiency virus from the domestic cat (Felis catus) and the African lion (Panthera leo) share usage of CD134 and CXCR4 as entry receptors

The env open reading frames of African lion (Panthera leo) lentivirus (feline immunodeficiency virus [FIV(Ple)]) subtypes B and E from geographically distinct regions of Africa suggest two distinct ancestries, with FIV(Ple)-E sharing a common ancestor with the domestic cat (Felis catus) lentivirus (FIV(Fca)). Here we demonstrate that FIV(Ple)-E and FIV(Fca) share the use of CD134 (OX40) and CXCR4 as a primary receptor and coreceptor, respectively, and that both lion CD134 and CXCR4 are functional receptors for FIV(Ple)-E. The shared usage of CD134 and CXCR4 by FIV(Fca) and FIV(Ple)-E may have implications for in vivo cell tropism and the pathogenicity of the E subtype among free-ranging lion populations.

Establishment of a feline astrocyte-derived cell line (G355-5 cells) expressing feline CD134 and a rapid quantitative assay for T-lymphotropic feline immunodeficiency viruses

Few laboratory strains of feline immunodeficiency virus (FIV) can infect Crandell feline kidney cells (an epithelial-type of cells), however, most primary isolates are T-lymphotropic. T-lymphotropic FIV requires both feline CD134 (an activation marker of helper T-lymphocytes) and CXCR4 (a chemokine receptor) in infection as primary and secondary receptors, respectively. Using feline T-lymphoblastoid cell lines, titration of primary FIV isolates was carried out, however the titration assay was laborious and time-consuming. In this study, using G355-5 cells (a feline astrocyte-derived cell line) transduced with a cDNA of feline CD134 as target cells, an assay system was developed to quantitate primary FIV isolates. With a previous method using a feline T-lymphoblastoid cell line (MYA-1 cells) highly sensitive to FIV, it took 12 days to complete the assay, however, it took only 2 days with the new method. The FIV-infected cells became in a state of persistent infection, producing a large amount of FIV, indicating that the cells will be useful for propagation of T-lymphotropic FIV strains.

Prevention of immunodeficiency virus induced CD4+ T-cell depletion by prior infection with a non-pathogenic virus

Immune dysregulation initiated by a profound loss of CD4+ T-cells is fundamental to HIV-induced pathogenesis. Infection of domestic cats with a non-pathogenic lentivirus prevalent in the puma (puma lentivirus, PLV or FIV(pco)) prevented peripheral blood CD4+ T-cell depletion caused by subsequent virulent FIV infection. Maintenance of this critical population was not associated with a significant decrease in FIV viremia, lending support to the hypothesis that direct viral cytopathic effect is not the primary cause of immunodeficiency. Although this approach was analogous to immunization with a modified live vaccine, correlates of immunity such as a serum-neutralizing antibody or virus-specific T-cell proliferative response were not found in protected animals. Differences in cytokine transcription profile, most notably in interferon gamma, were observed between the protected and unprotected groups. These data provide support for the importance of non-adaptive enhancement of the immune response in the prevention of CD4+ T-cell loss.

Transforming growth factor-beta/transforming growth factor-betaRII signaling may regulate CD4+CD25+ T-regulatory cell homeostasis and suppressor function in feline AIDS lentivirus infection

We have reported that CD4+CD25+ T-regulatory (Treg) cells are fully activated for suppressor function in feline immunodeficiency virus (FIV)-infected cats. Studies have suggested that surface transforming growth factor-beta (TGFbeta; membrane TGFbeta [mTGFbeta]) is a feature of activated CD4+CD25+ Treg cells and may play a role in Treg homeostasis and suppressor function. Herein, we explore the role of TGFbeta in feline Treg homeostasis and suppressor function and what effect FIV infection of cats might have on these processes. Stimulation of CD4+CD25- T helper (Th) cells with Concanavalin A (ConA) plus TGFbeta converts them to Treg-like cells capable of suppressor function. Reverse-transcription polymerase chain reaction and flow cytometry revealed that these ConA/TGFbeta-converted Treg cells upregulate Foxp3 and mTGFbeta. ConA stimulation of CD4+CD25- T cells upregulates TGFbeta receptor II (RII), and pretreatment of these cells with anti-TGFbeta-RII antibodies blocks the TGFbeta-induced conversion to Treg cells. Pretreatment of ConA/TGFbeta-converted Treg cells with anti-TGFbeta antibodies also abrogates their suppressor function, suggesting that Treg homeostasis and suppressor function may be mediated by mTGFbeta. Finally, we show that treatment of CD4+CD25+ mTGFbeta-positive Treg cells from FIV-infected cats with anti-TGFbeta antibodies or treatment of ConA-stimulated CD4+CD25- Th target cells with anti-TGFbeta-RII antibodies diminishes suppressor function. These data suggest that the recruitment of Treg cells from the Th pool and suppressor function of Treg cells are dependent on the TGFbeta/TGFbeta-RII signaling pathway and that this pathway is constitutively upregulated in asymptomatic chronically FIV-infected cats.

Genomic organization, sequence divergence, and recombination of feline immunodeficiency virus from lions in the wild

BACKGROUND

Feline immunodeficiency virus (FIV) naturally infects multiple species of cat and is related to human immunodeficiency virus in humans. FIV infection causes AIDS-like disease and mortality in the domestic cat (Felis catus) and serves as a natural model for HIV infection in humans. In African lions (Panthera leo) and other exotic felid species, disease etiology introduced by FIV infection are less clear, but recent studies indicate that FIV causes moderate to severe CD4 depletion.

RESULTS

In this study, comparative genomic methods are used to evaluate the full proviral genome of two geographically distinct FIV subtypes isolated from free-ranging lions. Genome organization of FIVPle subtype B (9891 bp) from lions in the Serengeti National Park in Tanzania and FIVPle subtype E (9899 bp) isolated from lions in the Okavango Delta in Botswana, both resemble FIV genome sequence from puma, Pallas cat and domestic cat across 5' LTR, gag, pol, vif, orfA, env, rev and 3'LTR regions. Comparative analyses of available full-length FIV consisting of subtypes A, B and C from FIVFca, Pallas cat FIVOma and two puma FIVPco subtypes A and B recapitulate the species-specific monophyly of FIV marked by high levels of genetic diversity both within and between species. Across all FIVPle gene regions except env, lion subtypes B and E are monophyletic, and marginally more similar to Pallas cat FIVOma than to other FIV. Sequence analyses indicate the SU and TM regions of env vary substantially between subtypes, with FIVPle subtype E more related to domestic cat FIVFca than to FIVPle subtype B and FIVOma likely reflecting recombination between strains in the wild.

CONCLUSION

This study demonstrates the necessity of whole-genome analysis to complement population/gene-based studies, which are of limited utility in uncovering complex events such as recombination that may lead to functional differences in virulence and pathogenicity. These full-length lion lentiviruses are integral to the advancement of comparative genomics of human pathogens, as well as emerging disease in wild populations of endangered species.

[Feline immunodeficiency virus tropism]

Feline immunodeficiency virus (FIV) induces a disease similar to acquired immunodeficiency syndrome (AIDS) in cats, yet in contrast to human immunodeficiency virus (HIV), CD4 is not the viral receptor. We identified a primary receptor for FIV as CD134 (OX40), a T cell activation antigen and costimulatory molecule. CD134 expression promotes viral binding and renders cells permissive for viral entry, productive infection, and syncytium formation. Infection is CXCR4-dependent, analogous to infection with X4 strains of HIV. Thus, despite the evolutionary divergence of the feline and human lentiviruses, both viruses use receptors that target the virus to a subset of cells that are pivotal to the acquired immune response. Further, we applied the new method for FIV receptor to Ebola virus entry factors with some modifications, and identified receptor-type tyrosine kinases, Axl and Dtk (members of Tyro3 family). Distribution of the molecules matches well with the Ebola virus tropism.

Frequent transmission of immunodeficiency viruses among bobcats and pumas

With the exception of human immunodeficiency virus (HIV), which emerged in humans after cross-species transmissions of simian immunodeficiency viruses from nonhuman primates, immunodeficiency viruses of the family Lentiviridae represent species-specific viruses that rarely cross species barriers to infect new hosts. Among the Felidae, numerous immunodeficiency-like lentiviruses have been documented, but only a few cross-species transmissions have been recorded, and these have not been perpetuated in the recipient species. Lentivirus seroprevalence was determined for 79 bobcats (Lynx rufus) and 31 pumas (Puma concolor) from well-defined populations in Southern California. Partial genomic sequences were subsequently obtained from 18 and 12 seropositive bobcats and pumas, respectively. Genotypes were analyzed for phylogenic relatedness and genotypic composition among the study set and archived feline lentivirus sequences. This investigation of feline immunodeficiency virus infection in bobcats and pumas of Southern California provides evidence that cross-species infection has occurred frequently among these animals. The data suggest that transmission has occurred in multiple locations and are most consistent with the spread of the virus from bobcats to pumas. Although the ultimate causes remain unknown, these transmission events may occur as a result of puma predation on bobcats, a situation similar to that which fostered transmission of HIV to humans, and likely represent the emergence of a lentivirus with relaxed barriers to cross-species transmission. This unusual observation provides a valuable opportunity to evaluate the ecological, behavioral, and molecular conditions that favor repeated transmissions and persistence of lentivirus between species.

Probing the interaction between feline immunodeficiency virus and CD134 by using the novel monoclonal antibody 7D6 and the CD134 (Ox40) ligand

The feline immunodeficiency virus (FIV) targets activated CD4-positive helper T cells preferentially, inducing an AIDS-like immunodeficiency in its natural host species, the domestic cat. The primary receptor for FIV is CD134, a member of the tumor necrosis factor receptor superfamily, and all primary viral strains tested to date use CD134 for infection. We examined the expression of CD134 in the cat using a novel anti-feline CD134 monoclonal antibody (MAb), 7D6, and showed that as in rats and humans, CD134 expression is restricted tightly to CD4+, and not CD8+, T cells, consistent with the selective targeting of these cells by FIV. However, FIV is also macrophage tropic, and in chronic infection the viral tropism broadens to include B cells and CD8+ T cells. Using 7D6, we revealed CD134 expression on a B220-positive (B-cell) population and on cultured macrophages but not peripheral blood monocytes. Moreover, macrophage CD134 expression and FIV infection were enhanced by activation in response to bacterial lipopolysaccharide. Consistent with CD134 expression on human and murine T cells, feline CD134 was abundant on mitogen-stimulated CD4+ T cells, with weaker expression on CD8+ T cells, concordant with the expansion of FIV into CD8+ T cells with progression of the infection. The interaction between FIV and CD134 was probed using MAb 7D6 and soluble CD134 ligand (CD134L), revealing strain-specific differences in sensitivity to both 7D6 and CD134L. Infection with isolates such as PPR and B2542 was inhibited well by both 7D6 and CD134L, suggesting a lower affinity of interaction. In contrast, GL8, CPG, and NCSU were relatively refractory to inhibition by both 7D6 and CD134L and, accordingly, may have a higher-affinity interaction with CD134, permitting infection of cells where CD134 levels are limiting.

FIV as a Model for HIV: An Overview

Animal models for human immunodeficiency virus (HIV) infection play a key role in understanding the pathogenesis of AIDS and the development of therapeutic agents and vaccines. As the only lentivirus that causes an immunodeficiency resembling that of HIV infection, in its natural host, feline immunodeficiency virus (FIV) has been a unique and powerful model for AIDS research. FIV was first described in 1987 by Niels Pedersen and co-workers as the causative agent for a fatal immunodeficiency syndrome observed in cats housed in a cattery in Petaluma, California. Since this landmark observation, multiple studies have shown that natural and experimental infection of cats with biological isolates of FIV produces an AIDS syndrome very similar in pathogenesis to that observed for human AIDS. FIV infection induces an acute viremia associated with Tcell alterations including depressed CD4 :CD8 T-cell ratios and CD4 T-cell depletion, peripheral lymphadenopathy, and neutropenia. In later stages of FIV infection, the host suffers from chronic persistent infections that are typically self-limiting in an immunocompetent host, as well as opportunistic infections, chronic diarrhea and wasting, blood dyscracias, significant CD4 T-cell depletion, neurologic disorders, and B-cell lymphomas. Importantly, chronic FIV infection induces a progressive lymphoid and CD4 T-cell depletion in the infected cat. The primary mode of natural FIV transmission appears to be blood-borne facilitated by fighting and biting. However, experimental infection through transmucosal routes (rectal and vaginal mucosa and perinatal) have been well documented for specific FIV isolates. Accordingly, FIV disease pathogenesis exhibits striking similarities to that described for HIV-1 infection.

Prime-boost vaccination using DNA and whole inactivated virus vaccines provides limited protection against virulent feline immunodeficiency virus

Protection against feline immunodeficiency virus (FIV) has been achieved using a variety of vaccines notably whole inactivated virus (WIV) and DNA. However protection against more virulent isolates, typical of those encountered in natural infections, has been difficult to achieve. In an attempt to improve protection against virulent FIV(GL8), we combined both DNA and WIV vaccines in a "prime-boost" approach. Thirty cats were divided into four groups receiving vaccinations and one unvaccinated control group. Following viral challenge, two vaccinated animals, one receiving DNA alone and one the prime-boost vaccine remained free of viraemia, whilst all controls became viraemic. Animals vaccinated with WIV showed apparent early enhancement of infection at 2 weeks post challenge (pc) with higher plasma viral RNA loads than control animals or cats immunised with DNA alone. Despite this, animals vaccinated with WIV or DNA alone showed significantly lower proviral loads in peripheral blood mononuclear cells and mesenteric lymph node cells, whilst those receiving the DNA-WIV prime-boost vaccine showed significantly lower proviral loads in PBMC, than control animals, at 35 weeks pc. Therefore both DNA and WIV vaccines conferred limited protection against viral challenge but the combination of WIV and DNA in a prime-boost approach appeared to offer no significant advantage over either vaccine alone.

Differential utilization of CD134 as a functional receptor by diverse strains of feline immunodeficiency virus

The feline homologue of CD134 (fCD134) is the primary binding receptor for feline immunodeficiency virus (FIV), targeting the virus preferentially to activated CD4+ helper T cells. However, with disease progression, the cell tropism of FIV broadens such that B cells and monocytes/macrophages become significant reservoirs of proviral DNA, suggesting that receptor utilization may alter with disease progression. We examined the receptor utilization of diverse strains of FIV and found that all strains tested utilized CD134 as the primary receptor. Using chimeric feline x human CD134 receptors, the primary determinant of receptor function was mapped to the first cysteine-rich domain (CRD1) of fCD134. For the PPR and B2542 strains, the replacement of CDR1 of fCD134 (amino acids 1 to 64) with human CD134 (hCD134) alone was sufficient to confer nearly optimal receptor function. However, evidence of differential utilization of CD134 was revealed, since strains GL8, CPGammer (CPG41), TM2, 0827, and NCSU1 required determinants in the region spanning amino acids 65 to 85, indicating that these strains may require a more stringent interaction for infection to proceed.

Lentivirus envelope protein exerts differential neuropathogenic effects depending on the site of expression and target cell

We investigated the neuropathogenic effects of feline immunodeficiency virus (FIV) envelope proteins in the context of both extracellular exposure and intracellular expression in feline neural cells. The envelope from the neurovirulent CSF-derived FIV V1 strain (V1-CSF) conferred infectivity to pseudotyped viruses in peripheral blood mononuclear cells (P < 0.01) in contrast to other cell types. Intracellular V1-CSF envelope expression in macrophages and microglia but not astrocytes resulted in the induction of host inflammatory genes contributing to neurotoxicity including IL-1beta, TNF-alpha, and indolamine 2',3'-dioxygenase (IDO) (P < 0.05) with concurrent neuronal death (P < 0.05). Upregulation of the endoplasmic reticulum stress genes was evident in brains from FIV-infected animals (P < 0.05) and in FIV-infected macrophages (P < 0.05) relative to controls. Intrastriatal implantation of an FIV envelope pseudotyped virus led to marked neuroinflammation and neuronal injury associated with neurobehavioral deficits (P < 0.01). Thus, lentivirus envelope proteins exert differential neuropathogenic effects through mechanisms that depend on the infected or exposed cell type.

Feline lentiviruses demonstrate differences in receptor repertoire and envelope structural elements

Feline immunodeficiency virus (FIV) causes fatal disease in domestic cats via T cell depletion-mediated immunodeficiency. Pumas and lions are hosts for apparently apathogenic lentiviruses (PLV, LLV) distinct from FIV. We compared receptor use among these viruses by: (1) evaluating target cell susceptibility; (2) measuring viral replication following exposure to specific and non-specific receptor antagonists; and (3) comparing Env sequence and structural motifs. Most isolates of LLV and PLV productively infected domestic feline T cells, but differed from domestic cat FIV by infecting cells independent of CXCR4, demonstrating equivalent or enhanced replication following heparin exposure, and demonstrating substantial divergence in amino acid sequence and secondary structure in Env receptor binding domains. PLV infection was, however, inhibited by CD134/OX40 antibody. Thus, although PLV and LLV infection interfere with FIV superinfection, we conclude that LLV and PLV utilize novel, more promiscuous mechanisms for cell entry than FIV, underlying divergent tropism and biological properties of these viruses.

[Evolution of lentiviruses and receptor specificity]

Lentiviruses consist of primate lentiviruses, ungulate lentiviruses and feline immunodeficiency virus (FIV). The primate lentiviruses utilize CD4 and chemokine receptors as a primary receptor and coreceptors, respectively. Recently we found that FIV utilizes CD134 and CXCR4 as a primary receptor and a coreceptor, respectively. FIV utilizes feline CD134 but not human CD134, whereas it can utilize both feline and human CXCR4. Exceptionally an FIV laboratory strain can infect human cells via CXCR4 only by the CD134-independent manner. Similarly several strains of primate lentiviruses also infect cells by the CD4-independent manner. In this review, the evolution of the lentiviruses and possible mechanism for lentiviral cross-species transmission is discussed.

Development and validation of puma (Felis concolor) cytokine and lentivirus real-time PCR detection systems

Studies of immune correlates of disease outcome associate humoral immune response mediated by T-helper 2 cytokines (IL-4, IL-10) with more virulent disease relative to a cell-mediated response driven by T-helper 1 cytokines (IL-2, IFN-gamma), particularly in viral and other intra-cellular infections. Specifically, the kinetics of both human immunodeficiency virus (HIV) and feline immunodeficiency virus (FIV) infection are closely associated with Type 1 versus Type 2 cytokine profiles. Puma (Felis concolor) lentivirus (PLV) is closely related to FIV, but based on phylogenetic and clinical studies, is more ancient and less pathogenic. The aims of this study were to validate feline real-time PCR primer/probe systems for puma cytokines and PLV as sensitive, quantitative assays for use in investigations of PLV pathogenicity. We demonstrate that primer/probe systems for IL-4, IL-10, IFN-gamma, TNF-alpha, GAPDH, and the pol region of PLV-1695 amplify puma cytokines and PLV-1695 with high amplification efficiency and sensitivity. Detection of PLV-1695 provirus in experimentally inoculated domestic cats proved to be of equivalent sensitivity, specificity, and positive and negative predictive value to co-culture of one million peripheral blood mononuclear cells (PBMC). Evaluation of cytokine induction during naturally occurring PLV infection will allow insight into mechanisms of host control associated with apathogenic infection. In addition, determination of viral loads during different stages of PLV infection or in different tissues from domestic cats or pumas will further elucidate capacity of these viruses to replicate and establish infection.

Puma lentivirus is controlled in domestic cats after mucosal exposure in the absence of conventional indicators of immunity

A high percentage of free-ranging pumas (Felis concolor) are infected with feline lentiviruses (puma lentivirus, feline immunodeficiency virus Pco [FIV-Pco], referred to here as PLV) without evidence of disease. PLV establishes productive infection in domestic cats following parenteral exposure but, in contrast to domestic cat FIV, it does not cause T-cell dysregulation. Here we report that cats exposed to PLV oro-nasally became infected yet rapidly cleared peripheral blood mononuclear cell (PBMC) proviral load in the absence of a correlative specific immune response. Two groups of four specific-pathogen-free cats were exposed to PLV via the mucosal (oro-nasal) or parenteral (i.v.) route. All animals were PBMC culture positive and PCR positive within 3 weeks postinfection and seroconverted without exhibiting clinical disease; however, three or four oro-nasally infected animals cleared circulating proviral DNA within 3 months. Antibody titers reached higher levels in animals that remained persistently infected. PLV antigen-induced proliferation was slightly greater in mucosally inoculated animals, but no differences were noted in cytotoxic T-lymphocyte responses or cytokine profiles between groups. The distribution of virus was predominantly gastrointestinal as opposed to lymphoid in all animals in which virus was detected at necropsy. Possible mechanisms for viral clearance include differences in viral fitness required for crossing mucosal surfaces, a threshold dose requirement for persistence, or an undetected sterilizing host immune response. This is the first report of control of a productive feline or primate lentivirus infection in postnatally exposed, seropositive animals. Mechanisms underlying this observation will provide clues to containment of immunodeficiency disease and could prompt reexamination of vaccine-induced immunity against human immunodeficiency virus and other lentiviruses.

Vaccination with an inactivated virulent feline immunodeficiency virus engineered to express high levels of Env

An inactivated virus vaccine was prepared from a pathogenic isolate of feline immunodeficiency virus containing a mutation that eliminated an endocytic sorting signal in the envelope glycoprotein, increasing its expression on virions. Cats immunized with inactivated preparations of this modified virus exhibited strong titers of antibody to Env by enzyme-linked immunosorbent assay. Evidence of protection following challenge demonstrated the potential of this approach to lentiviral vaccination.

Functional relationship between the matrix proteins of feline and simian immunodeficiency viruses

To investigate the functional relationship between the matrix (MA) proteins of feline and simian immunodeficiency viruses (FIV and SIV, respectively), we generated chimeric proviruses in which the MA-coding region of an SIV infectious molecular clone was partially or fully replaced by its FIV counterpart. Chimeric SIV proviruses containing the amino-terminal 36 residues or the central and carboxy-terminal regions of the FIV MA assembled into virions as efficiently as wild-type SIV. However, the resulting virions were noninfectious in single-cycle infectivity assays. Furthermore, a chimeric SIV provirus containing the entire FIV MA was found to be severely impaired in virion production due to inefficient membrane binding of the chimeric Gag polyprotein. Interestingly, the assembly defective phenotype of this chimeric Gag precursor could be reversed either by introducing the G31K/G33K double amino acid substitution in the FIV-derived MA domain or by coexpression with wild-type SIV Gag. Of note, a chimeric FIV provirus expressing the SIV MA not only assembled into particles as efficiently as wild-type FIV, but also replicated in feline T cells with wild-type kinetics. Our results thus provide novel information about the functional homology between the MA proteins of distantly related lentiviruses.

Downmodulation of CD3epsilon expression in CD8alpha+beta- T cells of feline immunodeficiency virus-infected cats

Feline immunodeficiency virus (FIV) infection in cats is associated with an increase of feline CD (fCD)8alpha+beta- and fCD8alpha+beta low cells in peripheral blood. To investigate these cells in more detail, an anti-fCD3epsilon mAb, termed NZM1, was generated, which recognizes the extracellular epitope of the fCD3epsilon molecule. The anti-fCD3epsilon mAb proved to be more suitable for identifying feline T cells than the anti-fCD5 one, which has been used as a pan-T-cell reagent in cats, because of the presence of fCD5+fCD3epsilon- cells among lymphocytes. Although the fCD8alpha+beta- and fCD8alpha+beta low cells in the FIV-infected cats expressed fCD3epsilon, a subset of fCD8alpha+beta- cells expressed fCD3epsilon antigen at a lower level than the T cells whose phenotype was fCD4+, or fCD8alpha+beta low. The lower expression of fCD3epsilon may be associated with the immune status of fCD8alpha+beta- T cells.

T cell subpopulations mediating inhibition of feline immunodeficiency virus replication in mucosally infected cats

Feline immunodeficiency virus (FIV) infection induces an increase in two subpopulations (CD8alpha(+)beta(low) and CD8alpha(+)beta(-)) within CD8(+) peripheral blood lymphocytes (PBLs) of cats. It is known that depletion of CD8(+) cells often results in augmentation of FIV proliferation in PBL culture, similarly to the case of human immunodeficiency virus. In this study, we attempted to define PBL subpopulations mediating antiviral activity in five cats intravaginally infected with a molecularly cloned FIV isolate. Several subpopulations (CD8alpha(+)beta(+), CD8alpha(+)beta(-), and CD4(+) cells) were shown to participate in inhibition of the FIV replication, at least in part, in a major histocompatibility complex-unrestricted manner. Moreover, the subpopulations showing anti-FIV activity were different among the individual cats.

Use of CD134 as a primary receptor by the feline immunodeficiency virus

Feline immunodeficiency virus (FIV) induces a disease similar to acquired immunodeficiency syndrome (AIDS) in cats, yet in contrast to human immunodeficiency virus (HIV), CD4 is not the viral receptor. We identified a primary receptor for FIV as CD134 (OX40), a T cell activation antigen and costimulatory molecule. CD134 expression promotes viral binding and renders cells permissive for viral entry, productive infection, and syncytium formation. Infection is CXCR4-dependent, analogous to infection with X4 strains of HIV. Thus, despite the evolutionary divergence of the feline and human lentiviruses, both viruses use receptors that target the virus to a subset of cells that are pivotal to the acquired immune response.

Identification of the feline CD63 homologue using retrovirus-mediated expression cloning

Recently, we combined a retrovirus-mediated expression cloning with a simple screening method using non-adherent cells and panning [Anal. Biochem. 315 (2003) 138]. In this study, we applied this method to identify the antigen recognized by an uncharacterized monoclonal antibody raised against a feline cell line, and identified it as the feline homologue of CD63. This simple method is useful for characterizing unknown antibodies that recognize cell surface molecules. Furthermore, the monoclonal antibody identified as an anti-feline CD63 antibody will be useful for studying feline molecular function(s).

Phenotypic changes in CD8+ peripheral blood lymphocytes in cats infected with feline immunodeficiency virus

It is well documented that several cell surface molecules of T lymphocytes are altered by immune activation. We previously reported that feline immunodeficiency virus (FIV) infection induces a reduction in CD8beta chain expression of peripheral blood lymphocytes (PBLs) in cats. In this study, we performed three-color flow-cytometric analysis for activation-associated cell surface molecules (CD2, CD11a, CD45RA-like and major histocompatibility complex antigen class II (MHC II)) and light scatters (cellular size and complexity) to examine whether phenotypic changes also occurred in CD4(+) PBLs in addition to CD8(+) PBLs, of five FIV-infected cats and one uninfected cat. It was shown that (i) CD8alpha(+) PBLs, but not CD4(+) PBLs, had a distinct subpopulation with increased CD11a expression accompanying a reduced CD8beta chain and increased intracellular granules (ii) CD8alpha(+) PBLs, but not CD4(+) PBLs, expressed CD45RA-like antigen with diverse expression levels and (iii) MHC II expression was greater in CD8alpha(+) PBLs than CD4(+) PBLs, and the CD8beta chain reduction was correlated with the MHC II decrease within CD8alpha(+) PBLs. These results suggest that FIV infection induces phenotypically heterogeneous subpopulations in CD8(+) PBLs, including activated phenotypes, rather than in CD4(+) PBLs.

Experimental mucosal infection with molecularly cloned feline immunodeficiency viruses

Four of six specific pathogen-free cats were infected after intravaginal exposure to molecularly cloned lymphotropic but non-Crandell feline kidney (CRFK)-tropic feline immunodeficiency virus strain TM2 and its AP-1 deletion mutant. The sequences of the env V3-to-V5 region which defines the CRFK tropism were unchanged in the infected cats through the infection. These data suggest that the strain was transmitted across the mucosal epithelium without a broadening of cell tropism.

Upregulation of surface feline CXCR4 expression following ectopic expression of CCR5: implications for studies of the cell tropism of feline immunodeficiency virus

Feline CXCR4 and CCR5 were expressed in feline cells as fusion proteins with enhanced green fluorescent protein (EGFP). Expression of the EGFP fusion proteins was localized to the cell membrane, and surface expression of CXCR4 was confirmed by using a cross-species-reactive anti-CXCR4 monoclonal antibody. Ectopic expression of feline CCR5 enhanced expression of either endogenous feline CXCR4 or exogenous feline or human CXCR4 expressed from a retrovirus vector, indicating that experiments investigating the effect of CCR5 expression on feline immunodeficiency virus (FIV) infection must be interpreted with caution. Susceptibility to infection with cell culture-adapted strains of FIV or to syncytium formation following transfection with a eukaryotic vector expressing an env gene from a cell culture-adapted strain of virus correlated with expression of either human or feline CXCR4, whereas feline CCR5 had no effect. In contrast, neither CXCR4 nor CCR5 rendered cells permissive to either productive infection with primary strains of FIV or syncytium formation following transfection with primary env gene expression vectors. Screening a panel of Ghost cell lines expressing diverse human chemokine receptors confirmed that CXCR4 alone supported fusion mediated by the FIV Env from cell culture-adapted viruses. CXCR4 expression was upregulated in Ghost cells coexpressing CXCR4 and CCR5 or CXCR4, CCR5, and CCR3, and susceptibility to FIV infection could be correlated with the level of CXCR4 expression. The data suggest that beta-chemokine receptors may influence FIV infection by modulating the expression of CXCR4.

Evolution of replication efficiency following infection with a molecularly cloned feline immunodeficiency virus of low virulence

The development of an effective vaccine against human immunodeficiency virus is considered to be the most practicable means of controlling the advancing global AIDS epidemic. Studies with the domestic cat have demonstrated that vaccinal immunity to infection can be induced against feline immunodeficiency virus (FIV); however, protection is largely restricted to laboratory strains of FIV and does not extend to primary strains of the virus. We compared the pathogenicity of two prototypic vaccine challenge strains of FIV derived from molecular clones; the laboratory strain PET(F14) and the primary strain GL8(414). PET(F14) established a low viral load and had no effect on CD4(+)- or CD8(+)-lymphocyte subsets. In contrast, GL8(414) established a high viral load and induced a significant reduction in the ratio of CD4(+) to CD8(+) lymphocytes by 15 weeks postinfection, suggesting that PET(F14) may be a low-virulence-challenge virus. However, during long-term monitoring of the PET(F14)-infected cats, we observed the emergence of variant viruses in two of three cats. Concomitant with the appearance of the variant viruses, designated 627(W135) and 628(W135,) we observed an expansion of CD8(+)-lymphocyte subpopulations expressing reduced CD8 beta-chain, a phenotype consistent with activation. The variant viruses both carried mutations that reduced the net charge of the V3 loop (K409Q and K409E), giving rise to a reduced ability of the Env proteins to both induce fusion and to establish productive infection in CXCR4-expressing cells. Further, following subsequent challenge of naïve cats with the mutant viruses, the viruses established higher viral loads and induced more marked alterations in CD8(+)-lymphocyte subpopulations than did the parent F14 strain of virus, suggesting that the E409K mutation in the PET(F14) strain contributes to the attenuation of the virus.

Expression of chemokine receptors in the feline reproductive tract and large intestine

Feline immunodeficiency virus (FIV) is a lentivirus that causes feline acquired immunodeficiency syndrome. Infection can be transmitted experimentally via the vagina and rectum, making the cat a useful model for human immunodeficiency virus (HIV) infection. Some strains of FIV use the CXCR4 chemokine receptor in vitro to gain entry to feline cell lines, thymocytes and peripheral blood leucocytes (PBLs). In this study, the tissue expression of messenger ribonucleic acid (mRNA) encoding the CCR3, CXCR4 and CCR5 receptors was examined by reverse transcriptase polymerase chain reaction (RT-PCR). mRNA encoding each receptor was expressed by two feline T-cell lines (Mya-1 and FeTJ), a feline kidney fibroblast cell line (FKCU) and PBLs. Mesenteric lymph node, colon, rectum, uterus, cervix and vagina all expressed mRNA for CXCR4 and CCR5 whilst only lymph node expressed CCR3 mRNA. In order to locate this receptor mRNA expression, in-situ hybridization studies were performed with DNA probes specific for the chemokine receptor mRNAs. CCR5 and CXCR4 receptor mRNA was expressed by epithelial cells and some lamina propria cells of the colon and rectum. Epithelial cell expression of chemokine receptor mRNA was reduced in intensity towards the base of the crypts. Expression of CXCR4 receptor was also demonstrated immunohistochemically on some lamina propria and intraepithelial cells. The expression of these receptor molecules may be important in mucosal infection with FIV.

A feline CD2 homologue interacts with human red blood cells

A cDNA encoding a feline homologue of CD2 (fCD2) was identified. Several amino acids (aa) important for ligand interaction, molecular folding or signal transduction, found in other mammalian CD2, were found to be highly conserved in the predicted fCD2 aa sequence. fCD2-expressing cells were able to form rosettes with human red blood cells (probably via human CD58), and the rosette formation was inhibited by an anti-fCD2 monoclonal antibody. These results are indicative of the similarity of feline and human CD2 structures. fCD2 was found to be expressed in feline peripheral blood T lymphocytes, monocytes and cultured lymphoid cells.

Involvement of cytolytic and non-cytolytic T cells in the control of feline immunodeficiency virus infection

The appearance of non-cytolytic T cells that suppressed feline immunodeficiency virus (FIV) replication in vitro, and FIV-specific cytotoxic T cell (CTL) responses was compared in a group of seven, specific pathogen free (SPF) domestic cats following primary infection with the Glasgow(8) isolate of FIV (FIV(GL-8)). FIV proviral burdens were quantified in the blood and lymphoid tissues by real-time PCR. Non-cytolytic T cell suppression of FIV replication was measured by co-cultivating lymphoblasts prepared from the cats at different time-points during infection with FIV-infected MYA-1 cells in vitro. Non-cytolytic suppressor activity was detected as early as 1 week after infection, and was evident in all the lymphoid tissues examined. Further, this activity was present in subpopulations of T cells in the blood with normal (CD8(hi)) or reduced (CD8(lo)) expression of the CD8 molecule, and temporal modulations in non-cytolytic suppressor activity were unrelated to the circulating CD8(+) T cell numbers. Virus-specific CTL responses, measured by (51)Cr release assays, were not detected until 4 weeks after infection, with the emergence of FIV-specific effector CTLs in the blood. Throughout infection the response was predominantly directed towards FIV Gag-expressing target cells, and by 47 weeks after infection CTL responses had become localised in the lymph nodes and spleen. The results suggest that both non-cytolytic T cell suppression of FIV replication and FIV-specific CTL responses are important cellular immune mechanisms in the control of FIV replication in infected asymptomatic cats.

Protection against feline immunodeficiency virus using replication defective proviral DNA vaccines with feline interleukin-12 and -18

A molecular clone of the Glasgow-8 isolate of FIV (FIVGL8) was rendered replication defective by an in-frame deletion in either reverse transcriptase (deltaRT) or integrase (deltaIN) genes for use as DNA vaccines. To test the ability of these multi-gene vaccines to protect against two feline immunodeficiency virus (FIV) isolates of differing virulence, cats were immunized using either DNA vaccine alone or co-administered with interleukin-12 (IL-12) and/or interleukin-18 (IL-18) cytokine DNA. Animals were challenged sequentially with FIV-Petaluma (FIVPET) an FIV isolate of relatively low virulence and subsequently with the more virulent FIVGL8. A proportion of vaccinates (5/18 deltaIN and 2/12 deltaRT) were protected against primary challenge with FIV(PET). Five of the vaccinated-protected cats were re-challenged with FIV(PET); four (all deltaIN) remained free of viraemia whilst all naive controls became viraemic. Following subsequent challenge with the more virulent FIVGL8 these four vaccinated-protected animals all became viraemic but showed lower proviral loads than naive cats. This study suggests that while our current DNA vaccines may not produce sterilizing immunity against more virulent isolates of FIV, they may nevertheless significantly reduce the impact of infection.

Pathogenic potential of canine parvovirus types 2a and 2c in domestic cats

The in vivo pathogenicity of canine parvovirus (CPV) type 2c (strain V203) and of CPV type 2a (strain V154) against cats was investigated. Our results indicate that both types of CPV have the potential to induce disease in cats.

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.

Efficient expression of the envelope protein of feline immunodeficiency virus in a recombinant feline herpesvirus type 1 (FHV-1) using the gC promoter of FHV-1

We constructed two recombinant feline herpesvirus type 1 (FHV-1) expressing the envelope (Env) protein of feline immunodeficiency virus (FIV). One recombinant, designated dlTK-env, has the whole FIV env gene inserted at a thymidine kinase (TK) deletion site. The second recombinant, designated dlTK(gCp)-env, has a cassette containing a partial FIV env gene fused with the signal sequence of the gC protein of FHV-1 (under the control of the gC promoter) inserted at the same site. Growth kinetics of both the recombinants in Crandell feline kidney (CRFK) cells were similar to that of the parent strain of FHV-1. By indirect immunofluorescence assays and immunoblot analyses, we confirmed the expression of the FIV Env protein in CRFK cells infected with both recombinants. Enzyme-linked immunosorbent assays showed that the maximum Env expression level achieved by dlTK(gCp)-env was more than four times higher than that observed for dlTK-env. Flow cytometric analyses revealed that the Env protein produced by both recombinants was efficiently expressed on the cell surface. The dlTK(gCp)-env reported here may thus be a promising candidate for a live recombinant vaccine to protect against FIV infection.