Evidence for CD8+ antiviral activity in cats infected with feline immunodeficiency virus

The CD8+ T Cell Noncytotoxic Antiviral Responses

The CD8+ T cell noncytotoxic antiviral response (CNAR) was discovered during studies of asymptomatic HIV-infected subjects more than 30 years ago. In contrast to CD8+ T cell cytotoxic lymphocyte (CTL) activity, CNAR suppresses HIV replication without target cell killing. This activity has characteristics of innate immunity: it acts on all retroviruses and thus is neither epitope specific nor HLA restricted. The HIV-associated CNAR does not affect other virus families. It is mediated, at least in part, by a CD8+ T cell antiviral factor (CAF) that blocks HIV transcription. A variety of assays used to measure CNAR/CAF and the effects on other retrovirus infections are described. Notably, CD8+ T cell noncytotoxic antiviral responses have now been observed with other virus families but are mediated by different cytokines. Characterizing the protein structure of CAF has been challenging despite many biologic, immunologic, and molecular studies. It represents a low-abundance protein that may be identified by future next-generation sequencing approaches. Since CNAR/CAF is a natural noncytotoxic activity, it could provide promising strategies for HIV/AIDS therapy, cure, and prevention.

Large granular lymphocytes are universally increased in human, macaque, and feline lentiviral infection

Large granular lymphocytes (LGLs) have only been anecdotally reported in HIV infection. We previously reported an LGL lymphocytosis in FIV-infected cats associated with a rise in FIV proviral loads and a marked neutropenia that persisted during chronic infection. Extensive immunophenotyping of peripheral blood mononuclear cells in cats chronically infected with FIV were identified LGLs as CD8lo(+)FAS(+); this cell population expanded commensurate with viral load. CD8lo(+)FAS(+) cells expressed similar levels of interferon-γ compared to CD8lo(+)FAS(+) cells from FIV-naive control animals, yet CD3ɛ expression, which was increased on total CD8(+) T cells in FIV-infected cats, was decreased on CD8lo(+)FAS(+) cells. Down-modulation of CD3 expression was reversed after culturing PBMC for 3 days in culture with ConA/IL-2. We identified CD8lo(+)FAS(+) LGLs to be polyclonal T cells lacking CD56 expression. Blood smears from HIV-infected individuals and SIVmac239-infected rhesus macaques revealed increased LGLs compared to HIV/SIV negative counterparts. In humans, there was no correlation with viral load or treatment and in macaques the LGLs arose in acute SIV infection with increases in viremia. This is the first report describing and partially characterizing LGL lymphocytosis in association with lentiviral infections in three different species.

Dispelling myths and focusing on notable concepts in HIV pathogenesis

Since the discovery of HIV over three decades ago, major efforts have been made to control and perhaps eliminate HIV infection worldwide. During these studies, certain myths or misconceptions about this infectious disease have been emphasized and other potentially beneficial concepts have received less attention. A true long-term solution to HIV infection merits an appreciation of alternative ideas and findings that could be beneficial in the ultimate control of HIV/AIDS. Here, I discuss six issues and call for more attention to the science of HIV and well-designed clinical trials.

Ability of CD8+T Cell Anti-Feline Immunodeficiency Virus Activity Correlated with Peripheral CD4+T Cell Counts and Plasma Viremia

In the host defense mechanism against feline immunodeficiency virus (FIV) infection, CD8(+) T cells specifically attack virus-infected cells and suppress the replication of the virus in a non-cytolytic manner by secreting soluble factors. In this study, we measured CD8(+) T cell anti-FIV activity in 30 FIV-infected cats. We investigated its relationship with the number of peripheral blood lymphocytes, particularly the CD4(+) T cell and CD8(+) T cell counts, and the relationship between anti-FIV activity and the number of T cells of CD8alpha(+)beta(lo) and CD8alpha(+)beta(-) phenotypes. A clearly significant correlation was observed between anti-FIV activity and the number of CD4(+) T cells. A weaker anti-FIV activity was associated with a greater decrease in the number of CD4(+) T cells. However, there was no significant correlation between anti-FIV activity and the number of B or CD8(+) T cells. Compared with SPF cats, FIV-infected cats had significantly higher CD8alpha(+)beta(lo) T cell and CD8alpha(+)beta(-) T cell counts, but, no significant correlation was observed between these cell counts and anti-FIV activity. This anti-FIV activity significantly correlated with plasma viremia, which was detected in cats with a weak anti-FIV activity. These results suggest that the anti-FIV activity of CD8(+) T cells plays an important role in plasma viremia and the maintenance of CD4(+) T cells in the body. It is unlikely that CD8alpha(+)beta(lo) or CD8alpha(+)beta(-) T cells appearing after FIV infection represent a phenotype of CD8(+) cells with anti-FIV activity.

Prior mucosal exposure to heterologous cells alters the pathogenesis of cell-associated mucosal feline immunodeficiency virus challenge

BACKGROUND

Several lines of research suggest that exposure to cellular material can alter the susceptibility to infection by HIV-1. Because sexual contact often includes exposure to cellular material, we hypothesized that repeated mucosal exposure to heterologous cells would induce an immune response that would alter the susceptibility to mucosal infection. Using the feline immunodeficiency virus (FIV) model of HIV-1 mucosal transmission, the cervicovaginal mucosa was exposed once weekly for 12 weeks to 5,000 heterologous cells or media (control) and then cats were vaginally challenged with cell-associated or cell-free FIV.

RESULTS

Exposure to heterologous cells decreased the percentage of lymphocytes in the mucosal and systemic lymph nodes (LN) expressing L-selectin as well as the percentage of CD4+ CD25+ T cells. These shifts were associated with enhanced ex-vivo proliferative responses to heterologous cells. Following mucosal challenge with cell-associated, but not cell-free, FIV, proviral burden was reduced by 64% in cats previously exposed to heterologous cells as compared to media exposed controls.

CONCLUSIONS

The pathogenesis and/or the threshold for mucosal infection by infected cells (but not cell-free virus) can be modulated by mucosal exposure to uninfected heterologous cells.

CD4+CD25+ regulatory T cells are infected and activated during acute FIV infection

HIV-induced AIDS may be mediated by the activation of immunosuppressive CD4+CD25+ T regulatory cells (Treg cells). Treg cells have been shown to regulate CD4+ and CD8+ immune responses to HIV and FIV antigens in vitro. We tested the hypothesis that Treg cells become infected and activated during the acute infection with FIV leading to the suppression of CD4+ T helper cell responses. Cats were experimentally infected with FIV-NCSU1 and blood and lymph node cells were collected at weekly intervals following inoculation. Real-time RT-PCR was used to determine plasma viremia and the relative expression of FIV, FoxP3, TGF-beta, and GAPDH mRNA copies in CD4+CD25+ and CD4+CD25- T cell subsets. Flow cytometry was used to assess the absolute numbers of each cell type and the expression of surface TGF-beta and intracellular FoxP3 in CD4+CD25+ and CD4+CD25- T cells at each time-point. Treg suppression of IL-2 production in CD4+ T helper cells was assessed by ELISPOT assays. Our results showed that peak viremia occurred at 2 weeks post infection and correlated with maximal infectivity in CD4+CD25+ T cell populations. FIV-gag-mRNA levels were higher in CD4+CD25+ T cells than CD4+CD25- T cells throughout the acute phase of infection. Induction of FoxP3 and TGF-beta indicated activation of Treg cells during the acute stage infection, which was confirmed by Treg cell suppression of IL-2 production by CD4+ Th cells in an ELISPOT assay. Our findings support the hypothesis that early activation of Treg immunosuppressor function may limit an effective anti-FIV response, contributing to the establishment of chronic infection and the immunodeficiency caused by this virus.

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.

Ability of CD8+ T Cell Anti-Feline Immunodeficiency Virus (FIV) Activity and FIV Proviral DNA Load in Mononuclear Cells in FIV-Infected Cats

We investigated the relationship between CD8+ T cell anti-feline immunodeficiency virus (FIV) activity and FIV proviral DNA load integrated in mononuclear cells. The anti-FIV activity and the proviral DNA load were correlated, and the number of proviral DNA copies was high in cats with decreased anti-FIV activity. Particularly, no anti-FIV activity was detected in the cats staged as having an acquired immunodeficiency syndrome (AIDS)-related complex or AIDS, and the number of proviral DNA copies was obviously increased compared to those in the cats in the asymptomatic stage. These results suggest that decreased anti-FIV activity destroys the control of in vivo FIV replication, which leads to an increased proviral DNA load with the progression of the clinical stage of disease.

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.

The role of inducer cells in mediating in vitro suppression of feline immunodeficiency virus replication

CD8+ T-cell-mediated suppression of feline immunodeficiency virus (FIV) replication has been described by several groups, although the mechanisms of activation and conditions for viral suppression vary with the methodologies. We have previously reported that CD8+ T-cell-mediated suppression of FIV replication required inducer cell stimulation of the effector cells. The focus of the present study was to examine the essential role of inducer cells required for the induction of this soluble anti-FIV activity. Both FIV-PPR-infected T cells and feline skin fibroblasts (FSF) infected with an alphavirus vector expressing FIV capsid or the irrelevant antigen lacZ, stimulated autologous or heterologous effector cells to produce supernatants that suppressed FIV replication. Thus, induction of this suppression of FIV replication did not strictly require autologous inducer cells and did not require the presence of FIV antigen. Anti-viral activity correlated with the presence of CD8+ T cells. Suppression was maximal when the inducer cells and the effector cells were in contact with each other, because separation of the inducer and effector cells by a 0.45-microm membrane reduced FIV suppression by approximately 50%. These findings emphasize the importance for membrane antigen interactions and cytokines in the optimal induction of effector cell synthesis of the soluble anti-FIV activity.

The search for the CD8+ cell anti-HIV factor (CAF)

Efforts to control HIV infection have led to the development of several antiretroviral drugs that can limit virus replication, however, these therapies do not offer a long-term solution to the infection. We can learn a great deal from HIV-infected individuals who have lived for more than ten years and remain healthy without receiving antiviral drugs. These long-term survivors or long-term non-progressors have an immune system that can control HIV infection. A major component of this immune response is innate immunity, particularly the CD8(+) cell antiviral non-cytotoxic response (CNAR), mediated by a novel CD8(+) cell antiviral factor (CAF). The characteristics of CNAR and CAF will be described and progress made toward identifying CAF will be reviewed. These studies have uncovered several potentially important natural anti-HIV factors and their relationship to the originally described CAF is considered.

Covert vertical transmission of feline immunodeficiency virus

Covert vertical transmission of feline immunodeficiency virus (FIV), the feline counterpart of human immunodeficiency virus type 1 (HIV-1), was identified in kittens born to FIV-infected cats. DNA PCR detected FIV gag and env sequences in tissues from kittens nonviable at birth, and in viable kittens monitored postnatally and necropsied at either 11 weeks or 1 year of age. Although FIV DNA was detected in initial blood samples from all 16 viable kittens, viral DNA became increasingly difficult to detect over time and infectious virus could rarely be demonstrated. Only maternal FIV antibody was detected in kitten plasma during the entire postnatal observation period, and kittens remained healthy, with normal CD4:CD8 T cell ratios at >14 months of age. Thus, mother-to-offspring FIV exposure, occurring in utero and postnatally, can result in covert infection in kittens with virus sequestered and contained in tissue sites. These findings appear directly relevant to suspected transient HIV infections and reports of HIV-specific cellular immune responses in highly exposed seronegative adults and uninfected infants born to HIV-positive women.

AIDS vaccination studies using an ex vivo feline immunodeficiency virus model: failure to protect and possible enhancement of challenge infection by four cell-based vaccines prepared with autologous lymphoblasts

Immunogenicity and protective activity of four cell-based feline immunodeficiency virus (FIV) vaccines prepared with autologous lymphoblasts were investigated. One vaccine was composed of FIV-infected cells that were paraformaldehyde fixed at the peak of viral expression. The other vaccines were attempts to maximize the expression of protective epitopes that might become exposed as a result of virion binding to cells and essentially consisted of cells mildly fixed after saturation of their surface with adsorbed, internally inactivated FIV particles. The levels of FIV-specific lymphoproliferation exhibited by the vaccinees were comparable to the ones previously observed in vaccine-protected cats, but antibodies were largely directed to cell-derived constituents rather than to truly viral epitopes and had very poor FIV-neutralizing activity. Moreover, under one condition of testing, some vaccine sera enhanced FIV replication in vitro. As a further limit, the vaccines proved inefficient at priming animals for anamnestic immune responses. Two months after completion of primary immunization, the animals were challenged with a low dose of homologous ex vivo FIV. Collectively, 8 of 20 vaccinees developed infection versus one of nine animals mock immunized with fixed uninfected autologous lymphoblasts. After a boosting and rechallenge with a higher virus dose, all remaining animals became infected, thus confirming their lack of protection.

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.

Constitutive expression of types 1 and 2 cytokines by alveolar macrophages from feline immunodeficiency virus-infected cats

Evidence suggests that feline immunodeficiency virus (FIV), causes pulmonary immunodeficiency. The overall objective of this study was to explore FIV-induced alterations in cell counts and cytokine gene expression in the pulmonary compartment during the acute stage infection. Bronchoalveolar lavage (BAL) cells were collected from FIV-infected and control cats at 0, 4, 10, and 16 weeks post-FIV infection for phenotype and cytokine analysis. The major change in BAL cellular populations following FIV-infection was the development of a neutrophilia. Total BAL cell counts and relative numbers of alveolar macrophages (AM), eosinophils, and lymphocytes remained similar in both groups. The RT-qcPCR analyses of AM purified from BAL showed constitutive expression of TNFα, IL6 and IL10 mRNAs that peaked during the acute stage of infection then declined. The TNFα and IL6 bioactive protein secretion showed a similar response. In contrast, IFNγ expression increased progressively with time after infection and paralleled a progressive increase in FIV-gag mRNA in AM. The IL12 p40 expression also differed from the other cytokines in that there was a progressive decrease in the number of cats with AM IL12 expression following FIV infection. Infection of AM in vitro with FIV also caused an increase in TNFα and IL6 mRNA and bioactive protein suggesting that the increased cytokine response by AM following infection of cats with FIV is an intrinsic characteristic of FIV-infected AM. In summary, pulmonary immune changes seen in FIV-infected cats are similar to those seen in HIV-infected human patients.

The importance of the innate immune system in controlling HIV infection and disease

The innate immune system is the first line of defense against invading pathogens and is particularly important in warding off bacterial and viral infections presenting at the mucosal cell surface. From this primitive immune response, the more sophisticated adaptive immune system was derived. Despite nearly two decades of research directed at inducing adaptive immune responses to HIV, no successful immunological therapy or vaccine has been developed. On the basis of recent observations, it is suggested that instead emphasis should now be placed on the alternative arm of the immune system, the innate immune response. Novel approaches should be developed to elicit this rapidly responding immune activity in HIV infection.

Immunogenicity of an anti-clade B feline immunodeficiency fixed-cell virus vaccine in field cats

Attempts at vaccine development for feline immunodeficiency virus (FIV) have been extensive, both because this is a significant health problem for cats and because FIV may be a useful vaccine model for human immunodeficiency virus. To date, only modest success, producing only short-term protection, has been achieved for vaccine trials in controlled laboratory settings. It is unclear how relevant such experiments are to prevention of natural infection. The current study used a vaccine that employs cell-associated FIV-M2 strain fixed with paraformaldehyde. Subject cats were in a private shelter where FIV was endemic, a prevalence of 29 to 58% over an 8-year observation period. Cats roamed freely from the shelter through the surrounding countryside but returned for food and shelter. After ensuring that cats were FIV negative, they were immunized using six doses of vaccine over a 16-month period and observed for 28 months after the initiation of immunization. Twenty-six cats (12 immunized and 14 nonimmunized controls) were monitored for a minimum of 22 months. Immunized cats did not experience significant adverse effects from immunization and developed both antibodies and cellular immunity to FIV, although individual responses varied greatly. At the conclusion of the study, 0 of 12 immunized cats had evidence of FIV infection, while 5 of 14 control cats were infected. Thus, the vaccine was safe and immunogenic and did not transmit infection. Furthermore, vaccinated cats did not develop FIV infection in a limited clinical trial over an extended time period. Thus, the data suggest that a fixed, FIV-infected cell vaccine has potential for preventing natural FIV infection in free-roaming cats.

CD8+ thymic lymphocytes express reduced levels of CD8beta and increased interferon gamma in cats perinatally infected with the JSY3 molecular clone of feline immunodeficiency virus

Biological isolates of feline immunodeficiency virus (FIV) cause a relative expansion of activated single-positive CD8(+) (SP CD8(+)) lymphocytes within the thymus of infected cats. In this study, thymic SP CD8(+) lymphocytes were analyzed from cats inoculated as neonates with a pathogenic molecular clone of FIV, JSY3, which was previously derived from the wild-type biological isolate FIV(NCSU-1) (NCSU-1). Four cats were inoculated intraperitoneally with NCSU-1 and compared with 11 cats inoculated with JSY3. Five control cats matched in litter and age were administered an intraperitoneal sham inoculum. Between 12 and 16 weeks postinoculation, interferon-gamma (IFN-gamma) mRNA was quantified by RT-PCR in freshly isolated thymocytes and peripheral blood mononuclear cells (PBMCs). The quantity of IFN-gamma mRNA was increased more than 10-fold in thymocytes and PBMCs of 13 of 13 FIV-inoculated cats as compared with the sham-inoculated controls. IFN-gamma mRNA coenriched with magnetically sorted CD8(+) PBMCs and single-positive (SP) CD8(+) thymocytes. Cells expressing IFN-gamma mRNA were located within the thymic perivascular zone, along the corticomedullary junction, and adjacent to lymphoid follicles. The expansion of thymic SP CD8(+) cells was associated with an increase in CD8alpha(+)/beta(neg) and CD8alpha(+)/beta(lo) phenotypes, the latter population resembling a previously reported memory/effector peripheral blood cell with FIV suppressor activity. From these data we conclude that JSY3 and NCSU-1 induce similar phenotypic changes in thymic and peripheral blood CD8(+) cells. Thus, JSY3 is pathogenic for the thymus in vivo and will be useful for defining determinants of the CD8(+) cell response in this pediatric AIDS model.

Class I–unrestricted noncytotoxic anti–HTLV-I activity of CD8+ T cells

Although it is widely believed that viral clearance is mediated principally by the destruction of infected cells by cytotoxic T cells, noncytolytic antiviral activity of CD8+ T cells may play a role in preventing the progression to disease in infections with immunodeficiency viruses and hepatitis B virus. We demonstrate here that (1) replication of human T-lymphotropic virus type I (HTLV-I) is more readily detected from CD8+ T-cell–depleted (CD8−) peripheral blood mononuclear cells (PBMCs) of healthy HTLV-I carriers than from unfractionated PBMCs, (2) cocultures of CD8− PBMCs with autologous or allogeneic CD8+ T cells suppressed HTLV-I replication, and (3) CD8+ T-cell anti-HTLV-I activity is not abrogated intrans-well cultures in which CD8+ cells are separated from CD8− PBMCs by a permeable membrane filter. These results suggest that class I-unrestricted noncytolytic anti–HTLV-I activity is mediated, at least in part by a soluble factor(s), and may play a role in the pathogenesis of HTLV-I infection.

Class I–unrestricted noncytotoxic anti–HTLV-I activity of CD8+ T cells

Although it is widely believed that viral clearance is mediated principally by the destruction of infected cells by cytotoxic T cells, noncytolytic antiviral activity of CD8+ T cells may play a role in preventing the progression to disease in infections with immunodeficiency viruses and hepatitis B virus. We demonstrate here that (1) replication of human T-lymphotropic virus type I (HTLV-I) is more readily detected from CD8+ T-cell–depleted (CD8−) peripheral blood mononuclear cells (PBMCs) of healthy HTLV-I carriers than from unfractionated PBMCs, (2) cocultures of CD8− PBMCs with autologous or allogeneic CD8+ T cells suppressed HTLV-I replication, and (3) CD8+ T-cell anti-HTLV-I activity is not abrogated intrans-well cultures in which CD8+ cells are separated from CD8− PBMCs by a permeable membrane filter. These results suggest that class I-unrestricted noncytolytic anti–HTLV-I activity is mediated, at least in part by a soluble factor(s), and may play a role in the pathogenesis of HTLV-I infection.

Protection against FIV challenge infection by genetic vaccination using minimalistic DNA constructs for FIV env gene and feline IL-12 expression

OBJECTIVE

To evaluate the efficacy of a genetic vaccination protocol based on minimalistic, immunogenic defined gene expression (MIDGE) vectors coding for domains of the feline immunodeficiency virus (FIV) env gene and feline IL-12.

METHODS

Three groups of four cats each were immunized three times within 6 weeks by the ballistic transfer of gold particles coated with MIDGE vectors. Group 1 received non-coated gold beads, groups 2 and 3 MIDGE vectors expressing FIV surface plus part of the transmembrane protein. In addition, group 3 received feline IL-12 DNA. All cats were challenged by intraperitoneal injection of 25 TCID50 of infectious FIV Z2. The following criteria were monitored: clinical signs, antibodies to transmembrane protein, antibodies to whole FIV, haematological parameters and kinetics of CD4 and CD8 cells, FIV proviral load (determined by quantitative polymerase chain reaction; PCR) and cytotoxic T lymphocyte (CTL) activity (in selected cats).

RESULTS

None of the cats developed a detectable antibody response during immunizations. Four weeks after challenge exposure, all cats in group 1 (control) and group 2 (FIV surface-transmembrane protein) had seroconverted and showed a high proviral load until week 19 (end of experiment). In contrast, only one of four cats in group 3 (surface-transmembrane protein and IL-12) showed antibodies; it was provirus positive at reduced virus load. Short-lived CTL activity was found in two cats in group 3.

CONCLUSION

Genetic vaccination using a MIDGE-based construct for the expression of the surface-transmembrane protein domain of FIV env and feline IL-12 DNA led to protection against homologous virus challenge in three out of four vaccinated cats.

Protective immunity against feline immunodeficiency virus induced by inoculation with vif-deleted proviral DNA

To determine whether live-attenuated feline immunodeficiency virus (FIV) proviral DNA will induce protective immunity, a plasmid clone constructed with a FIV provirus containing a deletion in the viral accessory gene vif (FIV-pPPR-Deltavif) was inoculated as proviral DNA into four cats by the intramuscular route. After 43 weeks, these cats were boosted with the same proviral plasmid. Analysis of peripheral blood mononuclear cells at several time points after the primary and booster inoculations revealed no detectable virus or proviral DNA. At 6 weeks after the booster, immunized cats and additional naive control cats were challenged with a cell-free preparation of the infectious biological isolate FIV-PPR by the intraperitoneal route. Virus was detected after challenge in unvaccinated control cats but not in any of the FIV-pPPR-Deltavif-immunized cats. Both FIV Gag- and Env-specific cytotoxic T lymphocyte (CTL) activities were detected in peripheral blood cells of control cats after challenge infection, whereas only one of four cats immunized with FIV-pPPR-Deltavif DNA exhibited a measurable CTL response to Env following challenge. Although anti-Gag antibodies were not detected after both proviral DNA inoculation and challenge, anti-Env antibodies were found in FIV-pPPR-Deltavif-immunized cats after vaccination as well as after challenge. These findings indicate that inoculation with FIV-pPPR-Deltavif proviral DNA induced resistance to challenge with infectious FIV and that a vif deletion mutant may provide a relatively safe attenuated lentiviral vaccine.

Anti-feline immunodeficiency virus (FIV) soluble factor(s) produced from antigen-stimulated feline CD8(+) T lymphocytes suppresses FIV replication

Feline immunodeficiency virus (FIV) causes AIDS-like symptoms in infected cats. Concanavalin A (ConA)-stimulated peripheral blood mononuclear cells (PBMC) from chronically FIV strain PPR-infected cats readily expressed FIV. In contrast, when PBMC from these animals were stimulated with irradiated, autologous antigen-presenting cells (APC), at least a 10-fold drop in viral production was observed. In addition to FIV-specific cytotoxic T lymphocytes, anti-FIV activity was demonstrated in the cell-free supernatants of effector T lymphocytes stimulated with APC. The FIV-suppressive activity was induced from APC-stimulated PBMC of either FIV-infected or uninfected cats but not from ConA-stimulated PBMC. Suppression of FIV strain PPR replication was observed for both autologous and heterologous feline PBMC, was dose dependent, and demonstrated cross-reactivity and cell specificity. It was also demonstrated that the anti-FIV activity originated from CD8(+) T lymphocytes and was mediated by a noncytolytic mechanism.

Attempt to modify the immune response developed against FIV gp120 protein by preliminary FIV DNA injection

Following inactivated virus vaccination trials, the surface glycoprotein gp120 (SU) of the feline immunodeficiency virus (FIV) was considered as one of the determinants for protection. However, several vaccination trials using recombinant Env protein or some Env-derived peptides failed to induce protection. To study the influence of the environment in which the surface protein (SU) is injected. we analyzed the impact of a nucleocapsid (NC) DNA immunization on the presentation of the recSU protein to the immune system. Cats were vaccinated either with the recSU protein alone or with NC DNA followed by the recSU protein. Two routes of nucleocapsid DNA vaccination were tested: intramuscular and mucosal injections. Cats immunized with the recSU protein showed a facilitation of infection, since they presented the earliest and the highest humoral response correlating with the highest proviral load. They also showed an acceleration of the appearance of IL4 mRNA signal. Preliminary injection of the DNA coding for NC protein, regardless the route of inoculation, seemed to inhibit the facilitation induced by vaccination with the recSU protein alone. The previously nucleocapsid DNA immunized cats had infectious status similar to those of the control cats, but with lower proviral load and less developed anti-FIV humoral response. Cat No. 2, belonging to the group vaccinated with NC protein by the mucosal route, had a protected-like status which did not correlate with the humoral response. This cat was the only one to have a persisting IFN mRNA signal after challenge specific for the p10 nucleocapsid and recSU proteins. However, no NC specific cytotoxic cells were observed throughout the experiment in this cat. The role of nucleocapsid DNA vaccination is still unknown nevertheless we did demonstrate that the facilitation observed in vaccination trial with recombinant proteins could be modified and that recombinant proteins could be a component of an effective vaccine.

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

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

Horizontal transmission of feline immunodeficiency virus with semen from seropositive cats

The AIDS virus of cat species, feline immunodeficiency virus (FIV), has been used extensively as an animal model of HIV-1 infection. This felid lentivirus shares many molecular and biochemical traits with HIV-1 and causes similar immunologic and clinical perturbations, most notably CD4+ cell loss, impaired cell-mediated immunity and increased susceptibility to opportunistic pathogens. Previous reports have shown that FIV is transmitted horizontally by biting and vertically in utero and through nursing. Our objective was to determine whether FIV could be venereally transmitted in domestic cats. In the first experiment, susceptibility of the female reproductive tract to mucosal transmission of the FIV isolate, NCSU1, was demonstrated via intravaginal inoculation with infected cultured cells. We next identified virus in electroejaculates from asymptomatic, chronically FIV-NCSU1-infected, adult males. A fragment of FIV gag provirus DNA was detected by nested polymerase chain reaction (PCR) in nonfractionated seminal cells and in swim-up sperm preparations. Additionally, replication-competent virus was isolated from cell-free seminal plasma and seminal cells by co-cultivation with a feline CD4+ T-cell line. In the third study, queens were artificially inseminated via an intrauterine laparoscopic technique with electroejaculates from FIV-NCSU1-infected males. Of six inseminations carried out with fresh semen, three resulted in infection of queens. Lastly, immunohistochemical studies identified potential virus target cell populations in normal female reproductive tissues. In conclusion, these experiments indicate that FIV infection in domestic cats may provide a unique small animal model of sexual transmission of HIV-1.

Neurovirulence in feline immunodeficiency virus-infected neonatal cats is viral strain specific and dependent on systemic immune suppression

Feline immunodeficiency virus (FIV) is a lentivirus that causes immune suppression and neurological disease in cats. Among animal viruses, individual viral strains have been shown to be neurovirulent, but the role of viral strain specificity among lentiviruses and its relationship to systemic immune suppression in the development of neurological disease remains uncertain. To determine the extent to which different FIV strains caused neurological disease, FIV V1CSF and Petaluma were compared in ex vivo assays and in vivo. Both viruses infected and replicated in macrophage and mixed glial cell cultures at similar levels, but V1CSF induced significantly greater neuronal death than Petaluma in a neurotoxicity assay. V1CSF-infected animals showed significant neurodevelopmental delay compared to the Petaluma-infected and uninfected animals. Magnetic resonance spectroscopy studies of frontal cortex revealed significantly reduced N-acetyl aspartate/creatine ratios in the V1CSF group compared to the other groups. Cyclosporin A treatment of Petaluma-infected animals caused neurodevelopmental delay and reduced N-acetyl aspartate/creatine ratios in the brain. Reduced CD4(+) and CD8(+) cell counts were observed in the V1CSF-infected group compared to the uninfected and Petaluma-infected groups. These findings suggest that neurodevelopmental delay and neuronal injury is FIV strain specific but that systemic immune suppression is also an important determinant of FIV-induced neurovirulence.

Studies of AIDS vaccination using an ex vivo feline immunodeficiency virus model: protection conferred by a fixed-cell vaccine against cell-free and cell-associated challenge differs in duration and is not easily boosted

Cats immunized with cells infected with a primary isolate of feline immunodeficiency virus (FIV) and fixed with paraformaldehyde were challenged with cell-free or cell-associated homologous virus obtained ex vivo. Complete protection was observed in animals challenged with cell-free virus 4 months after completion of vaccination (p.v.) or with cell-associated virus 12 months p.v. In contrast, no protection was observed in cats challenged with cell-free virus 12 or 28 months p.v. or with cell-associated virus 37.5 months p.v. Prior to the 28- and 37.5-month challenges, the animals had received a booster dose of vaccine that had elicited a robust anamnestic immune response. These results show that vaccine-induced protection against ex vivo FIV is achievable but is relatively short-lived and can be difficult to boost.