Experimental vaccine protection against feline immunodeficiency virus

Developing a Feline Immunodeficiency Virus Subtype B Vaccine Prototype Using a Recombinant MVA Vector

The feline immunodeficiency virus (FIV) is a retrovirus with global impact and distribution, affecting both domestic and wild cats. This virus can cause severe and progressive immunosuppression culminating in the death of felids. Since the discovery of FIV, only one vaccine has been commercially available. This vaccine has proven efficiency against FIV subtypes A and D, whereas subtype B (FIV-B), found in multiple continents, is not currently preventable by vaccination. We, therefore, developed and evaluated a vaccine prototype against FIV-B using the recombinant viral vector modified vaccinia virus Ankara (MVA) expressing the variable region V1-V3 of the FIV-B envelope protein. We conducted preclinical tests in immunized mice (C57BL/6) using a prime-boost protocol with a 21 day interval and evaluated cellular and humoral responses as well the vaccine viability after lyophilization and storage. The animals immunized with the recombinant MVA/FIV virus developed specific splenocyte proliferation when stimulated with designed peptides. We also detected cellular and humoral immunity activation with IFN-y and antibody production. The data obtained in this study support further development of this immunogen and testing in cats.

Lessons Learned in Developing a Commercial FIV Vaccine: The Immunity Required for an Effective HIV-1 Vaccine

The feline immunodeficiency virus (FIV) vaccine called Fel-O-Vax^® FIV is the first commercial FIV vaccine released worldwide for the use in domestic cats against global FIV subtypes (A⁻E). This vaccine consists of inactivated dual-subtype (A plus D) FIV-infected cells, whereas its prototype vaccine consists of inactivated dual-subtype whole viruses. Both vaccines in experimental trials conferred moderate-to-substantial protection against heterologous strains from homologous and heterologous subtypes. Importantly, a recent case-control field study of Fel-O-Vax-vaccinated cats with outdoor access and ≥3 years of annual vaccine boost, resulted in a vaccine efficacy of 56% in Australia where subtype-A viruses prevail. Remarkably, this protection rate is far better than the protection rate of 31.2% observed in the best HIV-1 vaccine (RV144) trial. Current review describes the findings from the commercial and prototype vaccine trials and compares their immune correlates of protection. The studies described in this review demonstrate the overarching importance of ant-FIV T-cell immunity more than anti-FIV antibody immunity in affording protection. Thus, future efforts in developing the next generation FIV vaccine and the first effective HIV-1 vaccine should consider incorporating highly conserved protective T-cell epitopes together with the conserved protective B-cell epitopes, but without inducing adverse factors that eliminate efficacy.

An initial examination of the potential role of T-cell immunity in protection against feline immunodeficiency virus (FIV) infection

The importance of vaccine-induced T-cell immunity in conferring protection with prototype and commercial FIV vaccines is still unclear. Current studies performed adoptive transfer of T cells from prototype FIV-vaccinated cats to partial-to-complete feline leukocyte antigen (FLA)-matched cats a day before either homologous FIVPet or heterologous-subtype pathogenic FIVFC1 challenge. Adoptive-transfer (A-T) conferred a protection rate of 87% (13 of 15, p < 0.001) against FIVPet using the FLA-matched T cells, whereas all 12 control cats were unprotected. Furthermore, A-T conferred protection rate of 50% (6 of 12, p<0.023) against FIVFC1 using FLA-matched T cells, whereas all 8 control cats were unprotected. Transfer of FLA-matched T and B cells demonstrated that T cells are needed to confer A-T protection. In addition, complete FLA-matching and addition of T-cell numbers > 13 × 10(6) cells were required for A-T protection against FIVFC1 strain, reported to be a highly pathogenic virus resistant to vaccine-induced neutralizing-antibodies. The addition of FLA-matched B cells alone was not protective. The poor quality of the anti-FIV T-cell immunity induced by the vaccine likely contributed to the lack of protection in an FLA-matched recipient against FIVFC1. The quality of the immune response was determined by the presence of high mRNA levels of cytolysin (perforin) and cytotoxins (granzymes A, B, and H) and T helper-1 cytokines (interferon-γ [IFNγ] and IL2). Increased cytokine, cytolysin and cytotoxin production was detected in the donors which conferred protection in A-T studies. In addition, the CD4(+) and CD8(+) T-cell proliferation and/or IFNγ responses to FIV p24 and reverse transcriptase increased with each year in cats receiving 1X-3X vaccine boosts over 4 years. These studies demonstrate that anti-FIV T-cell immunity induced by vaccination with a dual-subtype FIV vaccine is essential for prophylactic protection against AIDS lentiviruses such as FIV and potentially HIV-1.

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.

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.

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.

Alloimmunity does not protect from challenge with the feline immunodeficiency virus

Immune responses against polymorphic host molecules incorporated into lentiviral envelopes during cell budding have induced protection against primate immunodeficiency virus infection. Dendritic cells (DCs) express high levels of MHC molecules and are infectable by lentiviruses. Therefore, in this pilot study we addressed the hypothesis that immunization of cats with allogeneic DC would induce immune responses that protect against challenge with the feline immunodeficiency virus. Two groups of 3 cats each received 3 subcutaneous injections of allogeneic or autologous DC, and were then challenged with viruses propagated in the immunizing DC. Infection status and lymphocyte parameters of cats were assessed during 6 weeks after challenge. MHC II antigens were incorporated into viral particles as identified by Western blot; and antibodies reactive with MHC class II antigens were detected in the serum of cats immunized with allogeneic but not autologous DC. After challenge, all cats had proviral DNA in blood leukocytes from 2 weeks post-challenge onward and seroconverted. Cats immunized with allogeneic DC maintained higher total and CD21(+) lymphocyte concentrations, and higher CD4(+)/CD8(+) lymphocyte ratios; however, these differences were not significantly different from cats that received autologous DC immunizations. Plasma viral load was not significantly different between groups of cats (p=0.204). These results suggest that immunization of cats with allogeneic DC does not induce protective immunity against FIV infection.

Evaluation of feline monocyte-derived dendritic cells loaded with internally inactivated virus as a vaccine against feline immunodeficiency virus

Dendritic cells are the only antigen-presenting cells that can present exogenous antigens to both helper and cytolytic T cells and prime Th1-type or Th2-type cellular immune responses. Given their unique immune functions, dendritic cells are considered attractive "live adjuvants" for vaccination and immunotherapy against cancer and infectious diseases. The present study was carried out to assess whether the reinjection of autologous monocyte-derived dendritic cells loaded with an aldithriol-2-inactivated primary isolate of feline immune deficiency virus (FIV) was able to elicit protective immune responses against the homologous virus in naive cats. Vaccine efficacy was assessed by monitoring immune responses and, finally, by challenge with the homologous virus of vaccinated, mock-vaccinated, and healthy cats. The outcome of challenge was followed by measuring cellular and antibody responses and viral and proviral loads and quantitating FIV by isolation and a count of CD4(+)/CD8(+) T cells in blood. Vaccinated animals exhibited clearly evident FIV-specific peripheral blood mononuclear cell proliferation and antibody titers in response to immunization; however, they became infected with the challenge virus at rates comparable to those of control animals.

Advances in FIV vaccine technology

Advances in vaccine technology are occurring in the molecular techniques used to develop vaccines and in the assessment of vaccine efficacy, allowing more complete characterization of vaccine-induced immunity correlating to protection. FIV vaccine development has closely mirrored and occasionally surpassed the development of HIV-1 vaccine, leading to first licensed technology. This review will discuss technological advances in vaccine designs, challenge infection assessment, and characterization of vaccine immunity in the context of the protection detected with prototype and commercial dual-subtype FIV vaccines and in relation to HIV-1.

Lentivirus-induced immune dysregulation

FIV/HIV infections are associated with an early robust humoral and cellular anti-viral immune response followed by a progressive immune suppression that eventually results in AIDS. Several mechanisms responsible for this immune dysfunction have been proposed including cytokine dysregulation, immunologic anergy and apoptosis, and inappropriate activation of immune regulatory cells. Studies on FIV infection provide evidence for all three. Cytokine alterations include decreases in IL2 and IL12 production and increases in IFNgamma and IL10 in FIV(+) cats compared to normal cats. The elevated IL10:IL12 ratio is associated with the inability of FIV(+) cats to mount a successful immune response to secondary pathogens. Additionally, chronic antigenic (FIV) stimulation results in an increase in the percent of activated T cells expressing B7 and CTLA4 co-stimulatory molecules in infected cats. The expression of these molecules is associated with T cells that are undergoing apoptosis in the lymph nodes. As ligation of CTLA4 by B7 transduces a signal for induction of anergy, one can speculate that the activated T cells are capable of T cell-T cell interactions resulting in anergy and apoptosis. The inability of CD4(+) cells from FIV(+) cats to produce IL2 in response to recall antigens and the gradual loss of CD4(+) cell numbers could be due to B7-CTLA4 interactions. The chronic antigenemia may also lead to activation of CD4(+)CD25(+) T regulatory cells. Treg cells from FIV(+) cats are chronically activated and inhibit the mitogen-induced proliferative response of CD4(+)CD25(-) by down-regulating IL2 production. Although Treg cell activation can be antigen-specific, the suppressor function is not, and thus activated Treg cells would suppress responses to secondary pathogens as well as to FIV. Concomitant with the well-known virus-induced immune suppression is a progressive immune hyper-activation. Evidence for immune hyper-activation includes polyclonal B cell responses, gradual replacement of naïve CD4(+) and CD8(+) T cell phenotypes with activation phenotypes (CD62L(-), B7(+), CTLA4(+)), and the chronic activation of CD4(+)CD25(+) Treg cells. Thus lentivirus infections lead to severe immune dysregulation manifested as both chronic immune suppression and chronic immune activation. FIV infection of cats provides a number of advantages over other lentivirus infections as a model to study this immune dysregulation. It is a natural infection that has existed in balance with the cat's immune system for thousands of years. As such, the natural history and pathogenesis provides an excellent model to study the long-term relationships between AIDS lentivirus and host immune system function/dysregulation.

Vaccination against the feline immunodeficiency virus: the road not taken

Natural infection of domestic cats by the feline immunodeficiency virus (FIV) causes acquired immunodeficiency syndrome (AIDS). FIV is genetically related to human immunodeficiency virus (HIV), and the clinical and biological features of infections caused by feline and human viruses in their respective hosts are highly analogous. Although the obstacles to vaccinating against FIV and HIV would seem to be of comparable difficulty, a licensed vaccine against feline AIDS is already in widespread use in several countries. While this seemingly major advance in prevention of AIDS would appear to be highly instructive for HIV vaccine development, its message has not been heeded by investigators in the HIV field. This review endeavours to relate what has been learned about vaccination against feline AIDS, and to suggest what this may mean for HIV vaccine development.

Vaccine protection against feline immunodeficiency virus: setting the challenge

Since feline immunodeficiency virus (FIV) was first isolated, international research efforts have been directed towards developing a protective vaccine, not least because it may provide a model for a candidate human immunodeficiency virus (HIV) vaccine. This article reviews the challenges facing vaccine development, the current state of knowledge and future prospects for FIV vaccination.

FIV as a Model for AIDS Vaccine Studies

Many experimental strategies have been adopted in experiments to protect cats from FIV infection by vaccination, and some have been successful. The interest in developing a vaccine arose both because FIV is a common cause of morbidity and mortality in pet cats and because the feline virus provides a model for its counterpart in man, human immunodeficiency virus (HIV), for which an effective vaccine is urgently required to halt the current tragic pandemic of acquired immunodeficiency syndrome (AIDS). Shortly after the discovery of FIV and its characterization as a lentivirus, attempts were made to produce a vaccine and success was soon achieved with relatively simple inactivated virus or inactivated virus-infected cell vaccines.82 Further development of this approach led to the introduction in 2002 of the first commercial vaccine against FIV.59 With an estimated prevalence of the infection of up to 25% in populations of pet cats, an effective FIV vaccine could have a significant influence on animal welfare. In addition, this success poses the question of whether a similar strategy might produce an effective vaccine against HIV.

Efficacy and safety of a feline immunodeficiency virus vaccine

Fel-O-Vax FIV is an inactivated virus vaccine designed as an aid in the prevention of infection of cats, 8 weeks or older, by feline immunodeficiency virus (FIV). It contains two genetically distinct FIV strains. The efficacy of this vaccine was demonstrated in a vaccination–challenge study designed to meet various regulatory requirements for registering the vaccine. Eight-week-old kittens were vaccinated with an immunogenicity vaccine which contained minimal release levels of FIV antigens formulated with a proprietary adjuvant system. Twelve months later, all vaccinates and controls were challenged with a heterologous FIV strain. Following the vigorous challenge exposure, cats were monitored for FIV viremia. It was found that 16% of the vaccinated cats developed viremia while 90% of the controls became persistently infected with FIV, which demonstrated that the vaccine was efficacious and the protective immunity lasted for at least 12 months. The safety of the vaccine was demonstrated by a field safety trial in which only 22 mild reactions of short duration were observed following administering 2051 doses of two pre-licensing serials of Fel-O-Vax FIV to cats of various breeds, ages and vaccination histories. Thus, Fel-O-Vax FIV is safe and efficacious for the prevention of FIV infection in cats.

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.

Lessons from the cat: development of vaccines against lentiviruses

Feline immunodeficiency virus (FIV) is a natural infection of domestic cats, which produces a disease with many similarities to human immunodeficiency virus (HIV) infection in man. The virus is an important cause of morbidity and mortality in pet cats worldwide. As such an effective vaccine is desirable both for its use in veterinary medicine and also as a model for the development of an HIV vaccine. A large number of candidate vaccines have been tested against feline immunodeficiency virus. These include inactivated virus and infected cell vaccines, DNA and viral vectored vaccines, subunit and peptide vaccines and vaccines using bacterial vectors. Ultimately, the development of inactivated virus and infected cell vaccines led to the release of the first licensed vaccine against FIV, in 2002. This review highlights some of the difficulties associated with the development of lentiviral vaccines and some of the lessons that have been learned in the FIV model that are of particular relevance to the development of HIV vaccines.

Induction of human immunodeficiency virus neutralizing antibodies using fusion complexes

Human immunodeficiency virus-1 (HIV-1) infects cells by membrane fusion that is mediated by the envelope proteins gp120/gp41 and the cellular receptors CD4 and CCR5. During this process, some conserved viral epitopes are temporarily exposed and may induce a neutralizing antibody response when fixed in the fusogenic conformation. These transient structures are conserved and may be effective antigens for use in an anti-HIV-1 vaccine. In this study we tested different conditions of preparation of fusion complexes inducing neutralizing antibodies against both R5 and X4 tropic HIV-1 strains. Cell lines expressing HIV-1 gp120/gp41 and CD4-CCR5 were prepared and conditions for producing fusion complexes were tested. Complexes produced at different temperature and fixative combinations were used to immunize mice. Results indicated that (a) fusion complexes prepared at either 21 degrees C, 30 degrees C or 37 degrees C were immunogenic and induced neutralizing antibodies against both R5 and X4 HIV-1 heterologous isolates; (b) after extensive purification of antibodies there was no cytotoxic effect; (c) complexes prepared at 37 degrees C were more immunogenic and induced higher titers of neutralizing antibodies than complexes prepared at either 21 degrees C or 30 degrees C; (d) the fixative used did not affect the titer of neutralizing antibodies except for glutaraldehyde which was ineffective; (e) the neutralizing activity was retained after CD4-CCR5 antibody removal. The production of higher titers of neutralizing antibody with fusion complexes prepared at 37 degrees C, as compared to lower temperatures, may be related to the induction of antibodies against many different conformation intermediates that subsequently act synergistically at different steps in the fusion process.

Formaldehyde-treated, heat-inactivated virions with increased human immunodeficiency virus type 1 env can be used to induce high-titer neutralizing antibody responses

The lack of success of subunit human immunodeficiency virus (HIV) type 1 vaccines to date suggests that multiple components or a complex virion structure may be required. We hypothesized that the failure of current vaccine strategies to induce protective antibodies is linked to the inability of native envelope structures to readily elicit these types of antibodies. We have previously reported on the ability of a formaldehyde-treated, heat-inactivated vaccine to induce modest antibody responses in animal vaccine models. We investigated here whether immunization for HIV with an envelope-modified, formaldehyde-stabilized, heat-inactivated virion vaccine could produce higher-titer and/or broader neutralizing antibody responses. Thus, a clade B vaccine which contains a single point mutation in gp41 (Y706C) that results in increased incorporation of oligomeric Env into virions was constructed. This vaccine was capable of inducing high-titer antibodies that could neutralize heterologous viruses, including those of clades A and C. These results further support the development of HIV vaccines with modifications in native Env structures for the induction of neutralizing antibody responses.

FIV-infected cats respond to short-term rHuG-CSF treatment which results in anti-G-CSF neutralizing antibody production that inactivates drug activity

The hematological and virological effects of recombinant human granulocyte colony-stimulating factor (rHuG-CSF) were evaluated in feline immunodeficiency virus (FIV)-infected cats. Six age-matched, FIV-infected cats used in this cross-over study were injected subcutaneously with 5 microg/kg of rHuG-CSF daily for 3 weeks, while six control cats received a placebo. Five of six rHuG-CSF-treated cats had significant increases in neutrophil counts that peaked on days 11-21 of treatment. All rHuG-CSF-treated cats exhibited an increase in myeloid:erythroid ratios of the bone marrow cells without significant changes in lymphocyte, CD4 counts, CD4/CD8 ratios, RBC counts, FIV antibody titers, and FIV loads in peripheral blood, and without clinical and hematological toxicities. Five of six rHuG-CSF-treated cats developed antibodies to rHuG-CSF by 14-21 days of treatment, which correlated with decreasing neutrophil counts and increasing neutralizing antibodies to rHuG-CSF. Three cats re-treated with rHuG-CSF rapidly developed neutralizing antibodies to rHuG-CSF, while one cat also developed neutralizing antibodies to recombinant feline G-CSF (rFeG-CSF). Overall, rHuG-CSF treatment increased neutrophil counts in FIV-infected cats without affecting the infection status of cats. However, long-term use of rHuG-CSF is not recommended in cats because of the neutralizing antibody production to rHuG-CSF that affects the drug activity. In addition, a preliminary finding suggests that repeated treatment cycle can also induce cross-neutralizing antibodies to rFeG-CSF, which may potentially affect the homeostasis of endogenous FeG-CSF.

Dual-subtype vaccine (Fel-O-Vax FIV) protects cats against contact challenge with heterologous subtype B FIV infected cats

Fel-O-Vax FIV is a dual-subtype vaccine consisting of inactivated whole viruses of subtype A (Petaluma strain) and subtype D (Shizuoka strain). The efficacy of this vaccine against heterologous subtype A strain challenge was demonstrated, but it is unclear whether the result reflects efficacy in the field. In this study, we evaluated the efficacy of this vaccine against contact challenge by exposing both vaccinated and unvaccinated control animals with cats infected with Aomori-2 strain belonging to subtype B, a subtype prevalent in many regions of the world. Nineteen specific-pathogen-free (SPF) cats were divided into a vaccinated group (six cats), an unvaccinated control group (eight cats), and a challenge group (five cats), and maintained in the same room. Cats were monitored for FIV proviral DNA by nested PCR and for FIV-specific antibody levels by ELISA. After 1 year of commingling, each cat in the vaccinated group was given a booster dose. In addition, the original challenge group was removed and replaced with another challenge group of SPF cats, which were inoculated with the Aomori-2 strain. FIV infection was confirmed in four of the eight animals in the unvaccinated control group by the 29th week in the second year of commingling. In contrast, all of the animals were negative in the vaccinated group. These findings confirmed the efficacy of this vaccine against heterologous stains classified as subtype B, and suggested that the vaccine exhibits broad efficacy against genetically diverse FIV.

Cellular immune responses to feline immunodeficiency virus (FIV) induced by dual-subtype FIV vaccine

Vaccine-induced T cell responses to FIV were assessed by measuring FIV-specific cytokine and cytotoxic-effector molecule production. A total of 22 cats at 10-12 weeks of age received either dual-subtype FIV vaccine (n=12), uninfected cell lysate (n=5) consisting of cells used to produce vaccine viruses, or no immunization (n=5). Significant increases in mRNA and protein production of T-helper 1 (Th1) cytokines (IL-2, IFNgamma), mRNA production of a cytotoxic-effector molecule (perforin), and lymphoproliferation response were observed in peripheral blood mononuclear cells (PBMC) from dual-subtype FIV-vaccinated cats after in vitro stimulation with inactivated FIV. In contrast, no statistically significant increase in FIV-stimulated mRNA production of Th2 cytokines (IL-4, IL-6) or other cytotoxic-effector molecules (TNFalpha, FasL) was observed in the PBMC from dual-subtype vaccinated cats. Moreover, no FIV-specific increases in the IFNgamma, IL-2, and perforin mRNA productions and in the IFNgamma bioactivity and lymphoproliferation responses were observed in the PBMC from cell-immunized cats. These observations suggest that IFNgamma induction, lymphoproliferation, and significant portion of IL-2 and perforin productions in the PBMC from dual-subtype vaccinated cats are clearly specific for viral antigens. Overall, dual-subtype FIV vaccine elicited strong Th1 response (IFN(, IL-2), which may contribute to the vaccine protection by enhancing the perforin-mediated cytotoxic-cell activity against FIV.

Antibodies specific for hypervariable regions 3 to 5 of the feline immunodeficiency virus envelope glycoprotein are not solely responsible for vaccine-induced acceleration of challenge infection in cats

In a previous vaccination study in cats, the authors reported on accelerated feline immunodeficiency virus (FIV) replication upon challenge in animals vaccinated with a candidate envelope subunit vaccine. Plasma transfer studies as well as antibody profiles in vaccinated cats indicated a causative role for antibodies directed against the hypervariable regions HV3, HV4 and HV5 (HV3-5) of the envelope glycoprotein. The present study was designed to investigate further the contribution of antibodies in envelope vaccine-induced acceleration of FIV infection. To this end, regions HV3-5 of the envelope glycoprotein were deleted from the original vaccine, thus addressing the contributing role of antibodies directed against these hypervariable regions. Interestingly, this approach did not prevent acceleration of challenge infection. Analysis of the antibody responses in the respective groups suggested that removal of HV3-5 redirected the humoral immune response towards other regions of the envelope glycoprotein, indicating that these regions can also induce antibodies that accelerate virus replication.

Domestic cats infected with lion or puma lentivirus develop anti-feline immunodeficiency virus immune responses

Attenuated live viral strains have afforded significant protection against virus challenge in HIV vaccine models. Although both cellular and humoral immunity are assumed to be vital for protection, specific parameters consistently associated with control of infection have been elusive. Our previous studies have shown that lentiviruses from 2 nondomestic feline species--lion (Pathera leo) and puma (Felis concolor)--persistently but nonpathogenetically infect domestic cats (Felis domestica). Moreover, infection with either the puma lentivirus (PLV) or lion lentivirus (LLV) conferred partial protection against superinfection with virulent feline immunodeficiency virus (FIV), the feline equivalent of HIV. To determine whether domestic cats infected by the lentiviruses of pumas or lions generate cross-reactive immune responses, we infected groups of 5 domestic cats with PLV, LLV, or a sham control and then monitored virus load, hematologic parameters, antibody protection, proliferative responses, and the ability of blood mononuclear cells to inhibit LLV, PLV, and FIV replication in vitro. All cats inoculated with LLV or PLV developed persistent infection, and low-level cell-associated viremia has been previously described. Infected cats also generated robust antibody titers and lymphocytes that proliferated in response to viral antigens and downregulated PLV, LLV, and FIV replication in vitro. This latter activity was CD8 cell associated for PLV and LLV inhibition but not for FIV inhibition. Thus, cats infected with the phylogenetically more ancient and less pathogenic feline lentiviruses generated humoral and cell-mediated immune responses reactive against both the homologous viruses and the heterologous FIV of domestic cats, which correlated with decreased viral load. These results are analogous to protection studies with attenuated primate immunodeficiency viruses and provide a system by which to examine adaptation, interference, and cross protection among lentiviruses.

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.

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.

Is AZT/3TC therapy effective against FIV infection or immunopathogenesis?

In vitro and in vivo prophylactic and therapeutic efficacy of AZT/3TC treatment was evaluated against feline immunodeficiency virus (FIV) infection. In vitro studies utilized FIV-infected peripheral blood mononuclear cells (PBMCs) or FIV-infected T-cell lines treated with AZT (azidothymidine) alone, 3TC alone, or AZT/3TC combination and tested for anti-FIV activity and drug toxicity. AZT/3TC combination had additive to synergistic anti-FIV activities in primary PBMC but not in chronically infected cell lines. In vivo studies consisted of four treatment groups (n=15) of SPF cats receiving AZT/3TC combination (5-75 mg/kg/drug PO BID for 8 or 11 weeks) and one control group (n=9) receiving oral placebo. Group I (n=6, 150 mg/kg/drug/day) was treated starting 3 days pre-FIV inoculation, whereas Group II (n=3, 150 mg/kg/drug/day) and Group III (n=3, 100 mg/kg/drug/day) treatments were simultaneous with FIV inoculation. Group IV treatment (n=3, 100 mg/kg/drug/day) was initiated 2 weeks post-FIV inoculation. All cats were monitored for drug toxicity and FIV infection. Eighty-three percent of cats in Group I and 33% of cats in Groups II and III were completely protected from FIV infection. A significant delay in infection and antibody seroconversion was observed in all unprotected cats from Groups I, II and III. Group IV cats had only a slight delay in FIV antibody seroconversion. Adverse drug reactions (anemia and neutropenia) were observed at high doses (100-150 mg/kg/drug/day) were reversible upon lowering the dose (20 mg/kg/drug/day). In contrast, AZT/3TC treatment had no anti-FIV activity in chronically infected cats. Furthermore, severe clinical symptoms caused by adverse drug reactions were observed in some of these cats. Overall, AZT/3TC treatment is effective for prophylaxis but not for therapeutic use in chronically FIV-infected cats.

Targeted lymph node immunization can protect cats from a mucosal challenge with feline immunodeficiency virus

With the rapid spread of human immunodeficiency virus (HIV) infection worldwide it is clear that effective strategies for mucosal vaccination against lentiviruses are urgently required. The aim of the present study is to determine whether protective immune responses against a mucosal challenge by feline immunodeficiency virus (FIV) can be elicited by targeting the immunization to the medial iliac lymph nodes--the principal site of migration of cells from the genital and rectal mucosa. Cats were challenged with homologous FIV via the rectal route. Targeted lymph node immunization was found to be an effective route of immunization eliciting both humoral and proliferative responses to peptide-based and fixed cell vaccines. Vaccination with fixed virus infected cells elicited protection against a cell-free mucosal FIV challenge. In addition, some cats vaccinated with fixed uninfected cells also remained uninfected following a cell-associated FIV challenge.

Dual-subtype FIV vaccine protects cats against in vivo swarms of both homologous and heterologous subtype FIV isolates

OBJECTIVE

To evaluate the immunogenicity and efficacy of an inactivated dual-subtype feline immunodeficiency virus (FIV) vaccine.

DESIGN

Specific-pathogen-free cats were immunized with dual-subtype (subtype A FIV(Pet) and subtype D FIV(Shi)) vaccine and challenged with either in vivo- or in vitro-derived FIV inocula.

METHODS

Dual-subtype vaccinated, single-subtype vaccinated, and placebo-immunized cats were challenged within vivo-derived heterologous subtype B FIV(Bang) [10--100 50% cat infectious doses (CID(50))], in vivo-derived homologous FIV(Shi)(50 CID(50)), and in vitro- and in vivo-derived homologous FIV(Pet)(20--50 CID(50)). Dual-subtype vaccine immunogenicity and efficacy were evaluated and compared to single-subtype strain vaccines. FIV infection was determined using virus isolation and proviral PCR of peripheral blood mononuclear cells and lymphoid tissues.

RESULTS

Four out of five dual-subtype vaccinated cats were protected against low-dose FIV(Bang) (10 CID(50)) and subsequently against in vivo-derived FIV(Pet) (50 CID(50)) challenge, whereas all placebo-immunized cats became infected. Furthermore, dual-subtype vaccine protected two out of five cats against high-dose FIV(Bang) challenge (100 CID(50)) which infected seven out of eight single-subtype vaccinated cats. All dual-subtype vaccinated cats were protected against in vivo-derived FIV(Pet), but only one out of five single-subtype vaccinated cats were protected against in vivo-derived FIV(Pet). Dual-subtype vaccination induced broad-spectrum virus-neutralizing antibodies and FIV-specific interferon-gamma responses along with elevated FIV-specific perforin mRNA levels, suggesting an increase in cytotoxic cell activities.

CONCLUSION

Dual-subtype vaccinated cats developed broad-spectrum humoral and cellular immunity which protected cats against in vivo-derived inocula of homologous and heterologous FIV subtypes. Thus, multi-subtype antigen vaccines may be an effective strategy against AIDS viruses.

AIDS vaccination studies using an ex vivo feline immunodeficiency virus model: reevaluation of neutralizing antibody levels elicited by a protective and a nonprotective vaccine after removal of antisubstrate cell antibodies

In the feline immunodeficiency virus system, immunization with a fixed-infected-cell vaccine conferred protection against virulent homologous challenge but the immune effectors involved remained elusive. In particular, few or no neutralizing antibodies were detected in sera from vaccinated cats. Here we show that, when preadsorbed with selected feline cells, the same sera revealed clearly evident virus-neutralizing activity. Because high titers of neutralizing antibody in cell-adsorbed sera from 23 cats immunized with fixed-infected-cell or whole-inactivated-virus vaccines correlated with protection, it is likely that they were more important for protection than formerly realized. In vitro, the fixed-cell vaccine efficiently removed neutralizing antibody from immune sera while the whole-inactivated-virus vaccine was much less effective.

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.

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.

Feline leukaemia virus: protective immunity is mediated by virus-specific cytotoxic T lymphocytes

Feline leukaemia virus (FeLV) nucleic acid vaccination of domestic cats affords protection against viraemia and the development of latency without inducing antiviral antibodies.1 To determine the contribution of cell-mediated immunity to the control of virus replication and clearance from the host, FeLV-specific cytotoxic T lymphocyte (CTL) responses were compared in vaccine-protected, transiently viraemic, and persistently viraemic cats. Vaccinal immunity was associated with the detection of higher levels of virus-specific effector CTL in the peripheral blood and lymphoid organs to FeLV Gag/Pro and Env antigens than those observed in unvaccinated control, persistently viraemic cats (P<0.001). Likewise, higher levels of virus-specific CTLs were also observed in transiently viraemic cats which recovered following exposure to FeLV. In cats that controlled their infection, recognition of Gag/Pro antigens was significantly higher than the recognition of Env antigens. This is the first report highlighting the very significant role that virus-specific CTL have in determining the outcome of FeLV infection in either vaccinated cats or cats recovering naturally from FeLV exposure.

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.

Mucosal immunization with experimental feline immunodeficiency virus (FIV) vaccines induces both antibody and T cell responses but does not protect against rectal FIV challenge

Feline immunodeficiency virus (FIV) is a natural lentiviral pathogen of cats which can be experimentally transmitted via rectal and vaginal routes--the major routes of human immunodeficiency virus type 1 transmission in man. An important objective for lentiviral research is the development of vaccine strategies which generate good mucosal immune responses capable of giving protection from a mucosal virus challenge. The experimental vaccines employed in this study were based on (a) a peptide from the third variable region of the FIV envelope glycoprotein and (b) fixed whole FIV, Glasgow-8 strain. Adjuvants used were Quil A and cholera toxin for mucosal administration and incomplete Freund's adjuvant and immune stimulating complexes for subcutaneous injection. Mucosal immunization was given by rectal and intranasal routes. Both antibody and proliferative responses were elicited by mucosal immunization and cholera toxin was found to be a good mucosal adjuvant. The addition of a lipo thioester to the FIV peptide improved IgG and IgA responses upon parenteral administration. However, no protection from a rectal FIV challenge was achieved.

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.

MHC-restricted protection of cats against FIV infection by adoptive transfer of immune cells from FIV-vaccinated donors

The role of cellular immunity in vaccine protection against FIV infection was evaluated using adoptive cell transfer studies. Specific-pathogen-free cats received two adoptive transfers of washed blood cells from either vaccinated or unvaccinated donors with varying MHC compatibility at 1-week intervals, and a homologous FIV(Pet) challenge 1 day after the first adoptive transfer. FIV-specific CTL, IFN-gamma production, and proliferation responses were detected in the PBMC from the vaccinated donors. Seven of eleven (64%) recipients of cells from half-matched/vaccinated donors remained negative for FIV-antibodies after FIV challenge and four of those were completely protected. Two of two recipients of cells from MHC-identical/vaccinated donors were completely protected. All recipients of cells from unrelated/vaccinated, half-matched/unvaccinated, or unrelated/unvaccinated donors were unprotected. Thus, protection mediated by adoptive transfer of immunocytes from vaccinated cats was MHC-restricted, occurred in the absence of antiviral humoral immunity, and correlated with the transfer of cells with FIV-specific CTL and T-helper activities.

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.

Fusion-competent vaccines: broad neutralization of primary isolates of HIV

Current recombinant human immunodeficiency virus (HIV) gp120 protein vaccine candidates are unable to elicit antibodies capable of neutralizing infectivity of primary isolates from patients. Here, "fusion-competent" HIV vaccine immunogens were generated that capture the transient envelope-CD4-coreceptor structures that arise during HIV binding and fusion. In a transgenic mouse immunization model, these formaldehyde-fixed whole-cell vaccines elicited antibodies capable of neutralizing infectivity of 23 of 24 primary HIV isolates from diverse geographic locations and genetic clades A to E. Development of these fusion-dependent immunogens may lead to a broadly effective HIV vaccine.

AIDS vaccination studies using an ex vivo feline immunodeficiency virus model: detailed analysis of the humoral immune response to a protective vaccine

The feline immunodeficiency virus (FIV) cat model is extensively used to investigate possible vaccination approaches against AIDS in humans. Although consistent levels of protection have been achieved with FIV, as with other model systems, by immunizing with whole inactivated virus or fixed infected cells, the mechanisms responsible for protection are elusive. In previous studies we showed that cats immunized with a vaccine consisting of fixed infected cells were protected or unprotected against cell-free or cell-associated FIV challenge depending on the time interval between completion of vaccination and challenge. In an attempt to define possible humoral immune correlates of protection, selected sera harvested at the times of challenge from such cats were examined for anti-FIV-antibody titers and properties by using binding and functional immunological assays. Binding assays included quantitative Western blotting, enzyme-linked tests for antibodies to FIV glycoproteins and immunodominant linear epitopes, and tests for measuring conformation dependence and avidity of anti-viral-envelope antibodies. Functional assays included virus neutralization performed with two different cell substrates, complement- and antibody-dependent virolysis, blocking of reverse transcriptase, and an assay that measured the ability of sera to prevent FIV growth in cocultures of infected and uninfected cells. Despite the wide spectrum of parameters investigated, no correlation between vaccine-induced protection and the humoral parameters measured was noted.

Feline bone marrow transplantation: its use in FIV-infected cats

The use of autologous and allogenic bone marrow transplantations (BMT) in FIV-infected and uninfected cats is a novel therapy for feline hematopoietic diseases and retroviral infections. A total of 13 specific pathogen-free (SPF) cats received either autologous or allogenic BMT and seven of these cats were also infected with FIV before autologous or allogenic BMT. All BMT recipients received total body irradiation of 900 cGy just before BMT. Two FIV-infected and four uninfected cats received autologous uninfected BM cells cryopreserved before BMT. Five infected and two uninfected cats received BM cells from allogenic uninfected donors (RBC-, MHC-, and cross-matched). MHC-matching was based on mixed leucocyte reaction (MLR) and the donor-recipient combination which was compatible by MLR analysis, was used in this study. Recipients were monitored for hematology, immunology, virology, and clinical signs. All FIV-infected and uninfected recipients of autologous BMT had complete engraftment with minimal complications. Uninfected recipients of allogenic BMT had a more severe clinical episode with slower rate of engraftment. None of these BMT groups had mortality. In contrast, only two of the five infected recipients of allogenic BMT survived for a significant period of time (23 and 50 weeks) and rest of the cats succumbed to transfusion reactions. Both infected BMT groups had persistent CD4/CD8 inversion, low CD4+ cell counts, and FIV infection of engrafted peripheral blood mononuclear cells (PBMC). Overall, successful autologous and allogenic BMTs were performed in FIV-free cats. All infected recipients of autologous BMT had compete engraftment and are currently alive, with thelongest survival time being over 1 year. Thus, BMT in combination with antiviral drug therapies may be an alternative therapy against retroviral infection.

Accelerated viraemia in cats vaccinated with fixed autologous FIV-infected cells

We have vaccinated cats with fixed autologous FIV infected PBMC to determine whether autologous presentation of antigen is capable of inducing a protective immune response against homologous challenge. To this end autologous PBMC were infected with a FIV molecular clone (19k1). When infection was established, cells were inactivated by dialysis against paraformaldehyde. Upon vaccination, cats developed a virus specific immune response as measured by ELISA against the Gag protein of FIV. No antibodies against the envelope protein were detected with a peptide ELISA. Virus neutralizing antibodies however could be detected with a neutralization assay based on infection of CrFK cells, but not in an assay based on infection of primary T-cells. Although vaccination led to the induction of these virus-specific immune responses, vaccinated cats were not protected against homologous challenge but showed an accelerated viraemia upon infection. This was shown both by PCR and cell-associated viral load. The possible mechanisms underlying this observation are discussed in this paper.

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

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

Delayed infection after immunization with a peptide from the transmembrane glycoprotein of the feline immunodeficiency virus

Recent advances in the quantitative assessment of viral burden, by permitting the extension of criteria applied to assess the efficacy of vaccines from all-or-none protection to diminution of the viral burden, may allow the identification of original immunogens of value in combined vaccines. Peptides corresponding to three domains of the envelope glycoproteins of feline immunodeficiency virus that are recognized during natural infection were used to immunize cats. After challenge with a primary isolate of feline immunodeficiency virus, the development of acute infection was monitored by quantitative assessment of the viral burden in plasma and tissues by competitive reverse transcription-PCR, by measurement of the humoral response developed to viral components, and by lymphocyte subset analysis. Whereas immunization with two peptides derived from the surface glycoprotein had no effect on the early course of infection, immunization with a peptide derived from the transmembrane glycoprotein delayed infection, as reflected by a diminished viral burden in the early phase of primary infection and delayed seroconversion. This peptide, located in the membrane-proximal region of the extracellular domain, has homology to an epitope of human immunodeficiency virus type 1 recognized by a broadly neutralizing monoclonal antibody. These results suggest that lentivirus transmembrane glycoproteins share a determinant in the juxtamembrane ectodomain which could be of importance in the design of vaccines against AIDS.

Partial protection by vaccination with recombinant feline immunodeficiency virus surface glycoproteins

In an effort to induce a strong immune response that might protect against feline immunodeficiency virus (FIV) challenge infection, three groups of five specified pathogen-free (spf) cats each were immunized subcutaneously with different FIV antigen preparations. Immunizations were done at weeks 0, 2, and 4 with 100 microg of recombinant SU from an FIV Zurich 2 (FIV Z2) strain expressed by E. coli (group 1) or the baculovirus expression system (groups 2 and 3) adsorbed on aluminum hydroxyde and administered with QS-21 (groups 1 and 2) or Freund's adjuvant together with the recombinant nucleocapsid protein (protein NC) of rabies virus (group 3). Protein NC was described to act as an exogenous superantigen. Group 3 cats demonstrated the highest detectable antibody response to the vaccine antigen as determined by ELISA and Western blot analysis. All immunized cats together with seven control animals were challenged with 20 CID50 of cat lymphocyte-grown FIV Z2 3 weeks following the last immunization. Whereas virus was readily recovered from peripheral blood lymphocytes of seven of seven nonvaccinated control cats following this challenge dose, virus was not recovered from two cats of groups 1 and 2. All cats in groups 2 and 3 showed a provirus load significantly decreased to 3% of that of controls up to week 8 after challenge infection. Eleven of 15 vaccinated cats and 5 of 7 control cats developed virus-neutralizing antibodies by week 8 after challenge infection. The two cats negative on virus isolation remained seronegative, developed no detectable virus-neutralizing activities, but were repeatedly positive in provirus PCR. Moreover, starting at week 1 after challenge, both cats showed the lowest provirus load in their respective groups. These results indicate that immunization with recombinant FIV SU in conjunction with appropriate adjuvants may lead to partial protection against FIV challenge infection.

Efficacy evaluation of prime-boost protocol: canarypoxvirus-based feline immunodeficiency virus (FIV) vaccine and inactivated FIV-infected cell vaccine against heterologous FIV challenge in cats

OBJECTIVE

To evaluate the immunogenicity and prophylactic efficacy of immunization schemes employing a recombinant canarypoxvirus ('ALVAC')-based feline immunodeficiency virus (FIV) vaccine alone or in combination with an inactivated FIV-infected cell vaccine against homologous and heterologous FIV challenges in cats.

METHODS

Specific pathogen-free cats were given a total of three immunizations with subtype A vaccines and challenged 4 weeks after the final immunization with 50 median animal infectious doses (ID50) of FIV-Petaluma, a subtype A isolate. Following the initial challenge, protected cats received a second challenge with 75 ID50 of FIV-Bangston, a subtype B isolate. FIV-specific humoral and cell-mediated responses were measured to determine the immune correlates of protection.

RESULTS

Two of three cats immunized with the ALVAC FIV recombinants alone were protected from homologous FIV challenge in the presence of FIV-specific cytotoxic T-lymphocyte (CTL) responses but in the absence of FIV-specific humoral responses. All three cats immunized with the ALVAC-FIV recombinant and boosted with FIV-infected cell vaccine were also protected from homologous FIV challenge in the presence of both FIV-specific CTL and humoral responses. Partial to full protection was observed in ALVAC-FIV/FIV-infected cell vaccine-immunized cats against a heterologous FIV challenge given 8 months after the initial challenge. Two out of three cats had transient infection and the remaining cat had no sign of FIV infection at a dose at which all three control cats were readily infected.

CONCLUSIONS

Immunization schemes employing ALVAC-based FIV vaccines in combination with inactivated FIV-infected cell vaccine generate protective immune responses that can cross-react with FIV isolates that are genetically distinct from the vaccine strains.

Feline immunodeficiency virus subunit vaccines that induce virus neutralising antibodies but no protection against challenge infection

Three experimental vaccines against feline immunodeficiency virus (FIV), all based on viral antigens presented via immune stimulating complexes (iscoms), were tested for their capacity to induce protection in cats from FIV infection. The respective vaccines consisted of FIV propagated in Crandell feline kidney (CrFK) cells (FIV-iscoms); FIV-iscoms spiked with recombinant vaccinia virus expressed FIV envelope glycoprotein incorporated into iscoms (FIV-iscoms + vGR657x15-iscoms) and vGR657x15-iscoms spiked with recombinant FIV Gag protein incorporated into iscoms (vGR657x15-iscoms + FIV-Gag-iscoms). Simian immunodeficiency virus envelope glycoprotein incorporated into iscoms, iscoms prepared with uninfected CrFK cells, and PBS served as controls. All cats vaccinated with vGR657x15-iscoms combined with FIV-iscoms or FIV-Gag-iscoms developed Env-specific plasma antibody responses. These antibodies neutralised FIV infection in CrFK cells, but failed to neutralise FIV infection in primary feline thymocytes. FIV-iscoms induced poor Env-specific responses and only one out of six cats developed antibodies that neutralised FIV in the CrFK cell based assay. Four weeks after challenge all cats proved to be infected, showing that none of the vaccine preparations provided protection. In contrast, 2 weeks after infection, virus infected peripheral blood mononuclear cells were only observed in cats vaccinated with FIV-iscoms + vGR657x15-iscoms or CrFK-iscoms and to a lesser extent in cats vaccinated with FIV-iscoms and vGR657x15-iscoms + FIV-Gag-iscoms, but not in cats vaccinated with SIV-iscoms or PBS. The differences found in cell associated virus loads amongst the respective groups are discussed in the light of antibody mediated enhancement of infectivity and protective effects provided by Gag-specific T cell responses.

Human rhinovirus type 14:human immunodeficiency virus type 1 (HIV-1) V3 loop chimeras from a combinatorial library induce potent neutralizing antibody responses against HIV-1

In an effort to develop a useful AIDS vaccine or vaccine component, we have generated a combinatorial library of chimeric viruses in which the sequence IGPGRAFYTTKN from the V3 loop of the MN strain of human immunodeficiency virus type 1 (HIV-1) is displayed in many conformations on the surface of human rhinovirus 14 (HRV14). The V3 loop sequence was inserted into a naturally immunogenic site of the cold-causing HRV14, bridged by linkers consisting of zero to three randomized amino acids on each side. The library of chimeric viruses obtained was subjected to a variety of immunoselection schemes to isolate viruses that provided the most useful presentations of the V3 loop sequence for potential use in a vaccine against HIV. The utility of the presentations was assessed by measures of antigenicity and immunogenicity. Most of the immunoselected chimeras examined were potently neutralized by each of the four different monoclonal anti-V3 loop antibodies tested. Seven of eight chimeric viruses were able to elicit neutralizing antibody responses in guinea pigs against the MN and ALA-1 strains of HIV-1. Three of the chimeras elicited HIV neutralization titers that exceeded those of all but a small number of previously described HIV immunogens. These results indicate that HRV14:HIV-1 chimeras may serve as useful immunogens for stimulating immunity against HIV-1. This method can be used to flexibly reconstruct varied immunogens on the surface of a safe and immunogenic vaccine vehicle.