Lymphoid activation: a confounding factor in AIDS vaccine development?

Vaccine-associated enhanced disease in humans and animal models: Lessons and challenges for vaccine development

The fight against infectious diseases calls for the development of safe and effective vaccines that generate long-lasting protective immunity. In a few situations, vaccine-mediated immune responses may have led to exacerbated pathology upon subsequent infection with the pathogen targeted by the vaccine. Such vaccine-associated enhanced disease (VAED) has been reported, or at least suspected, in animal models, and in a few instances in humans, for vaccine candidates against the respiratory syncytial virus (RSV), measles virus (MV), dengue virus (DENV), HIV-1, simian immunodeficiency virus (SIV), feline immunodeficiency virus (FIV), severe acute respiratory syndrome coronavirus 1 (SARS-CoV-1), and the Middle East respiratory syndrome coronavirus (MERS-CoV). Although alleviated by clinical and epidemiological evidence, a number of concerns were also initially raised concerning the short- and long-term safety of vaccines against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), which is causing the ongoing COVID-19 pandemic. Although the mechanisms leading to this phenomenon are not yet completely understood, the individual and/or collective role of antibody-dependent enhancement (ADE), complement-dependent enhancement, and cell-dependent enhancement have been highlighted. Here, we review mechanisms that may be associated with the risk of VAED, which are important to take into consideration, both in the assessment of vaccine safety and in finding ways to define models and immunization strategies that can alleviate such concerns.

Failed Disruption of Tick Feeding, Viability, and Molting after Immunization of Mice and Sheep with Recombinant Ixodes ricinus Salivary Proteins IrSPI and IrLip1

To identify potential vaccine candidates against Ixodes ricinus and tick-borne pathogen transmission, we have previously sequenced the salivary gland transcriptomes of female ticks infected or not with Bartonella henselae. The hypothesized potential of both IrSPI (I. ricinus serine protease inhibitor) and IrLip1 (I. ricinus lipocalin 1) as protective antigens decreasing tick feeding and/or the transmission of tick-borne pathogens was based on their presumed involvement in dampening the host immune response to tick feeding. Vaccine endpoints included tick larval and nymphal mortality, feeding, and molting in mice and sheep. Whether the antigens were administered individually or in combination, the vaccination of mice or sheep elicited a potent antigen-specific antibody response. However, and contrary to our expectations, vaccination failed to afford protection against the infestation of mice and sheep by I. ricinus nymphs and larvae, respectively. Rather, vaccination with IrSPI and IrLip1 appeared to enhance tick engorgement and molting and decrease tick mortality. To the best of our knowledge, these observations represent the first report of induction of vaccine-mediated enhancement in relation to anti-tick vaccination.

FIV vaccine with receptor epitopes results in neutralizing antibodies but does not confer resistance to challenge

Feline immunodeficiency virus (FIV) is the feline analogue to human immunodeficiency virus (HIV) and utilizes parallel modes of receptor-mediated entry. The FIV surface glycoprotein (SU) is an important target for induction of neutralizing antibodies, and autoantibodies to the FIV binding receptor (CD134) block infection ex vivo; thus highlighting the potential for immunotherapies which utilize anti-receptor antibodies to block viral infection. To determine whether vaccination with CD134-SU complexes could induce protection against FIV infection, cats (n = 5 per group) were immunized with soluble CD134, recombinant FIV-SU protein, and/or CD134+SU complexes. Two trials were performed with different antigen combinations and vaccination schedules. In vivo generation of anti-CD134 and anti-SU IgG antibodies was measured, and in vitro neutralization assays were conducted. Immunization induced production of anti-CD134 and anti-SU antibodies that significantly inhibited FIV infection in vitro. However, no vaccine combination protected cats from FIV infection, and neat serum from vaccinated cats enhanced FIV growth in vitro. CD134+SU vaccinated cats exhibited increased CD4:CD8 ratio immediately prior to challenge, and antibodies were much more efficiently generated against vaccine by-products versus target antigens. Results suggest vaccination against viral and cryptic receptor epitopes yields neutralizing antibodies that synergistically inhibit FIV infection in vitro. Factors contributing to vaccine failure may include: (1) Heat-labile serum factors that enhance viral replication, (2) changes in circulating target cell populations induced by vaccination, and (3) weak immunogenicity of neutralizing epitopes compared to off-target vaccine components. Results reinforce the need to monitor vaccine preparation components and avoid non-specific immune stimulation during vaccination.

A vaccine candidate for feline immunodeficiency virus elicits strong immunological reaction in vitro, but no protection to live cats. The feline analog to human immunodeficiency virus, FIV shares a similar infection paradigm and has only one partially effective vaccine. A US team, led by Colorado State University’s Susan VandeWoude, immunized cats using a complex of an FIV surface protein and a feline cell-surface protein known to facilitate FIV’s entry into immune cells. Tissue culture assays yielded promising results; however, this did not translate to live-animal protection. The researchers highlighted multiple factors that could explain the lack of success, including circulatory pro-infection factors, and immune responses generated against vaccine by-products rather than intended targets. While the vaccine candidate failed, the research provides invaluable guidance for future efforts into FIV vaccination with implications for HIV vaccine trials.

Applications of the FIV Model to Study HIV Pathogenesis

Feline immunodeficiency virus (FIV) is a naturally-occurring retrovirus that infects domestic and non-domestic feline species, producing progressive immune depletion that results in an acquired immunodeficiency syndrome (AIDS). Much has been learned about FIV since it was first described in 1987, particularly in regard to its application as a model to study the closely related lentivirus, human immunodeficiency virus (HIV). In particular, FIV and HIV share remarkable structure and sequence organization, utilize parallel modes of receptor-mediated entry, and result in a similar spectrum of immunodeficiency-related diseases due to analogous modes of immune dysfunction. This review summarizes current knowledge of FIV infection kinetics and the mechanisms of immune dysfunction in relation to opportunistic disease, specifically in regard to studying HIV pathogenesis. Furthermore, we present data that highlight changes in the oral microbiota and oral immune system during FIV infection, and outline the potential for the feline model of oral AIDS manifestations to elucidate pathogenic mechanisms of HIV-induced oral disease. Finally, we discuss advances in molecular biology, vaccine development, neurologic dysfunction, and the ability to apply pharmacologic interventions and sophisticated imaging technologies to study experimental and naturally occurring FIV, which provide an excellent, but often overlooked, resource for advancing therapies and the management of HIV/AIDS.

Phylogenetic analysis of feline immunodeficiency virus strains from naturally infected cats in Belgium and The Netherlands

Feline immunodeficiency virus (FIV) is a major pathogen in feline populations worldwide, with seroprevalences up to 26%. Virus strains circulating in domestic cats are subdivided into different phylogenetic clades (A-E), based on the genetic diversity of the V3-V4 region of the env gene. In this report, a phylogenetic analysis of the V3-V4 env region, and a variable region in the gag gene was made for 36 FIV strains isolated in Belgium and The Netherlands. All newly generated gag sequences clustered together with previously known clade A FIV viruses, confirming the dominance of clade A viruses in Northern Europe. The same was true for the obtained env sequences, with only one sample of an unknown env subtype. Overall, the genetic diversity of FIV strains sequenced in this report was low. This indicates a relatively recent introduction of FIV in Belgium and The Netherlands. However, the sample with an unknown env subtype indicates that new introductions of FIV from unknown origin do occur and this will likely increase genetic variability in time.

TRANSPARENT TESTA2 regulates embryonic fatty acid biosynthesis by targeting FUSCA3 during the early developmental stage of Arabidopsis seeds

TRANSPARENT TESTA2 (TT2) regulates the biosynthesis of proanthocyanidins in the seed coat of Arabidopsis. We recently found that TT2 also participates in inhibition of fatty acid (FA) biosynthesis in the seed embryo. However, the mechanism by which TT2 suppresses the accumulation of seed FA remains unclear. In this study, we show that TT2 is expressed in embryos at an early developmental stage. TT2 is directly bound to the regulatory region of FUSCA3 (FUS3), and mediates the expression of numerous genes in the FA biosynthesis pathway. These genes include BCCP2, CAC2, MOD1 and KASII, which encode proteins involved in the initial steps of FA chain formation, FAD2 and FAD3, which are responsible for FA desaturation, and FAE1, which catalyzes very-long-chain FA elongation. Loss of function of TT2 results in reduced expression of GLABRA2 but does not cause a significant reduction in the mucilage attached to the seed coats, which competes with FA for photosynthates. TT2 is expressed in both maternal seed coats and embryonic tissues, but proanthocyanidins are only found in wild-type seed coats and not in embryonic tissues. The amount of proanthocyanidins in the seed coat is negatively correlated with the amount of FAs in the embryo.

Time-course analysis of main markers of primary infection in cats with the feline immunodeficiency virus

Studies of the response of the immune system to feline immunodeficiency virus (FIV) during primary infection have shown that a subpopulation of CD8⁺ T-cells with an activated phenotype and reduced expression of the CD8β chain (denoted CD8β^low T cells) expands to reach up to 80% of the total CD8⁺ T cell count. The expansion of this subpopulation is considered to be a signature of FIV and an indicator of immune system alteration. We use a simple mathematical formalism to study the relationships over time between the dose of infection, the size of the CD8β^low population, and the circulating viral load in cats infected with FIV. Viremia profiles are described using a combination of two exponential laws, whereas the CD8β^low percentage (out of the total CD8⁺ population) is represented by a Gompertz law including an expansion phase and a saturation phase. Model parameters are estimated with a population approach using data from 102 experimentally infected cats. We examine the dose of infection as a potential covariate of parameters. We find that the rates of increase of viral load and of CD8β^low percentage are both correlated with the dose of infection. Cats that develop strong acute viremia also show the largest degree of CD8β^low expansion. The two simple models are robust tools for analysing the time course of CD8β^low percentage and circulating viral load in FIV-infected cats and may be useful for generating new insights on the disease and on the design of therapeutic strategies, potentially applicable to HIV infection.

HIV/AIDS vaccines: a need for new concepts?

HIV vaccine research is at a crossroads carefully contemplating on the next path. The unexpected results of the Merck vaccine trial, while providing a stunning blow to a field in dire need of a protective vaccine, has also raised several fundamental questions regarding the candidate immunogen itself, preexisting immunity to vaccine vectors, surrogate assays and animal models used for assessing preclinical protective responses, as well as relevant endpoints to be measured in a clinical trial. As a result, the research community is faced with the daunting task of identifying novel vaccine concepts and products to continue the search. This review highlights and addresses some of the scientific and practical concerns.

Vaccination reduces the viral load and the risk of transmission of Jembrana disease virus in Bali cattle

The efficacy of a tissue-derived vaccine, which is currently used in Indonesia to control the spread of Jembrana disease in Bali cattle, was determined by quantifying the viral load in plasma following experimental infection with Jembrana disease virus. Virus transmission is most likely to occur during the acute phase of infection when viral titers are greater than 10(6) genomes/ml. Vaccinated cattle were found to have a 96% reduction in viral load above this threshold compared to control cattle. This would reduce the chance of virus transmission as the number of days above the threshold in the vaccinated cattle was reduced by 33%. Viral loads at the onset and resolution of fever were significantly lower in the vaccinated cattle and immune function was maintained with the development of antibody responses to Env proteins within 10-24 days post challenge. There was, however, no significant reduction in the duration of the febrile period in vaccinated animals. The duration and severity of clinical parameters were found to be variable within each group of cattle but the quantification of viral load revealed the benefits of vaccinating to reduce the risk of virus transmission as well as to ameliorate disease.

Vaccine-induced enhancement of viral infections

Examples of vaccine-induced enhancement of susceptibility to virus infection or of aberrant viral pathogenesis have been documented for infections by members of different virus families. Several mechanisms, many of which still are poorly understood, are at the basis of this phenomenon. Vaccine development for lentivirus infections in general, and for HIV/AIDS in particular, has been little successful. Certain experimental lentiviral vaccines even proved to be counterproductive: they rendered vaccinated subjects more susceptible to infection rather than protecting them. For vaccine-induced enhanced susceptibility to infection with certain viruses like feline coronavirus, Dengue virus, and feline immunodeficiency virus, it has been shown that antibody-dependent enhancement (ADE) plays an important role. Other mechanisms may, either in the absence of or in combination with ADE, be involved. Consequently, vaccine-induced enhancement has been a major stumble block in the development of certain flavi-, corona-, paramyxo-, and lentivirus vaccines. Also recent failures in the development of a vaccine against HIV may at least in part be attributed to induction of enhanced susceptibility to infection. There may well be a delicate balance between the induction of protective immunity on the one hand and the induction of enhanced susceptibility on the other. The present paper reviews the currently known mechanisms of vaccine-induced enhancement of susceptibility to virus infection or of aberrant viral pathogenesis.

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.

Vaccination of sheep with Maedi-visna virus gag gene and protein, beneficial or harmful?

In spite of intense efforts no vaccine is yet available that protects against lentiviral infections. Sheep were immunised eight times over a period of 2.5 years with the maedi-visna (MVV) gag gene on two different vectors, 2 sheep with VR1012-gag-CTE and 2 sheep with pcDNA3.1-gag-CTE. All sheep responded to some of the mature MVV Gag proteins in Western blot (WB). Three of them responded to the virus in lymphocyte proliferation test. The sheep received a boost with recombinant Gag protein resulting in elevated antibody response. However, when they were challenged intratracheally with MVV they all became immediately infected as judged by a strong rise in antibody titer and virus isolation from blood. It is therefore clear that the vaccination gave no protection. It is even possible that it facilitated infectivity since virus was isolated earlier from all the vaccinated sheep than from any of the unvaccinated sheep infected in the same way with the same dose.

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.

Vaccination with a T-cell-priming Gag peptide of caprine arthritis encephalitis virus enhances virus replication transiently in vivo

CD4+ T cells are involved in several immune response pathways used to control viral infections. In this study, a group of genetically defined goats was immunized with a synthetic peptide known to encompass an immunodominant helper T-cell epitope of caprine arthritis encephalitis virus (CAEV). Fifty-five days after challenge with the molecularly cloned CAEV strain CO, the vaccinated animals had a higher proviral load than the controls. The measurement of gamma interferon and interleukin-4 gene expression showed that these cytokines were reliable markers of an ongoing immune response but their balance did not account for more or less efficient control of CAEV replication. In contrast, granulocyte–macrophage colony-stimulating factor appeared to be a key cytokine that might support virus replication in the early phase of infection. The observation of a potential T-cell-mediated enhancement of virus replication supports other recent findings showing that lentivirus-specific T cells can be detrimental to the host, suggesting caution in designing vaccine candidates.

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.

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.

AIDS vaccination studies with an ex vivo feline immunodeficiency virus model: analysis of the accessory ORF-A protein and DNA as protective immunogens

Determining which antigen must be included in AIDS vaccines to confer maximum protection is of utmost importance. In primate models, vaccines consisting of or including accessory viral proteins have yielded conflicting results. We investigated the protective potential of the accessory protein ORF-A of feline immunodeficiency virus (FIV) in cats. All three immunization strategies used (protein alone in alum adjuvant, DNA alone, or DNA prime-protein boost) clearly generated detectable immune responses. Upon challenge with ex vivo homologous FIV, ORF-A-immunized cats showed distinct enhancement of acute-phase infection relative to mock-immunized animals given alum or empty vector DNA. This effect was tentatively attributed to increased expression of the FIV receptor CD134 that was observed in the immunized cats. However, at subsequent sampling points that were continued for up to 10 months postchallenge, the average plasma viral loads of the ORF-A-immunized animals were slightly but consistently reduced relative to those of the control animals. In addition, CD4(+) T lymphocytes in the circulation system declined more slowly in immunized animals than in control animals. These findings support the contention that immunization with lentiviral accessory proteins can improve the host's ability to control virus replication and slow down disease progression but also draw attention to the fact that even simple immunogens that eventually contribute to protective activity can transiently exacerbate subsequent lentiviral infections.

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.

Evaluation of live feline immunodeficiency virus vaccines with modified antigenic properties

Live-attenuated viruses have typically been generated from pathogenic viruses by genetic modifications that modified their replicative capacity. The present study investigated whether modification of the antigenic properties of live-attenuated viruses might improve upon the protection that such vaccines afford against lentivirus infection. In a previous study, random amino acid substitutions were introduced into the transmembrane envelope glycoprotein of the feline immunodeficiency virus (FIV), within a highly conserved domain (principal immunodominant domain) bearing immunodominant B-cell epitopes. Amongst a wide set of mutants, mutations that modified antibody specificity without abolishing infectivity ex vivo were selected. In the present study, two such mutants, TN14 and TN92, were evaluated for their replicative capacities and pathogenic properties in vivo in comparison with the parental virus, FIV 34TF10. No significant differences in viral load were observed between mutant and parental viruses. After 1 year of infection, all animals were subjected to a heterologous intraclade superinfection with a primary strain of FIV. Whilst both parental and modified viruses protected cats from high viral loads after superinfection, the TN92 virus afforded a higher degree of protection (P=0·0079). Such improvement in protection might correlate with a decrease in the immunogenicity of a B-cell epitope potentially involved in antibody enhancement of infection.

Toward a detailed characterization of feline immunodeficiency virus-specific T cell immune responses and mediated immune disorders

Infection of domestic cats with feline immunodeficiency virus (FIV) is associated with the development of an acquired immunodeficiency syndrome (AIDS). The pathogenesis of FIV is not fully understood but it has been reported that the immune system is progressively impaired during disease progression. As a result, anti-FIV specific immune response will usually not clear the virus and the acute stage is followed by a chronic asymptomatic phase. The overall objective of this study was to characterized FIV-induced immune cellular responses and -mediated immune disorder following the first weeks post-infection. Using both cytokine ELISpot and intracellular staining assays, FIV-specific T cells were monitored at 6, 9 and 12 weeks post-infection. We demonstrated that both IFNgamma(+) and, CD4 and CD8 TNFalpha(+) T cells specifically respond to FIV antigens. These responses were found to reach a peak at 9 weeks post-infection. It was further shown that the TNFalpha(+)CD8(+) responding T cells were contained within a CD8beta(low)CD62L(-) T cell subpopulation, expanded in FIV-infected cats. This T cell subpopulation which present features of activated CD8 T cells was further shown to be susceptible to spontaneous apoptosis following a short-term in vitro culture. Moreover, it was observed that cell death by apoptosis of this T cell subset was increased following FIV antigen-recognition. Therefore, FIV might alter immune homeostasis in inducing chronic activation of TNFalpha(+)CD8(+) T cells which eventually will die following antigen contact while deleting CD4(+) T cells. Interestingly, this study confirmed the strong similarity between FIV and HIV pathogenesis.

Enhanced SIV replication and accelerated progression to AIDS in macaques primed to mount a CD4 T cell response to the SIV envelope protein

Given the dual role of CD4 T cells as both immune effectors and targets for HIV infection, the balance of CD4 versus CD8 T cell-mediated responses induced by candidate AIDS vaccines may be critical in determining postvaccination infection outcomes. An attenuated recombinant varicella-zoster virus vaccine expressing the simian immunodeficiency virus (SIV) envelope (Env) elicited nonneutralizing Env-binding antibodies and little if any cytotoxic T lymphocyte responses in rhesus macaques (Macaca mulatta). After challenge with SIV, Env vaccinees manifested increased levels of SIV replication, more rapid CD4 depletion, and accelerated progression to AIDS compared with controls. Enhanced SIV replication correlated with increased CD4 T cell proliferation soon after SIV challenge, apparently the result of an anamnestic response to SIV antigens. Thus activation of virus-specific CD4 T cells at the time of exposure to a CD4 T cell-tropic lentivirus, in the absence of an effective CD8 response, may enhance virus replication and disease. These data suggest suggest that candidate AIDS vaccines may not simply be either efficacious or neutral; they may also have the potential to be harmful.

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.

Mucosal AIDS vaccines

Debates are still being waged over what is the best strategy for developing a potent AIDS vaccine. All the obvious approaches to making AIDS vaccines have been tried in the past two decades without much success. It is clear that new thinking and a revision of prevailing dogmas needs to be in place if we really want a vaccine. Conventional envelope-based antibody-inducing vaccines do not appear to hold promise, and broadly-neutralizing antibodies are now being searched as an alternative to the failed approach with subunit vaccines. The current consensus is that cellular immune responses, especially those mediated by CD8 cytotoxic/suppressor (CTL) and CD4 helper T lymphocytes, are needed to control HIV. Vaccines capable of inducing cell-mediated responses are, therefore, considered critical for controlling the spread of HIV. DNA-based vaccines triggering CTL reaction are currently thought to be an answer, but will they fulfill the promise? In the following paragraphs, a critical assessment of the state of the art will be provided in an attempt to analyze what we know and still don't know. The focus of this review is primarily on mucosal vaccines-a relatively new area in AIDS research. The update on V-1 Immunitor, the first mucosal AIDS vaccine available commercially, is provided within this context. Some of the reviewed concepts may be disputable, but without departure from the uninspiring consensus no substantial progress in the AIDS vaccine field can be envisioned.