AIDS vaccination studies using feline immunodeficiency virus as a model: immunisation with inactivated whole virus suppresses viraemia levels following intravaginal challenge with infected cells but not following intravenous challenge with cell-free virus

A novel method for producing target cells and assessing cytotoxic T lymphocyte activity in outbred hosts

BACKGROUND

Cytotoxic T lymphocytes play a crucial role in the immunological control of microbial infections and in the design of vaccines and immunotherapies. Measurement of cytotoxic T lymphocyte activity requires that the test antigen is presented by target cells having the same or compatible class I major histocompatibility complex antigens as the effector cells. Conventional assays use target cells labeled with 51chromium and infer cytotoxic T lymphocyte activity by measuring the isotope released by the target cells lysed following incubation with antigen-specific cytotoxic T lymphocytes. This assay is sensitive but needs manipulation and disposal of hazardous radioactive reagents and provides a bulk estimate of the reporter released, which may be influenced by spontaneous release of the label and other poorly controllable variables. Here we describe a novel method for producing target in outbred hosts and assessing cytotoxic T lymphocyte activity by flow cytometry.

RESULTS

The method consists of culturing skin fibroblasts, immortalizing them with a replication defective clone of simian virus 40, and finally transducing them with a bicistronic vector encoding the target antigen and the reporter green fluorescent protein. When used in a flow cytometry-based assay, the target cells obtained with this method proved valuable for assessing the viral envelope protein specific cytotoxic T lymphocyte activity in domestic cats acutely or chronically infected with feline immunodeficiency virus, a lentivirus similar to human immunodeficiency virus and used as animal model for AIDS studies.

CONCLUSION

Given the versatility of the bicistronic vector used, its ability to deliver multiple and large transgenes in target cells, and its extremely wide cell specificity when pseudotyped with the vesicular stomatitis virus envelope protein, the method is potentially exploitable in many animal species.

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.

Streamlined design of a self-inactivating feline immunodeficiency virus vector for transducing ex vivo dendritic cells and T lymphocytes

Background

Safe and efficient vector systems for delivering antigens or immunomodulatory molecules to dendritic cells (DCs), T lymphocytes or both are considered effective means of eliciting adaptive immune responses and modulating their type, extent, and duration. As a possible tool toward this end, we have developed a self-inactivating vector derived from feline immunodeficiency virus (FIV) showing performance characteristics similar to human immunodeficiency virus-derived vectors but devoid of the safety concerns these vectors have raised.

Methods

The pseudotyped FIV particles were generated with a three-plasmid system consisting of: the packaging construct, providing Gag, Pol and the accessory proteins; the vector(s), basically containing FIV packaging signal (ψ), Rev responsive element, R-U5 region at both ends, and the green fluorescent protein as reporter gene; and the Env plasmid, encoding the G protein of vesicular stomatitis virus (VSV-G) or the chimeric RD114 protein. Both packaging and vector constructs were derived from p34TF10, a replication competent molecular clone of FIV. The pseudotyped particles were produced by transient transfection in the Crandell feline fibroblast kidney (CrFK) or the human epithelial (293T) cell line.

Results

To broaden its species tropism, the final vector construct was achieved through a series of intermediate constructs bearing a longer ψ, the FIV central polypurin tract sequence (cPPT), or the woodchuck hepatitis post-regulatory element (WPRE). These constructs were compared for efficiency and duration of transduction in CrFK or 293T cells and in the murine fibroblast cell line NIH-3T3. Whereas ψ elongation and cPPT addition did not bring any obvious benefit, insertion of WPRE downstream GFP greatly improved vector performances. To maximize the efficiency of transduction for ex-vivo murine DCs and T-lymphocytes, this construct was tested with VSV-G or RD114 and using different transduction protocols. The results indicated that the FIV construct derived herein stably transduced both cell types, provided that appropriate vector makeup and transduction protocol were used. Further, transduced DCs underwent changes suggestive of an induced maturation.

Conclusion

In contrast to previously described FIV vectors that were poorly efficient in delivering genetic material to DCs and T lymphocytes, the vector developed herein has potential for use in experimental immunization strategies.

Mucosal challenge with cell-associated or cell-free feline immunodeficiency virus induces rapid and distinctly different patterns of phenotypic change in the mucosal and systemic immune systems

The majority of human immunodeficiency virus type 1 (HIV-1) infections occur via mucosal transmission through contact with genital secretions containing cell-associated and cell-free virus. However, few studies have assessed whether exposure to cells, HIV-1 infected or uninfected, plays a role in the sexual transmission of HIV-1. This study examined phenotypic changes in mucosal and systemic lymphoid tissue 24 hr after vaginal exposure to in vitro equilibrated infectious doses of cell-associated or cell-free feline immunodeficiency virus, uninfected heterologous cells, or medium alone. We found that even at this early time-point, mucosal exposure to virus induced substantial alterations in the phenotype and distribution of leucocytes, particularly in the tissues of the mucosal immune system. Second, we found that the type of virus inoculum directly influenced the phenotypic changes seen. Vaginal exposure to cell-free virus tended to induce more generalized phenotypic changes, typically in the peripheral immune system (blood and systemic lymph nodes). In contrast, exposure to cell-associated virus was primarily associated with phenotypic shifts in the mucosal immune system (gut and mucosal/draining lymph nodes). In addition, we found that exposure to uninfected heterologous cells also induced alterations in the mucosal immune system. These data suggest that significant immune changes occur within the first 24 hr of virus exposure, well before substantial replication would be anticipated. As the mucosal immune system, and particularly the gut, is an early and persistent target for lentiviral replication, these findings have substantial implications for HIV-1 pathogenesis and vaccine development.

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.

Vaccination of cats with attenuated feline immunodeficiency virus proviral DNA vaccine expressing gamma interferon

A feline immunodeficiency virus (FIV) provirus with a vif gene deletion (FIVDelta vifATGgamma) that coexpresses feline gamma interferon (IFN-gamma) was tested as a proviral DNA vaccine to extend previous studies showing efficacy with an FIV-pPPRDelta vif DNA vaccine. Cats were vaccinated with either FIVDelta vifATGgamma or FIV-pPPRDelta vif proviral plasmid DNA or with both FIV-pPPRDelta vif DNA and a feline IFN-gamma expression plasmid (pCDNA-IFNgamma). A higher frequency of FIV-specific T-cell proliferation responses was observed in cats immunized with either FIVDelta vifATGgamma or FIV-pPPRDelta vif plus pCDNA-IFNgamma, while virus-specific cytotoxic-T-lymphocyte responses were comparable between vaccine groups. Antiviral antibodies were not observed postvaccination. Virus-specific cellular and humoral responses were similar between vaccine groups after challenge with a biological FIV isolate (FIV-PPR) at 13 weeks postimmunization. All vaccinated and unvaccinated cats were infected after FIV-PPR challenge and exhibited similar plasma virus loads. Accordingly, inclusion of plasmids containing IFN-gamma did not enhance the efficacy of FIV-pPPRDelta vif DNA immunization. Interestingly, the lack of protection associated with FIV-pPPRDelta vif DNA immunization contrasted with findings from a previous study and suggested that multiple factors, including timing of FIV-pPPRDelta vif inoculations and challenge, as well as route of challenge virus delivery, may significantly impact vaccine efficacy.

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.

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.

Evaluation of feline immunodeficiency virus ORF-A mutants as candidate attenuated vaccine

Feline immunodeficiency virus (FIV) made defective in the accessory gene ORF-A were previously shown to be greatly attenuated in its ability to replicate in lymphocytes but to grow normally or near normally in other cell types. Here, we examined whether FIV thus mutated could protect specific pathogen-free cats against challenge with ex vivo fully virulent homologous virus. No reversion of the vaccinating infections to wild type ORF-A was noted over 22 months of in vivo infection. Following challenge, 6/6 unvaccinated control cats became readily and heavily infected. In contrast, 3/9 vaccinees showed no evidence of the challenge virus over a 15-month observation period. In the other vaccinees, the challenge virus was predominant for various periods of time, but pre-existing viral loads and CD4 lymphocyte counts were either unaffected or altered only marginally and transiently. These findings show that ORF-A-defective FIV should be further examined as a candidate live attenuated vaccine.

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.

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

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

AIDS vaccination studies using an ex vivo feline immunodeficiency virus model: protection from an intraclade challenge administered systemically or mucosally by an attenuated vaccine

Feline immunodeficiency virus (FIV) infection of domestic cats represents a valuable system through which to investigate criteria for antilentiviral vaccines in a natural host species. Here, we examined whether vaccination with a strain of FIV attenuated as a result of prolonged growth in vitro could protect against a fully virulent, highly heterologous intraclade challenge. The results indicated that the vaccine virus produced a low-grade infection with no detectable pathological effects and afforded a long-lasting sterilizing immunity if the challenge was delivered intraperitoneally as cell-free virus but not against a cell-associated intravaginal challenge. In the latter case, however, the replication and pathological consequences of the challenge virus were markedly suppressed. Together with similar results obtained in rhesus monkey models, these findings should give impulse to the development of attenuated FIV vaccines to be tested in controlled studies in field cats. Field studies may provide answers to some of the existing safety concerns surrounding attenuated AIDS vaccines in humans.

FIV vaccine development and its importance to veterinary and human medicine: a review FIV vaccine 2002 update and review

Feline immunodeficiency virus (FIV) is a natural infection of domestic cats that results in acquired immunodeficiency syndrome resembling human immunodeficiency virus (HIV) infection in humans. The worldwide prevalence of FIV infection in domestic cats has been reported to range from 1 to 28%. Hence, an effective FIV vaccine will have an important impact on veterinary medicine in addition to being used as a small animal AIDS model for humans. Since the discovery of FIV reported in 1987, FIV vaccine research has pursued both molecular and conventional vaccine approaches toward the development of a commercial product. Published FIV vaccine trial results from 1998 to the present have been compiled to update the veterinary clinical and research communities on the immunologic and experimental efficacy status of these vaccines. A brief report is included on the outcome of the 10 years of collaborative work between industry and academia which led to recent USDA approval of the first animal lentivirus vaccine, the dual-subtype FIV vaccine. The immunogenicity and efficacy of the experimental prototype, dual-subtype FIV vaccine and the efficacy of the currently approved commercial, dual-subtype FIV vaccine (Fel-O-Vax FIV) are discussed. Potential cross-reactivity complications between commercial FIV diagnostic tests, Idexx Snap Combo Test and Western blot assays, and sera from previously vaccinated cats are also discussed. Finally, recommendations are made for unbiased critical testing of new FIV vaccines, the currently USDA approved vaccine, and future vaccines in development.

Vaccine development against HIV-1: current perspectives and future directions

The development of an efficacious vaccine against the human immunodeficiency virus (HIV) is of great urgency, because it is accepted that vaccination is the only means capable of controlling the AIDS pandemic. The foundation of HIV vaccine development is the analysis of immune responses during natural infection and the utilization of this knowledge for the development of protective immunization strategies. Initial vaccine development and experimentation are usually in animal models, including murine, feline, and nonhuman primates. Experimental vaccine candidates are closely studied for both efficacy and safety before proceeding to human clinical trials. There are a number of different therapeutic and prophylactic vaccine strategies currently being studied in human clinical trials. Vaccine strategies that are being tested, or have previously been tested, in humans include subunit, DNA plasmid, and viral vector, and combinations of these various strategies. Some of the results of these trials are promising, and additional research has focused on the development of appropriate chemical and genetic adjuvants as well as methods of vaccine delivery to improve the host immune response. This review summarizes the vaccine strategies that have been tested in both animal models and human clinical trials.

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.

Development of feline immunodeficiency virus ORF-A (tat) mutants: in vitro and in vivo characterization

A functional ORF-A is essential for efficient feline immunodeficiency virus replication in lymphocytes. We have characterized a series of mutants of the Petaluma strain, derived from p34TF10 and having different combinations of stop codons and increasingly long deletions in ORF-A. Six clones proved fully replicative in fibroblastoid Crandell feline kidney cells and monocyte-derived macrophage cultures but failed to replicate in T cell lines and primary lymphoblasts. Cats inoculated with three selected mutants had considerably milder infections than controls given intact ORF-A virus. In vivo, the mutants maintained growth properties similar to those in vitro for at least 7 months, except that replication in lymphoid cells was strongly reduced but not ablated. One mutant underwent extensive ORF-A changes without, however, reverting to wild-type. Antiviral immune responses were feeble in all cats, suggesting that viral loads were too low to represent a sufficiently powerful antigenic stimulus.

Detection of feline immunodeficiency virus RNA by two nucleic acid sequence based amplification (NASBA) formats

Feline immunodeficiency virus (FIV) is an AIDS-inducing lentivirus that infects domestic cats worldwide. Because of its clinicopathologic similarities to human immunodeficiency virus type 1 (HIV-1) infection, the FIV/cat infection system is a valuable animal model for investigating comparative aspects of HIV-1 biology. An assay that detects quickly and efficiently FIV RNA in relatively small volume samples of feline blood or other body fluids would be of benefit in studies of viral transmission and antiviral interventions. Nucleic acid sequence based amplification (NASBA) technology is particularly suited for the detection of RNA in a variety of body fluids. In this report, the development of two rapid, sensitive and versatile NASBA formats is described for the detection of FIV gag RNA in plasma from infected cats. RNA detection by either format was unaffected by the presence of feline plasma. The limits of detection were at least 200 copies of input RNA for both formats. Results from seropositive and seronegative feline plasma samples were clearly distinguishable. These results demonstrate that NASBA provides a rapid and sensitive alternative to RT-PCR and culture isolation for detecting FIV RNA in infected feline plasma.

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.

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.