Neutralising antibody response in domestic cats immunised with a commercial feline immunodeficiency virus (FIV) vaccine

Across human and veterinary medicine, vaccines against only two retroviral infections have been brought to market successfully, the vaccines against feline leukaemia virus (FeLV) and feline immunodeficiency virus (FIV). FeLV vaccines have been a global success story, reducing virus prevalence in countries where uptake is high. In contrast, the more recent FIV vaccine was introduced in 2002 and the degree of protection afforded in the field remains to be established. However, given the similarities between FIV and HIV, field studies of FIV vaccine efficacy are likely to advise and inform the development of future approaches to HIV vaccination.Here we assessed the neutralising antibody response induced by FIV vaccination against a panel of FIV isolates, by testing blood samples collected from client-owned vaccinated Australian cats. We examined the molecular and phenotypic properties of 24 envs isolated from one vaccinated cat that we speculated might have become infected following natural exposure to FIV. Cats vaccinated against FIV did not display broadly neutralising antibodies, suggesting that protection may not extend to some virulent recombinant strains of FIV circulating in Australia.     

Feline immunodeficiency virus (FIV) vaccine efficacy and FIV neutralizing antibodies

A HIV-1 tier system has been developed to categorize the various subtype viruses based on their sensitivity to vaccine-induced neutralizing antibodies (NAbs): tier 1 with greatest sensitivity, tier 2 being moderately sensitive, and tier 3 being the least sensitive to NAbs (Mascola et al., J Virol 2005; 79:10103–7). Here, we define an FIV tier system using two related FIV dual-subtype (A + D) vaccines: the commercially available inactivated infected-cell vaccine (Fel-O-Vax^® FIV) and its prototype vaccine solely composed of inactivated whole viruses. Both vaccines afforded combined protection rates of 100% against subtype-A tier-1 FIV_Pet, 89% against subtype-B tier-3 FIV_FC1, 61% against recombinant subtype-A/B tier-2 FIV_Bang, 62% against recombinant subtype-F′/C tier-3 FIV_NZ1, and 40% against subtype-A tier-2 FIV_UK8 in short-duration (37–41 weeks) studies. In long-duration (76–80 weeks) studies, the commercial vaccine afforded a combined protection rate of at least 46% against the tier-2 and tier-3 viruses. Notably, protection rates observed here are far better than recently reported HIV-1 vaccine trials (Sanou et al., The Open AIDS J 2012; 6:246–60). Prototype vaccine protection against two tier-3 and one tier-2 viruses was more effective than commercial vaccine. Such protection did not correlate with the presence of vaccine-induced NAbs to challenge viruses. This is the first large-scale (228 laboratory cats) study characterizing short- and long-duration efficacies of dual-subtype FIV vaccines against heterologous subtype and recombinant viruses, as well as FIV tiers based on in vitro NAb analysis and in vivo passive-transfer studies. These studies demonstrate that not all vaccine protection is mediated by vaccine-induced NAbs.     

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.     

Evaluation of a prime-boost vaccine schedule with distinct adenovirus vectors against malaria in rhesus monkeys

A vaccine that elicits both specific antibodies and IFN-γ-producing T cells is required to protect against pre-erythrocytic malaria. Among the most promising approaches to induce such complex immunity are heterologous prime-boost vaccination regimens, in particular ones containing live viral vector. We have demonstrated previously that adenovectors serotype 35 (Ads35) encoding the circumsporozoite (CS) antigen or liver-stage antigen-1 (LSA-1) are highly effective in improving the T-cell responses induced by immunizations with protein-based vaccines in a heterologous prime-boost schedule. Here we evaluated the potential of a heterologous prime-boost vaccination that combines the Ad35.CS vector with the serologically distinct adenovector Ad5.CS, in rhesus macaques, after establishing the potency in mice. We show that the heterologous Ad35.CS/Ad5.CS prime-boost regimen elicits both antibody responses and robust IFN-γ-producing CD8⁺ T-cell responses against the CS antigen. Analysis of the quality of the antibody responses in rhesus macaques, using indirect immunofluorescence assay (IFA) with Plasmodium falciparum-coated slides, demonstrated that this heterologous prime-boost regimen elicits a high titer of antibodies that are able to bind to P. falciparum sporozoites. Level of the IFA response was superior to the response measured with sera of an adult human population living in endemic malaria region. In conclusion, the combination of Ad35.CS, a vaccine based on a rare serotype adenovirus, with Ad5.CS or possibly another adenovector of a distinct serotype, induces a complex immune response that is required for protection against malaria, and is thus a highly promising approach for pediatric vaccination.     

Viral vectors for malaria vaccine development

A workshop on viral vectors for malaria vaccine development, organized by the PATH Malaria Vaccine Initiative, was held in Bethesda, MD on October 20, 2005. Recent advancements in viral-vectored malaria vaccine development and emerging vector technologies were presented and discussed. Classic viral vectors such as poxvirus, adenovirus and alphavirus vectors have been successfully used to deliver malaria antigens. Some of the vaccine candidates have demonstrated their potential in inducing malaria-specific immunity in animal models and human trials. In addition, emerging viral-vector technologies, such as measles virus (MV), vesicular stomatitis virus (VSV) and yellow fever (YF) virus, may also be useful for malaria vaccine development. Studies in animal models suggest that each viral vector is unique in its ability to induce humoral and/or cellular immune responses. Those studies have also revealed that optimization of Plasmodium genes for mammalian expression is an important aspect of vaccine design. Codon-optimization, surface-trafficking, de-glycosylation and removal of toxic domains can lead to improved immunogenicity. Understanding the vector's ability to induce an immune response and the expression of malaria antigens in mammalian cells will be critical in designing the next generation of viral-vectored malaria vaccines.     

Development of nonhuman adenoviruses as vaccine vectors

Human adenoviral (HAd) vectors have demonstrated great potential as vaccine vectors. Preclinical and clinical studies have demonstrated the feasibility of vector design, robust antigen expression and protective immunity using this system. However, clinical use of adenoviral vectors for vaccine purposes is anticipated to be limited by vector immunity that is either preexisting or develops rapidly following the first inoculation with adenoviral vectors. Vector immunity inactivates the vector particles and rapidly removes the transduced cells, thereby limiting the duration of transgene expression. Due to strong vector immunity, subsequent use of the same vector is usually less efficient. In order to circumvent this limitation, nonhuman adenoviral vectors have been proposed as alternative vectors. In addition to eluding HAd immunity, these vectors possess most of the attractive features of HAd vectors. Several replication-competent or replication-defective nonhuman adenoviral vectors have been developed and investigated for their potential as vaccine-delivery vectors. Here, we review recent advances in the design and characterization of various nonhuman adenoviral vectors, and discuss their potential applications for human and animal vaccination.     

Advances in vaccinia virus-based vaccine vectors,with applications in flavivirus vaccine development

Historically, virulent variola virus infection caused hundreds of millions of deaths. The smallpox pandemic in human beings has spread for centuries until the advent of the attenuated vaccinia virus (VV) vaccine, which played a crucial role in eradicating the deadly contagious disease. Decades of exploration and utilization have validated the attenuated VV as a promising vaccine vehicle against various lethal viruses. In this review, we focus on the advances in VV-based vaccine vector studies, including construction approaches of recombinant VV, the impact of VV-specific pre-existing immunity on subsequent VV-based vaccines, and antigen-specific immune responses. More specifically, the recombinant VV-based flaviviruses are intensively discussed. Based on the publication data, this review aims to provide valuable insights and guidance for future VV-based vaccine development.     

Evaluation of attenuated VSVs with mutated M or/and G proteins as vaccine vectors

Vesicular stomatitis virus (VSV) is a promising vector for vaccine and oncolysis, but it can also produce acute diseases in cattle, horses, and swine characterized by vesiculation and ulceration of the tongue, oral tissues, feet, and teats. In experimental animals (primates, rats, and mice), VSV has been shown to lead to neurotoxicities, such as hind limb paralysis. The virus matrix protein (M) and glycoprotein (G) are both major pathogenic determinants of wild-type VSV and have been the major targets for the production of attenuated strains. Existing strategies for attenuation included: (1) deletion or M51R substitution in the M protein (VSVΔM51 or VSVM51R, respectively); (2) truncation of the C-terminus of the G protein (GΔ28). Despite these mutations, recombinant VSV with mutated M protein is only moderately attenuated in animals, whereas there are no detailed reports to determine the pathogenicity of recombinant VSV with truncated G protein at high dose. Thus, a novel recombinant VSV (VSVΔM51-GΔ28) as well as other attenuated VSVs (VSVΔM51, VSV-GΔ28) were produced to determine their efficacy as vaccine vectors with low pathogenicity. In vitro studies indicated that truncated G protein (GΔ28) could play a more important role than deletion of M51 (ΔM51) for attenuation of recombinant VSV. VSVΔM51-GΔ28 was determined to be the most attenuated virus with low pathogenicity in mice, with VSV-GΔ28 also showing relatively reduced pathogenicity. Further, neutralizing antibodies stimulated by VSV-GΔ28 proved to be significantly higher than in mice treated with VSVΔM51-GΔ28. In conclusion, among different attenuated VSVs with mutated M and/or G proteins, recombinant VSV with only truncated G protein (VSV-GΔ28) demonstrated ideal balance between pathogenesis and stimulating a protective immune response. These properties make VSV-GΔ28 a promising vaccine vector and vaccine candidate for preventing vesicular stomatitis disease.     

Development of an AIDS vaccine using Sendai virus vectors

Development of an effective AIDS vaccine is crucial for the control of global human immunodeficiency virus type 1 (HIV-1) prevalence. We have developed a novel AIDS vaccine using a Sendai virus (SeV) vector and investigated its efficacy in a macaque AIDS model of simian immunodeficiency virus (SIV) infection. Its immunogenicity and protective efficacy have been shown, indicating that the SeV vector is a promising delivery tool for AIDS vaccines. Here, we describe the potential of SeV vector as a vaccine antigen delivery tool to induce effective immune responses against HIV-1 infection.     

Comparison of multiple vaccine vectors in a single heterologous prime-boost trial

The prevention of infectious disease via prophylactic immunization is a mainstay of global public health efforts. Vaccine design would be facilitated by a better understanding of the type and durability of immune responses generated by different vaccine vectors. We report here the results of a comparative immunogenicity trial of six different vaccine vectors expressing the same insert antigen, cowpox virus B5 (CPXV-B5). Of those vectors tested, recombinant adenovirus (rAd5) was the most immunogenic, inducing the highest titer anti-B5 antibodies and conferring protection from sublethal vaccinia virus challenge in mice after a single immunization. We tested select heterologous prime-boost combinations and identified recombinant vesicular stomatitis virus (rVSV) and recombinant Venezuelan equine encephalitis virus replicons (VRP) as the most synergistic regimen. Comparative data such as those presented here are critical to efforts to generate protective vaccines for emerging infectious diseases as well as for biothreat agents.     

Feline herpesvirus vectored-rabies vaccine in cats: A dual protection

In China, cats cause about 5% of human rabies cases. Rabies control in cats plays a role in achieving the ultimate goal of elimination of dog rabies-mediated human deaths. However, there is no cat-specific rabies vaccine in China yet. In this study, we constructed a recombinant rabies vaccine by using a felid herpesvirus 1 (FHV-1) isolate, and deleted the gI/E in the FHV-1 and replaced the region with a glycoprotein (G) of rabies virus (RABV) strain BD06 through homologous recombination. The recombinant virus FHV-RVG was recovered and purified, and the expression of RABV glycoprotein was verified by indirect immunofluorescent assay. For potency in cats, each animal was inoculated intranasally with 1?ml FHV-RVG at 10^6.5 TCID₅₀. Blood samples were collected at defined intervals for antibody titration. The animals were challenged by herpes and rabies after completion of vaccination on day 180 and day 194, respectively. Our results demonstrated all vaccinated cats generated antibodies against both FHV-1 and RABV, and reached an arbitrary protective titer?>?0.5?IU/ml for rabies viral neutralizing antibody (VNA) by day 14 post inoculation (dpi) and titer peaked on 30 dpi with VNA at 24.5?±?10.23?IU/ml. All vaccinated cats presented no clinical signs of FHV-1 infection and survived rabies challenge, while the control cats had severe rhinotracheitis and died from rabies after challenge. All this demonstrated that the FHV-based recombinant vaccine is effective in protection against both FHV-1 and RABV infections.     

Non-replicating expression vectors: applications in vaccine development and gene therapy

This review presents experimental, preclinical and clinical data illustrating the multiple uses of recombinant non-replicating virus vectors in the fields of immunoprophylaxis and gene therapy.     

Cross-clade immunity in cats vaccinated with a canarypox-vectored avian influenza vaccine

Several felid species have been shown to be susceptible to infection with highly pathogenic avian influenza (HPAI) viruses of the H5N1 subtype. Infection of felids by H5N1 HPAI virus is often fatal, and cat-to-cat transmission has been documented. Domestic cats may then be involved in the transmission of infection to other animals but also to humans. A particular concern is the hypothetical role of the cat in the adaptation of the virus to mammalian species, thus increasing the pandemic risk. Therefore, the development of a HPAI vaccine for domestic cats should be considered a veterinary and also a public health priority. Here we show that vaccination of cats with a recombinant canarypox (ALVAC^®1) virus, expressing the hemagglutinin (HA) of influenza virus A/chicken/Indonesia/03 (H5N1) confers protection against challenge infection with two antigenically distinct H5N1 virus isolates from humans. Despite low hemagglutination inhibiting (HI) antibody titers at the time of challenge, all vaccinated cats were protected against mortality and had reduced histopathological changes in the lungs. Importantly, viral shedding was reduced in vaccinated cats as compared to controls, suggesting that vaccination of cats could reduce the risk of viral transmission. In conclusion this study showed that the recombinant canarypox virus protected cats against homologous and heterologous H5N1 HPAI virus challenges.     

Stabilisation of Salmonella vaccine vectors by the induction of trehalose biosynthesis

The growth of an aroA mutant of Salmonella typhimurium (SL3261) in minimal medium containing 0.5 M NaCl resulted in the intracellular accumulation of 2.2 micromol trehalose/mg total protein. The vacuum drying of these bacteria in the presence of trehalose allowed the recovery of 35% of the viable cells that were present before drying. In contrast, bacteria cultured in control medium accumulated 0.4 micromol trehalose/mg total protein and only 5% of the viable cells were recovered after vacuum drying with trehalose. Similar results were obtained when S. typhimurium SL3261, expressing the vaccine antigen (F1-antigen) of Yersinia pestis, was cultured in minimal medium with or without 0.5 M NaCl and dried in the presence of trehalose. Although these results indicate the potential for trehalose stabilisation of vaccine strains of S. typhimiurium, growth in minimal medium containing 0.5 M NaCl resulted in the loss of invasion competence of the bacteria.     

Effect of diethylcarbamazine on cats given feline leukaemia virus vaccine

Leukocell (Norden Laboratories, Lincoln, NE, USA), a commercially available retroviral vaccine, is given to protect susceptible cats against feline leukaemia virus. Oral diethylcarbamazine, an immunomodulator used in the treatment of filariasis, when given with 1 ml i.m. Leukocell, increased titres and duration of serum antibodies to feline oncornavirus-associated cell membrane antigen and transient average monocyte counts in treated/vaccinated cats as compared to cats that received vaccine without diethylcarbamazine. These results may have implications for the development of more effective retroviral vaccination techniques for humans as well as for cats.     

Novel adenovirus vaccine vectors based on the enteric-tropic serotype 41

Replication-defective adenovirus vectors, primarily developed from serotype 5 (Ad5) viruses, have been widely used for gene transfer and vaccination approaches. Vectors based on other serotypes of adenovirus could be used in conjunction with, or in place of, Ad5 vectors. In this study, Ad41, an enteric adenovirus usually described as 'non-cultivable' or 'fastidious,' has been successfully cloned, rescued and propagated on 293-ORF6 cells. The complementation capabilities of this cell line allow generation of Ad41 vectors at titers comparable to those obtained for Ad5 vectors. Mice immunized with an Ad41 vector containing an HIV envelope (Env) gene mounted anti-Env cellular and humoral immune responses. Ad41-Env vectors appear to be particularly attractive when used in heterologous prime-boost regimens, where they induce significantly higher cellular immune responses to HIV-Env than Ad5-based regimens. Ad41-based constructs are attractive vaccine vectors alone or in combination with Ad5 adenovectors, since each vector type can provide circumvention of pre-existing immunity to the other.     

Combination recombinant simian or chimpanzee adenoviral vectors for vaccine development

Recombinant adenoviral vector (rAd)-based vaccines are currently being developed for several infectious diseases and cancer therapy, but pre-existing seroprevalence to such vectors may prevent their use in broad human populations. In this study, we investigated the potential of low seroprevalence non-human primate rAd vectors to stimulate cellular and humoral responses using HIV/SIV Env glycoprotein (gp) as the representative antigen. Mice were immunized with novel simian or chimpanzee rAd (rSAV or rChAd) vectors encoding HIV gp or SIV gp by single immunization or in heterologous prime/boost combinations (DNA/rAd; rAd/rAd; rAd/NYVAC or rAd/rLCM), and adaptive immunity was assessed. Among the rSAV and rChAd tested, rSAV16 or rChAd3 vector alone generated the most potent immune responses. The DNA/rSAV regimen also generated immune responses similar to the DNA/rAd5 regimen. rChAd63/rChAd3 and rChAd3 /NYVAC induced similar or even higher levels of CD4+ and CD8+ T-cell and IgG responses as compared to rAd28/rAd5, one of the most potent combinations of human rAds. The optimized vaccine regimen stimulated improved cellular immune responses and neutralizing antibodies against HIV compared to the DNA/rAd5 regimen. Based on these results, this type of novel rAd vector and its prime/boost combination regimens represent promising candidates for vaccine development.     

Evaluation of invalid vaccine doses

BACKGROUND: The 2002 Recommended Childhood Immunization Schedule clarified the definition of an invalid dose of vaccine as any dose administered >/=5 days before the minimum age or interval had elapsed. Any invalid dose of vaccine should be repeated. OBJECTIVE: Determine the proportion of U.S. children who received an invalid dose of vaccine, evaluate the impact on vaccination coverage levels if invalid doses were not counted, and determine the vaccine purchase cost if at least one invalid dose is repeated. METHODS: Provider-reported vaccination histories of children aged 19 to 35 months sampled by the 2000 National Immunization Survey were evaluated. Analyses were performed in 2002 after the 2002 Advisory Committee on Immunization Practices (ACIP) Recommended Immunization Schedule was released. Any vaccine dose administered >/=5 days before the recommended minimum age or interval was classified as invalid. Change in vaccination coverage was determined by subtracting estimated valid-dose coverage (based on number of valid doses received) from the estimated up-to-date coverage (based on number of doses received regardless of age or spacing). RESULTS: Overall, 10.5% (+/-0.6%) of children had received at least one invalid dose of vaccine. Of the invalid doses, 51% were hepatitis B, 100% of which were the third dose; 19% were diphtheria-tetanus-pertussis (DTP/DTaP), 92% of which were the fourth dose; 12% were measles-containing vaccine (MCV); 15% were varicella vaccine; and 4% were polio vaccine, 96% of which were the first dose. Excluding invalid doses resulted in a small change in vaccination coverage: 2.2% for DTP/DTaP, 0.7% for polio, 6.5% for hepatitis B, 1.4% for MCV, and 1.7% for varicella. The vaccine purchase cost to repeat at least one invalid dose ranged from approximately $10 million (public-purchased) to approximately $18 million (private-purchased). CONCLUSIONS: Nationally about 595,000 of children aged 19 to 35 months, born between February 1997 and May 1999, received at least one invalid dose of vaccine. The cost of revaccinating these children is substantial and may have a negative impact on parents, physicians, and vaccine purchasers. Educating immunization providers regarding proper immunization timing should be conducted to reduce the administration of invalid doses of vaccines.     

Efficacy of an inactivated FeLV vaccine compared to a recombinant FeLV vaccine in minimum age cats following virulent FeLV challenge

The aim of the study was to determine the efficacy of an inactivated feline leukemia virus (FeLV) vaccine (Versifel^® FeLV, Zoetis.) compared to a recombinant FeLV vaccine (Purevax^® FeLV, Merial Animal Health) in young cats, exposed under laboratory conditions to a highly virulent challenge model. The study was designed to be consistent with the general immunogenicity requirements of the European Pharmacopoeia 6.0 Monograph 01/2008:1321—Feline Leukaemia Vaccine (Inactivated) with the exception that commercial-strength vaccines were assessed. Fifty seronegative cats (8–9 weeks old) were vaccinated subcutaneously on two occasions, three weeks apart, with either placebo (treatment group T01), Versifel FeLV Vaccine (treatment group T02), or Purevax FeLV Vaccine (treatment group T03) according to the manufacturer's directions. Cats were challenged three weeks after the second vaccination with a virulent FeLV isolate (61E strain). Persistent FeLV antigenemia was determined from 3 to 15 weeks postchallenge. Bone marrow samples were tested for the presence of FeLV proviral DNA to determine FeLV latent infection. At week 15 after challenge with the virulent FeLV 61E strain, the Versifel FeLV Vaccine conferred 89.5% protection against FeLV persistent antigenemia and 94.7% protection against FeLV proviral DNA integration in bone marrow cells. In comparison, the Purevax FeLV Vaccine conferred 20% protection against FeLV persistent antigenemia and 35% protection against FeLV proviral DNA integration in bone marrow cells following challenge. The data from this study show that the Versifel FeLV Vaccine was efficacious in preventing both FeLV persistent p27 antigenemia and FeLV proviral DNA integration in bone marrow cells of cats challenged with this particular challenge model under laboratory conditions and provided better protection than Purevax FeLV in this experimental challenge model with highly virulent FeLV.     

Serological responses in cats vaccinated with FeLV ISCOM and an inactivated FeLV vaccine

Various approaches have been considered for generation of effective and safe vaccines against retroviruses, including HIV, with limited success. In the present vaccination study, encompassing 137 household cats, we have composed an experimental ISCOM subunit vaccine containing gp70 of feline leukaemia virus (FeLV)--the external glycosylated envelope protein, and the transmembrane protein p15E, with a commercial available inactivated FeLV vaccine (Leukocell). The two vaccines were estimated to contain approximately the same amount of gp70 antigen and the cats were immunized three times according to the recommendations of the commercial vaccine. A control preparation not containing gp70 or p15E was also included. During the observation period of 200 days all cats remained healthy and no virus was isolated during the isolation attempts. The serological responses were measured in ELISA, membrane immunofluorescence (MIF) and virus neutralization (VN) tests. In contrast to the cats in the other groups almost all ISCOM-vaccinated cats responded by seroconversion or increased titres in the three tests. The development of specific antibodies to gp70 and p15E were confirmed in Western blot. These results clearly illustrate the potential of the ISCOM structure for the development of safe and effective vaccines against retroviruses.