Enhanced cellular immune response against SIV Gag induced by immunization with DNA vaccines expressing assembly and release-defective SIV Gag proteins

Evaluation of in ovo vaccination of DNA vaccines for Campylobacter control in broiler chickens

Campylobacter is the leading bacterial cause of human enteritis in developed countries. Chicken is a major natural host of Campylobacter. Thus, on-farm control of Campylobacter load in poultry would reduce the risk of human exposure to this pathogen. Vaccination is an attractive intervention measure to mitigate Campylobacter in poultry. Our previous studies have demonstrated that Campylobacter outer membrane proteins CmeC (a component of multidrug efflux pump) and CfrA (ferric enterobactin receptor) are feasible and promising candidates for vaccine development. In this study, by targeting these two attractive vaccine candidates, we explored and evaluated a new vaccination strategy, which combines the in ovo vaccination route and novel DNA vaccine formulation, for Campylobacter control in broilers. We observed that direct cloning of cfrA or cmeC gene into the eukaryotic expression vector pCAGGS did not lead to sufficient level of production of the target proteins in the eukaryotic HEK-293 cell line. However, introduction of the Kozak consensus sequence (ACCATGG) in the cloned bacterial genes greatly enhanced production of inserted gene in eukaryotic cells, creating desired DNA vaccines. Subsequently, the validated DNA vaccines were prepared and used for two independent in ovo vaccination trials to evaluate their immune response and protective efficacy. However, single in ovo injection of specific DNA vaccine at 18th day of embryonation, regardless using neutral lipid-protected vector or not, failed to trigger significant IgG and IgA immune responses and did not confer protection against C. jejuni colonization in the intestine of chickens. In conclusion, this study demonstrates that the Kozak sequence is critically important for construction of the DNA vaccine expressing prokaryotic gene. The optimal regimen for in ovo vaccination of DNA vaccine for Campylobacter control in poultry needs to be determined in future studies.

The immune response to a vesicular stomatitis virus vaccine vector is independent of particulate antigen secretion and protein turnover rate

Vesicular stomatitis virus (VSV) is a highly cytopathic virus being developed as a vaccine vector due to its ability to induce strong protective T cell and antibody responses after a single dose. However, little is known regarding the mechanisms underlying the potent immune responses elicited by VSV. We previously generated a VSV vector expressing the hepatitis B virus middle envelope surface glycoprotein (MS) that induces strong MS-specific T cell and antibody responses in mice. After synthesis in the cytoplasm, the MS protein translocates to the endoplasmic reticulum, where it forms subviral particles that are secreted from the cell. To better understand the contributions of secreted and intracellular protein to the VSV-induced immune response, we produced a vector expressing a secretion-deficient MS mutant (MS(C69A)) and compared the immunogenicity of this vector to that of the wild-type VSV-MS vector in mice. As expected, the MS(C69A) protein was not secreted from VSV-infected cells and displayed enhanced proteasome-mediated degradation. Surprisingly, despite these differences in intracellular protein processing, the T cell and antibody responses generated to MS(C69A) were comparable to those elicited by virus expressing wild-type MS protein. Therefore, when it is expressed from VSV, the immune responses to MS are independent of particulate antigen secretion and the turnover rate of cytoplasmic protein. These results are consistent with a model in which the immune responses to VSV are strongly influenced by the replication cycle of the vector and demonstrate that characteristics of the vector have the capacity to affect vaccine efficacy more than do the properties of the antigen itself.

Intranasal immunization with chitosan/pCAGGS-flaA nanoparticles inhibits Campylobacter jejuni in a White Leghorn model

Campylobacter jejuni is the most common zoonotic bacterium associated with human diarrhea, and chickens are considered to be one of the most important sources for human infection, with no effective prophylactic treatment available. We describe here a prophylactic strategy using chitosan-DNA intranasal immunization to induce specific immune responses. The chitosan used for intranasal administration is a natural mucus absorption enhancer, which results in transgenic DNA expression in chicken nasopharynx. Chickens immunized with chitosan-DNA nanoparticles, which carried a gene for the major structural protein FlaA, produced significantly increased levels of serum anti-Campylobacter jejuni IgG and intestinal mucosal antibody (IgA), compared to those treated with chitosan-DNA (pCAGGS). Chitosan-pCAGGS-flaA intranasal immunization induced reductions of bacterial expellation by 2-3 log₁₀ and 2 log₁₀ in large intestine and cecum of chickens, respectively, when administered with the isolated C. jejuni strain. This study demonstrated that intranasal delivery of chitosan-DNA vaccine successfully induced effective immune response and might be a promising vaccine candidate against C. jejuni infection.

A naturally occurring deletion in its NS gene contributes to the attenuation of an H5N1 swine influenza virus in chickens

In 2001 and 2003, we isolated two H5N1 viruses, A/swine/Fujian/1/01 (SW/FJ/01) and A/swine/Fujian/1/03 (SW/FJ/03), from pigs in Fujian Province, southern China. Genetically, these two viruses are similar, although the NS gene of the SW/FJ/03 virus has a 15-nucleotide deletion at coding positions 612 to 626. The SW/FJ/01 virus is highly lethal for chickens, whereas the SW/FJ/03 virus is nonpathogenic for chickens when administrated intravenously or intranasally. To understand the molecular basis for the difference in virulence, we used reverse genetics to create a series of single-gene recombinants of both viruses. We found that a recombinant virus containing the mutated NS gene from the SW/FJ/03 virus in the SW/FJ/01 virus background was completely attenuated in chickens. We also found that viruses expressing the mutant NS1 protein of SW/FJ/03 did not antagonize the induction of interferon (IFN) protein. Conversely, only the recombinant virus containing the wild-type SW/FJ/01 NS gene in the SW/FJ/03 background was lethal in chickens and antagonized IFN protein levels. Further, we proved that the NS1 genes of the two viruses differ in their stabilities in the host cells and in their abilities to interact with the chicken cleavage and polyadenylation specificity factor. These results indicate that the deletion of amino acids 191 to 195 of the NS1 protein is critical for the attenuation of the SW/FJ/03 virus in chickens and that this deletion affects the ability of the virus to antagonize IFN induction in host cells.

Enhanced protective efficacy of H5 subtype avian influenza DNA vaccine with codon optimized HA gene in a pCAGGS plasmid vector

H5N1 influenza viruses have caused significant disease and deaths in various parts of the world in several species, including humans. Vaccination combined with culling can provide an attractive method for outbreak containment. Using synthesized oligos and overlapping extension PCR techniques, we constructed an H5 HA gene, optiHA, containing chicken biased codons based on the HA amino acid sequence of the highly pathogenic H5N1 virus A/goose/Guangdong/1/96 (GS/GD/96). The optiHA and wild-type HA genes were inserted into plasmids pCI or pCAGGS, and designated as pCIoptiHA, pCAGGoptiHA, pCIHA and pCAGGHA, respectively. To evaluate vaccine efficacy, groups of 3-week-old specific pathogen free (SPF) chickens were intramuscularly injected with the four plasmids. Sera were collected on a weekly basis post-vaccination (p.v.) for hemagglutination inhibition (HI) assays and neutralization (NT) antibody detection. All chickens receiving pCAGGoptiHA and pCAGGHA developed high levels of HI and NT antibodies at 3 weeks p.v., and were completely protected from lethal H5 virus challenge, while only partial protection was induced by inoculation with the other two plasmids. A second experiment was conducted to evaluate if a lower dose of the pCAGGoptiHA vaccine could be effective, results indicated that two doses of 10 microg of pCAGGoptiHA could induce complete protection in chickens against H5 lethal virus challenge. Based on our results, we conclude that construction optimization could dramatically increase the H5 HA gene DNA vaccine efficacy in chickens, and therefore, greatly decrease the dose necessary for inducing complete protection in chickens.

Structure and immunogenicity of alternative forms of the simian immunodeficiency virus gag protein expressed using Venezuelan equine encephalitis virus replicon particles

Venezuelan equine encephalitis virus replicon particles (VRP) were engineered to express different forms of SIV Gag to compare expression in vitro, formation of intra- and extracellular structures and induction of humoral and cellular immunity in mice. The three forms examined were full-length myristylated SIV Gag (Gagmyr+), full-length Gag lacking the myristylation signal (Gagmyr-) or a truncated form of Gagmyr- comprising only the matrix and capsid domains (MA/CA). Comparison of VRP-infected primary mouse embryo fibroblasts, mouse L929 cells and primate Vero cells showed comparable expression levels for each protein, as well as extracellular virus-like particles (VRP-Gagmyr+) and distinctive cytoplasmic aggregates (VRP-Gagmyr-) with each cell type. VRP were used to immunize BALB/c mice, and immune responses were compared using an interferon (IFN)-gamma ELISPOT assay and a serum antibody ELISA. Although all three VRP generated similar levels of IFN-gamma-producing cells at 1 week post-boost, at 10 weeks post-boost the MA/CA-VRP-induced response was maintained at a significantly higher level relative to that induced by Gagmyr+-VRP. Antibody responses to MA/CA-VRP and Gagmyr+-VRP were not significantly different.

Efficacy of DNA Immunization with F and G Protein Genes of Nipah Virus

We investigated the antibody response of DNA immunization with two mammalian codon optimized envelope glycoprotein genes, F and G, of Nipah virus in a mouse model. The results indicated that G gene immunization elicited more significant specific serum IgG response and neutralization antibody response than F gene did, suggesting that the G gene DNA immunization is a potential vaccine strategy against Nipah virus.

Mucosal immunization of sheep with a Maedi-Visna virus (MVV) env DNA vaccine protects against early MVV productive infection

Gene gun mucosal DNA immunization of sheep with a plasmid expressing the env gene of Maedi-Visna virus (MVV) was used to examine the protection against MVV infection in sheep from a naturally infected flock. For immunization, sheep were primed with a pcDNA plasmid (pcDNA-env) encoding the Env glycoproteins of MVV and boosted with combined pcDNA-env and pCR3.1-IFN-gamma plasmid inoculations. The pcDNA plasmid used in the control group contained the lacZ coding sequences instead of the env gene. Within a month post-challenge, the viral load in the vaccinated group was lower (p < or = 0.05) and virus was only detected transiently compared with the control group. Furthermore, 2 months later, neutralizing antibodies (NtAb) were detected in all the control animals and none of the vaccinated animals (p < or = 0.01). These results demonstrated a significant early protective effect of this immunization strategy against MVV infection that restricts the virus replication following challenge in the absence of NtAb production. This vaccine protective effect against MVV infection disappeared after two years post-challenge, when active replication of MVV challenge strain was observed. Protection conferred by the vaccine could not be explained by OLA DRB1 allele or genotype differences. Most of the individuals were DRB1 heterozygous and none was totally resistant to infection.

An Evaluation of Enforced Rapid Proteasomal Degradation as a Means of Enhancing Vaccine-Induced CTL Responses

The HIV-1 Gag protein is an attractive target for CTL-based vaccine strategies because it shows less sequence variability than other HIV-1 proteins. In an attempt to increase the immunogenicity of HIV-1 Gag, we created Gag variants that were targeted to the proteasomal pathway for rapid degradation. This enhanced rate of degradation was associated with increased presentation of MHC class I-associated antigenic peptides on the cell surface. Despite this, immunizing mice with either plasmid DNA or recombinant vaccinia vectors expressing unstable Gag failed to produce significant increases in bulk CTL responses or Ag-specific production of IFN-gamma by CD8(+) T cells compared with mice immunized with stable forms of Gag. Production of IFN-gamma by CD4(+) T cells was also impaired, and we speculate that the abrogation of CD4(+) T cell help was responsible for the impaired CTL response. These results suggest that vaccine strategies designed to increase the density of peptide-MHC class I complexes on the surfaces of APC may not necessarily enhance immunogenicity with respect to CTL responses.