DNA vaccine construct incorporating intercellular trafficking and intracellular targeting motifs effectively primes and induces memory B- and T-cell responses in outbred animals

Recent advances in antigen targeting to antigen-presenting cells in veterinary medicine

Advances in antigen targeting in veterinary medicine have gained traction over the years as an alternative approach for diseases that remain a challenge for traditional vaccines. In addition to the nature of the immunogen, antigen-targeting success relies heavily on the chosen receptor for its direct influence on the elicited response that will ensue after antigen uptake. Different approaches using antibodies, natural or synthetic ligands, fused proteins, and DNA vaccines have been explored in various veterinary species, with pigs, cattle, sheep, and poultry as the most frequent models. Antigen-presenting cells can be targeted using a generic approach, such as broadly expressed receptors such as MHC-II, CD80/86, CD40, CD83, etc., or focused on specific cell populations such as dendritic cells or macrophages (Langerin, DC-SIGN, XCR1, DC peptides, sialoadhesin, mannose receptors, etc.) with contrasting results. Interestingly, DC peptides show high specificity to DCs, boosting activation, stimulating cellular and humoral responses, and a higher rate of clinical protection. Likewise, MHC-II targeting shows consistent results in enhancing both immune responses; an example of this strategy of targeting is the approved vaccine against the bovine viral diarrhea virus in South America. This significant milestone opens the door to continuing efforts toward antigen-targeting vaccines to benefit animal health. This review discusses the recent advances in antigen targeting to antigen-presenting cells in veterinary medicine, with a special interest in pigs, sheep, cattle, poultry, and dogs.

Bovine Anaplasmosis: Will there ever be an almighty effective vaccine?

Bovine anaplasmosis is a tick-borne bacterial disease with a worldwide distribution and the cause of severe economic losses in the livestock industry in many countries, including México. In the present work, we first review the elements of the immune response of the bovine, which allows ameliorating the clinical signs while eliminating the majority of the blood forms and generating an immunologic memory such that future confrontations with the pathogen will not end in disease. On the other hand, many vaccine candidates have been evaluated for the control of bovine anaplasmosis yet without no commercial worldwide effective vaccine. Lastly, the diversity of the pathogen and how this diversity has impaired the many efforts to control the disease are reviewed.

Adenovirus-Vectored African Swine Fever Virus pp220 Induces Robust Antibody, IFN-γ, and CTL Responses in Pigs

African Swine Fever Virus (ASFV) poses a serious threat to the pork industry worldwide; however, there is no safe vaccine or treatment available. The development of an efficacious subunit vaccine will require the identification of protective antigens. The ASFV pp220 polyprotein is essential for virus structural integrity. This polyprotein is processed to generate p5, p34, p14, p37, and p150 individual proteins. Immunization of pigs with a cocktail of adenoviruses expressing the proteins induced significant IgG, IFN-γ-secreting cells, and cytotoxic T lymphocyte responses. Four predicted SLA-I binding nonamer peptides, namely p34161−169, p37859−867, p1501363−1371, and p1501463−1471, recalled strong IFN-γ+ PBMC and splenocyte responses. Notably, peptide p34161−169 was recognized by PBMCs isolated from 7/10 pigs and by splenocytes isolated from 8/10 pigs. Peptides p37859−867 and p1501363−1371 stimulated recall IFN-γ+ responses in PBMCs and splenocytes isolated from 8/10 pigs, whereas peptide p1501463−1471 recalled responses in PBMCs and splenocytes isolated from 7/10 to 9/10 pigs, respectively. The results demonstrate that the pp220 polyprotein contains multiple epitopes that induce robust immune responses in pigs. Importantly, these epitopes are 100% conserved among different ASFV genotypes and were predicted to bind multiple SLA-I alleles. The outcomes suggest that pp220 is a promising candidate for inclusion in a prototype subunit vaccine.

A multi-epitope DNA vaccine co-administered with monophosphoryl lipid A adjuvant provides protection against tick transmitted Ehrlichia ruminantium in sheep

Previously, a heartwater experimental DNA vaccine provided 100% protection following laboratory challenge with Ehrlichia ruminantium administered by needle but not against an E. ruminantium tick challenge in the field. A multi-epitope DNA vaccine incorporating both CD4⁺ and CD8⁺ cytotoxic T lymphocytes epitopes could provide a better alternative. In this study, we investigated the use of multi-epitope DNA vaccines against an E. ruminantium experimental tick challenge in sheep. The multi-epitope DNA vaccines were delivered via the intramuscular route and intradermal route using the gene gun in the presence of monophosphoryl lipid A (MPL) adjuvant, which was either applied topically to the gene gun inoculation site or co-administered with the vaccine via the intramuscular route. Initially two constructs namely, pSignal plus and pLamp were tested with MPL applied topically only and no protection was obtained in this formulation. However, when pLamp was co-administered with MPL via the intramuscular route in addition to topical application, its protective efficiency improved to protect 60% of the sheep against tick challenge. In this formulation, the vaccine induced enhanced activation of memory T cell responses both before and after challenge with variations amongst the different sheep possibly due to their different genetic backgrounds. In conclusion, this study showed that a heartwater multi-epitope DNA vaccine, co-administered with MPL adjuvant can protect sheep following a laboratory E. ruminantium tick challenge.

Adenovirus-vectored novel African Swine Fever Virus antigens elicit robust immune responses in swine

African Swine Fever Virus (ASFV) is a high-consequence transboundary animal pathogen that often causes hemorrhagic disease in swine with a case fatality rate close to 100%. Lack of treatment or vaccine for the disease makes it imperative that safe and efficacious vaccines are developed to safeguard the swine industry. In this study, we evaluated the immunogenicity of seven adenovirus-vectored novel ASFV antigens, namely A151R, B119L, B602L, EP402RΔPRR, B438L, K205R and A104R. Immunization of commercial swine with a cocktail of the recombinant adenoviruses formulated in adjuvant primed strong ASFV antigen-specific IgG responses that underwent rapid recall upon boost. Notably, most vaccinees mounted robust IgG responses against all the antigens in the cocktail. Most importantly and relevant to vaccine development, the induced antibodies recognized viral proteins from Georgia 2007/1 ASFV-infected cells by IFA and by western blot analysis. The recombinant adenovirus cocktail also induced ASFV-specific IFN-γ-secreting cells that were recalled upon boosting. Evaluation of local and systemic effects of the recombinant adenovirus cocktail post-priming and post-boosting in the immunized animals showed that the immunogen was well tolerated and no serious negative effects were observed. Taken together, these outcomes showed that the adenovirus-vectored novel ASFV antigen cocktail was capable of safely inducing strong antibody and IFN-γ⁺ cell responses in commercial swine. The data will be used for selection of antigens for inclusion in a multi-antigen prototype vaccine to be evaluated for protective efficacy.

Induction of Robust Immune Responses in Swine by Using a Cocktail of Adenovirus-Vectored African Swine Fever Virus Antigens

The African swine fever virus (ASFV) causes a fatal hemorrhagic disease in domestic swine, and at present no treatment or vaccine is available. Natural and gene-deleted, live attenuated strains protect against closely related virulent strains; however, they are yet to be deployed and evaluated in the field to rule out chronic persistence and a potential for reversion to virulence. Previous studies suggest that antibodies play a role in protection, but induction of cytotoxic T lymphocytes (CTLs) could be the key to complete protection. Hence, generation of an efficacious subunit vaccine depends on identification of CTL targets along with a suitable delivery method that will elicit effector CTLs capable of eliminating ASFV-infected host cells and confer long-term protection. To this end, we evaluated the safety and immunogenicity of an adenovirus-vectored ASFV (Ad-ASFV) multiantigen cocktail formulated in two different adjuvants and at two immunizing doses in swine. Immunization with the cocktail rapidly induced unprecedented ASFV antigen-specific antibody and cellular immune responses against all of the antigens. The robust antibody responses underwent rapid isotype switching within 1 week postpriming, steadily increased over a 2-month period, and underwent rapid recall upon boost. Importantly, the primed antibodies strongly recognized the parental ASFV (Georgia 2007/1) by indirect fluorescence antibody (IFA) assay and Western blotting. Significant antigen-specific gamma interferon-positive (IFN-γ⁺) responses were detected postpriming and postboosting. Furthermore, this study is the first to demonstrate induction of ASFV antigen-specific CTL responses in commercial swine using Ad-ASFV multiantigens. The relevance of the induced immune responses in regard to protection needs to be evaluated in a challenge study.

Loss of Immunization-Induced Epitope-Specific CD4 T-Cell Response following Anaplasma marginale Infection Requires Presence of the T-Cell Epitope on the Pathogen and Is Not Associated with an Increase in Lymphocytes Expressing Known Regulatory Cell Phenotypes

We have shown that in cattle previously immunized with outer membrane proteins, infection with Anaplasma marginale induces a functionally exhausted CD4 T-cell response to the A. marginale immunogen. Furthermore, T-cell responses following infection in nonimmunized cattle had a delayed onset and were sporadic and transient during persistent infection. The induction of an exhausted T-cell response following infection presumably facilitates pathogen persistence. In the current study, we hypothesized that the loss of epitope-specific T-cell responses requires the presence of the immunizing epitope on the pathogen, and T-cell dysfunction correlates with the appearance of regulatory T cells. In limited studies in cattle, regulatory T cells have been shown to belong to γδ T-cell subsets rather than be CD4 T cells expressing forkhead box protein P3 (FoxP3). Cattle expressing the DRB3*1101 haplotype were immunized with a truncated A. marginale major surface protein (MSP) 1a that contains a DRB3*1101-restricted CD4 T-cell epitope, F2-5B. Cattle either remained unchallenged or were challenged with A. marginale bacteria that express the epitope or with A. marginale subsp. centrale that do not. Peripheral blood and spleen mononuclear cells were monitored for MSP1a epitope F2-5B-specfic T-cell proliferative responses and were stained for γδ T-cell subsets or CD4(+) CD25(+) FoxP3(+) T cells before and during infection. As hypothesized, the induction of T-cell exhaustion occurred only following infection with A. marginale, which did not correlate with an increase in either CD4(+) CD25(+) FoxP3(+) T cells or any γδ T-cell subset examined.

Fusion to the Lysosome Targeting Signal of the Invariant Chain Alters the Processing and Enhances the Immunogenicity of HIV-1 Reverse Transcriptase

Intracellular processing of the antigen encoded by a DNA vaccine is one of the key steps in generating an immune response. Immunization with DNA constructs targeted to the endosomal-lysosomal compartments and to the MHC class II pathway can elicit a strong immune response. Herein, the weakly immunogenic reverse transcriptase of HIV-1 was fused to the minimal lysosomal targeting motif of the human MHC class II invariant chain. The motif fused to the N-terminus shifted the enzyme intracellular localization and accelerated its degradation. Degradation of the chimeric protein occurred predominantly in the lysosomal compartment. BALB/c mice immunized with the plasmid encoding the chimeric protein demonstrated an enhanced immune response, in the form of an increased antigen-specific production of Th1 cytokines, INF-γ and IL-2, by mouse splenocytes. Moreover, the majority of the splenocytes secreted both cytokines; i.e., were polyfunctional. These findings suggest that retargeting of the antigen to the lysosomes enhances the immune response to DNA vaccine candidates with low intrinsic immunogenicity.

Adaptive immunity to Anaplasma pathogens and immune dysregulation: implications for bacterial persistence

Anaplasma marginale is an obligate intraerythrocytic bacterium that infects ruminants, and notably causes severe economic losses in cattle worldwide. Anaplasma phagocytophilum infects neutrophils and causes disease in many mammals, including ruminants, dogs, cats, horses, and humans. Both bacteria cause persistent infection - infected cattle never clear A. marginale and A. phagocytophilum can also cause persistent infection in ruminants and other animals for several years. This review describes correlates of the protective immune response to these two pathogens as well as subversion and dysregulation of the immune response following infection that likely contribute to long-term persistence. I also compare the immune dysfunction observed with intraerythrocytic A. marginale to that observed in other models of chronic infection resulting in high antigen loads, including malaria, a disease caused by another intraerythrocytic pathogen.

The synthetic peptides bovine enteric β-defensin (EBD), bovine neutrophil β-defensin (BNBD) 9 and BNBD 3 are chemotactic for immature bovine dendritic cells

Human and murine immature DCs (iDCs) are highly efficient in antigen capture and processing, while as mature cells they present antigen and are potent initiators of cell-mediated immune responses. Consequently, iDCs are logical targets for vaccine antigens. Originally discovered for their antimicrobial activity, and thought of as strictly part of the innate immune system, studies with defensins such as human β (beta)-defensin 2 (hBD2) and murine β-defensin 2 (mBD2) have shown that they can function as chemo-attractant for iDCs and, in vaccination strategies, can enhance antigen-specific adaptive immune responses. Most studies to date have been conducted in mice. In contrast, little is known about defensins in cattle. To expand our understanding of the role of defensins in modulating immune responses in cattle, DCs were generated from bovine monocytes and the immature state of these bovine DCs was characterized phenotypically and through functional assays. By day 3 (DC3), bovine monocyte-derived DCs stained positively for DC-specific receptors CD1, CD80/86, CD205, DC-Lamp and MMR. When compared to conventional 6-day DC cultures or DCs cultured for 10 days with and without maturation factors, these DC3 were functionally at their most immature stage. Fourteen of the 16 known bovine β-defensins were synthesized and the synthetic peptides were screened for their ability to attract bovine iDCs. Bovine DC3 were consistently attracted to BNBD3, an analog of BNBD3 (aBNBD3), BNBD9 and bovine EBD in vitro and to aBNBD3 in vivo. These results are the first to describe chemotactic ability of synthetic bovine β-defensins for immature bovine monocyte-derived DCs.

Regulation of Immunogen Processing: Signal Sequences and Their Application for the New Generation of DNA-Vaccines

Immunization with naked genes (DNA-immunization) is a perspective modern approach to prophylactic as well as therapeutic vaccination against pathogens, as well as cancer and allergy. A panel of DNA immunogens has been developed, some are already in the clinical trials. However, the immunogenicity of DNA vaccines, specifically of those applied to humans, needs a considerable improvement. There are several approaches to increase DNA vaccine immunogenicity. One approach implies the modifications of the encoded immunogen that change its processing and presentation, and thus the overall pattern of anti-immunogen response. For this, eukaryotic expression vectors are constructed that encode the chimeric proteins composed of the immunogen and specialized targeting or signal sequences. The review describes a number of signals that if fused to immunogen, target it into the predefined subcellular compartments. The review gives examples of their application for DNA-immunization.

Enhanced immune response and protection efficacy of a DNA vaccine constructed by linkage of the Mycobacterium tuberculosis Ag85B-encoding gene with the BVP22-encoding gene

Plasmid DNA vaccines have been widely explored for use in tuberculosis immunization but their immunogenicity needs improvement. In the present study, we incorporated the bovine herpesvirus 1 VP22 (BVP22)-encoding gene, which encodes a protein that demonstrates a capability for disseminating the expressed antigen to neighbouring cells, into a DNA vector in which it was fused to the Ag85B-encoding gene of Mycobacterium tuberculosis (Mtb), and investigated whether this linkage could enhance immune response and protective efficacy in C57BL/6 mice compared to plasmid DNA encoding Ag85B alone. After immunization in mice, Ag85B-specific ELISA antibodies and spleen lymphocyte proliferative responses induced by DNA co-expressing BVP22 and Ag85B were significantly higher than those obtained in mice immunized with Ag85B-encoding DNA alone, except for the number of gamma interferon secreting cells. In addition, based on histopathological examination and bacterial-load determination in lung and spleen, protection against intravenous Mtb H37Rv challenge evoked by the BVP22–Ag85B DNA immunization exceeded the response elicited by Ag85B DNA alone, which was not significantly different from that provided by Bacillus Calmette–Guérin (BCG). These results suggested that DNA vaccine consisting of BVP22 and Ag85B-encoding DNA enhanced immune response and protection against intravenous Mtb H37Rv challenge in mice, indicating that BVP22-encoding DNA might be a promising tool to enhance TB DNA vaccine efficacy.

Rapid deletion of antigen-specific CD4+ T cells following infection represents a strategy of immune evasion and persistence for Anaplasma marginale

Acquired T cell immunity is central for protection against infection. However, the immunological consequences of exposing memory T cells to high Ag loads during acute and persistent infection with systemic pathogens are poorly understood. We investigated this by using infection with Anaplasma marginale, a ruminant pathogen that replicates to levels of 10(9) bacteria per ml of blood during acute infection and maintains mean bacteremia levels of 10(6) per ml during long-term persistent infection. We established that immunization-induced Ag-specific peripheral blood CD4(+) T cell responses were rapidly and permanently lost following infection. To determine whether these T cells were anergic, sequestered in the spleen, or physically deleted from peripheral blood, CD4(+) T lymphocytes from the peripheral blood specific for the major surface protein (MSP) 1a T cell epitope were enumerated by DRB3*1101 tetramer staining and FACS analysis throughout the course of immunization and challenge. Immunization induced significant epitope-specific T lymphocyte responses that rapidly declined near peak bacteremia to background levels. Concomitantly, the mean frequency of tetramer(+)CD4(+) cells decreased rapidly from 0.025% before challenge to a preimmunization level of 0.0003% of CD4(+) T cells. Low frequencies of tetramer(+)CD4(+) T cells in spleen, liver, and inguinal lymph nodes sampled 9-12 wk postchallenge were consistent with undetectable or unsustainable Ag-specific responses and the lack of T cell sequestration. Thus, infection of cattle with A. marginale leads to the rapid loss of Ag-specific T cells and immunologic memory, which may be a strategy for this pathogen to modulate the immune response and persist.

Bovine immune response to inoculation with Neospora caninum surface antigen SRS2 lipopeptides mimics immune response to infection with live parasites

Infection of cattle with Neospora caninum protozoa, the causative agent of bovine protozoal abortion, results in robust cellular and humoral immune responses, particularly CD4(+) T-lymphocyte activation and gamma interferon (IFN-gamma) secretion. In the present study, N. caninum SRS2 (NcSRS2) T-lymphocyte-epitope-bearing subunits were incorporated into DNA and peptide preparations to assess CD4(+) cell proliferation and IFN-gamma T-lymphocyte-secretion immune responses in cattle with predetermined major histocompatibility complex (MHC) genotypes. In order to optimize dendritic-cell processing, NcSRS2 DNA vaccine was delivered with granulocyte macrophage-colony-stimulating factor and Flt3 ligand adjuvant. The synthesized NcSRS2 peptides were coupled with a palmitic acid molecule (lipopeptide) and delivered with Freund's adjuvant. Cattle vaccinated with NcSRS2 DNA vaccine alone did not induce T-lymphocyte activation or IFN-gamma secretion, whereas subsequent booster inoculation with NcSRS2-lipopeptides induced robust NcSRS2-specific immune responses. Compared to the response in control animals, NcSRS2-lipopeptide-immunized cattle had significantly increased NcSRS2-specific T-lymphocyte proliferation, numbers of IFN-gamma-secreting peripheral blood mononuclear cells, and immunoglobulin G1 (IgG1) and IgG2a antibody levels. The findings show that N. caninum NcSRS2 subunits bearing T-lymphocyte epitopes induced cell-mediated immune responses similar to the protective immune responses previously described against live parasite infection, namely T-lymphocyte activation and IFN-gamma secretion. The findings support the investigation of NcSRS2 immunogens for protection against N. caninum-induced fetal infection and abortion in cattle.