Immune response and protective efficacy of live attenuated Salmonella vaccine expressing antigens of Mycobacterium avium subsp. paratuberculosis against challenge in mice

Characterization and Protective Efficacy of a Salmonella Typhimurium ATCC 14028 sptP Mutant as a Live Attenuated Vaccine Candidate

BACKGROUND

Salmonella Typhimurium poses a substantial health risk to both humans and animals. This study evaluated the potential of using the Salmonella Typhimurium ΔsptP mutant as a live-attenuated vaccine candidate by constructing it through homologous recombination and assessing its key biological properties, including growth characteristics, immunogenicity, and protective efficacy.

METHODS

We generated the ΔsptP mutant through targeted gene deletion, ensuring the preservation of the bacterial strain's growth and stability. In vitro and in vivo assays were performed to compare the invasive capabilities between the mutant and the wild-type strains. Specifically, we examined the invasion into RAW264.7 murine macrophages and mice. Furthermore, the virulence of the mutant was evaluated by determining the median lethal dose (LD50). To evaluate immunogenicity and protection, mice were immunized with 2 × 104 CFUs of the ΔsptP mutant, followed by a booster immunization, and then challenged with a virulent strain.

RESULTS

The ΔsptP mutant exhibited no significant changes in growth characteristics or genetic stability compared to the wild-type strain. However, it demonstrated a significantly diminished capacity for invasion in both murine macrophages and mice. The LD50 for the mutant was 39.92-fold higher than that of the wild-type, indicating a marked reduction in virulence. Mice immunized with the ΔsptP mutant and administered a booster immunization exhibited 87.5% protection against challenge with a virulent strain, as compared to the PBS control group. Moreover, the mutant induced IgG antibody levels comparable to those induced by the wild-type strain.

CONCLUSIONS

The ΔsptP mutant of Salmonella Typhimurium exhibits markedly reduced virulence while retaining robust immunogenicity and protective efficacy. These findings suggest that the ΔsptP mutant is a promising candidate for a live-attenuated vaccine, potentially providing an effective strategy to prevent Salmonella Typhimurium infections.

A candidate subunit vaccine induces protective immunity against Mycobacterium avium subspecies paratuberculosis in mice

Mycobacterium avium subspecies paratuberculosis (MAP) causes paratuberculosis (PTB), which is a granulomatous enteritis in ruminants that threatens the dairy industry's healthy development and public health safety worldwide. Because the commercial inactivated vaccines are not completely protective and interfere with bovine tuberculosis diagnostics, we tested four fusion proteins, namely 66NC, 66CN, 90NC, and 90CN, which were constructed with MAP3527, Ag85B, and Hsp70 of MAP in different tandem combinations. Notably, 66NC, which encodes a 66 kDa fusion protein that combines in linear order MAP3527_N40-232, Ag85B_41-330, and MAP3527_C231-361, induced a powerful and specific IFN-γ response. Immunization of C57BL/6 mice with the 66NC fusion protein formulated in Montanide ISA 61 VG adjuvant generated robust Th1, Th2, and Th17 type immune responses and strong antibody responses. The 66NC vaccine protected C57BL/6 mice against virulent MAP K-10 infection. This resulted in a reduction of bacterial load and improvement of pathological damage in the liver and intestine, in addition to a reduction of body weight loss; significantly better protection than the reported 74 F vaccine was also induced. Furthermore, vaccine efficacy correlated with the levels of IFN-γ-, TNF-α-, and IL-17A-secreting antigen-specific CD4⁺ and CD8⁺ T lymphocytes as well as with serum IFN-γ and TNF-α levels after vaccination. These results demonstrate that recombinant protein 66NC is an efficient candidate for further development into a protective vaccine in terms of inducing specific protection against MAP.

Mycobacterium avium subsp. paratuberculosis antigens induce cellular immune responses in cattle without causing reactivity to tuberculin in the tuberculosis skin test

Mycobacterium avium subspecies paratuberculosis (MAP) causes chronic progressive granulomatous enteritis leading to diarrhea, weight-loss, and eventual death in ruminants. Commercially available vaccine provides only partial protection against MAP infection and can interfere with the use of current diagnostic tests for bovine tuberculosis in cattle. Here, we characterized immune responses in calves to vaccines containing four truncated MAP antigens as a fusion (Ag85A^202-347-SOD^1-72-Ag85B^173-330-74F^{1-148+669-786}), either displayed on protein particles, or expressed as a soluble recombinant MAP (rMAP) fusion protein as well as to commercially available Silirum^® vaccine. The rMAP fusion protein elicited the strongest antigen-specific antibody responses to both PPDA and recombinant antigen and strong and long-lasting T-cell immune responses to these antigens, as indicated by increased production of IFN-γ and IL-17A in antigen-stimulated whole blood cultures. The MAP fusion protein particle vaccine induced minimal antibody responses and weak IFN-γ responses but stimulated IL-17A responses to recombinant antigen. The immune response profile of Silirum^® vaccine was characterized by weak antibodies and strong IFN-γ and IL-17A responses to PPDA. Transcription analysis on antigen-stimulated leukocytes from cattle vaccinated with rMAP fusion protein showed differential expression of several immune response genes and genes involved in costimulatory signaling, TLR4, TLR2, PTX3, PTGS2, PD-L1, IL1B, IL2, IL6, IL12B, IL17A, IL22, IFNG, CD40, and CD86. Moreover, the expression of several genes of immune pathways correlated with cellular immune responses in the rMAP fusion protein vaccinated group. These genes have key roles in pathways of mycobacterial immunity, including autophagy, manipulation of macrophage-mediated killing, Th17- and regulatory T cells- (Treg) mediated responses. Calves vaccinated with either the rMAP fusion protein or MAP fusion protein particle vaccine did not induce reactivity to PPDA and PPDB in a comparative cervical skin test, whereas Silirum^® induced reactivity to these tuberculins in most of the vaccinated animals. Overall, our results suggest that a combination of recombinant MAP antigens in the form of a soluble fusion protein vaccine are capable of inducing strong antigen-specific humoral and a balanced Th1/Th17-cell immune response. These findings, together with the absence of reactivity to tuberculin, suggest this subunit vaccine could provide protective immunity against intracellular MAP infection in cattle without compromising the use of current bovine tuberculosis surveillance test.

Reduced tissue colonization of Mycobacterium avium subsp. paratuberculosis in neonatal calves vaccinated with a cocktail of recombinant proteins

An increasing prevalence of paratuberculosis supports the need for new efficacious vaccines as an essential management tool. Two separate studies were performed in neonatal calves to evaluate the effectiveness of pooled recombinant Mycobacterium avium subsp. paratuberculosis (MAP) proteins (MAP1087, MAP1204, MAP1272c, MAP2077c) as a potential vaccine. In the first study vaccinated calves were immunized with 400 µg protein cocktail per dose, whereas the second study compared doses of 400 µg and 800 µg of protein cocktail, followed by challenge with live MAP for both vaccinated and nonvaccinated control calves 28 days post-vaccination. At the end of 12 months, tissue colonization with MAP was significantly reduced for the vaccinated calves compared to control animals. A higher dose of vaccine improved protection, with further reductions of MAP burden. Antigen-specific IFN-γ responses and serum antibody responses were similar regardless of vaccination, indicating exposure to MAP invoked conventional host immune responses. Host immunity differed due to vaccination, resulting in increased percentages of CD4+ T cells and B cells after stimulation of PBMCs with antigen. Interestingly, gene expression in PBMCs was similar for both control and vaccinated calves except for significant increases in IFN-γ, IL-12, and IL-17 expression observed in vaccinated calves. Vaccination with a cocktail of immunogenic recombinant MAP proteins was efficacious in reducing the level of infection and fecal shedding of neonatal calves and may be a potential tool for curtailing the spread of Johne's disease.

Self-assembled particulate vaccine elicits strong immune responses and reduces Mycobacterium avium subsp. paratuberculosis infection in mice

Mycobacterium avium subspecies paratuberculosis (MAP) causes chronic progressive granulomatous enteritis leading to diarrhoea, weight loss, and eventual death in ruminants. Commercially available vaccines provide only partial protection against MAP infection and can compromise the use of bovine tuberculosis diagnostic tests. Here, we report the development of a protein-particle-based vaccine containing MAP antigens Ag85A202-347-SOD1-72-Ag85B173-330-74F1-148+669-786 as a fusion ('MAP fusion protein particle'). The fusion antigen displayed on protein particles was identified using mass spectrometry. Surface exposure and accessibility of the fusion antigen was confirmed by flow cytometry and ELISA. The MAP fusion protein particle vaccine induced strong antigen-specific T-cell immune responses in mice, as indicated by increased cytokine (IFN-γ and IL-17A) and costimulatory signals (CD40 and CD86) in these animals. Following MAP-challenge, a significant reduction in bacterial burden was observed in multiple organs of the mice vaccinated with the MAP fusion protein particle vaccine compared with the PBS group. The reduction in severity of MAP infection conferred by the MAP fusion protein particle vaccine was similar to that of Silirum and recombinant protein vaccines. Overall, the results provide evidence that MAP antigens can be engineered as a protein particulate vaccine capable of inducing immunity against MAP infection. This utility offers an attractive platform for production of low-cost particulate vaccines against other intracellular pathogens.

Differentially regulated promoters for antigen expression in Salmonella vaccine strains

In most attenuated Salmonella enterica vaccines, heterologous antigens are expressed under the control of strong inducible promoters to ensure a high level of synthesis. Although high expression levels of the antigen can improve the immunogenicity of the vaccine, they might be toxic to the Salmonella carrier. Expression problems could be avoided by the use of promoters with specific characteristics with respect to strength and timing of expression. To study the expression of ten selected promoters, translational promoter-green fluorescent protein (GFP) fusions were analyzed in three attenuated Salmonella strains, Ty21a, SL3261 and PhoP^C. Promoter expression was evaluated both in vitro and in intracellular conditions using flow cytometry and confocal microscopy, with specific focus on the levels and timing of expression. We identified one major candidate promoter (Pasr) that could be used to express antigens specifically during in vivo conditions, without impairing bacterial growth during in vitro vaccine production.

A Novel Approach to Deliver a Mycobacterium avium subsp. paratuberculosis Antigen in Eukaryotic Cells

This study was aimed to express and deliver a Mycobacterium avium subsp. paratuberculosis antigen to macrophages using salmonella as carrier. The coding sequence of a fibronectin attachment protein which is expressed by Mycobacterium avium subsp. paratuberculosis was cloned into pcDNA3.1 (+) plasmid. The construct was introduced into the attenuated Salmonella typhimurium strain SL7207 (ΔhisG, ΔaroA) as carrier. In order to evaluate the delivery capacity of Salmonella and gene expression by antigen-presenting cells, the THP-1 derived macrophages were infected with the salmonella carrier. SDS-PAGE and western blot analysis showed the successful delivery and expression of targeted gene in THP-1 cell line. Although, in vitro stimulation of peripheral blood mononuclear cells with Salmonella containing plasmid did not trigger IFNγ production significantly. But it seems that this carrier can increase plasmid uptake and antigen expression by host intestinal antigen-presenting cells after mucosal administration. So, the construct can be used for further in vivo studies on the Salmonella carrier's efficiency in mycobacterial DNA vaccines.

Antigen Delivery System II

This chapter reviews papers mostly written since 2005 that report results using live attenuated bacterial vectors to deliver after administration through mucosal surfaces, protective antigens, and DNA vaccines, encoding protective antigens to induce immune responses and/or protective immunity to pathogens that colonize on or invade through mucosal surfaces. Papers that report use of such vaccine vector systems for parenteral vaccination or to deal with nonmucosal pathogens or do not address induction of mucosal antibody and/or cellular immune responses are not reviewed.

A rational framework for evaluating the next generation of vaccines against Mycobacterium avium subspecies paratuberculosis

Since the early 1980s, several investigations have focused on developing a vaccine against Mycobacterium avium subspecies paratuberculosis (MAP), the causative agent of Johne's disease in cattle and sheep. These studies used whole-cell inactivated vaccines that have proven useful in limiting disease progression, but have not prevented infection. In contrast, modified live vaccines that invoke a Th1 type immune response, may improve protection against infection. Spurred by recent advances in the ability to create defined knockouts in MAP, several independent laboratories have developed modified live vaccine candidates by transpositional mutation of virulence and metabolic genes in MAP. In order to accelerate the process of identification and comparative evaluation of the most promising modified live MAP vaccine candidates, members of a multi-institutional USDA-funded research consortium, the Johne's disease integrated program (JDIP), met to establish a standardized testing platform using agreed upon protocols. A total of 22 candidates vaccine strains developed in five independent laboratories in the United States and New Zealand voluntarily entered into a double blind stage gated trial pipeline. In Phase I, the survival characteristics of each candidate were determined in bovine macrophages. Attenuated strains moved to Phase II, where tissue colonization of C57/BL6 mice were evaluated in a challenge model. In Phase III, five promising candidates from Phase I and II were evaluated for their ability to reduce fecal shedding, tissue colonization and pathology in a baby goat challenge model. Formation of a multi-institutional consortium for vaccine strain evaluation has revealed insights for the implementation of vaccine trials for Johne's disease and other animal pathogens. We conclude by suggesting the best way forward based on this 3-phase trial experience and challenge the rationale for use of a macrophage-to-mouse-to native host pipeline for MAP vaccine development.

Cellular and humoral immune responses in sheep vaccinated with candidate antigens MAP2698c and MAP3567 from Mycobacterium avium subspecies paratuberculosis

Control of Johne's disease, caused by Mycobacterium avium subspecies paratuberculosis (MAP) in ruminants using commercially available vaccine reduces production losses, mortality, fecal shedding and histopathological lesions but does not provide complete protection from infection and interferes with serological diagnosis of Johne's disease and bovine tuberculosis. At this time no recombinant antigens have been found to provide superior protection compared to whole killed or live-attenuated MAP vaccines. Therefore, there is a need to evaluate more candidate MAP antigens. In this study recombinant MAP antigens MAP2698c and MAP3567 were formulated with four different MONTANIDE™ (ISA 50V2, 61VG, 71VG, and 201VG) adjuvants and evaluated for their ability to produce specific immune responses in vaccinated sheep. The cellular immune response was measured with an interferon-gamma (IFN-γ) release assay and the humoral immune response was measured by antibody detection enzyme linked immunosorbent assay. Recombinant vaccine formulation with the antigen MAP2698c and MONTANIDE™ ISA 201VG adjuvant produced strong whole-MAP as well as MAP2698c-specific IFN-γ responses in a high proportion of the vaccinated sheep. The formulation caused less severe injection site lesions in comparison to other formulations. The findings from this study suggest that the MAP2698c + 201VG should be evaluated in a challenge trial to determine the efficacy of this vaccine candidate.

Screening of Mycobacterium avium subsp. paratuberculosis mutants for attenuation in a bovine monocyte-derived macrophage model

Vaccination remains a major tool for prevention and progression of Johne's disease, a chronic enteritis of ruminants worldwide. Currently there is only one licensed vaccine within the United States and two vaccines licensed internationally against Johne's disease. All licensed vaccines reduce fecal shedding of Mycobacterium avium subsp. paratuberculosis (MAP) and delay disease progression. However, there are no available vaccines that prevent disease onset. A joint effort by the Johne's Disease Integrated Program (JDIP), a USDA-funded consortium, and USDA—APHIS/VS sought to identify transposon insertion mutant strains as vaccine candidates in part of a three phase study. The focus of the Phase I study was to evaluate MAP mutant attenuation in a well-defined in vitro bovine monocyte-derived macrophage (MDM) model. Attenuation was determined by colony forming unit (CFUs) counts and slope estimates. Based on CFU counts alone, the MDM model did not identify any mutant that significantly differed from the wild-type control, MAP K-10. Slope estimates using mixed models approach identified six mutants as being attenuated. These were enrolled in protection studies involving murine and baby goat vaccination-challenge models. MDM based approach identified trends in attenuation but this did not correlate with protection in a natural host model. These results suggest the need for alternative strategies for Johne's disease vaccine candidate screening and evaluation.

Evaluation of a Salmonella vectored vaccine expressing Mycobacterium avium subsp. paratuberculosis antigens against challenge in a goat model

Johnes disease (JD), caused by Mycobacterium avium subsp paratuberculosis (MAP), occurs worldwide as chronic granulomatous enteritis of domestic and wild ruminants. To develop a cost effective vaccine, in a previous study we constructed an attenuated Salmonella strain that expressed a fusion product made up of partial fragments of MAP antigens (Ag85A, Ag85B and SOD) that imparted protection against challenge in a mouse model. In the current study we evaluated the differential immune response and protective efficacy of the Sal-Ag vaccine against challenge in a goat model as compared to the live attenuated vaccine MAP316F. PBMCs from goats vaccinated with Sal-Ag and challenged with MAP generated significantly lower levels of IFN-γ, following in vitro stimulation with either Antigen-mix or PPD jhonin, than PBMC from MAP316F vaccinated animals. Flow cytometric analysis showed the increase in IFN-γ correlated with a significantly higher level of proliferation of CD4, CD8 and γδT cells and an increased expression of CD25 and CD45R0 in MAP316F vaccinated animals as compared to control animals. Evaluation of a range of cytokines involved in Th1, Th2, Treg, and Th17 immune responses by quantitative PCR showed low levels of expression of Th1 (IFN-γ, IL-2, IL-12) and proinflammatory cytokines (IL-6, IL-8, IL-18, TNF-α) in the Sal-Ag immunized group. Significant levels of Th2 and anti-inflammatory cytokines transcripts (IL-4, IL-10, IL-13, TGF-β) were expressed but their level was low and with a pattern similar to the control group. Over all, Sal-Ag vaccine imparted partial protection that limited colonization in tissues of some animals upon challenge with wild type MAP but not to the level achieved with MAP316F. In conclusion, the data indicates that Sal-Ag vaccine induced only a low level of protective immunity that failed to limit the colonization of MAP in infected animals. Hence the Sal-Ag vaccine needs further refinement to increase its efficacy.