Listeria-Vectored Vaccine Expressing the Mycobacterium tuberculosis 30-Kilodalton Major Secretory Protein via the Constitutively Active prfA* Regulon Boosts Mycobacterium bovis BCG Efficacy against Tuberculosis

Listeria-vectored multi-antigenic tuberculosis vaccine protects C57BL/6 and BALB/c mice and guinea pigs against Mycobacterium tuberculosis challenge

Mycobacterium tuberculosis (Mtb) infects one-third of the world's population and is a leading cause of death from a single infectious agent. New TB vaccines are urgently needed to augment immunity conferred by the current modestly protective BCG vaccine. We have developed live attenuated recombinant Listeria monocytogenes (rLm)-vectored TB vaccines expressing five [Mpt64/23.5-EsxH/TB10.4-EsxA/ESAT6-EsxB/CFP10-Ag85B/r30] (rLmMtb5Ag) or nine (additionally EsxN-PPE68-EspA-TB8.4) immunoprotective Mtb antigens (rLmMtb9Ag) and evaluated them for safety, immunogenicity and efficacy as standalone vaccines in two mouse models and an outbred guinea pig model. In immunogenicity studies, rLmMtb5Ag administered subcutaneously induces significantly enhanced antigen-specific CD4+ and CD8+ T-cell responses in C57BL/6 and BALB/c mice, and rLmMtb9Ag induces antigen-specific CD4+ and CD8+ T-cell proliferation in guinea pigs. In efficacy studies, both rLmMtb5Ag and rLmMtb9Ag are safe and protect C57BL/6 and BALB/c mice and guinea pigs against aerosol challenge with highly virulent Mtb. Hence, multi-antigenic rLm vaccines hold promise as new vaccines against TB.

Listeria-Vectored Multiantigenic Tuberculosis Vaccine Enhances Protective Immunity against Aerosol Challenge with Virulent Mycobacterium tuberculosis in BCG-Immunized C57BL/6 and BALB/c Mice

Mycobacterium tuberculosis infects approximately one-third of the world's population, causing active tuberculosis (TB) in ~10 million people and death in ~1.5 million people annually. A potent vaccine is needed to boost the level of immunity conferred by the current Mycobacterium bovis BCG vaccine that provides moderate protection against childhood TB but variable protection against adult pulmonary TB. Previously, we developed a recombinant attenuated Listeria monocytogenes (rLm)-vectored M. tuberculosis vaccine expressing the M. tuberculosis 30-kDa major secretory protein (r30/Ag85B), recombinant attenuated L. monocytogenes ΔactA ΔinlB prfA*30 (rLm30), and showed that boosting BCG-primed mice and guinea pigs with rLm30 enhances immunoprotection against challenge with aerosolized M. tuberculosis Erdman strain. To broaden the antigen repertoire and robustness of rLm30, we constructed 16 recombinant attenuated L. monocytogenes vaccine candidates expressing 3, 4, or 5 among 15 selected M. tuberculosis antigens, verified their protein expression, genetic stability, and growth kinetics in macrophages, and evaluated them for capacity to boost protective efficacy in BCG-primed mice. We found that boosting BCG-primed C57BL/6 and BALB/c mice with recombinant attenuated L. monocytogenes multiantigenic M. tuberculosis vaccines, especially the rLm5Ag(30) vaccine expressing a fusion protein of 23.5/Mpt64, TB10.4/EsxH, ESAT6/EsxA, CFP10/EsxB, and r30, enhances BCG-induced protective immunity against M. tuberculosis aerosol challenge. In immunogenicity studies, rLm5Ag(30) strongly boosts M. tuberculosis antigen-specific CD4-positive (CD4⁺) and CD8⁺ T cell-mediated TH1-type immune responses in the spleens and lungs of BCG-primed C57BL/6 mice but does so only weakly in BCG-primed BALB/c mice. Hence, rLm5Ag(30) boosts BCG-primed immunoprotection against M. tuberculosis aerosol challenge in both C57BL/6 and BALB/c mice despite major differences in the magnitude of the vaccine-induced Th1 response in these mouse strains. Given the consistency with which recombinant attenuated L. monocytogenes vaccines expressing the 5 M. tuberculosis antigens in rLm5Ag(30) are able to boost the already high level of protection conferred by BCG alone in two rigorous mouse models of pulmonary TB and the broad CD4⁺ and CD8⁺ T cell immunity induced by rLm5Ag(30), this vaccine holds considerable promise as a new vaccine to combat the TB pandemic, especially for the majority of the world’s population immunized with BCG in infancy.

Hacking Commensal Bacteria to Consolidate the Adaptive Mucosal Immune Response in the Gut–Lung Axis: Future Possibilities for SARS-CoV-2 Protection

Infectious diseases caused by mucosal pathogens significantly increase mortality and morbidity. Thus, the possibility to target these pathogens at their primary entry points can consolidate protective immunity. Regarding SARS-CoV-2 infection, it has been observed that the upper respiratory mucosa is highly affected and that dysregulation of resident microbiota in the gut–lung axis plays a crucial role in determining symptom severity. Thus, understanding the possibility of eliciting various mucosal and adaptive immune responses allows us to effectively design bacterial mucosal vaccine vectors. Such design requires rationally selecting resident bacterial candidates as potential host carriers, evaluating effective carrier proteins for stimulating an immune response, and combining these two to improve antigenic display and immunogenicity. This review investigated mucosal vaccine vectors from 2015 to present, where a few have started to utilize Salmonella and lactic acid bacteria (LAB) to display SARS-CoV-2 Spike S proteins or fragments. Although current literature is still lacking for its studies beyond in vitro or in vivo efficiency, decades of research into these vectors show promising results. Here, we discuss the mucosal immune systems focusing on the gut–lung axis microbiome and offer new insight into the potential use of alpha streptococci in the upper respiratory tract as a vaccine carrier.

Preclinical Progress of Subunit and Live Attenuated Mycobacterium tuberculosis Vaccines: A Review following the First in Human Efficacy Trial

Tuberculosis (TB) is the global leading cause of death from an infectious agent with approximately 10 million new cases of TB and 1.45 million deaths in 2018. Bacille Calmette-Guérin (BCG) remains the only approved vaccine for Mycobacterium tuberculosis (M. tb, causative agent of TB), however clinical studies have shown BCG has variable effectiveness ranging from 0–80% in adults. With 1.7 billion people latently infected, it is becoming clear that vaccine regimens aimed at both post-exposure and pre-exposure to M. tb will be crucial to end the TB epidemic. The two main strategies to improve or replace BCG are subunit and live attenuated vaccines. However, following the failure of the MVA85A phase IIb trial in 2013, more varied and innovative approaches are being developed. These include recombinant BCG strains, genetically attenuated M. tb and naturally attenuated mycobacteria strains, novel methods of immunogenic antigen discovery including for hypervirulent M. tb strains, improved antigen recognition and delivery strategies, and broader selection of viral vectors. This article reviews preclinical vaccine work in the last 5 years with focus on those tested against M. tb challenge in relevant animal models.

Heterologous Boosting With Listeria-Based Recombinant Strains in BCG-Primed Mice Improved Protection Against Pulmonary Mycobacterial Infection

While Baccillus Calmette-Guerin (BCG) is used worldwide, tuberculosis (TB) is still a global concern due to the poor efficacy of BCG. Novel vaccine candidates are therefore urgently required. In this study, two attenuated recombinant Listeria strains, LMΔ-msv and LIΔ-msv were constructed by deletion of actA and plcB and expression of a fusion protein consisting of T cell epitopes from four Mycobacterium tuberculosis (Mtb) antigens (Rv2460c, Rv2660c, Rv3875, and Rv3804c). The safety and immunogenicity of the two recombinant strains were evaluated in C57BL/6J mice. After intravenous immunization individually, both recombinant strains entered liver and spleen but eventually were eliminated from these organs after several days. Simultaneously, they induced antigen-specific cell-mediated immunity, indicating that the recombinant Listeria strains were immunogenic and safe in vivo. LMΔ-msv immunization induced stronger cellular immune responses than LIΔ-msv immunization, and when boosted with LIΔ-msv, antigen-specific IFN-γ CD8⁺ T cell responses were notably magnified. Furthermore, we evaluated the protection conferred by the vaccine candidates against mycobacterial infection via challenging the mice with 1 × 10⁷ CFU of BCG. Especially, we tested the feasibility of application of them as heterologous BCG supplement vaccine by immunization of mice with BCG firstly, and boosted with LMΔ-msv and LIΔ-msv sequentially before challenging. Combination immune strategy (LMΔ-msv prime-LIΔ-msv boost) conferred comparable protection efficacy as BCG alone. More importantly, BCG-vaccinated mice acquired stronger resistance to Mycobacterial challenge when boosted with LMΔ-msv and LIΔ-msv sequentially. Our results inferred that heterologous immunization with Listeria-based recombinant strains boosted BCG-primed protection against pulmonary mycobacterial infection.

Inactivated Lactobacillus plantarum Carrying a Surface-Displayed Ag85B-ESAT-6 Fusion Antigen as a Booster Vaccine Against Mycobacterium tuberculosis Infection

Vaccination is considered the most effective strategy for controlling tuberculosis (TB). The existing vaccine, the Bacille Calmette-Guérin (BCG), although partially protective, has a number of limitations. Therefore, there is a need for developing new TB vaccines and several strategies are currently exploited including the use of viral and bacterial delivery vectors. We have previously shown that Lactobacillus plantarum (Lp) producing Ag85B and ESAT-6 antigens fused to a dendritic cell-targeting peptide (referred to as Lp_DC) induced specific immune responses in mice. Here, we analyzed the ability of two Lp-based vaccines, Lp_DC and Lp_HBD (in which the DC-binding peptide was replaced by an HBD-domain directing the antigen to non-phagocytic cells) to activate antigen-presenting cells, induce specific immunity and protect mice from Mycobacterium tuberculosis infection. We tested two strategies: (i) Lp as BCG boosting vaccine (a heterologous regimen comprising parenteral BCG immunization followed by intranasal Lp boost), and (ii) Lp as primary vaccine (a homologous regimen including subcutaneous priming followed by intranasal boost). The results showed that both Lp constructs applied as a BCG boost induced specific cellular immunity, manifested in T cell proliferation, antigen-specific IFN-γ responses and multifunctional T cells phenotypes. More importantly, intranasal boost with Lp_DC or Lp_HBD enhanced protection offered by BCG, as shown by reduced M. tuberculosis counts in lungs. These findings suggest that Lp constructs could be developed as a potential mucosal vaccine platform against mycobacterial infections.

Innate and Adaptive Immune Responses during Listeria monocytogenes Infection

It could be argued that we understand the immune response to infection with Listeria monocytogenes better than the immunity elicited by any other bacteria. L. monocytogenes are Gram-positive bacteria that are genetically tractable and easy to cultivate in vitro, and the mouse model of intravenous (i.v.) inoculation is highly reproducible. For these reasons, immunologists frequently use the mouse model of systemic listeriosis to dissect the mechanisms used by mammalian hosts to recognize and respond to infection. This article provides an overview of what we have learned over the past few decades and is divided into three sections: "Innate Immunity" describes how the host initially detects the presence of L. monocytogenes and characterizes the soluble and cellular responses that occur during the first few days postinfection; "Adaptive Immunity" discusses the exquisitely specific T cell response that mediates complete clearance of infection and immunological memory; "Use of Attenuated Listeria as a Vaccine Vector" highlights the ways that investigators have exploited our extensive knowledge of anti-Listeria immunity to develop cancer therapeutics.

Listeria monocytogenes as a Vector for Cancer Immunotherapy: Current Understanding and Progress

Listeria monocytogenes, a Gram-positive facultative anaerobic bacterium, is becoming a popular vector for cancer immunotherapy. Indeed, multiple vaccines have been developed utilizing modified Listeria as a tool for generating immune responses against a variety of cancers. Moreover, over a dozen clinical trials testing Listeria cancer vaccines are currently underway, which will help to understand the utility of Listeria vaccines in cancer immunotherapy. This review aims to summarize current views on how Listeria-based vaccines induce potent antitumor immunity and the current state of Listeria-based cancer vaccines in clinical trials.

Heterologous vaccination targeting prostatic acid phosphatase (PAP) using DNA and Listeria vaccines elicits superior anti-tumor immunity dependent on CD4+ T cells elicited by DNA priming

Background. Sipuleucel T, an autologous cell-based vaccine targeting prostatic acid phosphatase (PAP), has demonstrated efficacy for the treatment of advanced prostate cancer. DNA vaccines encoding PAP and live attenuated Listeria vaccines have entered clinical trials for patients with prostate cancer, and have advantages in terms of eliciting predominantly Th1-biased immunity. In this study, we investigated whether the immunogenicity and anti-tumor efficacy of a DNA and Listeria vaccine, each encoding PAP, could be enhanced by using them in a heterologous prime/boost approach. Methods. Transgenic mice expressing HLA-A2.01 and HLA-DRB1*0101 were immunized alone or with a heterologous prime/boost strategy. Splenocytes were evaluated for MHC class I and II-restricted, PAP-specific immune responses by IFNγ ELISPOTs. Anti-tumor activity to a syngeneic, PAP-expressing tumor line was evaluated. Results. PAP-specific cellular immunity and anti-tumor activity were elicited in mice after immunization with DNA- or listeria-based vaccines. Greater CD4+ and CD8+ responses, and anti-tumor responses, were elicited when mice were immunized first with DNA and boosted with Listeria, but not when administered in the opposite order. This was found to be dependent on CD4+ T cells elicited with DNA priming, and was not due to inflammatory signals by Listeria itself or due to B cells serving as antigen-presenting cells for DNA during priming. Conclusions. Heterologous prime/boost vaccination using DNA priming with Listeria boosting may provide better anti-tumor immunity, similar to many reports evaluating DNA priming with vaccines targeting foreign microbial antigens. These findings have implications for the design of future clinical trials.

Single vector platform vaccine protects against lethal respiratory challenge with Tier 1 select agents of anthrax, plague, and tularemia

Bacillus anthracis, Yersinia pestis, and Francisella tularensis are the causative agents of Tier 1 Select Agents anthrax, plague, and tularemia, respectively. Currently, there are no licensed vaccines against plague and tularemia and the licensed anthrax vaccine is suboptimal. Here we report F. tularensis LVS ΔcapB (Live Vaccine Strain with a deletion in capB)- and attenuated multi-deletional Listeria monocytogenes (Lm)-vectored vaccines against all three aforementioned pathogens. We show that LVS ΔcapB- and Lm-vectored vaccines express recombinant B. anthracis, Y. pestis, and F. tularensis immunoprotective antigens in broth and in macrophage-like cells and are non-toxic in mice. Homologous priming-boosting with the LVS ΔcapB-vectored vaccines induces potent antigen-specific humoral and T-cell-mediated immune responses and potent protective immunity against lethal respiratory challenge with all three pathogens. Protection against anthrax was far superior to that obtained with the licensed AVA vaccine and protection against tularemia was comparable to or greater than that obtained with the toxic and unlicensed LVS vaccine. Heterologous priming-boosting with LVS ΔcapB- and Lm-vectored B. anthracis and Y. pestis vaccines also induced potent protective immunity against lethal respiratory challenge with B. anthracis and Y. pestis. The single vaccine platform, especially the LVS ΔcapB-vectored vaccine platform, can be extended readily to other pathogens.

Next-Generation Vaccines Based on Bacille Calmette-Guérin

Tuberculosis (TB), caused by the intracellular bacterium Mycobacterium tuberculosis (Mtb), remains a major health threat. A live, attenuated mycobacterium known as Bacille Calmette-Guérin (BCG), derived from the causative agent of cattle TB, Mycobacterium bovis, has been in clinical use as a vaccine for 90 years. The current incidence of TB demonstrates that BCG fails to protect sufficiently against pulmonary TB, the major disease manifestation and source of dissemination. The protective efficacy of BCG is on average 50% but varies substantially with geographical location and is poorer in those with previous exposure to mycobacteria. BCG can also cause adverse reactions in immunocompromised individuals. However, BCG has contributed to reduced infant TB mortality by protecting against extrapulmonary TB. In addition, BCG has been associated with reduced general childhood mortality by stimulating immune responses. In order to improve the efficacy of BCG, two major strategies have been employed. The first involves the development of recombinant live mycobacterial vaccines with improved efficacy and safety. The second strategy is to boost BCG with subunit vaccines containing Mtb antigens. This article reviews recombinant BCG strains that have been tested against TB in animal models. This includes BCG strains that have been engineered to induce increased immune responses by the insertion of genes for Mtb antigens, mammalian cytokines, or host resistance factors, the insertion of bacterial toxin-derived adjuvants, and the manipulation of bacterial genes in order to increase antigen presentation and immune activation. Subunit vaccines for boosting BCG are also briefly discussed.