Recombinant BCG vaccines: molecular features and their influence in the expression of foreign genes

Recent Advances in Bacterium-Based Therapeutic Modalities for Melanoma Treatment

Melanoma is one of the most severe skin cancer indications with rapid progression and a high risk of metastasis. However, despite the accumulated advances in melanoma treatment including adjuvant radiation, chemotherapy, and immunotherapy, the overall melanoma treatment efficacy in the clinics is still not satisfactory. Interestingly, bacterial therapeutics have demonstrated unique properties for tumor-related therapeutic applications, such as tumor-targeted motility, tailorable cytotoxicity, and immunomodulatory capacity of the tumor microenvironment, which have emerged as a promising platform for melanoma therapy. Indeed, the recent advances in genetic engineering and nanotechnologies have boosted the application potential of bacterium-based therapeutics for treating melanoma by further enhancing their tumor-homing, cell-killing, drug delivery, and immunostimulatory capacities. This review provides a comprehensive summary of the state-of-the-art bacterium-based anti-melanoma modalities, which are categorized according to their unique functional merits, including tumor-specific cytotoxins, tumor-targeted drug delivery platforms, and immune-stimulatory agents. Furthermore, a perspective is provided discussing the potential challenges and breakthroughs in this area. The insights in this review may facilitate the development of more advanced bacterium-based therapeutic modalities for improved melanoma treatment efficacy.

Genetic engineering employing MPB70 and its promoter enables efficient secretion and expression of foreign antigen in bacillus Calmette Guérin (BCG) Tokyo

Vaccination is an important factor in public health. The recombinant bacillus Calmette Guérin (rBCG) vaccine, which expresses foreign antigens, is expected to be a superior vaccine against infectious diseases. Here, we report a new recombination platform in which the BCG Tokyo strain is transformed with nucleotide sequences encoding foreign protein fused with the MPB70 immunogenic protein precursor. By RNA-sequencing, mpb70 was found to be the most transcribed among all known genes of BCG Tokyo. Small oligopeptide, namely, polyhistidine tag, was able to be expressed in and secreted from rBCG through a process in which polyhistidine tag fused with intact MPB70 were transcribed by an mpb70 promoter. This methodology was applied to develop an rBCG expressing the receptor binding domain (RBD) of severe acute respiratory syndrome coronavirus 2. Immunoblotting images and mass spectrometry data showed that RBD was also secreted from rBCG. Sera from mice vaccinated with the rBCG showed a tendency of weak neutralizing capacity. The secretion was retained even after a freeze-drying process. The freeze-dried rBCG was administered to and recovered from mice. Recovered rBCG kept secreting RBD. Collectively, our recombination platform offers stable secretion of foreign antigens and can be applied to the development of practical rBCGs.

Characterization of cellular immune response in hamsters immunized with recombinant vaccines against leptospirosis based on LipL32:LemA:LigAni chimeric protein

In the last 20 years, various research groups have endeavored to develop recombinant vaccines against leptospirosis to overcome the limitations of commercially available bacterins. Numerous antigens and vaccine formulations have been tested thus far. However, the analysis of cellular response in these vaccine formulations is not commonly conducted, primarily due to the scarcity of supplies and kits for the hamster animal model. Our research group has already tested the Q1 antigen, a chimeric protein combining the immunogenic regions of LipL32, LemA, and LigANI, in recombinant subunit and BCG-vectored vaccines. In both strategies, 100 % of the hamsters were protected against clinical signs of leptospirosis. However, only the recombinant BCG-vectored vaccine provided protection against renal colonization. Thus, the objective of this study is to characterize the cellular immune response in hamsters immunized with different vaccine formulations based on the Q1 antigen through transcriptional analysis of cytokines. The hamsters were allocated into groups and vaccinated as follows: recombinant subunit (rQ1), recombinant BCG (rBCG:Q1), and saline and BCG Pasteur control vaccines. To assess the cellular response induced by the vaccines, we cultured and stimulated splenocytes, followed by RNA extraction from the cells and analysis of cytokines using real-time PCR. The results revealed that the recombinant subunit vaccine elicited a Th2-type response, characterized by the expression of cytokines IL-10, IL-1α, and TNF-α. This pattern closely resembles the cytokines expressed in severe cases of leptospirosis. On the other hand, the rBCG-vectored vaccine induced a Th1-type response with significant up-regulation of IFN-γ. These findings suggest the involvement of the cellular response and the IFN-γ mediated inflammatory response in the sterilizing immunity mediated by rBCG. Therefore, this study may assist future investigations in characterizing the cellular response in hamsters, aiming to elucidate the mechanisms of efficacy and establish potential correlates of protection.

Auxotrophic Mycobacterium bovis BCG: Updates and Perspectives

Mycobacterium bovis BCG has been used for a century as the only licensed vaccine against tuberculosis. Owing to its strong adjuvant properties, BCG has also been employed as an oncological immunotherapeutic as well as a live vaccine vector against other pathogens. However, BCG vaccination has limited efficacy in protecting against adult forms of tuberculosis (TB), raises concerns about its safety in immunocompromised populations, compromises the diagnosis of TB through the tuberculin test and lacks predictability for successful antigen expression and immune responses to heterologous antigens. Together, these factors propelled the construction and evaluation of auxotrophic BCG strains. Auxotrophs of BCG have been developed from mutations in the genes required for their growth using different approaches and have shown the potential to provide a model to study M. tuberculosis, a more stable, safe, and effective alternative to BCG and a vector for the development of recombinant live vaccines, especially against HIV infection. In this review, we provide an overview of the strategies for developing and using the auxotrophic BCG strains in different scenarios.

TonB-dependent receptor epitopes expressed in M. bovis BCG induced significant protection in the hamster model of leptospirosis

Abstract

Leptospirosis is an emerging infectious disease caused by pathogenic Leptospira spp. A universal vaccine against leptospirosis is likely to require highly conserved epitopes from pathogenic leptospires that are exposed on the bacterial surface and that generate a protective and sterilizing immune response. Our group recently identified several genes predicted to encode TonB-dependent receptors (TBDR) in Leptospira interrogans using a reverse vaccinology approach. Three leptospiral TBDRs were previously described and partially characterized as ferric-citrate, hemin, and cobalamin transporters. In the current study, we designed a fusion protein composed of predicted surface-exposed epitopes from three conserved leptospiral TBDRs. Based on their three-dimensional structural models and the prediction of immunogenic regions, nine putative surface-exposed fragments were selected to compose a recombinant chimeric protein. A Mycobacterium bovis BCG strain expressing this chimeric antigen encoded in the pUP500/P_pAN mycobacterial expression vector was used to immunize Syrian hamsters. All animals (20/20) vaccinated with recombinant BCG survived infection with an endpoint dose of L. interrogans (p < 0.001). No animal survived in the negative control group. Immunization with our recombinant BCG elicited a humoral immune response against leptospiral TBDRs, as demonstrated by ELISA and immunoblot. No leptospiral DNA was detected by lipL32 qPCR in the kidneys of vaccinated hamsters. Similarly, no growth was observed in macerated kidney cultures from the same animals, suggesting the induction of a sterilizing immune response. Design of new vaccine antigens based on the structure of outer membrane proteins is a promising approach to overcome the impact of leptospirosis by vaccination.

Key points

• Predicted surface-exposed epitopes were identified in three leptospiral TBDRs.

• An M. bovis BCG strain expressing a chimeric protein (rTBDRchi) was constructed.

• Hamsters vaccinated with rBCG:TBDRchi were protected from lethal leptospirosis.

Graphical abstract

Supplementary Information

The online version contains supplementary material available at 10.1007/s00253-021-11726-9.

The Diverse Applications of Recombinant BCG-Based Vaccines to Target Infectious Diseases Other Than Tuberculosis: An Overview

Live attenuated Bacillus Calmette-Guérin (BCG) is the world's most widely used vaccine which is mainly administered for its protection against tuberculosis (TB), particularly in young children. However, since its initial use over 100years ago, it has also proven to offer a level of protection against various other pathogens, as a consequence of its non-specific immune enhancing effects. Thus, over the past few decades, recombinant BCG (rBCG) technology has been used as a vector to create rBCG vaccines expressing heterologous antigens that elicit immunity against a range of bacterial, viral, and parasitic diseases. Our goal with this mini-review is to provide an up-to-date survey of the various techniques, approaches, and applications of rBCG-based vaccines for targeting infectious diseases other than TB.

Thirty years of recombinant BCG: new trends for a centenary vaccine

Introduction: Global perception of the potential for Bacille Calmette-Guérin (BCG), and consequently recombinant BCG (rBCG), in a variety of prophylactic and therapeutic applications has been increasing. A century of information on BCG, and three decades of experience with rBCG, has generated solid knowledge in this field.Area covered: Here, we review the current state of knowledge of BCG and rBCG development. Molecular tools have facilitated the expression of a variety of molecules in BCG, with the aim of improving its efficacy as a tuberculosis vaccine, generating polyvalent vaccines against other pathogens, including viruses, bacteria, and parasites, and developing immunotherapy approaches against noninvasive bladder cancer. BCG's recently appraised heterologous effects and prospects for expanding its application to other diseases are also addressed.Expert opinion: There are high expectations for new tuberculosis vaccines currently undergoing advanced clinical trials, which could change the prospects of the field. Systems biology could reveal effective biomarkers of protection, which would greatly support vaccine development. The development of appropriate large-scale production processes would further support implementation of new vaccines and rBCG products. The next few years should consolidate the broader applications of BCG and produce insights into improvements using the recombinant BCG technology.

A New Mouse Model of Chronic Myocarditis Induced by Recombinant Bacille Calmette-Guèrin Expressing a T-Cell Epitope of Cardiac Myosin Heavy Chain-α

Dilated cardiomyopathy (DCM) is a potentially lethal disorder characterized by progressive impairment of cardiac function. Chronic myocarditis has long been hypothesized to be one of the causes of DCM. However, owing to the lack of suitable animal models of chronic myocarditis, its pathophysiology remains unclear. Here, we report a novel mouse model of chronic myocarditis induced by recombinant bacille Calmette-Guérin (rBCG) expressing a CD4+ T-cell epitope of cardiac myosin heavy chain-α (rBCG-MyHCα). Mice immunized with rBCG-MyHCα developed chronic myocarditis, and echocardiography revealed dilation and impaired contraction of ventricles, similar to those observed in human DCM. In the heart, CD62L-CD4+ T cells were increased and produced significant amounts of IFN-γ and IL-17 in response to cardiac myosin. Adoptive transfer of CD62L-CD4+ T cells induced myocarditis in the recipient mice, which indicated that CD62L-CD4+ T cells were the effector cells in this model. rBCG-MyHCα-infected dendritic cells produced proinflammatory cytokines and induced MyHCα-specific T-cell proliferation and Th1 and Th17 polarization. This novel chronic myocarditis mouse model may allow the identification of the central pathophysiological and immunological processes involved in the progression to DCM.

Protection against leptospirosis conferred by Mycobacterium bovis BCG expressing antigens from Leptospira interrogans

Leptospirosis is a zoonotic disease worldwide and caused by the pathogenic spirochetes of the genus Leptospira. Bacterins make up the vaccines used against leptospirosis, but they only succeed in providing short-term and serovar-specific protection. The use of Mycobacterium bovis BCG as a live vaccine vector expressing leptospiral antigens is a promising alternative, particularly due to its adjuvant properties. Four distinct portions P₁ (lipL32), P₂ (ligAni), P₃ (lemA:ligAni) and P₄ (lipL32:lemA) of a recombinant chimera composed of the lipL32, lemA and ligANI genes from Leptospira interrogans were cloned individually according to the BioBricks® strategy in the plasmid pUP500/P_pAN. These constructs were individually transformed into a BCG Pasteur strain, and protein expression was detected by Western blot. For vaccination, 5 groups of 10 Golden Syrian hamsters were used, aged 4-6 weeks - group 1, rBCG (LipL32); group 2, rBCG (LigAni); group 3, rBCG (LemA:LigAni); group 4, (LipL32:LemA); group 5, wild-type BCG Pasteur (negative control). Two doses containing 10⁶ CFU of rBCG were administered subcutaneously, the challenge was performed with 5 × LD₅₀ of Leptospira interrogans serovar Copenhageni L1-130, and the animals were observed for a 30-day period until the endpoint was reached. Humoral immunity was assessed via indirect ELISA, while renal colonisation was assessed by culture and quantitative real-time PCR. All vaccinated groups were protected against lethal challenge and renal colonisation, in comparison with negative control group (P < 0.05). Recombinant vaccines were not effective at inducing significant humoral immunity, which suggests the induction of cellular immunity - a characteristic of M. bovis BCG. In conclusion, all formulations provide 100% significant protection against leptospirosis in hamsters with no renal colonisation. The use of rBCG as a vaccine vector represents a promising alternative for the control of animal leptospirosis, allowing for protection against clinical signs of leptospirosis and renal colonisation.

Characterisation of alternative expression vectors for recombinant Bacillus Calmette-Guérin as live bacterial delivery systems

BACKGROUND

Bacillus Calmette-Guérin (BCG) is considered a promising live bacterial delivery system. However, several proposals for rBCG vaccines have not progressed, mainly due to the limitations of the available expression systems.

OBJECTIVES

To obtain a set of mycobacterial vectors using a range of promoters with different strengths based on a standard backbone, previously shown to be stable.

METHODS

Mycobacterial expression vectors based on the pLA71 vector as backbone, were obtained inserting different promoters (P_AN, P_αAg, P_Hsp60, P_BlaF* and P_L5) and the green fluorescence protein (GFP) as reporter gene, to evaluate features such as their relative strengths, and the in vitro (inside macrophages) and in vivo stability.

FINDINGS

The relative fluorescence observed with the different vectors showed increasing strength of the promoters: P_AN was the weakest in both Mycobacterium smegmatis and BCG and P_BlaF* was higher than P_Hsp60 in BCG. The relative fluorescence observed in a macrophage cell line showed that P_BlaF* and P_Hsp60 were comparable. It was not possible to obtain strains transformed with the extrachromosomal expression vector containing the P_L5 in either species.

MAIN CONCLUSION

We have obtained a set of potentially stable mycobacterial vectors with a arrange of expression levels, to be used in the development of rBCG vaccines.

BCG-Induced Cross-Protection and Development of Trained Immunity: Implication for Vaccine Design

The Bacillus Calmette-Guérin (BCG) is a live attenuated tuberculosis vaccine that has the ability to induce non-specific cross-protection against pathogens that might be unrelated to the target disease. Vaccination with BCG reduces mortality in newborns and induces an improved innate immune response against microorganisms other than Mycobacterium tuberculosis, such as Candida albicans and Staphylococcus aureus. Innate immune cells, including monocytes and natural killer (NK) cells, contribute to this non-specific immune protection in a way that is independent of memory T or B cells. This phenomenon associated with a memory-like response in innate immune cells is known as "trained immunity." Epigenetic reprogramming through histone modification in the regulatory elements of particular genes has been reported as one of the mechanisms associated with the induction of trained immunity in both, humans and mice. Indeed, it has been shown that BCG vaccination induces changes in the methylation pattern of histones associated with specific genes in circulating monocytes leading to a "trained" state. Importantly, these modifications can lead to the expression and/or repression of genes that are related to increased protection against secondary infections after vaccination, with improved pathogen recognition and faster inflammatory responses. In this review, we discuss BCG-induced cross-protection and acquisition of trained immunity and potential heterologous effects of recombinant BCG vaccines.

Mycobacterium bovis BCG in metastatic melanoma therapy

Melanoma is the most aggressive form of skin cancer, with a high mortality rate and with 96,480 new cases expected in 2019 in the USS. BRAF^V600E, the most common driver mutation, is found in around 50% of melanomas, contributing to tumor growth, angiogenesis, and metastatic progression. Dacarbazine (DTIC), an alkylate agent, was the first chemotherapeutic agent approved by the US Food and Drug Administration (FDA) used as a standard treatment. Since then, immunotherapies have been approved for metastatic melanoma (MM) including ipilimumab and pembrolizumab checkpoint inhibitors that help decrease the risk of progression. Moreover, Mycobacterium bovis Bacillus Calmette-Guerin (BCG) serves as an adjuvant therapy that induces the recruitment of natural killer NK, CD4⁺, and CD8⁺ T cells and contributes to antitumor immunity. BCG can be administered in combination with chemotherapeutic and immunotherapeutic agents and can be genetically manipulated to produce recombinant BCG (rBCG) strains that express heterologous proteins or overexpress immunogenic proteins, increasing the immune response and improving patient survival. In this review, we highlight several studies utilizing rBCG immunotherapy for MM in combination with other therapeutic agents.

Advances and challenges in recombinant Mycobacterium bovis BCG-based HIV vaccine development: lessons learned

INTRODUCTION

Human Immunodeficiency Virus/Acquired Immune Deficiency Syndrome, tuberculosis, and malaria are responsible for most human deaths produced by infectious diseases worldwide. Vaccination against HIV requires generation of memory T cells and neutralizing antibodies, mucosal immunity, and stimulation of an innate immune responses. In this context, the use of Mycobacterium bovis bacillus Calmette-Guérin (BCG) as a live vaccine vehicle is a promising approach for T-cell induction.

AREAS COVERED

In this review, we provide a comprehensive summary of the literature regarding immunogenicity studies in animal models performed since 2005. Furthermore, we provide expert commentary and 5-year view on how the development of potential recombinant BCG-based HIV vaccines involves careful selection of the HIV antigen, expression vectors, promoters, BCG strain, preclinical animal models, influence of preexisting immunity, and safety issues, for the rational design of recombinant BCG:HIV vaccines to prevent HIV transmission in the general population.

EXPERT COMMENTARY

The three critical issues to be considered when developing a rBCG:HIV vaccine are codon optimization, antigen localization, and plasmid stability in vivo. The use of integrative expression vectors are likely to improve the mycobacterial vaccine stability and immunogenicity to develop not only recombinant BCG-based vaccines expressing second generation of HIV-1 immunogens but also other major pediatric pathogens to prime protective responses shortly following birth.