Bacillus Calmette-Guérin Overexpressing an Endogenous Stimulator of Interferon Genes Agonist Provides Enhanced Protection Against Pulmonary Tuberculosis

A modified BCG with depletion of enzymes associated with peptidoglycan amidation induces enhanced protection against tuberculosis in mice

Mechanisms by which Mycobacterium tuberculosis (Mtb) evades pathogen recognition receptor activation during infection may offer insights for the development of improved tuberculosis (TB) vaccines. Whilst Mtb elicits NOD-2 activation through host recognition of its peptidoglycan-derived muramyl dipeptide (MDP), it masks the endogenous NOD-1 ligand through amidation of glutamate at the second position in peptidoglycan side-chains. As the current BCG vaccine is derived from pathogenic mycobacteria, a similar situation prevails. To alleviate this masking ability and to potentially improve efficacy of the BCG vaccine, we used CRISPRi to inhibit expression of the essential enzyme pair, MurT-GatD, implicated in amidation of peptidoglycan side-chains. We demonstrate that depletion of these enzymes results in reduced growth, cell wall defects, increased susceptibility to antibiotics, altered spatial localization of new peptidoglycan and increased NOD-1 expression in macrophages. In cell culture experiments, training of a human monocyte cell line with this recombinant BCG yielded improved control of Mtb growth. In the murine model of TB infection, we demonstrate that depletion of MurT-GatD in BCG, which is expected to unmask the D-glutamate diaminopimelate (iE-DAP) NOD-1 ligand, yields superior prevention of TB disease compared to the standard BCG vaccine. In vitro and in vivo experiments in this study demonstrate the feasibility of gene regulation platforms such as CRISPRi to alter antigen presentation in BCG in a bespoke manner that tunes immunity towards more effective protection against TB disease.

Improved bladder cancer antitumor efficacy with a recombinant BCG that releases a STING agonist

Despite the introduction of several new agents for the treatment of bladder cancer (BC), intravesical BCG remains a first line agent for the management of non-muscle invasive bladder cancer. In this study we evaluated the antitumor efficacy in animal models of BC of a recombinant BCG known as BCG-disA-OE that releases the small molecule STING agonist c-di-AMP. We found that compared to wild-type BCG (BCG-WT), in both the orthotopic, carcinogen-induced rat MNU model and the heterotopic syngeneic mouse MB-49 model BCG-disA-OE afforded improved antitumor efficacy. A mouse safety evaluation further revealed that BCG-disA-OE proliferated to lesser degree than BCG-WT in BALB/c mice and displayed reduced lethality in SCID mice. To probe the mechanisms that may underlie these effects, we found that BCG-disA-OE was more potent than BCG-WT in eliciting IFN-β release by exposed macrophages, in reprogramming myeloid cell subsets towards an M1-like proinflammatory phenotypes, inducing epigenetic activation marks in proinflammatory cytokine promoters, and in shifting monocyte metabolomic profiles towards glycolysis. Many of the parameters elevated in cells exposed to BCG-disA-OE are associated with BCG-mediated trained innate immunity suggesting that STING agonist overexpression may enhance trained immunity. These results indicate that modifying BCG to release high levels of proinflammatory PAMP molecules such as the STING agonist c-di-AMP can enhance antitumor efficacy in bladder cancer.

Nucleotides as Bacterial Second Messengers

In addition to comprising monomers of nucleic acids, nucleotides have signaling functions and act as second messengers in both prokaryotic and eukaryotic cells. The most common example is cyclic AMP (cAMP). Nucleotide signaling is a focus of great interest in bacteria. Cyclic di-AMP (c-di-AMP), cAMP, and cyclic di-GMP (c-di-GMP) participate in biological events such as bacterial growth, biofilm formation, sporulation, cell differentiation, motility, and virulence. Moreover, the cyclic-di-nucleotides (c-di-nucleotides) produced in pathogenic intracellular bacteria can affect eukaryotic host cells to allow for infection. On the other hand, non-cyclic nucleotide molecules pppGpp and ppGpp are alarmones involved in regulating the bacterial response to nutritional stress; they are also considered second messengers. These second messengers can potentially be used as therapeutic agents because of their immunological functions on eukaryotic cells. In this review, the role of c-di-nucleotides and cAMP as second messengers in different bacterial processes is addressed.

Multidimensional futuristic approaches to address the pandemics beyond COVID-19

Globally, the impact of the coronavirus disease 2019 (COVID-19) pandemic has been enormous and unrelenting with ∼6.9 million deaths and ∼765 million infections. This review mainly focuses on the recent advances and potentially novel molecular tools for viral diagnostics and therapeutics with far-reaching implications in managing the future pandemics. In addition to briefly highlighting the existing and recent methods of viral diagnostics, we propose a couple of potentially novel non-PCR-based methods for rapid, cost-effective, and single-step detection of nucleic acids of viruses using RNA mimics of green fluorescent protein (GFP) and nuclease-based approaches. We also highlight key innovations in miniaturized Lab-on-Chip (LoC) devices, which in combination with cyber-physical systems, could serve as ideal futuristic platforms for viral diagnosis and disease management. We also discuss underexplored and underutilized antiviral strategies, including ribozyme-mediated RNA-cleaving tools for targeting viral RNA, and recent advances in plant-based platforms for rapid, low-cost, and large-scale production and oral delivery of antiviral agents/vaccines. Lastly, we propose repurposing of the existing vaccines for newer applications with a major emphasis on Bacillus Calmette-Guérin (BCG)-based vaccine engineering.

Fighting Tuberculosis: In Search of a BCG Replacement

Tuberculosis is one of the most threatening infectious diseases and represents an important and significant reason for mortality in high-burden regions. The only licensed vaccine, BCG, is hardly capable of establishing long-term tuberculosis protection and is highly variable in its effectiveness. Even after 100 years of BCG use and research, we still cannot unequivocally answer the question of which immune correlates of protection are crucial to prevent Mycobacterium tuberculosis (Mtb) infection or the progression of the disease. The development of a new vaccine against tuberculosis arises a nontrivial scientific challenge caused by several specific features of the intracellular lifestyle of Mtb and the ability of the pathogen to manipulate host immunity. The purpose of this review is to discuss promising strategies and the possibilities of creating a new vaccine that could replace BCG and provide greater protection. The considered approaches include supplementing mycobacterial strains with immunodominant antigens and genetic engineering aimed at altering the interaction between the bacterium and the host cell, such as the exit from the phagosome. Improved new vaccine strains based on BCG and Mtb undergoing clinical evaluation are also overviewed.

Recent Developments in Mycobacteria-Based Live Attenuated Vaccine Candidates for Tuberculosis

Vaccination is an excellent approach to stimulating the host immune response and reducing human morbidity and mortality against microbial infections, such as tuberculosis (TB). Bacillus Calmette-Guerin (BCG) is the most widely administered vaccine in the world and the only vaccine approved by the World Health Organization (WHO) to protect against TB. Although BCG confers "protective" immunity in children against the progression of Mycobacterium tuberculosis (Mtb) infection into active TB, this vaccine is ineffective in protecting adults with active TB manifestations, such as multiple-, extensive-, and total-drug-resistant (MDR/XDR/TDR) cases and the co-existence of TB with immune-compromising health conditions, such as HIV infection or diabetes. Moreover, BCG can cause disease in individuals with HIV infection or other immune compromises. Due to these limitations of BCG, novel strategies are urgently needed to improve global TB control measures. Since live vaccines elicit a broader immune response and do not require an adjuvant, developing recombinant BCG (rBCG) vaccine candidates have received significant attention as a potential replacement for the currently approved BCG vaccine for TB prevention. In this report, we aim to present the latest findings and outstanding questions that we consider worth investigating regarding novel mycobacteria-based live attenuated TB vaccine candidates. We also specifically discuss the important features of two key animal models, mice and rabbits, that are relevant to TB vaccine testing. Our review emphasizes that the development of vaccines that block the reactivation of latent Mtb infection (LTBI) into active TB would have a significant impact in reducing the spread and transmission of Mtb. The results and ideas discussed here are only based on reports from the last five years to keep the focus on recent developments.

Highlights of the 3rd international BCG symposium: 100th anniversary of the first administration of BCG

2021 was the year of the 100th anniversary of the first administration of the Bacillus Calmette-Guérin (BCG) to a human being. It was the start of a long journey of the world’s most widely used vaccine and the oldest vaccine still in use. More than 4 billion children have been vaccinated with BCG for protection against tuberculosis. However, over the years it became apparent that BCG also has beneficial non-specific effects. As such, it provides protection against various heterologous infectious and non-infectious diseases and is used to treat non-muscle-invasive bladder cancer. As BCG was developed at the Institut Pasteur de Lille by Albert Calmette and Camille Guérin, the Institute has celebrated this important anniversary with an international scientific symposium on all aspects of BCG, held from November 17 to 19, 2021 at the Institut Pasteur de Lille. It covered BCG against tuberculosis and described novel vaccine approaches, the effect of BCG against heterologous infections, including BCG and COVID-19, the effect of BCG against cancer, and BCG against auto-immune and inflammatory diseases. To discuss these areas, the symposium gathered close to 200 participants from all five continents, 2/3 on-line. This article presents the highlights of this 3^rd International Symposium on BCG.

The role of bacterial cyclic di-adenosine monophosphate in the host immune response

Cyclic di-adenosine monophosphate (c-di-AMP) is a second messenger which is widely used in signal transduction in bacteria and archaea. c-di-AMP plays an important role in the regulation of bacterial physiological activities, such as the cell cycle, cell wall stability, environmental stress response, and biofilm formation. Moreover, c-di-AMP produced by pathogens can be recognized by host cells for the activation of innate immune responses. It can induce type I interferon (IFN) response in a stimulator of interferon genes (STING)-dependent manner, activate the nuclear factor kappa B (NF-κB) pathway, inflammasome, and host autophagy, and promote the production and secretion of cytokines. In addition, c-di-AMP is capable of triggering a host mucosal immune response as a mucosal adjuvant. Therefore, c-di-AMP is now considered to be a new pathogen-associated molecular pattern in host immunity and has become a promising target in bacterial/viral vaccine and drug research. In this review, we discussed the crosstalk between bacteria and host immunity mediated by c-di-AMP and addressed the role of c-di-AMP as a mucosal adjuvant in boosting evoked immune responses of subunit vaccines. The potential application of c-di-AMP in immunomodulation and immunotherapy was also discussed in this review.

A century of BCG vaccination: Immune mechanisms, animal models, non-traditional routes and implications for COVID-19

Bacillus Calmette-Guerin (BCG) has been used as a vaccine against tuberculosis since 1921 and remains the only currently approved vaccine for this infection. The recent discovery that BCG protects against initial infection, and not just against progression from latent to active disease, has significant implications for ongoing research into the immune mechanisms that are relevant to generate a solid host defense against Mycobacterium tuberculosis (Mtb). In this review, we first explore the different components of immunity that are augmented after BCG vaccination. Next, we summarize current efforts to improve the efficacy of BCG through the development of recombinant strains, heterologous prime-boost approaches and the deployment of non-traditional routes. These efforts have included the development of new recombinant BCG strains, and various strategies for expression of important antigens such as those deleted during the M. bovis attenuation process or antigens that are present only in Mtb. BCG is typically administered via the intradermal route, raising questions about whether this could account for its apparent failure to generate long-lasting immunological memory in the lungs and the inconsistent level of protection against pulmonary tuberculosis in adults. Recent years have seen a resurgence of interest in the mucosal and intravenous delivery routes as they have been shown to induce a better immune response both in the systemic and mucosal compartments. Finally, we discuss the potential benefits of the ability of BCG to confer trained immunity in a non-specific manner by broadly stimulating a host immunity resulting in a generalized survival benefit in neonates and the elderly, while potentially offering benefits for the control of new and emerging infectious diseases such as COVID-19. Given that BCG will likely continue to be widely used well into the future, it remains of critical importance to better understand the immune responses driven by it and how to leverage these for the design of improved vaccination strategies against tuberculosis.

Cyclic di-AMP as endogenous adjuvant enhanced BCG-induced trained immunity and protection against Mycobacterium tuberculosis in mice

Bacillus Calmette-Guérin (BCG) is a licensed prophylactic vaccine against tuberculosis (TB). Current TB vaccine efforts focus on improving BCG effects through recombination or genetic attenuation and/or boost with different vaccines. Recent years, it was revealed that BCG could elicit non-specific heterogeneous protection against other pathogens such as viruses through a process termed trained immunity. Previously, we constructed a recombinant BCG (rBCG-DisA) with elevated c-di-AMP as endogenous adjuvant by overexpressing di-adenylate cyclase of Mycobacterium tuberculosis DisA, and found that rBCG-DisA induced enhanced immune responses by subcutaneous route in mice after M. tuberculosis infection. In this study, splenocytes from rBCG-DisA immunized mice by intravenous route (i.v) elicited greater proinflammatory cytokine responses to homologous and heterologous re-stimulations than BCG. After M. tuberculosis infection, rBCG-DisA immunized mice showed hallmark responses of trained immunity including potent proinflammatory cytokine responses, enhanced epigenetic changes, altered lncRNA expressions and metabolic rewiring in bone marrow cells and other tissues. Moreover, rBCG-DisA immunization induced higher levels of antibodies and T cells responses in the lung and spleen of mice after M. tuberculosis infection. It was found that rBCG-DisA resided longer than BCG in the lung of M. tuberculosis infected mice implying prolonged duration of vaccine efficacy. Then, we found that rBCG-DisA boosting could prolong survival of BCG-primed mice over 90 weeks against M. tuberculosis infection. Our findings provided in vivo experimental evidence that rBCG-DisA with c-di-AMP as endogenous adjuvant induced enhanced trained immunity and adaptive immunity. What’s more, rBCG-DisA showed promising potential in prime-boost strategy against M. tuberculosis infection in adults.

Cyclic-di-AMP Phosphodiesterase Elicits Protective Immune Responses Against Mycobacterium tuberculosis H37Ra Infection in Mice

Many antigens from Mycobacterium tuberculosis (M. tuberculosis) have been demonstrated as strong immunogens and proved to have application potential as vaccine candidate antigens. Cyclic di-AMP (c-di-AMP) as a bacterial second messenger regulates various bacterial processes as well as the host immune responses. Rv2837c, the c-di-AMP phosphodiesterase (CnpB), was found to be relative to virulence of M. tuberculosis and interference with host innate immune response. In this study, recombinant CnpB was administered subcutaneously to mice. We found that CnpB had strong immunogenicity and induced high levels of humoral response and lung mucosal immunity after M. tuberculosis intranasally infection. CnpB immunization stimulated splenocyte proliferation and the increasing number of activated NK cells but had little effects on Th1/Th2 cellular immune responses in spleens. However, CnpB induced significant Th1/Th2 cellular immune responses with a decreased number of T and B cells in the lungs, and significantly recruits of CD4+ and CD8+ T cells after M. tuberculosis attenuated strain H37Ra infection. Besides, we first reported that CnpB could stimulate IFN-β expression transitorily and inhibit the autophagy of macrophages in vitro. In mice intranasally infection model, CnpB immunization alleviated pathological changes and reduced M. tuberculosis H37Ra loads in the lungs. Thus, our results suggested that CnpB interferes with host innate and adaptive immune responses and confers protection against M. tuberculosis respiratory infection, which should be considered in vaccine development as well as a drug target.

c-di-AMP Accumulation Regulates Growth, Metabolism, and Immunogenicity of Mycobacterium smegmatis

Cyclic dimeric adenosine monophosphate (c-di-AMP) is a ubiquitous second messenger of bacteria involved in diverse physiological processes as well as host immune responses. MSMEG_2630 is a c-di-AMP phosphodiesterase (cnpB) of Mycobacterium smegmatis, which is homologous to Mycobacterium tuberculosis Rv2837c. In this study, cnpB-deleted (ΔcnpB), -complemented (ΔcnpB::C), and -overexpressed (ΔcnpB::O) strains of M. smegmatis were constructed to investigate the role of c-di-AMP in regulating mycobacterial physiology and immunogenicity. This study provides more precise evidence that elevated c-di-AMP level resulted in smaller colonies, shorter bacteria length, impaired growth, and inhibition of potassium transporter in M. smegmatis. This is the first study to report that elevated c-di-AMP level could inhibit biofilm formation and induce porphyrin accumulation in M. smegmatis by regulating associated gene expressions, which may have effects on drug resistance and virulence of mycobacterium. Moreover, the cnpB-deleted strain with an elevated c-di-AMP level could induce enhanced Th1 immune responses after M. tuberculosis infection. Further, the pathological changes and the bacteria burden in ΔcnpB group were comparable with the wild-type M. smegmatis group against M. tuberculosis venous infection in the mouse model. Our findings enhanced the understanding of the physiological role of c-di-AMP in mycobacterium, and M. smegmatis cnpB-deleted strain with elevated c-di-AMP level showed the potential for a vaccine against tuberculosis.

Recombinant BCG to Enhance Its Immunomodulatory Activities

The bacillus Calmette-Guérin (BCG) is an attenuated Mycobacterium bovis derivative that has been widely used as a live vaccine against tuberculosis for a century. In addition to its use as a tuberculosis vaccine, BCG has also been found to have utility in the prevention or treatment of unrelated diseases, including cancer. However, the protective and therapeutic efficacy of BCG against tuberculosis and other diseases is not perfect. For three decades, it has been possible to genetically modify BCG in an attempt to improve its efficacy. Various immune-modulatory molecules have been produced in recombinant BCG strains and tested for protection against tuberculosis or treatment of several cancers or inflammatory diseases. These molecules include cytokines, bacterial toxins or toxin fragments, as well as other protein and non-protein immune-modulatory molecules. The deletion of genes responsible for the immune-suppressive properties of BCG has also been explored for their effect on BCG-induced innate and adaptive immune responses. Most studies limited their investigations to the description of T cell immune responses that were modified by the genetic modifications of BCG. Some studies also reported improved protection by recombinant BCG against tuberculosis or enhanced therapeutic efficacy against various cancer forms or allergies. However, so far, these investigations have been limited to mouse models, and the prophylactic or therapeutic potential of recombinant BCG strains has not yet been illustrated in other species, including humans, with the exception of a genetically modified BCG strain that is now in late-stage clinical development as a vaccine against tuberculosis. In this review, we provide an overview of the different molecular engineering strategies adopted over the last three decades in order to enhance the immune-modulatory potential of BCG.

The Mycobacterium tuberculosis PhoPR virulence system regulates expression of the universal second messenger c-di-AMP and impacts vaccine safety and efficacy

Cyclic (di)nucleotides act as universal second messengers endogenously produced by several pathogens. Specifically, the roles of c-di-AMP in Mycobacterium tuberculosis immunity and virulence have been largely explored, although its contribution to the safety and efficacy of live tuberculosis vaccines is less understood. In this study, we demonstrate that the synthesis of c-di-AMP is negatively regulated by the M. tuberculosis PhoPR virulence system. Accordingly, the live attenuated tuberculosis vaccine candidate M. tuberculosis vaccine (MTBVAC), based on double phoP and fadD26 deletions, produces more than 25- and 45-fold c-di-AMP levels relative to wild-type M. tuberculosis or the current vaccine bacille Calmette-Guérin (BCG), respectively. Secretion of this second messenger was exclusively detected in MTBVAC but not in M. tuberculosis or in BCG. We also demonstrate that c-di-AMP synthesis during in vitro cultivation of M. tuberculosis is a growth-phase- and medium-dependent phenotype. To uncover the role of this metabolite in the vaccine properties of MTBVAC, we constructed and validated knockout and overproducing/oversecreting derivatives by inactivating the disA or cnpB gene, respectively. All MTBVAC derivatives elicited superior interleukin-1β (IL-1β) responses compared with BCG during an in vitro infection of human macrophages. However, both vaccines failed to elicit interferon β (IFNβ) activation in this cellular model. We found that increasing c-di-AMP levels remarkably correlated with a safer profile of tuberculosis vaccines in the immunodeficient mouse model. Finally, we demonstrate that overproduction of c-di-AMP due to cnpB inactivation resulted in lower protection of MTBVAC, while the absence of c-di-AMP in the MTBVAC disA derivative maintains the protective efficacy of this vaccine in mice.

Evolution of immunotherapy in the treatment of non-muscle-invasive bladder cancer

INTRODUCTION

Immunotherapy with intravesical bacillus Calmette-Guérin (BCG) has been the gold standard treatment for intermediate- and high-risk non-muscle-invasive bladder cancer (NMIBC) for nearly half a century. Yet, many patients with high-risk disease will experience recurrence, including those who progress and eventually become unresponsive to BCG. For decades, apart from radical cystectomy, few therapeutic options existed for this at-risk population. However, the advent of novel immunotherapeutic agents has transformed treatment in a range of tumour types, including urothelial carcinoma. These immunotherapies have yielded promising results in the treatment of metastatic urothelial carcinoma and, as such, are also being investigated for use in NIMIBC.

AREAS COVERED

This article provides an overview of the evolution of immunotherapy for NMIBC, beginning from the original immunotherapy- BCG - to current agents including checkpoint inhibitors, IL-15 agonists, viral gene therapies and therapeutic cancer vaccines.

EXPERT OPINION

The KEYNOTE-057 trial represented a pivotal moment for immunotherapy in NMIBC, but patient selection and the development of biomarkers to guide the identification of patients who will benefit most from a particular immunotherapy remains critical. As research efforts come to fruition, novel immunotherapies may become integrated into the standard treatment paradigm for intermediate- and high-risk NMBIC.

Re-engineered BCG overexpressing cyclic di-AMP augments trained immunity and exhibits improved efficacy against bladder cancer

In addition to its role as a TB vaccine, BCG has been shown to elicit heterologous protection against many other pathogens including viruses through a process termed trained immunity. Despite its potential as a broadly protective vaccine, little has been done to determine if BCG-mediated trained immunity levels can be optimized. Here we re-engineer BCG to express high levels of c-di-AMP, a PAMP recognized by stimulator of interferon genes (STING). We find that BCG overexpressing c-di-AMP elicits more potent signatures of trained immunity including higher pro-inflammatory cytokine responses, greater myeloid cell reprogramming toward inflammatory and activated states, and enhances epigenetic and metabolomic changes. In a model of bladder cancer, we also show that re-engineered BCG induces trained immunity and improved functionality. These results indicate that trained immunity levels and antitumor efficacy may be increased by modifying BCG to express higher levels of key PAMP molecules.

BCG invokes superior STING-mediated innate immune response over radiotherapy in a carcinogen murine model of urothelial cancer

Radiation and bacillus Calmette-Guérin (BCG) instillations are used clinically for treatment of urothelial carcinoma, but the precise mechanisms by which they activate an immune response remain elusive. The role of the cGAS-STING pathway has been implicated in both BCG and radiation-induced immune response; however, comparison of STING pathway molecules and the immune landscape following treatment in urothelial carcinoma has not been performed. We therefore comprehensively analyzed the local immune response in the bladder tumor microenvironment following radiotherapy and BCG instillations in a well-established spontaneous murine model of urothelial carcinoma to provide insight into activation of STING-mediated immune response. Mice were exposed to the oral carcinogen, BBN, for 12 weeks prior to treatment with a single 15 Gy dose of radiation or three intravesical instillations of BCG (1 × 108  CFU). At sacrifice, tumors were staged by a urologic pathologist and effects of therapy on the immune microenvironment were measured using the NanoString Myeloid Innate Immunity Panel and immunohistochemistry. Clinical relevance was established by measuring immune biomarker expression of cGAS and STING on a human tissue microarray consisting of BCG-treated non-muscle-invasive urothelial carcinomas. BCG instillations in the murine model elevated STING and downstream STING-induced interferon and pro-inflammatory molecules, intratumoral M1 macrophage and T-cell accumulation, and complete tumor eradication. In contrast, radiotherapy caused no changes in STING pathway or innate immune gene expression; rather, it induced M2 macrophage accumulation and elevated FoxP3 expression characteristic of immunosuppression. In human non-muscle-invasive bladder cancer, STING protein expression was elevated at baseline in patients who responded to BCG therapy and increased further after BCG therapy. Overall, these results show that STING pathway activation plays a key role in effective BCG-induced immune response and strongly indicate that the effects of BCG on the bladder cancer immune microenvironment are more beneficial than those induced by radiation. © 2021 The Pathological Society of Great Britain and Ireland.

Impact of STING Inflammatory Signaling during Intracellular Bacterial Infections

The early detection of bacterial pathogens through immune sensors is an essential step in innate immunity. STING (Stimulator of Interferon Genes) has emerged as a key mediator of inflammation in the setting of infection by connecting pathogen cytosolic recognition with immune responses. STING detects bacteria by directly recognizing cyclic dinucleotides or indirectly by bacterial genomic DNA sensing through the cyclic GMP-AMP synthase (cGAS). Upon activation, STING triggers a plethora of powerful signaling pathways, including the production of type I interferons and proinflammatory cytokines. STING activation has also been associated with the induction of endoplasmic reticulum (ER) stress and the associated inflammatory responses. Recent reports indicate that STING-dependent pathways participate in the metabolic reprogramming of macrophages and contribute to the establishment and maintenance of a robust inflammatory profile. The induction of this inflammatory state is typically antimicrobial and related to pathogen clearance. However, depending on the infection, STING-mediated immune responses can be detrimental to the host, facilitating bacterial survival, indicating an intricate balance between immune signaling and inflammation during bacterial infections. In this paper, we review recent insights regarding the role of STING in inducing an inflammatory profile upon intracellular bacterial entry in host cells and discuss the impact of STING signaling on the outcome of infection. Unraveling the STING-mediated inflammatory responses can enable a better understanding of the pathogenesis of certain bacterial diseases and reveal the potential of new antimicrobial therapy.

Recombinant BCGs for tuberculosis and bladder cancer

Bacillus Calmette-Guérin (BCG) vaccine is an attenuated live strain of Mycobacterium bovis. It may be the most widely used vaccine in human history and is the only licensed human tuberculosis (TB) vaccine available. Despite its excellent safety history, a century of use in global vaccination programs, and its significant contribution to reducing TB mortality among children, the efficacy of BCG continues to be disputed due to its incomplete protection against pulmonary TB in adults. Still vaccines offer the best chance to contain the ongoing spread of multi-drug resistance TB and disease dissemination. The development of improved vaccines against TB therefore remains a high global priority. Interestingly, recent studies indicate that genetically modified BCG, or administration of existing BCG through alternate routes, or revaccination, offers improved protection, suggesting that BCG is well poised to make a comeback. Intravesical BCG is also the only approved microbial immunotherapy for any form of cancer, and is the first-line therapy for treatment-naïve non-muscle invasive bladder cancer (NMBIC), which represents a majority of the new bladder cancer cases diagnosed. However, almost a third of patients with NMIBC are either BCG unresponsive or have tumor recurrence, leading to a higher risk of disease progression. With very few advances in intravesical therapy over the past two decades for early-stage disease, and a limited pipeline of therapeutics in Phase 3 or late Phase 2 development, there is a major unmet need for improved intravesical therapies for NMIBC. Indeed, genetically modified candidate BCG vaccines engineered to express molecules that confer stronger protection against pulmonary TB or induce potent anti-tumor immunity in NMIBC have shown promise in both pre-clinical and clinical settings. This review discusses the development of second generation, genetically modified BCG candidates as TB vaccines and as anti-tumor adjuvant therapy for NMIBC.

100 years of Bacillus Calmette-Guérin immunotherapy: from cattle to COVID-19

Bacillus Calmette-Guérin (BCG) is the most widely used vaccine worldwide and has been used to prevent tuberculosis for a century. BCG also stimulates an anti-tumour immune response, which urologists have harnessed for the treatment of non-muscle-invasive bladder cancer. A growing body of evidence indicates that BCG offers protection against various non-mycobacterial and viral infections. The non-specific effects of BCG occur via the induction of trained immunity and form the basis for the hypothesis that BCG vaccination could be used to protect against the severity of coronavirus disease 2019 (COVID-19). This Perspective article highlights key milestones in the 100-year history of BCG and projects its potential role in the COVID-19 pandemic.

The Promise and Challenges of Cyclic Dinucleotides as Molecular Adjuvants for Vaccine Development

Cyclic dinucleotides (CDNs), originally discovered as bacterial second messengers, play critical roles in bacterial signal transduction, cellular processes, biofilm formation, and virulence. The finding that CDNs can trigger the innate immune response in eukaryotic cells through the stimulator of interferon genes (STING) signalling pathway has prompted the extensive research and development of CDNs as potential immunostimulators and novel molecular adjuvants for induction of systemic and mucosal innate and adaptive immune responses. In this review, we summarize the chemical structure, biosynthesis regulation, and the role of CDNs in enhancing the crosstalk between host innate and adaptive immune responses. We also discuss the strategies to improve the efficient delivery of CDNs and the recent advance and future challenges in the development of CDNs as potential adjuvants in prophylactic vaccines against infectious diseases and in therapeutic vaccines against cancers.

Bacillus Calmette-Guérin Immunotherapy for Cancer

Bacillus Calmette–Guérin (BCG), an attenuated vaccine from Mycobacterium bovis, was initially developed as an agent for vaccination against tuberculosis. BCG proved to be the first successful immunotherapy against established human bladder cancer and other neoplasms. The use of BCG has been shown to induce a long-lasting antitumor response over all other forms of treatment against intermediate, non-invasive muscle bladder cancer Several types of tumors may now be treated by releasing the immune response through the blockade of checkpoint inhibitory molecules, such as CTLA-4 and PD-1. In addition, Toll-Like Receptor (TLR) agonists and BCG are used to potentiate the immune response against tumors. Studies concerning TLR-ligands combined with BCG to treat melanoma have demonstrated efficacy in treating mice and patients This review addresses several interventions using BCG on neoplasms, such as Leukemia, Bladder Cancer, Lung Cancer, and Melanoma, describing treatments and antitumor responses promoted by this attenuated bacillus. Of essential importance, BCG is described recently to participate in an adequate microbiome, establishing an effective response during cell-target therapy when combined with anti-PD-1 antibody, which stimulates T cell responses against the melanoma. Finally, trained immunity is discussed, and reprogramming events to shape innate immune responses are addressed.

Subunit Vaccine ESAT-6:c-di-AMP Delivered by Intranasal Route Elicits Immune Responses and Protects Against Mycobacterium tuberculosis Infection

Tuberculosis (TB), caused by Mycobacterium tuberculosis (Mtb) infection, remains the most common cause of death from a single infectious disease. More safe and effective vaccines are necessary for preventing the prevalence of TB. In this study, a subunit vaccine of ESAT-6 formulated with c-di-AMP (ESAT-6:c-di-AMP) promoted mucosal and systemic immune responses in spleen and lung. ESAT-6:c-di-AMP inhibited the differentiations of CD8⁺ T cells as well as macrophages, but promoted the differentiations of ILCs in lung. The co-stimulation also enhanced inflammatory cytokines production in MH-S cells. It was first revealed that ESAT-6 and c-di-AMP regulated autophagy of macrophages in different stages, which together resulted in the inhibition of Mtb growth in macrophages during early infection. After Mtb infection, the level of ESAT-6-specific immune responses induced by ESAT-6:c-di-AMP dropped sharply. Finally, inoculation of ESAT-6:c-di-AMP led to significant reduction of bacterial burdens in lungs and spleens of immunized mice. Our results demonstrated that subunit vaccine ESAT-6:c-di-AMP could elicit innate and adaptive immune responses which provided protection against Mtb challenge, and c-di-AMP as a mucosal adjuvant could enhance immunogenicity of antigen, especially for innate immunity, which might be used for new mucosal vaccine against TB.

A decade of research on the second messenger c-di-AMP

Cyclic dimeric adenosine 3',5'-monophosphate (c-di-AMP) is an emerging second messenger in bacteria and archaea that is synthesized from two molecules of ATP by diadenylate cyclases and degraded to pApA or two AMP molecules by c-di-AMP-specific phosphodiesterases. Through binding to specific protein- and riboswitch-type receptors, c-di-AMP regulates a wide variety of prokaryotic physiological functions, including maintaining the osmotic pressure, balancing central metabolism, monitoring DNA damage and controlling biofilm formation and sporulation. It mediates bacterial adaptation to a variety of environmental parameters and can also induce an immune response in host animal cells. In this review, we discuss the phylogenetic distribution of c-di-AMP-related enzymes and receptors and provide some insights into the various aspects of c-di-AMP signaling pathways based on more than a decade of research. We emphasize the key role of c-di-AMP in maintaining bacterial osmotic balance, especially in Gram-positive bacteria. In addition, we discuss the future direction and trends of c-di-AMP regulatory network, such as the likely existence of potential c-di-AMP transporter(s), the possibility of crosstalk between c-di-AMP signaling with other regulatory systems, and the effects of c-di-AMP compartmentalization. This review aims to cover the broad spectrum of research on the regulatory functions of c-di-AMP and c-di-AMP signaling pathways.

Augmentation of the Riboflavin-Biosynthetic Pathway Enhances Mucosa-Associated Invariant T (MAIT) Cell Activation and Diminishes Mycobacterium tuberculosis Virulence

Mucosa-associated invariant T (MAIT) cells play a critical role in antimicrobial defense. Despite increased understanding of their mycobacterial ligands and the clinical association of MAIT cells with tuberculosis (TB), their function in protection against Mycobacterium tuberculosis infection remains unclear. Here, we show that overexpressing key genes of the riboflavin-biosynthetic pathway potentiates MAIT cell activation and results in attenuation of M. tuberculosis virulence in vivo. Further, we observed greater control of M. tuberculosis infection in MAIT^hi CAST/EiJ mice than in MAIT^lo C57BL/6J mice, highlighting the protective role of MAIT cells against TB. We also endogenously adjuvanted Mycobacterium bovis BCG with MR1 ligands via overexpression of the lumazine synthase gene ribH and evaluated its protective efficacy in the mouse model of M. tuberculosis infection. Altogether, our findings demonstrate that MAIT cells confer host protection against TB and that overexpression of genes in the riboflavin-biosynthetic pathway attenuates M. tuberculosis virulence. Enhancing MAIT cell-mediated immunity may also offer a novel approach toward improved vaccines against TB. IMPORTANCE Mucosa-associated invariant T (MAIT) cells are an important subset of innate lymphocytes that recognize microbial ligands derived from the riboflavin biosynthesis pathway and mediate antimicrobial immune responses. Modulated MAIT cell responses have been noted in different forms of tuberculosis. However, it has been unclear if increased MAIT cell abundance is protective against TB disease. In this study, we show that augmentation of the mycobacterial MAIT cell ligands leads to higher MAIT cell activation with reduced M. tuberculosis virulence and that elevated MAIT cell abundance confers greater control of M. tuberculosis infection. Our study also highlights the potential of endogenously adjuvanting the traditional BCG vaccine with MR1 ligands to augment MAIT cell activation. This study increases current knowledge on the roles of the riboflavin-biosynthetic pathway and MAIT cell activation in M. tuberculosis virulence and host immunity against TB.

Mycobacterium tuberculosis MmsA (Rv0753c) Interacts with STING and Blunts the Type I Interferon Response

It is unclear how the type I IFN response is regulated by mycobacterial determinants. Here, we characterized the previously unreported role of M. tuberculosis MmsA in immunological regulation of type I IFN response by targeting the central adaptor STING in the DNA sensing pathway. We identified STING-interacting MmsA by coimmunoprecipitation-mass spectrometry-based (IP-MS) proteomic analysis and showed MmsA interacting with STING and autophagy receptor p62 via its N terminus and C terminus, respectively. We also showed that MmsA downregulated type I IFN by promoting p62-mediated STING degradation. Moreover, the MmsA mutant R138W is potentially associated with the virulence of M. tuberculosis clinical strains owing to the modulation of STING protein. Our results provide novel insights into the regulatory mechanism of type I IFN response manipulated by mycobacterial MmsA and the additional cross talk between autophagy and STING in M. tuberculosis infection, wherein a protein from microbial pathogens induces autophagic degradation of host innate immune molecules.

Type I interferon (IFN) plays an important role in Mycobacterium tuberculosis persistence and disease pathogenesis. M. tuberculosis has evolved a number of mechanisms to evade host immune surveillance. However, it is unclear how the type I IFN response is tightly regulated by the M. tuberculosis determinants. Stimulator of interferon genes (STING) is an essential adaptor for type I IFN production triggered by M. tuberculosis genomic DNA or cyclic dinucleotides upon infection. To investigate how the type I IFN response is regulated by M. tuberculosis determinants, immunoprecipitation-mass spectrometry-based (IP-MS) proteomic analysis was performed to screen proteins interacting with STING in the context of M. tuberculosis infection. Among the many predicted candidates interacting with STING, the M. tuberculosis coding protein Rv0753c (MmsA) was identified. We confirmed that MmsA binds and colocalizes with STING, and the N-terminal regions of MmsA (amino acids [aa] 1 to 251) and STING (aa 1 TO 190) are responsible for MmsA-STING interaction. Type I IFN production was impaired with exogenous expression of MmsA in RAW264.7 cells. MmsA inhibited the STING-TBK1-IRF3 pathway, as evidenced by reduced STING levelS and subsequent IRF3 activation. Furthermore, MmsA facilitated p62-mediated STING autophagic degradation by binding p62 with its C terminus (aa 252 to 455), which may account for the negative regulation of M. tuberculosis MmsA in STING-mediated type I IFN production. Additionally, the M. tuberculosismmsA R138W mutation, detected in a hypervirulent clinical isolate, enhanced the degradation of STING, implying the important relevance of MmsA in disease outcome. Together, we report a novel mechanism where M. tuberculosis MmsA serves as an antagonist of type I IFN response by targeting STING with p62-mediated autophagic degradation.

Kawasaki-like diseases and thrombotic coagulopathy in COVID-19: delayed over-activation of the STING pathway?

We previously made the hypothesis that STING contributes to COVID-19. The present review detail new arguments for over-activation of STING pathways in COVID-19, following the description of hyper-coagulability and Kawasaki-like diseases in children. Indeed, Kawasaki disease is induced by overreaction of innate cells following exposition to various viruses, including herpes viruses which trigger STING. It predisposes to diffuse vasculitis and aneurysms, whereas STING is over-expressed in arterial aneurisms. The redness at the inoculation site of bacillus Calmette-Guérin, a specific feature of Kawasaki disease, is reproduced by activation of the STING pathway, which is inhibited upstream by aspirin, intravenous immunoglobulins, and Vitamin-D. SARS-CoV2 binding to ACE2 can lead to excessive angiotensin II signaling, which activates the STING pathway in mice. Over-activation of the STING-pathway promotes hyper-coagulability through release of interferon-β and tissue factor by monocytes-macrophages. Aspirin and dipyridamole, besides their anti-platelet activity, also reduce tissue factor procoagulant activity, and aspirin inhibits the STING pathway upstream of STING. Aspirin and dipyridamole may be used, in combination with drugs blocking downstream the activation of the STING pathway, like inhibitors of IL-6R and JAK/STAT pathways. The risk of bleeding should be low as bleeding has not been reported in severe COVID-19 patients.

Post translational modifications in tuberculosis: ubiquitination paradox

Innate immune signaling and xenophagy are crucial innate defense strategies exploited by the host to counteract intracellular pathogens with ubiquitination as a critical regulator of these processes. These pathogens, including Mycobacterium tuberculosis (M. tb), co-opt the host ubiquitin machinery by utilizing secreted or cell surface effectors to dampen innate host defenses. Inversely, the host utilizes ubiquitin ligase-mediated ubiquitination of intracellular pathogens and recruits autophagy receptors to induce xenophagy. In the current article, we discuss the co-option of the ubiquitin pathway by the M. tb virulence effectors.Abbreviations: ANAPC2: anaphase promoting complex subunit 2; IL: interleukin; Lys: lysine (K); MAPK: mitogen-activated protein kinase; MAP3K7/TAK1; mitogen-activated protein kinase kinase kinase 7; M. tb: Mycobacterium tuberculosis; NFKB/NF-κB: nuclear factor kappa B subunit; PtpA: protein tyrosine phosphatase; SQSTM1/p62: sequestosome 1; V-ATPase: vacuolar-type H+-ATPase; UBA: a eukaryotic-like ubiquitin-associated domain.

Vaccine strategies for the Mtb/HIV copandemic

One-third of world’s population is predicted to be infected with tuberculosis (TB). The resurgence of this deadly disease has been inflamed by comorbidity with human immunodeficiency virus (HIV). The risk of TB in people living with HIV (PLWH) is 15–22 times higher than people without HIV. Development of a single vaccine to combat both diseases is an ardent but tenable ambition. Studies have focused on the induction of specific humoral and cellular immune responses against HIV-1 following recombinant BCG (rBCG) expressing HIV-1 antigens. Recent advances in the TB vaccines led to the development of promising candidates such as MTBVAC, the BCG revaccination approach, H4:IC31, H56:IC31, M72/AS01 and more recently, intravenous (IV) BCG. Modification of these vaccine candidates against TB/HIV coinfection could reveal key correlates of protection in a representative animal model. This review discusses the (i) potential TB vaccine candidates that can be exploited for use as a dual vaccine against TB/HIV copandemic (ii) progress made in the realm of TB/HIV dual vaccine candidates in small animal model, NHP model, and human clinical trials (iii) the failures and promising targets for a successful vaccine strategy while delineating the correlates of vaccine-induced protection.

Immunodominant Mycobacterium tuberculosis Protein Rv1507A Elicits Th1 Response and Modulates Host Macrophage Effector Functions

Mycobacterium tuberculosis (M. tb) persists as latent infection in nearly a quarter of the global population and remains the leading cause of death among infectious diseases. While BCG is the only vaccine for TB, its inability to provide complete protection makes it imperative to engineer BCG such that it expresses immunodominant antigens that can enhance its protective potential. In-silico comparative genomic analysis of Mycobacterium species identified M. tb Rv1507A as a “signature protein” found exclusively in M. tb. In-vitro (cell lines) and in-vivo experiments carried out in mice, using purified recombinant Rv1507A revealed it to be a pro-inflammatory molecule, eliciting significantly high levels of IL-6, TNF-α, and IL-12. There was increased expression of activation markers CD69, CD80, CD86, antigen presentation molecules (MHC I/MHCII), and associated Th1 type of immune response. Rv1507A knocked-in M. smegmatis also induced significantly higher pro-inflammatory Th1 response and higher survivability under stress conditions, both in-vitro (macrophage RAW264.7 cells) and in-vivo (mice). Sera derived from human TB patients showed significantly enhanced B-cell response against M. tb Rv1507A. The ability of M. tb Rv1507A to induce immuno-modulatory effect, B cell response, and significant memory response, renders it a putative vaccine candidate that demands further exploration.