Aerosol immunisation for TB: matching route of vaccination to route of infection

Imprinting of Gut-Homing Receptors on Mtb-Specific Th1* Cells Is Associated with Reduced Lung Homing after Gavage BCG Vaccination of Rhesus Macaques

Alternative delivery routes of the current Mycobacterium tuberculosis (Mtb) vaccine, intradermally (ID) delivered BCG, may provide better protection against tuberculosis, and be more easily administered. Here, we use rhesus macaques to compare the airway immunogenicity of BCG delivered via either ID or intragastric gavage vaccination. Ag-specific CD4 T cell responses in the blood were similar after BCG vaccination via gavage or ID injection. However, gavage BCG vaccination induced significantly lower T cell responses in the airways compared to intradermal BCG vaccination. Examining T cell responses in lymph node biopsies showed that ID vaccination induced T cell priming in skin-draining lymph nodes, while gavage vaccination induced priming in the gut-draining nodes, as expected. While both delivery routes induced highly functional Ag-specific CD4 T cells with a Th1* phenotype (CXCR3⁺CCR6⁺), gavage vaccination induced the co-expression of the gut-homing integrin α₄β₇ on Ag-specific Th1* cells, which was associated with reduced migration into the airways. Thus, in rhesus macaques, the airway immunogenicity of gavage BCG vaccination may be limited by the imprinting of gut-homing receptors on Ag-specific T cells primed in intestinal lymph nodes. IMPORTANCE Mycobacterium tuberculosis (Mtb) is a leading cause of global infectious disease mortality. The vaccine for Mtb, Bacillus Calmette-Guérin (BCG), was originally developed as an oral vaccine, but is now given intradermally. Recently, clinical studies have reevaluated oral BCG vaccination in humans and found that it induces significant T cell responses in the airways. Here, we use rhesus macaques to compare the airway immunogenicity of BCG delivered intradermally or via intragastric gavage. We find that gavage BCG vaccination induces Mtb-specific T cell responses in the airways, but to a lesser extent than intradermal vaccination. Furthermore, gavage BCG vaccination induces the gut-homing receptor a4ß7 on Mtb-specific CD4 T cells, which was associated with reduced migration into the airways. These data raise the possibility that strategies to limit the induction of gut-homing receptors on responding T cells may enhance the airway immunogenicity of oral vaccines.

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.

Comparison of the Immunogenic Properties of Lactiplantibacillus plantarum Carrying the Mycobacterial Ag85B-ESAT-6 Antigen at Various Cellular Localizations

The bacille Calmette-Guèrin (BCG) vaccine has been used for a century; nonetheless, tuberculosis (TB) remains one of the deadliest diseases in the world. Thus, new approaches to developing a new, more efficient vaccine are desirable. Mucosal vaccines are of particular interest, considering that Mycobacterium tuberculosis first enters the body through the mucosal membranes. We have previously demonstrated the immunogenicity of a recombinant Lactiplantibacillus plantarum delivery vector with TB hybrid antigen Ag85B-ESAT-6 anchored to the cell membrane. The goal of the present study was to analyze the impact of antigen localization in the immune response. Thus, we assessed two novel vaccine candidates, with the TB antigen either non-covalently anchored to the cell wall (LysMAgE6) or located intracellularly (CytAgE6). In addition, we compared two expression systems, using an inducible (LipoAgE6) or a constitutive promoter (cLipoAgE6) for expression of covalently anchored antigen to the cell membrane. Following administration to mice, antigen-specific CD4⁺ T-cell proliferation and IFN-γ and IL-17A secretion were analyzed for lung cell and splenocyte populations. Generally, the immune response in lung cells was stronger compared to splenocytes. The analyses showed that the type of expression system did not significantly affect the immunogenicity, while various antigen localizations resulted in markedly different responses. The immune response was considerably stronger for the surface-displaying candidate strains compared to the candidate with an intracellular antigen. These findings emphasize the significance of antigen exposure and further support the potential of L. plantarum as a mucosal vaccine delivery vehicle in the fight against TB.

Particle Dynamics and Bioaerosol Viability of Aerosolized Bacillus Calmette-Guérin Vaccine Using Jet and Vibrating Mesh Clinical Nebulizers

Background: Bacillus Calmette-Guérin (BCG) is a vaccine used to protect against tuberculosis primarily in infants to stop early infection in areas of the world where the disease is endemic. Normally administered as a percutaneous injection, BCG is a live significantly attenuated bacteria that is now being investigated for its potential within an inhalable vaccine formulation. This study investigates the feasibility and performance of two jet and two vibrating mesh nebulizers aerosolizing BCG and the resulting particle characteristics and residual viability of the bacteria postaerosolization. Methods: A jet nebulizer (Collison), outfitted either with a 3- or 6-jet head, was compared with two clinical nebulizers, the vibrating mesh Omron MicroAir and Aerogen Solo devices. Particle characteristics, including aerodynamic particle sizing, was performed on all devices within a common aerosol chamber configuration and comparable BCG innocula concentrations. Integrated aerosol samples were collected for each generator and assayed for bacterial viability using conventional microbiological technique. Results: A batch lot of BCG (Danish) was grown to titer and used in all generator assessments. Aerosol particles within the respirable range were generated from all nebulizers at four different concentrations of BCG. The jet nebulizers produced a uniformly smaller particle size than the vibrating mesh devices, although particle concentrations by mass were similar across all devices tested with the exception of the Aerogen Solo, which resulted in a low concentration of BCG aerosols. Conclusions: The resulting measured viable BCG aerosol concentration fraction produced by each device approximated one another; however, a measurable decrease of efficiency and overall viability reduction in the jet nebulizer was observed in higher BCG inoculum starting concentrations, whereas the vibrating mesh nebulizer returned a remarkably stable viable aerosol fraction irrespective of inoculum concentration.

CD4+CCR6+ T cells dominate the BCG-induced transcriptional signature

BACKGROUND

The century-old Mycobacterium bovis Bacillus Calmette-Guerin (BCG) remains the only licensed vaccine against tuberculosis (TB). Despite this, there is still a lot to learn about the immune response induced by BCG, both in terms of phenotype and specificity.

METHODS

We investigated immune responses in adult individuals pre and 8 months post BCG vaccination. We specifically determined changes in gene expression, cell subset composition, DNA methylome, and the TCR repertoire induced in PBMCs and CD4 memory T cells associated with antigen stimulation by either BCG or a Mycobacterium tuberculosis (Mtb)-derived peptide pool.

FINDINGS

Following BCG vaccination, we observed increased frequencies of CCR6+ CD4 T cells, which includes both Th1* (CXCR3+CCR6+) and Th17 subsets, and mucosal associated invariant T cells (MAITs). A large number of immune response genes and pathways were upregulated post BCG vaccination with similar patterns observed in both PBMCs and memory CD4 T cells, thus suggesting a substantial role for CD4 T cells in the cellular response to BCG. These upregulated genes and associated pathways were also reflected in the DNA methylome. We described both qualitative and quantitative changes in the BCG-specific TCR repertoire post vaccination, and importantly found evidence for similar TCR repertoires across different subjects.

INTERPRETATION

The immune signatures defined herein can be used to track and further characterize immune responses induced by BCG, and can serve as reference for benchmarking novel vaccination strategies.

100 years of Mycobacterium bovis bacille Calmette-Guérin

Mycobacterium bovis bacille Calmette-Guérin (BCG), an experimental vaccine designed to protect cattle from bovine tuberculosis, was administered for the first time to a newborn baby in Paris in 1921. Over the past century, BCG has saved tens of millions of lives and has been given to more humans than any other vaccine. It remains the sole tuberculosis vaccine licensed for use in humans. BCG provides long-lasting strong protection against miliary and meningeal tuberculosis in children, but it is less effective for the prevention of pulmonary tuberculosis, especially in adults. Evidence mainly from the past two decades suggests that BCG has non-specific benefits against non-tuberculous infections in newborn babies and in older adults, and offers immunotherapeutic benefit in certain malignancies such as non-muscle invasive bladder cancer. However, as a live attenuated vaccine, BCG can cause localised or disseminated infections in immunocompromised hosts, which can also occur following intravesical installation of BCG for the treatment of bladder cancer. The legacy of BCG includes fundamental discoveries about tuberculosis-specific and non-specific immunity and the demonstration that tuberculosis is a vaccine-preventable disease, providing a foundation for new vaccines to hasten tuberculosis elimination.

The safety and efficacy of BCG encapsulated alginate particle (BEAP) against M.tb H37Rv infection in Macaca mulatta : A pilot study

Due to the limited utility of Bacillus Calmette–Guérin (BCG), the only approved vaccine available for tuberculosis, there is a need to develop a more effective and safe vaccine. We evaluated the safety and efficacy of a dry powder aerosol (DPA) formulation of BCG encapsulated alginate particle (BEAP) and the conventional intradermal BCG immunization in infant rhesus macaques (Macaca mulatta). The infant macaques were immunized intratracheally with DPA of BEAP into the lungs. Animals were monitored for their growth, behaviour, any adverse and allergic response. The protective efficacy of BEAP was estimated by the ex-vivo H37Rv infection method. Post-immunization with BEAP, granulocytes count, weight gain, chest radiography, levels of liver secreted enzymes, cytokines associated with inflammation like TNF and IL-6 established that BEAP is non-toxic and it does not elicit an allergic response. The T cells isolated from BEAP immunized animals’ blood, upon stimulation with M.tb antigen, secreted high levels of IFN-γ, TNF, IL-6 and IL-2. The activated T cells from BEAP group, when co-cultured with M.tb infected macrophages, eliminated largest number of infected macrophages compared to the BCG and control group. This study suggests the safety and efficacy of BEAP in Non-human primate model.

Changes in the Immune Phenotype and Gene Expression Profile Driven by a Novel Tuberculosis Nanovaccine: Short and Long-Term Post-immunization

Deciphering protection mechanisms against Mycobacterium tuberculosis (Mtb) remains a critical challenge for the development of new vaccines and therapies. We analyze the phenotypic and transcriptomic profile in lung of a novel tuberculosis (TB) nanoparticle-based boosting mucosal vaccine Nano-FP1, which combined to BCG priming conferred enhanced protection in mice challenged with low-dose Mtb. We analyzed the vaccine profile and efficacy at short (2 weeks), medium (7 weeks) and long term (11 weeks) post-vaccination, and compared it to ineffective Nano-FP2 vaccine. We observed several changes in the mouse lung environment by both nanovaccines, which are lost shortly after boosting. Additional boosting at long-term (14 weeks) recovered partially cell populations and transcriptomic profile, but not enough to enhance protection to infection. An increase in both total and resident memory CD4 and CD8 T cells, but no pro-inflammatory cytokine levels, were correlated with better protection. A unique gene expression pattern with differentially expressed genes revealed potential pathways associated to the immune defense against Mtb. Our findings provide an insight into the critical immune responses that need to be considered when assessing the effectiveness of a novel TB vaccine.

Therapeutic efficacy of pulmonary live tuberculosis vaccines against established asthma by subverting local immune environment

Background

Substantial recent advances in the comprehension of the molecular and cellular mechanisms behind asthma have evidenced the importance of the lung immune environment for disease outcome, making modulation of local immune responses an attractive therapeutic target against this pathology. Live attenuated mycobacteria, such as the tuberculosis vaccine BCG, have been classically linked with a type 1 response, and proposed as possible modulators of the type 2 response usually associated with asthma.

Methods

In this study we used different acute and chronic murine models of asthma to investigate the therapeutic efficacy of intranasal delivery of the live tuberculosis vaccines BCG and MTBVAC by regulating the lung immune environment associated with airway hyperresponsiveness (AHR).

Findings

Intranasal administration of BCG, or the novel tuberculosis vaccine candidate MTBVAC, abrogated AHR-associated hallmarks, including eosinophilia and lung remodeling. This correlated with the re-polarization of allergen-induced M2 macrophages towards an M1 phenotype, as well as with the induction of a strong allergen-specific Th1 response. Importantly, vaccine treatment was effective in a scenario of established chronic asthma where a strong eosinophil infiltration was already present prior to immunization. We finally compared the nebulization efficiency of clinical formulations of MTBVAC and BCG using a standard commercial nebulizer for potential aerosol application.

Interpretation

Our results demonstrate that pulmonary live tuberculosis vaccines efficiently revert established asthma in mice. These data support the further exploration of this approach as potential therapy against asthma.

Funding

Spanish Ministry of Science [grant numbers: BIO2014-5258P, RTI2018-097625-B-I00], Instituto de Salud Carlos III, Gobierno de Aragón/Fondo Social Europeo, University of Zaragoza [grant number: JIUZ-2018-BIO-01].

Developing New Anti-Tuberculosis Vaccines: Focus on Adjuvants

Tuberculosis (TB) is an infectious disease caused by Mycobacterium tuberculosis (Mtb) that sits in the top 10 leading causes of death in the world today and is the current leading cause of death among infectious diseases. Although there is a licensed vaccine against TB, the Mycobacterium bovis bacilli Calmette–Guérin (BCG) vaccine, it has several limitations, namely its high variability of efficacy in the population and low protection against pulmonary tuberculosis. New vaccines for TB are needed. The World Health Organization (WHO) considers the development and implementation of new TB vaccines to be a priority. Subunit vaccines are promising candidates since they can overcome safety concerns and optimize antigen targeting. Nevertheless, these vaccines need adjuvants in their formulation in order to increase immunogenicity, decrease the needed antigen dose, ensure a targeted delivery and optimize the antigens delivery and interaction with the immune cells. This review aims to focus on adjuvants being used in new formulations of TB vaccines, namely candidates already in clinical trials and others in preclinical development. Although no correlates of protection are defined, most research lines in the field of TB vaccination focus on T-helper 1 (Th1) type of response, namely polyfunctional CD4+ cells expressing simultaneously IFN-γ, TNF-α, and IL-2 cytokines, and also Th17 responses. Accordingly, most of the adjuvants reviewed here are able to promote such responses. In the future, it might be advantageous to consider a wider array of immune parameters to better understand the role of adjuvants in TB immunity and establish correlates of protection.

Is mapping the BCG vaccine-induced immune responses the key to improving the efficacy against tuberculosis?

In recent years, the century-old Mycobacterium bovis Bacillus Calmette-Guérin (BCG) vaccine against tuberculosis (TB) has been re-evaluated for its capacity to stem the global tide of TB. There is increasing evidence that the efficacy of BCG can be improved by the modified administration methods and schedules. Here, we first discuss recent approaches of vaccine administration, revaccination or boosting that have been used to try to improve the efficacy of BCG against TB. We then dive deeper into studies investigating the immune correlates of protection and describe studies that have investigated BCG-specific T-cell responses and the influence of environmental exposures. These studies all highlight that there is still a lot to learn about the immune response induced by BCG, both in terms of phenotype and specificity, which has been surprisingly understudied. We argue that several critical gaps in knowledge exist and must be addressed by future research to rationally improve the efficacy of BCG, including comprehensive, proteome-wide understanding of the epitopes derived from BCG recognized by BCG-vaccinated individuals, the phenotype of responding antigen-specific T cells and how previous exposure to environmental mycobacteria affect these parameters and thus influence vaccine efficacy. The development of modern techniques allows us to answer some of these questions to better understand how BCG works in terms of both protection against TB and the immune response that it triggers.

Vaccines Against Tuberculosis: Problems and Prospects (Review)

Despite the efforts of the global medical and scientific community, tuberculosis remains the leading cause of death from infectious diseases. The expectation of success associated with the development of new anti-TB drugs was not justified, and the attention of researchers was largely drawn to the creation of new mycobacterial strains for vaccination against tuberculosis. The proposed review contains current information on the existing vaccine strains and the development of new, genetically engineered strains for the prevention of tuberculosis and the prevention and treatment of other diseases. The review includes relevant information on the correlation between BCG vaccination and the frequency and severity of COVID-19 infection.

SARS-CoV-2 will constantly sweep its tracks: a vaccine containing CpG motifs in 'lasso' for the multi-faced virus

During the current COVID-19 pandemic, the global ratio between the dead and the survivors is approximately 1 to 10, which has put humanity on high alert and provided strong motivation for the intensive search for vaccines and drugs. It is already clear that if we follow the most likely scenario, which is similar to that used to create seasonal influenza vaccines, then we will need to develop improved vaccine formulas every year to control the spread of the new, highly mutable coronavirus SARS-CoV-2. In this article, using well-known RNA viruses (HIV, influenza viruses, HCV) as examples, we consider the main successes and failures in creating primarily highly effective vaccines. The experience accumulated dealing with the biology of zoonotic RNA viruses suggests that the fight against COVID-19 will be difficult and lengthy. The most effective vaccines against SARS-CoV-2 will be those able to form highly effective memory cells for both humoral (memory B cells) and cellular (cross-reactive antiviral memory T cells) immunity. Unfortunately, RNA viruses constantly sweep their tracks and perhaps one of the most promising solutions in the fight against the COVID-19 pandemic is the creation of 'universal' vaccines based on conservative SARS-CoV-2 genome sequences (antigen-presenting) and unmethylated CpG dinucleotides (adjuvant) in the composition of the phosphorothioate backbone of single-stranded DNA oligonucleotides (ODN), which can be effective for long periods of use. Here, we propose a SARS-CoV-2 vaccine based on a lasso-like phosphorothioate oligonucleotide construction containing CpG motifs and the antigen-presenting unique ACG-containing genome sequence of SARS-CoV-2. We found that CpG dinucleotides are the most rare dinucleotides in the genomes of SARS-CoV-2 and other known human coronaviruses, and hypothesized that their higher frequency could be responsible for the unwanted increased lethality to the host, causing a ‘cytokine storm’ in people who overexpress cytokines through the activation of specific Toll-like receptors in a manner similar to TLR9-CpG ODN interactions. Interestingly, the virus strains sequenced in China (Wuhan) in February 2020 contained on average one CpG dinucleotide more in their genome than the later strains from the USA (New York) sequenced in May 2020. Obviously, during the first steps of the microevolution of SARS-CoV-2 in the human population, natural selection tends to select viral genomes containing fewer CpG motifs that do not trigger a strong innate immune response, so the infected person has moderate symptoms and spreads SARS-CoV-2 more readily. However, in our opinion, unmethylated CpG dinucleotides are also capable of preparing the host immune system for the coronavirus infection and should be present in SARS-CoV-2 vaccines as strong adjuvants.

Towards new TB vaccines

Mycobacterium tuberculosis remains the leading cause of death attributed to a single infectious organism. Bacillus Calmette-Guerin (BCG), the standard vaccine against M. tuberculosis, is thought to prevent only 5% of all vaccine-preventable deaths due to tuberculosis, thus an alternative vaccine is required. One of the principal barriers to vaccine development against M. tuberculosis is the complexity of the immune response to infection, with uncertainty as to what constitutes an immunological correlate of protection. In this paper, we seek to give an overview of the immunology of M. tuberculosis infection, and by doing so, investigate possible targets of vaccine development. This encompasses the innate, adaptive, mucosal and humoral immune systems. Though MVA85A did not improve protection compared with BCG alone in a large-scale clinical trial, the correlates of protection this has revealed, in addition to promising results from candidate such as VPM1002, M72/ASO1E and H56:IC31 point to a brighter future in the field of TB vaccine development.

Adaption of the ex vivo mycobacterial growth inhibition assay for use with murine lung cells

In the absence of a correlate(s) of protection against human tuberculosis and a validated animal model of the disease, tools to facilitate vaccine development must be identified. We present an optimised ex vivo mycobacterial growth inhibition assay (MGIA) to assess the ability of host cells within the lung to inhibit mycobacterial growth, including Bacille Calmette-Guérin (BCG) and Mycobacterium tuberculosis (MTB) Erdman. Growth of BCG was reduced by 0.39, 0.96 and 0.73 log10 CFU following subcutaneous (s.c.) BCG, intranasal (i.n.) BCG, or BCG s.c. + mucosal boost, respectively, versus naïve mice. Comparatively, a 0.49 (s.c.), 0.60 (i.n.) and 0.81 (s.c. + mucosal boost) log10 reduction in MTB CFU was found. A BCG growth inhibitor, 2-thiophenecarboxylic acid hydrazide (TCH), was used to prevent quantification of residual BCG from i.n. immunisation and allow accurate MTB quantification. Using TCH, a further 0.58 log10 reduction in MTB CFU was revealed in the i.n. group. In combination with existing methods, the ex vivo lung MGIA may represent an important tool for analysis of vaccine efficacy and the immune mechanisms associated with vaccination in the organ primarily affected by MTB disease.

Mycobacterium indicus pranii Induced Memory T-Cells in Lung Airways Are Sentinels for Improved Protection Against M.tb Infection

The lungs are the most vulnerable site for air-borne infections. Immunologic compartmentalization of the lungs into airway lumen and interstitium has paved the way to determine the immune status of the site of pathogen entry, which is crucial for the outcome of any air-borne infections. Vaccination via the nasal route with Mycobacterium indicus pranii (MIP), a prospective candidate vaccine against tuberculosis (TB), has been reported to confer superior protection as compared to the subcutaneous (s.c.) route in small-animal models of TB. However, the immune mechanism remains only partly understood. Here, we showed that intranasal (i.n.) immunization of mice with MIP resulted in a significant recruitment of CD4⁺ and CD8⁺ T-cells expressing activation markers in the lung airway lumen. A strong memory T-cell response was observed in the lung airway lumen after i.n. MIP vaccination, compared with s.c. vaccination. The recruitment of these T-cells was regulated primarily by CXCR3–CXCL11 axis in “MIP i.n.” group. MIP-primed T-cells in the lung airway lumen effectively transferred protective immunity into naïve mice against Mycobacterium tuberculosis (M.tb) infection and helped reducing the pulmonary bacterial burden. These signatures of protective immune response were virtually absent or very low in unimmunized and subcutaneously immunized mice, respectively, before and after M.tb challenge. Our study provides mechanistic insights for MIP-elicited protective response against M.tb infection.

Characterization of local and circulating bovine γδ T cell responses to respiratory BCG vaccination

The Mycobacterium bovis Bacillus Calmette-Guerin (BCG) vaccine is administered parenterally to infants and young children to prevent tuberculosis (TB) infection. However, the protection induced by BCG is highly variable and the vaccine does not prevent pulmonary TB, the most common form of the illness. Until improved TB vaccines are available, it is crucial to use BCG in a manner which ensures optimal vaccine performance. Immunization directly to the respiratory mucosa has been shown to promote greater protection from TB in animal models. γδ T cells play a major role in host defense at mucosal sites and are known to respond robustly to mycobacterial infection. Their positioning in the respiratory mucosa ensures their engagement in the response to aerosolized TB vaccination. However, our understanding of the effect of respiratory BCG vaccination on γδ T cell responses in the lung is unknown. In this study, we used a calf model to investigate the immunogenicity of aerosol BCG vaccination, and the phenotypic profile of peripheral and mucosal γδ T cells responding to vaccination. We observed robust local and systemic M. bovis-specific IFN-γ and IL-17 production by both γδ and CD4 T cells. Importantly, BCG vaccination induced effector and memory cell differentiation of γδ T cells in both the lower airways and peripheral blood, with accumulation of a large proportion of effector memory γδ T cells in both compartments. Our results demonstrate the potential of the neonatal calf model to evaluate TB vaccine candidates that are to be administered via the respiratory tract, and suggest that aerosol immunization is a promising strategy for engaging γδ T cells in vaccine-induced immunity against TB.

The Humoral Immune Response to BCG Vaccination

Bacillus Calmette Guérin (BCG) is the only currently available vaccine against tuberculosis (TB), but it confers incomplete and variable protection against pulmonary TB in humans and bovine TB (bTB) in cattle. Insights into the immune response induced by BCG offer an underexploited opportunity to gain knowledge that may inform the design of a more efficacious vaccine, which is urgently needed to control these major global epidemics. Humoral immunity in TB and bTB has been neglected, but recent studies supporting a role for antibodies in protection against TB has driven a growing interest in determining their relevance to vaccine development. In this manuscript we review what is known about the humoral immune response to BCG vaccination and re-vaccination across species, including evidence for the induction of specific B cells and antibodies; and how these may relate to protection from TB or bTB. We discuss potential explanations for often conflicting findings and consider how factors such as BCG strain, manufacturing methodology and route of administration influence the humoral response. As novel vaccination strategies include BCG prime-boost regimens, the literature regarding off-target immunomodulatory effects of BCG vaccination on non-specific humoral immunity is also reviewed. Overall, reported outcomes to date are inconsistent, but indicate that humoral responses are heterogeneous and may play different roles in different species, populations, or individual hosts. Further study is warranted to determine whether a new TB vaccine could benefit from the targeting of humoral as well as cell-mediated immunity.

Mycobacterium bovis BCG Surface Antigens Expressed under the Granuloma-Like Conditions as Potential Inducers of the Protective Immunity

Bovine tuberculosis (bTB) is a highly transmissible infection and remains of great concern as a zoonosis. The worldwide incidence of bTB is in rise, creating potential reservoir and increased infection risk for humans and animals. In attempts to identify novel surface antigens of Mycobacterium bovis as a proof-of-concept for potential inducers of protective immunity, we investigated surface proteome of M. bovis BCG strain that was cultured under the granuloma-like condition. We also demonstrated that the pathogen exposed to the biologically relevant environment has greater binding and invasion abilities to host cells than those of bacteria incubated under regular laboratory conditions. A total of 957 surface-exposed proteins were identified for BCG cultured under laboratory condition, whereas 1,097 proteins were expressed under the granuloma-like condition. The overexpression of Mb1524, Mb01_03198, Mb1595_p3681 (PhoU1 same as phoY1_1), and Mb1595_p0530 (HbhA) surface proteins in Mycobacterium smegmatis leads to increased binding and invasion to mucosal cells. We also examined the immunogenicity of purified recombinant proteins and tested M. smegmatis overexpressing these surface antigens for the induction of protective immunity in mice. Significantly high levels of specific IgA and IgG antibodies were observed in recombinant protein immunized groups by both inhalation and intraperitoneal (IP) routes, but only IP delivery induced high total IgA and IgG levels. We did not detect major differences in antibody levels in the M. smegmatis group that overexpressed surface antigens. In addition, the bacterial load was significantly reduced in the lungs of mice immunized with the combination of inhaled recombinant proteins. Our findings suggest that the activation of the mucosal immunity can lead to increased ability to confer protection upon M. bovis BCG infection.

Mucosal boosting of H56:CAF01 immunization promotes lung-localized T cells and an accelerated pulmonary response to Mycobacterium tuberculosis infection without enhancing vaccine protection

T cell-mediated protection against Mycobacterium tuberculosis (Mtb) is dependent upon the ability to localize within the site of pulmonary infection and directly interact with infected cells. In turn, vaccine strategies to improve rapid T cell targeting of Mtb-infected cells after pulmonary exposure are being actively pursued. Given parenterally, the subunit vaccine H56:CAF01 elicits polyfunctional CD4 T cells that localize to the lung parenchyma and confer durable protection. Here, we find that airway mucosal boosting of parenteral H56:CAF01 immunization greatly enhances the population of long-lived lung-resident T cells (Trm) and increases early vaccine T cell responses to pulmonary Mtb challenge in multiple mouse models. However, mucosal boosting does not alter the Th1/17 vaccine signature typical of H56:CAF01 and does not further improve durable control of pulmonary infection following aerosol Mtb-challenge. Additional mucosal boosting with H56:CAF01 further enhances the Trm response without further improving protection, while blocking the recruitment of non-Trm with FTY720-treatment failed to exposed Trm-mediated protection in mucosally boosting animals. These results demonstrate the limitations of maximizing lung-localized Trm in vaccine control of pulmonary Mtb infection, especially within an immunization protocol that is already optimized for the induction of mucosal-homing Th17 cells.

Tuberculosis

Tuberculosis remains the leading cause of death from an infectious disease among adults worldwide, with more than 10 million people becoming newly sick from tuberculosis each year. Advances in diagnosis, including the use of rapid molecular testing and whole-genome sequencing in both sputum and non-sputum samples, could change this situation. Although little has changed in the treatment of drug-susceptible tuberculosis, data on increased efficacy with new and repurposed drugs have led WHO to recommend all-oral therapy for drug-resistant tuberculosis for the first time ever in 2018. Studies have shown that shorter latent tuberculosis prevention regimens containing rifampicin or rifapentine are as effective as longer, isoniazid-based regimens, and there is a promising vaccine candidate to prevent the progression of infection to the disease. But new tools alone are not sufficient. Advances must be made in providing high-quality, people-centred care for tuberculosis. Renewed political will, coupled with improved access to quality care, could relegate the morbidity, mortality, and stigma long associated with tuberculosis, to the past.

BCG Vaccination Prevents Reactivation of Latent Lymphatic Murine Tuberculosis Independently of CD4+ T Cells

Tuberculosis (TB) is a major global public health problem causing significant mortality and morbidity. In addition to ~10.4 million cases of active TB annually, it is estimated that about two billion people are latently infected with Mycobacterium tuberculosis (Mtb), the causative agent of TB. Reactivation of latent Mtb infection is the leading cause of death in patients with immunodeficiency virus (HIV) infection. The low efficiency of the only licensed anti-TB vaccine, Bacille Calmette–Guérin (BCG) to reduce pulmonary TB in adults contributes to this problem. Here we investigated if vaccination with conventional BCG or the genetically modified experimental BCGΔBCG1419c strain can prevent reactivation of latent lymphatic TB in a mouse model of induced reactivation, following the depletion of CD4⁺ T cells, as it occurs in HIV⁺ individuals. Vaccination with conventional BCG or BCGΔBCG1419c prevented reactivation of Mtb from the infected lymph node and the systemic spread of Mtb to spleen and lung. Prevention of reactivation was independent of vaccination route and was accompanied by reduced levels of circulating inflammatory cytokines and the absence of lung pathology. Our results demonstrate that vaccine-induced CD4⁺ T cells are not essential to prevent reactivation of latent lymphatic murine TB, and highlight the need to better understand how non-CD4⁺ immune cell populations participate in protective immune responses to control latent TB.

Designing tuberculosis vaccine efficacy trials - lessons from recent studies

INTRODUCTION

Tuberculosis (TB) is the leading infectious killer globally and new TB vaccines will be crucial to ending the epidemic. Since the introduction in 1921 of the only currently licensed TB vaccine, BCG, very few novel vaccine candidates or strategies have advanced into clinical efficacy trials. Areas covered: Recently, however, two TB vaccine efficacy trials with novel designs have reported positive results and are now driving new momentum in the field. They are the first Prevention of Infection trial, evaluating the H4:IC31 candidate or BCG revaccination in high-risk adolescents and a Prevention of Disease trial evaluating the M72/AS01_E candidate in M.tuberculosis-infected, healthy adults. These trials are briefly reviewed, and lessons learned are proposed to help inform the design of future efficacy trials. The references cited were chosen by the author based on PubMed searches to provide context for the opinions expressed in this Perspective article. Expert opinion: The opportunities created by these two trials for gaining critically important knowledge are game-changing for TB vaccine development. Their results clearly establish feasibility in the relatively near term of developing novel, effective vaccines that could be crucial to ending the TB epidemic.

BCG vaccination for bovine tuberculosis; conclusions from the Jerusalem One Health workshop

The global burden of bovine tuberculosis (bTB) remains poorly characterized, with spill-over impacts on multiple species. The "One Health" concept is especially relevant given the bidirectional risk of cattle infecting humans with Mycobacterium bovis and humans infecting cattle with Mycobacterium tuberculosis. "Test and cull" is the traditional bTB control method, but the strategy may not be economically feasible or culturally acceptable where cattle are highly prized or their killing is a religious taboo; it is also less effective when there are wildlife reservoirs of infection. Vaccination with M. bovis bacille Calmette-Guerin (BCG) provides protection against bTB, but its use in animals has been limited. The Jerusalem One Health workshop considered key bTB knowledge gaps and innovative solutions. Knowledge gaps identified included (a) the poorly quantified prevalence of M. bovis infection and disease in cattle, domestic camelids and human populations in developing countries, (b) the absence of alternatives to a "test and cull" strategy in settings where the killing of infected animals is culturally or economically unacceptable, or where affected species are protected and (c) an understanding of the induction of mucosal immunity against bTB. We summarize discussions on the use of BCG vaccination in domestic animals and wildlife and list potential projects to address the knowledge gaps identified.

Induction of Mycobacterium Tuberculosis Lipid-Specific T Cell Responses by Pulmonary Delivery of Mycolic Acid-Loaded Polymeric Micellar Nanocarriers

Mycolic acid (MA), a major lipid component of Mycobacterium tuberculosis (Mtb) cell wall, can be presented by the non-polymorphic antigen presenting molecule CD1b to T cells isolated from Mtb-infected individuals. These MA-specific CD1b-restricted T cells are cytotoxic, produce Th1 cytokines, and form memory populations, suggesting that MA can be explored as a potential subunit vaccine candidate for TB. However, the controlled elicitation of MA-specific T cell responses has been challenging due to difficulties in the targeted delivery of lipid antigens and a lack of suitable animal models. In this study, we generated MA-loaded micellar nanocarriers (MA-Mc) comprised of self-assembled poly(ethylene glycol)-bl-poly(propylene sulfide; PEG-PPS) copolymers conjugated to an acid sensitive fluorophore to enhance intracellular delivery of MA to phagocytic immune cells. Using humanized CD1 transgenic (hCD1Tg) mice, we found these nanobiomaterials to be endocytosed by bone marrow-derived dendritic cells (DCs) and localized to lysosomal compartments. Additionally, MA-Mc demonstrated superior efficacy over free MA in activating MA-specific TCR transgenic (DN1) T cells in vitro. Following intranasal immunization, MA-Mc were primarily taken up by alveolar macrophages and DCs in the lung and induced activation and proliferation of adoptively transferred DN1 T cells. Furthermore, intranasal immunization with MA-Mc induced MA-specific T cell responses in the lungs of hCD1Tg mice. Collectively, our data demonstrates that pulmonary delivery of MA via PEG-PPS micelles to DCs can elicit potent CD1b-restricted T cell responses both in vitro and in vivo and MA-Mc could be explored as subunit vaccines against Mtb infection.

Research and development of new tuberculosis vaccines: a review

Tuberculosis kills more people worldwide than any other single infectious disease agent, a threat made more dire by the spread of drug-resistant strains of Mycobacterium tuberculosis (Mtb). Development of new vaccines capable of preventing TB disease and new Mtb infection are an essential component of the strategy to combat the TB epidemic. Accordingly, the WHO considers the development of new TB vaccines a major public health priority. In October 2017, the WHO convened a consultation with global leaders in the TB vaccine development field to emphasize the WHO commitment to this effort and to facilitate creative approaches to the discovery and development of TB vaccine candidates. This review summarizes the presentations at this consultation, updated with scientific literature references, and includes discussions of the public health need for a TB vaccine; the status of efforts to develop vaccines to replace or potentiate BCG in infants and develop new TB vaccines for adolescents and adults; strategies being employed to diversify vaccine platforms; and new animal models being developed to facilitate TB vaccine development. A perspective on the status of these efforts from the major funders and organizational contributors also is included. This presentation highlights the extraordinary progress being made to develop new TB vaccines and provided a clear picture of the exciting development pathways that are being explored.

Research and development of new tuberculosis vaccines: a review

Tuberculosis kills more people worldwide than any other single infectious disease agent, a threat made more dire by the spread of drug-resistant strains of Mycobacterium tuberculosis (Mtb). Development of new vaccines capable of preventing TB disease and new Mtb infection are an essential component of the strategy to combat the TB epidemic. Accordingly, the WHO considers the development of new TB vaccines a major public health priority. In October 2017, the WHO convened a consultation with global leaders in the TB vaccine development field to emphasize the WHO commitment to this effort and to facilitate creative approaches to the discovery and development of TB vaccine candidates. This review summarizes the presentations at this consultation, updated with scientific literature references, and includes discussions of the public health need for a TB vaccine; the status of efforts to develop vaccines to replace or potentiate BCG in infants and develop new TB vaccines for adolescents and adults; strategies being employed to diversify vaccine platforms; and new animal models being developed to facilitate TB vaccine development. A perspective on the status of these efforts from the major funders and organizational contributors also is included. This presentation highlights the extraordinary progress being made to develop new TB vaccines and provided a clear picture of the exciting development pathways that are being explored.

Nonpathologic Infection of Macaques by an Attenuated Mycobacterial Vaccine Is Not Reactivated in the Setting of HIV Co-Infection

Failure to replace Bacille Calmette-Guerin vaccines with efficacious anti-tuberculosis (TB) vaccines have prompted outside-the-box thinking, including pulmonary vaccination to elicit local immunity. Inhalational MtbΔsigH, a stress-response-attenuated strain, protected against lethal TB in macaques. While live mycobacterial vaccines show promising efficacy, HIV co-infection and the resulting immunodeficiency prompts safety concerns about their use. We assessed the persistence and safety of MtbΔsigH, delivered directly to the lungs, in the setting of HIV co-infection. Macaques were aerosol-vaccinated with ΔsigH and subsequently challenged with SIVmac₂₃₉. Bronchoalveolar lavage and tissues were sampled for mycobacterial persistence, pathology, and immune correlates. Only 35% and 3.5% of lung samples were positive for live bacilli and granulomas, respectively. Our results therefore suggest that the nonpathologic infection of macaque lungs by ΔsigH was not reactivated by simian immunodeficiency virus, despite high viral levels and massive ablation of pulmonary CD4⁺ T cells. Protective pulmonary responses were retained, including vaccine-induced bronchus-associated lymphoid tissue and CD8⁺ effector memory T cells. Despite acute simian immunodeficiency virus infection, all animals remained asymptomatic of pulmonary TB. These findings highlight the efficacy of mucosal vaccination via this attenuated strain and will guide its further development to potentially combat TB in HIV-endemic areas. Our results also suggest that a lack of pulmonary pathology is a key correlate of the safety of live mycobacterial vaccines.

What Have We Learnt about BCG Vaccination in the Last 20 Years?

A number of new tuberculosis (TB) vaccines have been or are entering clinical trials, which include genetically modified mycobacteria, mycobacterial antigens delivered by viral vectors, or mycobacterial antigens in adjuvant. Some of these vaccines aim to replace the existing BCG vaccine but others will be given as a boosting vaccine following BCG vaccination given soon after birth. It is clear that the existing BCG vaccines provide incomplete and variable protection against pulmonary TB. This review will discuss what we have learnt over the last 20 years about how the BCG vaccine induces specific and non-specific immunity, what factors influence the immune responses induced by BCG, and progress toward identifying correlates of immunity against TB from BCG vaccination studies. There is still a lot to learn about the BCG vaccine and the insights gained can help the development of more protective vaccines.

Needle-free delivery of measles virus vaccine to the lower respiratory tract of non-human primates elicits optimal immunity and protection

Needle-free measles virus vaccination by aerosol inhalation has many potential benefits. The current standard route of vaccination is subcutaneous injection, whereas measles virus is an airborne pathogen. However, the target cells that support replication of live-attenuated measles virus vaccines in the respiratory tract are largely unknown. The aims of this study were to assess the in vivo tropism of live-attenuated measles virus and determine whether respiratory measles virus vaccination should target the upper or lower respiratory tract. Four groups of twelve cynomolgus macaques were immunized with 10⁴ TCID₅₀ of recombinant measles virus vaccine strain Edmonston-Zagreb expressing enhanced green fluorescent protein. The vaccine virus was grown in MRC-5 cells and formulated with identical stabilizers and excipients as used in the commercial MV^EZ vaccine produced by the Serum Institute of India. Animals were immunized by hypodermic injection, intra-tracheal inoculation, intra-nasal instillation, or aerosol inhalation. In each group six animals were euthanized at early time points post-vaccination, whereas the other six were followed for 14 months to assess immunogenicity and protection from challenge infection with wild-type measles virus. At early time-points, enhanced green fluorescent protein-positive measles virus-infected cells were detected locally in the muscle, nasal tissues, lungs, and draining lymph nodes. Systemic vaccine virus replication and viremia were virtually absent. Infected macrophages, dendritic cells and tissue-resident lymphocytes predominated. Exclusive delivery of vaccine virus to the lower respiratory tract resulted in highest immunogenicity and protection. This study sheds light on the tropism of a live-attenuated measles virus vaccine and identifies the alveolar spaces as the optimal site for respiratory delivery of measles virus vaccine.

Tuberculosis vaccines - perspectives from the NIH/NIAID Mycobacteria vaccine testing program

The development of novel vaccine candidates against infections with Mycobacterium tuberculosis has highlighted our limited understanding of immune mechanisms required to kill M. tuberculosis. The induction of a Th1 immunity is vital, but new studies are required to identify other mechanisms that may be necessary. Novel vaccines formulations that invoke effector cells such as innate lymphoid cells may provide an environment that promote effector mechanisms including T cell and B cell mediated immunity. Identifying pathways associated with killing this highly successful infectious agent has become critical to achieving the goal of reducing the global tuberculosis burden.

Diversity of Human and Macaque Airway Immune Cells at Baseline and during Tuberculosis Infection

Immune cells of the distal airways serve as "first responders" of host immunity to the airborne pathogen Mycobacterium tuberculosis (Mtb). Mtb infection of cynomolgus macaques recapitulates the range of human outcomes from clinically silent latent tuberculosis infection (LTBI) to active tuberculosis of various degrees of severity. To further advance the application of this model to human studies, we compared profiles of bronchoalveolar lavage (BAL) cells of humans and cynomolgus macaques before and after Mtb infection. A simple gating strategy effectively defined BAL T-cell and phagocyte populations in both species. BAL from Mtb-naive humans and macaques showed similar differential cell counts. BAL T cells of macaques were composed of fewer CD4⁺cells but more CD8⁺ and CD4⁺CD8⁺ double-positive cells than were BAL T cells of humans. The most common mononuclear phagocyte population in BAL of both species displayed coexpression of HLA-DR, CD206, CD11b, and CD11c; however, multiple phagocyte subsets displaying only some of these markers were observed as well. Macaques with LTBI displayed a marked BAL lymphocytosis that was not observed in humans with LTBI. In macaques, the prevalence of specific mononuclear phagocyte subsets in baseline BAL correlated with ultimate outcomes of Mtb infection (i.e., LTBI versus active disease). Overall, these findings demonstrate the comparability of studies of pulmonary immunity to Mtb in humans and macaques. They also indicate a previously undescribed complexity of airway mononuclear phagocyte populations that suggests further lines of investigation relevant to understanding the mechanisms of both protection from and susceptibility to the development of active tuberculosis within the lung.

Mucosal BCG Vaccination Induces Protective Lung-Resident Memory T Cell Populations against Tuberculosis

Mycobacterium bovis Bacille Calmette-Guérin (BCG) is the only licensed vaccine against tuberculosis (TB), yet its moderate efficacy against pulmonary TB calls for improved vaccination strategies. Mucosal BCG vaccination generates superior protection against TB in animal models; however, the mechanisms of protection remain elusive. Tissue-resident memory T (T_RM) cells have been implicated in protective immune responses against viral infections, but the role of T_RM cells following mycobacterial infection is unknown. Using a mouse model of TB, we compared protection and lung cellular infiltrates of parenteral and mucosal BCG vaccination. Adoptive transfer and gene expression analyses of lung airway cells were performed to determine the protective capacities and phenotypes of different memory T cell subsets. In comparison to subcutaneous vaccination, intratracheal and intranasal BCG vaccination generated T effector memory and T_RM cells in the lung, as defined by surface marker phenotype. Adoptive mucosal transfer of these airway-resident memory T cells into naive mice mediated protection against TB. Whereas airway-resident memory CD4⁺ T cells displayed a mixture of effector and regulatory phenotype, airway-resident memory CD8⁺ T cells displayed prototypical T_RM features. Our data demonstrate a key role for mucosal vaccination-induced airway-resident T cells in the host defense against pulmonary TB. These results have direct implications for the design of refined vaccination strategies.

IMPORTANCE

BCG remains the only licensed vaccine against TB. Parenterally administered BCG has variable efficacy against pulmonary TB, and thus, improved prevention strategies and a more refined understanding of correlates of vaccine protection are required. Induction of memory T cells has been shown to be essential for protective TB vaccines. Mimicking the natural infection route by mucosal vaccination has been known to generate superior protection against TB in animal models; however, the mechanisms of protection have remained elusive. Here we performed an in-depth analysis to dissect the immunological mechanisms associated with superior mucosal protection in the mouse model of TB. We found that mucosal, and not subcutaneous, BCG vaccination generates lung-resident memory T cell populations that confer protection against pulmonary TB. We establish a comprehensive phenotypic characterization of these populations, providing a framework for future vaccine development.

Novel lipopeptides of ESAT-6 induce strong protective immunity against Mycobacterium tuberculosis: Routes of immunization and TLR agonists critically impact vaccine's efficacy

Mycobacterium tuberculosis (Mtb), the bacterial cause of tuberculosis, is a leading infectious agent worldwide. The development of a new vaccine against Mtb is essential to control global spread of tuberculosis, since the current vaccine BCG is not very effective and antibiotic resistance is a serious, burgeoning problem. ESAT-6 is a secreted protein of Mtb, which is absent in BCG but has been implicated in inducing protective immunity against Mtb. Peptide based subunit vaccines are attractive due to their safety and high specificity in eliciting immune responses, but small synthetic peptides are usually not very immunogenic. We have designed a novel subunit vaccine for Mtb by using simple lipid (palmitic acid) modified derivatives of peptides from ESAT-6 protein corresponding to dominant human T cell epitopes and examined their ability to stimulate protective immunity against Mtb by intranasal and subcutaneous immunization in mice. We also investigated how individual TLR agonists as adjuvants (PolyI:C, MPL and GDQ) contribute to enhancing the induced immune responses and resulting protective efficacy of our vaccine. We observed that single C-terminal palmitoyl-lysine modified lipopeptides derived from ESAT-6 induce significant cellular immune responses on their own upon mucosal and subcutaneous immunizations. Intriguingly, a combination of immunogenic lipopeptides of ESAT-6 antigen exhibited local (pulmonary) and systemic immune responses along with efficient protective efficacy when administered intranasally or subcutaneously. Surprisingly, combination of ESAT-6 derived lipopeptides with a TLR-4 agonist (MPL) enhanced protection, whereas TLR-3 (Poly I:C) and TLR-7/8 agonists (gardiquimod, GDQ) led to reduced protection associated with specific local and systemic immune modulation. Our studies demonstrate the potential of ESAT-6 derived lipopeptides as a promising vaccine candidate against Mtb, and emphasize that selection of adjuvant is critical for the success of vaccines. These findings demonstrate the promise of synthetic lipopeptides as the basis of a subunit vaccine for TB.

Why don't we have an effective tuberculosis vaccine yet?

Mycobacterium tuberculosis (M.tb) has co-evolved with humans for thousands of years, to cause tuberculosis (TB). The success of M.tb as a pathogen is in part because of the ways in which M.tb evades and exploits different cell subsets, to persist and cause disease. M.tb expresses numerous molecules to prevent its recognition and destruction by immune cells. The only licensed vaccine against TB, Bacillle Calmette-Guerin (BCG), is effective at preventing disseminated disease in infants but confers highly variable efficacy against pulmonary TB in adults, particularly in the developing world. A greater understanding of the reasons for this variability, together with a better understanding of the early, innate, and non-antigen specific mechanisms of protection would facilitate the design and development of more effective vaccines.

A Mouse Model of Latent Tuberculosis Infection to Study Intervention Strategies to Prevent Reactivation

Infection with Mycobacterium tuberculosis (Mtb) is the leading cause of death in human immunodeficiency virus (HIV)⁺ individuals, particularly in Sub-Saharan Africa. Management of this deadly co-infection is a significant global health challenge that is exacerbated by the lack of efficient vaccines against both Mtb and HIV, as well as the lack of reliable and robust animal models for Mtb/HIV co-infection. Here we describe a tractable and reproducible mouse model to study the reactivation dynamics of latent Mtb infection following the loss of CD4⁺ T cells as it occurs in HIV-co-infected individuals. Whereas intradermally (i.d.) infected C57BL/6 mice contained Mtb within the local draining lymph nodes, depletion of CD4⁺ cells led to progressive systemic spread of the bacteria and induction of lung pathology. To interrogate whether reactivation of Mtb after CD4⁺ T cell depletion can be reversed, we employed interleukin (IL)-2/anti-IL-2 complex-mediated cell boost approaches. Although populations of non-CD4 lymphocytes, such as CD8⁺ memory T cells, natural killer (NK) cells and double-negative (DN) T cells significantly expanded after IL-2/anti-IL-2 complex treatment, progressive development of bacteremia and pathologic lung alterations could not be prevented. These data suggest that the failure to reverse Mtb reactivation is likely not due to anergy of the expanded cell subsets and rather indicates a limited potential for IL-2-complex-based therapies in the management of Mtb/HIV co-infection.

Boosting BCG with inert spores improves immunogenicity and induces specific IL-17 responses in a murine model of bovine tuberculosis

Tuberculosis (TB) remains a global pandemic, in both animals and man, and novel vaccines are urgently required. Heterologous prime-boost of BCG represents a promising strategy for improved TB vaccines, with respiratory delivery the most efficacious to date. Such an approach may be an ideal vaccination strategy against bovine TB (bTB), but respiratory vaccination presents a technical challenge in cattle. Inert bacterial spores represent an attractive vaccine vehicle. Therefore we evaluated whether parenterally administered spores are efficacious when used as a BCG boost in a murine model of immunity against Mycobacterium bovis. Here we report the use of heat-killed, TB10.4 adsorbed, Bacillus subtilis spores delivered via subcutaneous injection to boost immunity primed by BCG. We demonstrate that this approach improves the immunogenicity of BCG. Interestingly, this associated with substantial boosting of IL-17 responses; considered to be important in protective immunity against TB. These data demonstrate that parenteral delivery of spores represents a promising vaccine vehicle for boosting BCG, and identifies potential for optimisation for use as a vaccine for bovine TB.

Repeated Aerosolized-Boosting with Gamma-Irradiated Mycobacterium bovis BCG Confers Improved Pulmonary Protection against the Hypervirulent Mycobacterium tuberculosis Strain HN878 in Mice

Mycobacterium bovis bacillus Calmette-Guerin (BCG), the only licensed vaccine, shows limited protection efficacy against pulmonary tuberculosis (TB), particularly hypervirulent Mycobacterium tuberculosis (Mtb) strains, suggesting that a logistical and practical vaccination strategy is urgently required. Boosting the BCG-induced immunity may offer a potentially advantageous strategy for advancing TB vaccine development, instead of replacing BCG completely. Despite the improved protection of the airway immunization by using live BCG, the use of live BCG as an airway boosting agent may evoke safety concerns. Here, we analyzed the protective efficacy of γ-irradiated BCG as a BCG-prime boosting agent for airway immunization against a hypervirulent clinical strain challenge with Mycobacterium tuberculosis HN878 in a mouse TB model. After the aerosol challenge with the HN878 strain, the mice vaccinated with BCG via the parenteral route exhibited only mild and transient protection, whereas BCG vaccination followed by multiple aerosolized boosting with γ-irradiated BCG efficiently maintained long-lasting control of Mtb in terms of bacterial reduction and pathological findings. Further immunological investigation revealed that this approach resulted in a significant increase in the cellular responses in terms of a robust expansion of antigen (PPD and Ag85A)-specific CD4⁺ T cells concomitantly producing IFN-γ, TNF-α, and IL-2, as well as a high level of IFN-γ-producing recall response via both the local and systemic immune systems upon further boosting. Collectively, aerosolized boosting of γ-irradiated BCG is able to elicit strong Th1-biased immune responses and confer enhanced protection against a hypervirulent Mycobacterium tuberculosis HN878 infection in a boosting number-dependent manner.

Improvement of BCG protective efficacy with a novel chimpanzee adenovirus and a modified vaccinia Ankara virus both expressing Ag85A

A replication-deficient chimpanzee adenovirus expressing Ag85A (ChAdOx1.85A) was assessed, both alone and in combination with modified vaccinia Ankara also expressing Ag85A (MVA85A), for its immunogenicity and protective efficacy against a Mycobacterium tuberculosis (M.tb) challenge in mice. Naïve and BCG-primed mice were vaccinated or boosted with ChAdOx1.85A and MVA85A in different combinations. Although intranasally administered ChAdOx1.85A induced strong immune responses in the lungs, it failed to consistently protect against aerosol M.tb challenge. In contrast, ChAdOx1.85A followed by MVA85A administered either mucosally or systemically, induced strong immune responses and was able to improve the protective efficacy of BCG. This vaccination regime has consistently shown superior protection over BCG alone and should be evaluated further.

Mucosal vaccination with attenuated Mycobacterium tuberculosis induces strong central memory responses and protects against tuberculosis

Tuberculosis (TB) is a global pandaemic, partially due to the failure of vaccination approaches. Novel anti-TB vaccines are therefore urgently required. Here we show that aerosol immunization of macaques with the Mtb mutant in SigH (MtbΔsigH) results in significant recruitment of inducible bronchus-associated lymphoid tissue (iBALT) as well as CD4⁺ and CD8⁺ T cells expressing activation and proliferation markers to the lungs. Further, the findings indicate that pulmonary vaccination with MtbΔsigH elicited strong central memory CD4⁺ and CD8⁺ T-cell responses in the lung. Vaccination with MtbΔsigH results in significant protection against a lethal TB challenge, as evidenced by an approximately three log reduction in bacterial burdens, significantly diminished clinical manifestations and granulomatous pathology and characterized by the presence of profound iBALT. This highly protective response is virtually absent in unvaccinated and BCG-vaccinated animals after challenge. These results suggest that future TB vaccine candidates can be developed on the basis of MtbΔsigH.

BCG, the only vaccine currently used against tuberculosis, confers only limited protection. Here the authors show that mucosal immunization of macaques with an attenuated strain of Mycobacterium tuberculosis confers a high level of protection from a lethal challenge with the bacterium.