Prevention of tuberculosis in macaques after intravenous BCG immunization

Pathogenicity and virulence of Mycobacterium abscessus

Non-tuberculous mycobacteria (NTM), such as Mycobacterium abscessus (Mab) are an increasing cause of human disease. While the majority of immunocompetent hosts control Mab infections, the robust survival of Mab within the environment has shaped survival in human cells to help drive persistence and cause inflammatory damage in susceptible individuals. With high intrinsic resistance to antibiotics, there is an important need to fully understand how Mab causes infection, define protective host pathways that control disease, and develop new strategies to treat those at high risk. This review will examine the existing literature related to host-Mab interactions with a focus on virulence, the host response, and therapy development. The goal is to highlight key gaps in our understanding and describe novel approaches to encourage new research avenues that better define the pathogenesis and host response against this increasingly important human pathogen.

Antibody-Fab and -Fc features promote Mycobacterium tuberculosis restriction

Mycobacterium tuberculosis, the causative agent of tuberculosis (TB), a leading cause of death by an infectious disease globally, has no efficacious vaccine. Antibodies are implicated in M. tuberculosis control, but the mechanisms of action remain poorly understood. We assembled a library of monoclonal antibodies (mAb) and screened for M. tuberculosis-restrictive activity in mice, identifying protective antibodies targeting diverse antigens. To dissect the mechanism of mAb-mediated M. tuberculosis restriction, we optimized a protective lipoarabinomannan-specific mAb, generating Fc variants. In vivo analysis of these Fc variants revealed a role for Fc-effector function in M. tuberculosis restriction. Restrictive Fc variants altered distribution of M. tuberculosis across innate immune cells. Single-cell transcriptomics highlighted distinctly activated pathways within innate immune cell subpopulations, identifying early activation of neutrophils as a key signature of mAb-mediated M. tuberculosis restriction. Therefore, antibody-mediated restriction of M. tuberculosis is associated with reorganization of the tissue-level immune response to infection and depends on the collaboration of antibody Fab and Fc.

Early mucosal responses following a randomised controlled human inhaled infection with attenuated Mycobacterium bovis BCG

The development of an effective vaccine against Mycobacterium tuberculosis is hampered by an incomplete understanding of immunoprotective mechanisms. We utilise an aerosol human challenge model using attenuated Mycobacterium bovis BCG, in BCG-naïve UK adults. The primary endpoint of this study (NCT03912207) was to characterise the early immune responses induced by aerosol BCG infection, the secondary endpoint was to identify immune markers associated with in-vitro protection. Blinded volunteers were randomised to inhale 1 × 107 CFU aerosolised BCG or 0.9% saline (20:6); and sequentially allocated to bronchoscopy at day 2 or 7 post-inhalation (10 BCG, 3 saline each timepoint). In the bronchoalveolar lavage post-aerosol BCG infection, there was an increase in frequency of eosinophils, neutrophils, NK cells and Donor-Unrestricted T cells at day 7, and the frequency of antigen presenting cells decreased at day 7 compared with day 2. The frequency of interferon-gamma+ BCG-specific CD4+ T cells increased in the BAL and peaked in the blood at day 7 post-BCG infection compared to day 2. BAL cells at day 2 and day 7 upregulated gene pathways related to phagocytosis, MHC-II antigen loading, T cell activation and proliferation. BCG's lack of key virulence factors and its failure to induce granulomas, may mean the observed immune responses do not fully recapitulate Mycobacterium tuberculosis infection. However, human infection models can provide unique insights into early immune mechanisms, informing vaccine design for complex pathogens.

TnSeq identifies genetic requirements of Mycobacterium tuberculosis for survival under vaccine-induced immunity

Mycobacterium tuberculosis (Mtb), the etiologic agent of tuberculosis (TB), remains a persistent global health challenge due to the lack of an effective vaccine. The only licensed TB vaccine, Bacille Calmette-Guerin (BCG), is a live attenuated strain of Mycobacterium bovis that protects young children from severe disease but fails to provide protection through adulthood. It is unclear why BCG provides incomplete protection despite inducing a robust Th1 immune response. We set out to interrogate mycobacterial determinants of vaccine escape using a functional genomics approach, TnSeq, to define bacterial genes required for survival in mice vaccinated with BCG, the live attenuated Mtb vaccine strain, ΔLprG, and in mice with Mtb immunity conferred by prior infection. We find that critical virulence genes associated with acute infection and exponential growth are less essential in hosts with adaptive immunity, including genes encoding the Esx-1 and Mce1 systems. Genetic requirements for Mtb growth in vaccinated and previously Mtb-infected hosts mirror the genetic requirements reported for bacteria under in vitro conditions that reflect aspects of the adaptive immune response. Across distinct immunization conditions, differences in genetic requirements between live attenuated vaccines and vaccination routes are observed, suggesting that different immunization strategies impose distinct bacterial stressors. Collectively, these data support the idea that Mtb requires genes that enable stress adaptation and growth arrest upon encountering the restrictive host environment induced by the adaptive immune response. We demonstrate that TnSeq can be used to understand the bacterial genetic requirements for survival in vaccinated hosts across pre-clinical live attenuated vaccines and therefore may be applied to other vaccine modalities. Understanding how Mtb survives vaccine-induced immunity has the potential to inform the development of new vaccines or adjuvant therapies.

A recombinant BCG with surface-displayed antigen induces humoral and cellular immune responses

Bacillus Calmette-Guérin (BCG) is an attenuated vaccine widely used for tuberculosis prevention. While BCG has long been perceived as an intracellular candidate vector for delivering antigens against infectious diseases and cancers, challenges persist in inducing durable immune responses, particularly high-titer neutralizing antibodies (Nabs). Here we show that displaying antigens in the surface of BCG is a promising strategy to induce long-lasting Nabs production and T-cell responses. We constructed a recombinant BCG expressing the SARS-CoV-2 receptor-binding domain (RBD) antigen on its cell wall, termed CW-rBCG::RBD, which achieved an antigen yield approaching 850 nanograms per 107 colony-forming unit. Compared with both the parental BCG and the RBD protein subunit vaccine (RBDAS01), intravenous administration of CW-rBCG::RBD followed by a booster dose significantly enhanced Nab production and increased the frequencies of RBD-specific central memory T cells (Tcm) and T follicular helper (Tfh) cells in the spleen. In mice primed with a single dose of CW-rBCG::RBD and boosted with RBDAS01, we also observed elevated Nab titers and detectable levels of RBD-specific IgG2a antibodies at 8 weeks post-priming, responses that were not observed in the BCG-primed or RBDAS01-only groups. Furthermore, subcutaneous co-administration of CW-rBCG::RBD and RBDAS01 sustained Nab production for up to 31 weeks and maintained higher Tfh and Tcm cell frequencies compared to both BCG co-administration with RBDAS01 and RBDAS01 alone. These findings highlight an effective strategy for optimizing BCG-based vaccination and immunotherapy platforms. Subject terms: recombinant BCG; immune response; vaccines; cell wall; SARS-CoV-2 RBD.

Modulation of immune responses induced by recombinant BCG expressing LTAK63 adjuvant in an immunotherapeutic model vaccine

Tuberculosis (TB) remains a major global health issue, with current treatments relying on prolonged multidrug regimens that can reduce patient compliance, and lead to drug resistance. Immunotherapeutic vaccines against Mycobacterium tuberculosis (Mtb) offer a novel approach. We have previously shown that the recombinant BCG expressing LTAK63 adjuvant (rBCG-LTAK63) decreases bacillary load and lung inflammation in Mtb-infected mice. In this work, we further investigated specific immune mechanism induced in mice infected with Mtb and treated with rBCG-LTAK63 in combination with conventional chemotherapy; different routes of administration of rBCG-LTAK63 were evaluated, such as SC, IN, and IV. Immunotherapy with rBCG-LTAK63 induces early innate immune cells migration (predominantly NK cells and monocytes/macrophages) to distinct sites; increased IFN-γ, TNF-α, and IL-17 T cells, FoxP3 expressing regulatory T cells correlating with reduced bacillary load, particularly with IN administration. The findings highlight the potential of rBCG-LTAK63 to complement TB treatment.

Goals and strategies in vaccine development against tuberculosis

Tuberculosis (TB), caused by Mycobacterium tuberculosis (Mtb), continues to be a major health problem globally. The emergence of multi-drug-resistant TB and extensively drug-resistant TB has become a severe threat to TB control programs. Currently, the Bacille Calmette-Guerin (BCG) vaccine protects a child from disease dissemination efficiently, but its efficiency wanes in adults. Despite all the limitations of BCG and accelerated TB vaccine research, BCG remains the only approved vaccine available for TB. Anti-TB drug treatment has been successful in combating the disease, but it has various side effects and requires an extended drug treatment period. So, vaccination is the finest outlook that can surpass the above-mentioned limitations. Several vaccine candidates are in the pipeline, and the hope for a potential candidate to either boost the BCG vaccine or replace BCG is underway. This review discusses different approaches to TB vaccine development. It summarizes all the challenges and limitations in vaccine development, and its preclinical and clinical trials. Additionally, DNA vaccines and their vaccination techniques are also discussed. Furthermore, the immunoinformatics approach and nanomaterial-based vaccine delivery with practical and productive endpoints are also discussed. Lastly, the potential prospects are also suggested for further studies, which would help bring positive outcomes.

How macrophage heterogeneity affects tuberculosis disease and therapy

Macrophages are the primary host cell type for infection by Mycobacterium tuberculosis in vivo. Macrophages are also key immune effector cells that mediate the control of bacterial growth. However, the specific macrophage phenotypes that are required for optimal immune control of M. tuberculosis infection in vivo remain poorly defined. There are two distinct macrophage lineages in the lung, comprising embryonically derived, tissue-resident alveolar macrophages and recruited, blood monocyte-derived interstitial macrophages. Recent studies have shown that these lineages respond divergently to similar immune environments within the tuberculosis granuloma. Here, we discuss how the differing responses of macrophage lineages might affect the control or progression of tuberculosis disease. We suggest that the ability to reprogramme macrophage responses appropriately, through immunological or chemotherapeutic routes, could help to optimize vaccines and drug regimens for tuberculosis.

Evaluation of Immunogenicity of Mycobacterium tuberculosis ag85ab DNA Vaccine Delivered by Pulmonary Administration

Background: Tuberculosis (TB) is a respiratory infectious disease, and the current TB vaccine has low local lung protection. We aim to optimize immune pathways to improve the immunogenicity of vaccines. Methods: In the immunogenicity study, 50 BALB/c mice were randomly divided into the following: (1) phosphate buffered saline (PBS)+intramuscular injection combined with electroporation (EP) group (100 μL), (2) pVAX1+EP group (50 μg/100 μL), (3) ag85ab+EP group (50 μg/100 μL), (4) pVAX1+pulmonary delivery (PD) group (50 μg/50 μL), and (5) ag85ab+PD group (50 μg/50 μL). Immunization was given once every 2 weeks for a total of three times. The number of IFN-γ-secreting lung and spleen lymphocytes was determined by enzyme-linked immunospot assay (ELISPOT). The levels of Th1, Th2, and Th17 cytokines in the culture supernatants of lung and spleen lymphocytes were detected with the Luminex method. The proportion of FoxP3 regulatory T cells in splenocytes was determined by flow cytometry. The levels of IgG-, IgG1-, and IgG2a-specific antibodies in plasma and IgA antibody in bronchoalveolar lavage fluid (BALF) were determined by enzyme-linked immunosorbent assay (ELISA). Results: The PD and EP routes of Mycobacterium tuberculosis (M. tb) ag85ab DNA vaccine can effectively induce the responses of IFN-γ-secreting lung and spleen lymphocytes, and induce dominant Th1 and Th17 cell immune responses. The PD route can induce earlier, greater numbers and stronger responses of pulmonary effector T cells, with higher levels of the specific antibody IgA detected in BALF. High levels of the specific antibodies IgG, IgG1, and IgG2α were detected in the plasma of mice immunized by the EP route. Conclusions: The PD route of DNA vaccines can more effectively stimulate the body to produce strong cellular and mucosal immunity than the EP route, especially local cellular immunity in the lungs, which can provide early protection for the lungs. It can significantly improve the immunogenicity of the ag85ab DNA vaccine, suggesting a feasible and effective approach to DNA immunization.

Deconvoluting the interplay of innate and adaptive immunity in BCG-induced nonspecific and TB-specific host resistance

BCG is the oldest vaccine in continuous use. While current intradermal vaccination regimens confer limited protection outside the context of pediatric extrapulmonary tuberculosis (TB), promising new data indicate that when administered mucosally or intravenously at a higher dose, BCG can induce sterilizing immunity against pulmonary TB in nonhuman primates. BCG is also known to promote nonspecific host resistance against a variety of unrelated infections and is a standard immunotherapy for bladder cancer, suggesting that this innate immune function may contribute to its protective role against TB. Here, we propose that both the mycobacterial-specific and off-target effects of BCG depend on the interplay of adaptive and innate cells and the cytokines they produce, and that the elucidation of this interaction should be a major strategy in the development of more effective BCG-based vaccines and immunotherapies.

Intravenous BCG-mediated protection against tuberculosis requires CD4+ T cells and CD8α+ lymphocytes

Tuberculosis (TB) is a major health burden worldwide despite widespread intradermal (ID) BCG vaccination in newborns. We previously demonstrated that changing the BCG route and dose from 5 × 105 CFUs ID to 5 × 107 CFUs i.v. resulted in prevention of Mycobacterium tuberculosis (Mtb) infection and TB disease in highly susceptible nonhuman primates. Identifying immune mechanisms protection following i.v. BCG will facilitate development of more effective vaccines against TB. Here, we depleted lymphocyte subsets prior to and during Mtb challenge in i.v. BCG-vaccinated macaques to identify those necessary for protection. Depletion of adaptive CD4 T cells, but not adaptive CD8αβ T cells, resulted in loss of protection with increased Mtb burdens and dissemination, indicating that CD4 T cells are critical to i.v. BCG-mediated protection. Depletion of unconventional CD8α-expressing lymphocytes (NK cells, innate T cells, and CD4+CD8α+ double-positive T cells) abrogated protection in most i.v. BCG-immunized macaques, supporting further investigation into which of these cell subsets contribute to protection after vaccination.

Dynamic Alterations in DNA Methylation of CD4+ T Cells and Macrophages in a Murine Model of Tuberculous Pleural Infection Induced by BCG Vaccination

Tuberculous pleural effusion (TPE) is a prevalent form of extrapulmonary tuberculosis, and immune abnormalities play a crucial role in promoting its development. However, the dynamic changes and regulatory characteristics of immune cells during TPE progression remain incompletely understood. This study analyzed DNA methylation and transcriptome data from macrophages and CD4+ T cells from pleural lavage fluid of BCG-induced tuberculous pleurisy mouse models at specific time points (Days 0, 1, 7, and 14). The results revealed substantial alterations in DNA methylation patterns associated with inflammatory factors and interferon genes. Notably, macrophages exhibited the most pronounced differences in DNA methylation profiles on Day 1, while CD4+ T cells demonstrated gradual changes over time. The investigation further indicated that DNA methylation primarily regulated the differentiation of Th1, Th17, and Th22 cells but not Th9 cells. Additionally, single-cell RNA sequencing analysis revealed an increasing expression of C1q during infection, which was regulated by DNA methylation. Importantly, C1q+ and C1q- macrophages demonstrated distinct roles in modulating immune responses during infection. This research provides valuable insights into the DNA methylation profile of immune cells during Mycobacterium bovis infection-induced pleurisy in a mouse model, enhancing our understanding of the upstream regulatory mechanisms underlying immune response development in TPE.

Cellular and Molecular Mechanisms of Innate Memory Responses

There has been an increasing effort to understand the memory responses of a complex interplay among innate, adaptive, and structural cells in peripheral organs and bone marrow. Trained immunity is coined as the de facto memory of innate immune cells and their progenitors. These cells acquire epigenetic modifications and shift their metabolism to equip an imprinted signature to a persistent fast-responsive functional state. Recent studies highlight the contribution of noncoding RNAs and modulation of chromatin structures in establishing this epigenetic readiness for potential immune perturbations. In this review, we discuss recent studies that highlight trained immunity-mediated memory responses emerging intrinsically in innate immune cells and as a complex interplay with other cells at the organ level. Lastly, we survey epigenetic contributors to trained immunity phenotypes-specifically, a recently discovered regulatory circuit coordinating the regulation of a key driver of trained immunity.

Antigen-presenting innate lymphoid cells induced by BCG vaccination promote a respiratory antiviral immune response through the skin‒lung axis

The route of vaccine administration is associated with various immune outcomes, and the relationship between the route of administration and broad protection against heterologous pathogens remains unclear. Here, we found that subcutaneous vaccination with Bacillus Calmette-Guérin (BCG) promotes respiratory influenza clearance and T-cell responses. Group 1 innate lymphoid cells (ILC1s) express MHCII molecules and engage in antigen processing and presentation after BCG vaccination. During influenza virus infection, ILC1s in the lungs of BCG-vaccinated mice can present influenza virus antigens and prime Th1 cells. After subcutaneous vaccination with BCG, MHCII+ ILC1s migrate from the skin to the lungs and play an antigen-presenting role in influenza infection. Both the BCG and the BCG component lipomannan can induce MHCII expression and skin-to-lung migration of ILC1s via TLR2 signaling. Our study revealed an important regulatory mechanism by which subcutaneous vaccination with BCG promotes respiratory antiviral immune responses via the skin‒lung axis.

Insights into protection against Mycobacterium tuberculosis infection: time to officially confirm another phenotype?

Immune correlates of protection against infection with Mycobacterium tuberculosis (Mtb) remain elusive. In this issue of the JCI, Dallmann-Sauer and authors demonstrate that lack of tuberculin skin test (TST) and interferon γ release assay (IGRA) conversion among people with HIV despite years-long Mtb exposure is associated with alveolar lymphocytosis, including specific poly-cytotoxic T cells, and M1-type alveolar macrophages with a stronger ex vivo response to the pathogen. Studies in these rare individuals, termed "TB resisters" and in tuberculosis household contacts who are repeatedly IGRA negative in the months after a specific exposure event (known as "early clearers") help elucidate manipulatable mechanisms to boost protection against Mtb infection.

Targeting CCRL2 enhances therapeutic outcomes in a tuberculosis mouse model

Tuberculosis (TB) remains among the leading infectious causes of death. Due to the limited number of antimicrobials in the TB drug discovery pipeline, interest has developed in host-directed approaches to improve TB treatment outcomes. C-C motif chemokine-like receptor 2 (CCRL2) is a unique seven-transmembrane domain receptor that is upregulated by inflammatory signals and mediates leucocyte migration. However, little is known about its role in TB infection. Here, we show that Mycobacterium tuberculosis (Mtb) infection increases CCRL2 protein expression in macrophages in vitro and alveolar macrophages (AMs), dendritic cells (DCs) and neutrophils in mouse lungs. To target selectively CCRL2-expressing cells in vivo, we developed a novel mouse anti-CCRL2 antibody-drug conjugate (ADC) linked with the cytotoxic drug SG3249. We tested its adjunctive therapeutic efficacy against TB when combined with the first-line regimen for drug-susceptible TB (isoniazid, rifampin, pyrazinamide, ethambutol; RHZE). The anti-CCRL2 ADC treatment potentiated RHZE efficacy in Mtb-infected mice and decreased gross lung inflammation. CCRL2 expression in lung DCs and AMs was lower in mice receiving anti-CCRL2 ADC treatment+RHZE compared to those receiving RHZE alone or the control group, although the total innate cell populations did not differ across treatment groups. Interestingly, neutrophils were completely absent in the anti-CCRL2 ADC treatment + RHZE group, unlike in the other treatment groups. IFN-γ+-and IL17-α+-T-cell responses, which are associated with optimal TB control, were also elevated in the anti-CCRL2 ADC treatment + RHZE group. Our findings suggest that CCRL2-targeting approaches may improve TB treatment outcomes, possibly through selective killing of Mtb-infected innate immune cells.

Sensing mycobacteria through unconventional pathways

Approximately one-quarter of the global population is estimated to be infected with Mycobacterium tuberculosis. New developments in vaccine design and therapeutics are urgently needed, particularly in the face of multidrug-resistant tuberculosis (TB). In this issue of the JCI, Sakai and colleagues used a multidisciplinary approach to determine that trehalose-6-monomycolate (TMM), a mycobacterial cell wall lipid, serves as a T cell antigen presented by CD1b. CD1b-TMM-specific T cells were characterized by conserved T cell receptor features and were present at elevated frequencies in individuals with active TB disease. These findings highlight the dual role of TMM in stimulating both innate and adaptive immunity and broaden our understanding of CD1-mediated lipid recognition by unconventional T cells.

BCG-derived acellular membrane vesicles elicit antimycobacterial immunity and innate immune memory

Tuberculosis (TB) is one of the leading causes of death due to infectious disease. The sole established vaccine against TB is the Mycobacterium bovis Bacillus Calmette-Guerin (BCG) vaccine. However, owing to the lack of durable immunity with the BCG vaccine and its risk of infection, safer vaccines that can also be used as boosters are needed. Here, we examined whether membrane vesicles (MVs) from BCG (BCG-MVs) isolated from BCG statically cultured in nutrient-restricted Sauton's medium (s-MVs) and from BCG planktonically cultured in nutrient-rich medium commonly used in the laboratory (p-MVs) could be used as novel TB vaccines. MVs are extracellular vesicles produced by various bacteria, including mycobacteria. Differences in the culture conditions affected the morphology, contents, immunostimulatory activity and immunogenicity of BCG-MVs. s-MVs presented greater immunostimulatory activity than p-MVs via the induction of TLR2 signaling. Mouse immunization experiments revealed that s-MVs, but not p-MVs, induced mycobacterial humoral and mucosal immunity, especially when administered in combination with adjuvants. In a BCG challenge experiment using BCG Tokyo type I carrying pMV361-Km, subcutaneous vaccination with s-MVs reduced the bacterial burden in the mouse lung to a level similar to that after intradermal vaccination with live BCG. Furthermore, the administration of s-MVs induced a significant lipopolysaccharide-induced proinflammatory response in macrophages in vitro. These results indicate that BCG-MVs obtained from static culture in Sauton's medium induce not only humoral immunity against mycobacteria but also trained immunity, which can allow the clearance of infectious agents other than mycobacteria. Together, these findings highlight the immunological properties of BCG-MVs and the availability of acellular TB vaccines that confer broad protection against various infectious diseases.

Exploring BCG to deliver avidin fusion antigens from Schistosoma mansoni

BACKGROUND

Bacillus Calmette-Guérin (BCG) is one of the most successful vaccines in the world and evidence suggests it can be used as a bacterial vector to deliver heterologous antigens.

OBJECTIVES

We evaluated whether BCG could be biotinylated and used as a carrier of Schistosoma mansoni antigen tetraspanin-2 (TSP-2) fused with rhizavidin, an avidin analog.

METHODS

BCG was grown and biotinylated. The recombinant protein Rzv:TSP-2 was produced and purified from Escherichia coli. The biotinylation and antigen coupling was analysed by flow cytometry, enzyme-linked immunosorbent assay (ELISA) and Western blot. Vaccine immunogenicity was tested in immunised mice by the assessment of lung and splenic T cells.

FINDINGS

BCG can be biotinylated, which in turn, can be coupled with Rzv:TSP-2. After a series of optimisations which involved molarity of the biotin, ratio of BCG:reagent and the concentration of Rzv:TSP-2 used, almost 50% of the bacteria were biotinylated and 35% coupled with antigen. Although a clear adjuvant effect of BCG was observed, evaluation of immune response in immunised mice demonstrated an overall low immunogenicity of the BCG-Rzv:TSP-2.

MAIN CONCLUSION

These results demonstrated the use of BCG as a carrier of avidin-tagged antigens. Further optimisations are needed in order to strengthen the stability of tagged proteins in order to produce antigen-specific immune responses.

An LNP-mRNA vaccine modulates innate cell trafficking and promotes polyfunctional Th1 CD4+ T cell responses to enhance BCG-induced protective immunity against Mycobacterium tuberculosis

BACKGROUND

Mycobacterium tuberculosis remains the largest infectious cause of mortality worldwide, even with over a century of widespread administration of the only licenced tuberculosis (TB) vaccine, Bacillus Calmette-Guérin (BCG). mRNA technology remains an underexplored approach for combating chronic bacterial infections such as TB.

METHODS

We have developed a lipid nanoparticle (LNP)-mRNA vaccine, termed mRNACV2, encoding for the M. tuberculosis CysVac2 fusion protein, which we have previously formulated as an adjuvanted subunit vaccine. This LNP-mRNA vaccine was administered intramuscularly to female C57BL/6 mice as a standalone vaccine or as booster to BCG to assess immunogenicity and efficacy of the construct.

FINDINGS

Vaccination with mRNACV2 induced high frequencies of polyfunctional, antigen-specific Th1 CD4+ T cells in the blood and lungs, which was associated with the rapid recruitment of both innate and adaptive immune cells to lymph nodes draining the site of immunisation. mRNACV2 vaccination also provided significant pulmonary protection in M. tuberculosis-infected mice, reducing bacterial load and inflammatory infiltration in the lungs. Importantly, mRNACV2 enhanced immune responses and long-term protection when used to boost BCG-primed mice.

INTERPRETATION

These findings of a protective LNP-mRNA vaccine for TB highlight the potential of the LNP-mRNA platform for TB control and support further research to facilitate translation to humans.

FUNDING

This work was supported by the NHMRC Centre of Research Excellence in Tuberculosis Control to JAT and WJB (APP1153493), and MRFF mRNA Clinical Trial Enabling Infrastructure grant to CWP and HAW (MRFCTI000006).

Tuberculosis: An Update for the Clinician

Tuberculosis (TB) remains a significant global health threat with high mortality and efforts to meet WHO End TB Strategy milestones are off-track. It has become clear that TB is not a dichotomous infection with latent and active forms but presents along a disease spectrum. Subclinical TB plays a larger role in transmission than previously thought. Aerosol studies have shown that undiagnosed TB patients, even with paucibacillary disease, can be highly infectious and significantly contribute to TB spread. Encouraging clinical results have been seen with the M72/AS01E vaccine. If preliminary results can be confirmed in ongoing larger trials, modelling shows the vaccine can positively impact the epidemic. TB preventive therapy (TPT), especially for high-risk groups like people living with HIV and household contacts of drug-resistant TB patients, has shown efficacy but implementation is resource intensive. Treatment options for infectious patients have grown rapidly. New shorter, all-oral treatment regimens represent a breakthrough, but progress is threatened by rising resistance to bedaquiline. Many new chemical entities are entering clinical trials and raise hopes for all-new regimens that could overcome rising resistance rates to conventional agents. More research is needed on the management of complex cases, such as central nervous system TB and severe HIV-associated TB. Post-TB lung disease (PTLD) is an under-recognised but growing concern, affecting millions of survivors with lasting respiratory impairment and increased mortality. Continued investment in development of TB vaccines and therapeutics, treatment shortening, and management of TB sequelae is critical to combat this ongoing public health challenge.

Vax-Innate: improving therapeutic cancer vaccines by modulating T cells and the tumour microenvironment

T cells have a critical role in mediating antitumour immunity. The success of immune checkpoint inhibitors (ICIs) for cancer treatment highlights how enhancing endogenous T cell responses can mediate tumour regression. However, mortality remains high for many cancers, especially in the metastatic setting. Based on advances in the genetic characterization of tumours and identification of tumour-specific antigens, individualized therapeutic cancer vaccines targeting mutated tumour antigens (neoantigens) are being developed to generate tumour-specific T cells for improved therapeutic responses. Early clinical trials using individualized neoantigen vaccines for patients with advanced disease had limited clinical efficacy despite demonstrated induction of T cell responses. Therefore, enhancing T cell activity by improving the magnitude, quality and breadth of T cell responses following vaccination is one current goal for improving outcome against metastatic tumours. Another major consideration is how T cells can be further optimized to function within the tumour microenvironment (TME). In this Perspective, we focus on neoantigen vaccines and propose a new approach, termed Vax-Innate, in which vaccination through intravenous delivery or in combination with tumour-targeting immune modulators may improve antitumour efficacy by simultaneously increasing the magnitude, quality and breadth of T cells while transforming the TME into a largely immunostimulatory environment for T cells.

Prevention of tuberculosis in cynomolgus macaques by an attenuated Mycobacterium tuberculosis vaccine candidate

The need for novel vaccination strategies to control tuberculosis (TB) is underscored by the limited and variable efficacy of the currently licensed vaccine, Bacille Calmette-Guerin (BCG). SigH is critical for Mycobacterium tuberculosis (Mtb) to mitigate oxidative stress, and in its absence Mtb is unable to scavenge host oxidative/nitrosative bursts. The MtbΔsigH (ΔsigH) isogenic mutant induces signatures of the innate immunity in macrophages and protects rhesus macaques from a lethal Mtb challenge. To understand the immune mechanisms of protection via mucosal vaccination with ΔsigH, we employed the resistant cynomolgus macaque model; and our results show that ΔsigH vaccination significantly protects against lethal Mtb challenge in this species. ΔsigH-vaccinated macaques are devoid of granulomas and instead generate inducible bronchus associated lymphoid structures, and robust antigen-specific CD4+ and CD8+ T cell responses, driven by a hyper-immune, trained immunity-like phenotype in host macrophages with enhanced antigen presentation. Correlates of protection in ΔsigH-vaccinated macaques include gene signatures of T cell activation, IFNG production, including IFN-responsive, activated T cells, concomitant with IFNG production, and suppression of IDO+ Type I IFN-responsive macrophage recruitment. Thus, ΔsigH is a promising lead candidate for further development as an antitubercular vaccine.

Internal cap-initiated translation for efficient protein production from circular mRNA

Circular mRNA faces challenges in enhancing its translation potential as an RNA therapeutic. Here we introduce two molecular designs that bolster circular mRNA translation through an internal cap-initiated mechanism. The first consists of a circular mRNA with a covalently attached N7-methylguanosine (m7G) cap through a branching structure (cap-circ mRNA). This modification allows circular mRNA to recruit translation machinery and produce proteins more efficiently than internal ribosome entry site (IRES)-containing circular mRNAs. Combining with an N1-methylpseudouridine (m1Ψ) modification, cap-circ mRNA exhibits a lower acute immunostimulatory effect, maintaining high translation in mice. The second design features the non-covalent attachment of an m7G cap to a circular mRNA through hybridization with an m7G cap-containing oligonucleotide, enhancing translation by more than 50-fold. This setup allows circular mRNAs to synthesize reporter proteins upon hybridizing with capped mRNAs or long non-coding RNAs and to undergo rolling circle-type translation. These advancements broaden the therapeutic applications of circular mRNAs by minimizing their molecular size, elevating translation efficiency and facilitating cell-type-selective translation.

Engineered Mycobacterium tuberculosis triple-kill-switch strain provides controlled tuberculosis infection in animal models

Human challenge experiments could accelerate tuberculosis vaccine development. This requires a safe Mycobacterium tuberculosis (Mtb) strain that can both replicate in the host and be reliably cleared. Here we genetically engineered Mtb strains encoding up to three kill switches: two mycobacteriophage lysin operons negatively regulated by tetracycline and a degron domain-NadE fusion, which induces ClpC1-dependent degradation of the essential enzyme NadE, negatively regulated by trimethoprim. The triple-kill-switch (TKS) strain showed similar growth kinetics and antibiotic susceptibilities to wild-type Mtb under permissive conditions but was rapidly killed in vitro without trimethoprim and doxycycline. It established infection in mice receiving antibiotics but was rapidly cleared upon cessation of treatment, and no relapse was observed in infected severe combined immunodeficiency mice or Rag-/- mice. The TKS strain had an escape mutation rate of less than 10-10 per genome per generation. These findings suggest that the TKS strain could be a safe, effective candidate for a human challenge model.

A BCG kill switch strain protects against Mycobacterium tuberculosis in mice and non-human primates with improved safety and immunogenicity

Improved vaccination strategies for tuberculosis are needed. Intravenous (i.v.) delivery of live attenuated Mycobacterium bovis BCG provides protection against Mycobacterium tuberculosis (Mtb) in macaques but poses safety challenges. Here we genetically engineered two strains, BCG-TetON-DL and BCG-TetOFF-DL, to either induce or inhibit expression of two phage lysin operons, respectively, upon tetracycline exposure. We show that lysin expression kills BCG in vitro, in infected macrophages, and following infection of immunocompetent (C57BL/6) and immunocompromised (SCID) mice. Modified BCG elicited similar immune responses and provided similar protection against Mtb challenge as wild-type BCG in mice. In macaques, cessation of tetracycline treatment reduced i.v.-administered BCG-TetOFF-DL numbers. Intravenous BCG-TetOFF-DL increased pulmonary CD4 T-cell responses compared with wild-type BCG-induced responses and provided robust protection against Mtb challenge. Sterilizing immunity occurred in 6 of 8 macaques compared with 2 of 8 wild-type BCG-immunized macaques. Thus, a 'kill-switch' BCG strain provides additional safety and robust protection against Mtb infection.

Novel fusion protein REA induces robust prime protection against tuberculosis in mice

While many novel candidates for tuberculosis vaccines are presently undergoing pre-clinical or clinical trials, none of them have been able to eliminate infection entirely. In this study, we engineered a potent chimeric protein vaccine candidate, Rv2299cD2D3-ESAT-6-Ag85B (REA), which induced Th1 and Th17 responses via dendritic cell maturation. REA-activated macrophages operated the killing mechanisms of Mycobacterium tuberculosis (MTB), such as phagosomal maturation and phagolysosome fusion, through the (PI3K)-p38 MAPK-Ca2+-NADPH oxidase pathway. Dendritic cells and macrophages activated by REA elicited synergistic anti-mycobacterial responses. Notably, REA-immunized mice suppressed MTB growth to undetectable levels at 16 weeks post-infection, which was supported by gross and pathologic findings and acid-fast staining of the lung tissues, and maintained antigen-specific multifunctional IFN-γ+IL-2+TNF-α CD4+ T and long-lasting T cells producing cytokines in the tissues. Our findings suggest that REA is an outstanding prime prophylactic vaccine candidate against tuberculosis.

State of the Art and Emerging Technologies in Vaccine Design for Respiratory Pathogens

In this review, we present the efforts made so far in developing effective solutions to prevent infections caused by seven major respiratory pathogens: influenza virus, respiratory syncytial virus (RSV), the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), Bordetella pertussis, Streptococcus pneumoniae (pneumococcus), Mycobacterium tuberculosis, and Pseudomonas aeruginosa. Advancements driven by the recent coronavirus disease 2019 (COVID-19) crisis have largely focused on viruses, but effective prophylactic solutions for bacterial pathogens are also needed, especially in light of the antimicrobial resistance (AMR) phenomenon. Here, we discuss various innovative key technologies that can help address this critical need, such as (a) the development of Lung-on-Chip ex vivo models to gain a better understanding of the pathogenesis process and the host-microbe interactions; (b) a more thorough investigation of the mechanisms behind mucosal immunity as the first line of defense against pathogens; (c) the identification of correlates of protection (CoPs) which, in conjunction with the Reverse Vaccinology 2.0 approach, can push a more rational and targeted design of vaccines. By focusing on these critical areas, we expect substantial progress in the development of new vaccines against respiratory bacterial pathogens, thereby enhancing global health protection in the framework of the increasingly concerning AMR emergence.

Early and delayed STAT1-dependent responses drive local trained immunity of macrophages in the spleen

Trained immunity (TI) is the process wherein innate immune cells gain functional memory upon exposure to specific ligands or pathogens, leading to augmented inflammatory responses and pathogen clearance upon secondary exposure. While the differentiation of hematopoietic stem cells (HSCs) and reprogramming of bone marrow (BM) progenitors are well-established mechanisms underpinning durable TI protection, remodeling of the cellular architecture within the tissue during TI remains underexplored. Here, we study the effects of peritoneal Bacillus Calmette-Guérin (BCG) administration to find TI-mediated protection in the spleen against a subsequent heterologous infection by the Gram-negative pathogen Salmonella Typhimurium (S.Tm). Utilizing single cell RNA-sequencing and flow cytometry, we discerned STAT1-regulated genes in TI-associated resident and recruited splenic myeloid populations. The temporal dynamics of TI were further elucidated, revealing both early and delayed myeloid subsets with time-dependent, cell-type-specific STAT1 signatures. Using lineage tracing, we find that tissue-resident red pulp macrophages (RPM), initially depleted by BCG exposure, are restored from both tissue-trained, self-renewing macrophages and from bone marrow-derived progenitors, fostering long lasting local defense. Early inhibition of STAT1 activation, using specific JAK-STAT inhibitors, reduces both RPM loss and recruitment of trained monocytes. Our study suggests a temporal window soon after BCG vaccination, in which STAT1-dependent activation of long-lived resident cells in the tissue mediates localized protection.

Effects of different vaccination regimes on the immunodiagnosis of tuberculosis in goats and evaluation of defined antigens

Tuberculosis (TB) in goats is a chronic infectious disease caused by Mycobacterium tuberculosis complex (MTBC) organisms that pose a great health and economic challenge for the caprine industry in some European and developing countries. It is also a zoonotic disease posing a risk for public health. The control programs of the disease are based on a test-and-slaughter strategy, and vaccination is not feasible with available vaccines due to its interferences with the current TB immunodiagnosis. There is still a need for the development of an effective TB vaccine and, concurrently, diagnostic methods that allow differentiation between infected and vaccinated animals (DIVA approach). In this study, we investigated the interferences caused by the tuberculin (PPD)-based TB diagnostic tests in goats immunized by different mucosal and parenteral vaccination strategies: three single-dose strategies based on intranasal administration of BCG and two heat-inactivated M. bovis (HIMB) vaccines, and two prime-boost strategies based on parenteral BCG or HIMB priming and intranasal HIMB boosting. In addition, the defined antigens ESAT-6, CPF10, and EspC were evaluated as alternative diagnostic reagents to PPDs. At week 14 after prime vaccination of the animals, skin tests, IFN-γ release assay, and antibody detection assays were performed. The two prime-boosted and the single-dose intranasal BCG groups displayed greater cell-mediated immune responses to PPDs than the two single-dose intranasal HIMB vaccines. However, the use of reagents based on the defined antigens eliminated or reduced the vaccine-induced diagnostic interferences in all groups. Based on these results, the use of defined antigens in the current immunodiagnostic tests appears to be suitable in a future goat TB vaccination scenario.

Advancing veterinary vaccines design through trained immunity insights

Trained immunity, characterized by long-term functional reprogramming of innate immune cells, offers promising new directions for veterinary vaccine development. This perspective examines how trained immunity can be integrated into veterinary vaccine design through metabolic reprogramming and epigenetic modifications. We analyze key molecular mechanisms, including the shift to aerobic glycolysis and sustained epigenetic changes, that enable enhanced immune responses. Strategic approaches for vaccine optimization are proposed, focusing on selecting effective trained immunity inducers, developing innovative adjuvant systems, and achieving synergistic enhancement of immune responses. While implementation challenges exist, including individual response variations and safety considerations, trained immunity-based vaccines show potential for providing broader protection against emerging pathogens. This approach could revolutionize veterinary vaccinology by offering enhanced efficacy and cross-protection against heterologous infections, particularly valuable for zoonotic disease control.

Strong immune responses and robust protection following a novel protein in adjuvant tuberculosis vaccine candidate

BCG remains the only licensed vaccine for tuberculosis (TB), but its efficacy wanes over time. Subunit vaccines, aim to improve BCG immunity and protection, by inducing responses to a few mycobacterial antigens delivered with a specific platform. Since the platform shapes the immune response induced, selecting the right platform has been challenging due to the lack of immune correlates of protection. Recently, the protein-adjuvated subunit vaccine. M72/AS01E, demonstrated 49.7% efficacy in preventing active TB in latently infected adults, indicating that protective immunity through subunit vaccines is possible. In this study we evaluated the immunogenicity and efficacy of the promising mycobacterial antigen PPE15, formulated with five adjuvants developed by the Vaccine Formulation Institute. While all adjuvants were immunogenic, PPE15 with LMQ protected vaccinated mice against an in vivo Mycobacterium tuberculosis challenge, both as a standalone vaccine and as a boost to BCG. Vaccinated mice had enriched lung parenchymal antigen-specific CD4 + CXCR3 + KLRG1- T cells previously associated with TB protection. Heterologous vaccination strategies were also explored by combining intranasal ChAdOx1.PPE15 viral vector, with intramuscular PPE15-LMQ resulting in improved protection compared to individual vaccines. These findings support the progression of this vaccine candidate to the next stages of development.

High-dose intravenous BCG vaccination induces enhanced immune signaling in the airways

Intradermal Bacillus Calmette-Guérin (BCG) is the most widely administered vaccine, but it does not sufficiently protect adults against pulmonary tuberculosis. Recent studies in nonhuman primates show that intravenous BCG administration offers superior protection against Mycobacterium tuberculosis (Mtb). We used single-cell analysis of bronchoalveolar lavage cells from rhesus macaques vaccinated via different routes and doses of BCG to identify alterations in the immune ecosystem in the airway following vaccination. Our findings reveal that high-dose intravenous BCG induces an influx of polyfunctional T cells and macrophages in the airways, with alveolar macrophages from high-dose intravenous BCG displaying a basal activation state in the absence of purified protein derivative stimulation, defined in part by interferon signaling. Enhanced intercellular immune signaling and stronger T helper 1-T helper 17 transcriptional responses were observed following purified protein derivative stimulation. These results suggest that high-dose intravenous BCG vaccination creates a specialized immune environment that primes airway cells for effective Mtb clearance.

BCG's role in strengthening immune responses: Implications for tuberculosis and comorbid diseases

The BCG vaccine represents a significant milestone in the prevention of tuberculosis (TB), particularly in children. Researchers have been developing recombinant BCG (rBCG) variants that can trigger lasting memory responses, thereby enhancing protection against TB in adults. The breakdown of immune surveillance is a key link between TB and other communicable and non-communicable diseases. Notably, TB is more prevalent among people with comorbidities such as HIV, diabetes, cancer, influenza, COVID-19, and autoimmune disorders. rBCG formulations have the potential to address both TB and HIV co-pandemics. TB increases the risk of lung cancer and immunosuppression caused by cancer can reactivate latent TB infections. Moreover, BCG's efficacy extends to bladder cancer treatment and blood glucose regulation in patients with diabetes and TB. Additionally, BCG provides cross-protection against unrelated pathogens, emphasizing the importance of BCG-induced trained immunity in COVID-19 and other respiratory diseases. Furthermore, BCG reduced the severity of pulmonary TB-induced influenza virus infections. Recent studies have proposed innovations in BCG delivery, revaccination, and attenuation techniques. Disease-centered research has highlighted the immunomodulatory effects of BCG on TB, HIV, cancer, diabetes, COVID-19, and autoimmune diseases. The complex relationship between TB and comorbidities requires a nuanced re-evaluation to understand the shared attributes regulated by BCG. This review assessed the interconnected relationships influenced by BCG administration in TB and related disorders, recommending the expanded use of rBCG in healthcare. Collaboration among vaccine research stakeholders is vital to enhance BCG's efficacy against global health challenges.

Humoral correlates of protection against Mycobacterium tuberculosis following intravenous BCG vaccination in rhesus macaques

Altering Bacille Calmette-Guérin (BCG) immunization from low-dose intradermal (i.d.) to high-dose intravenous (i.v.) vaccination provides a high level of protection against Mycobacterium tuberculosis (Mtb). In addition to strong T cell immunity, i.v. BCG drives robust humoral immune responses that track with bacterial control. However, given the near-complete protection afforded by high-dose i.v. BCG immunization, a precise correlate of protection was difficult to define. Here we leveraged plasma and bronchoalveolar lavage fluid (BAL) from a cohort of rhesus macaques that received decreasing doses of i.v. BCG and aimed to define correlates of immunity following Mtb challenge. We show an i.v. BCG dose-dependent induction of mycobacterial-specific humoral immune responses. Antibody responses at peak immunogenicity predicted bacterial control post-challenge. Multivariate analyses revealed antibody-mediated complement and natural killer (NK) cell-activating humoral networks as key signatures of protective immunity. This work extends our understanding of humoral biomarkers and potential mechanisms of i.v. BCG-mediated protection against Mtb.

Markov field network model of multi-modal data predicts effects of immune system perturbations on intravenous BCG vaccination in macaques

Analysis of multi-modal datasets can identify multi-scale interactions underlying biological systems but can be beset by spurious connections due to indirect impacts propagating through an unmapped biological network. For example, studies in macaques have shown that Bacillus Calmette-Guerin (BCG) vaccination by an intravenous route protects against tuberculosis, correlating with changes across various immune data modes. To eliminate spurious correlations and identify critical immune interactions in a public multi-modal dataset (systems serology, cytokines, and cytometry) of vaccinated macaques, we applied Markov fields (MFs), a data-driven approach that explains vaccine efficacy and immune correlations via multivariate network paths, without requiring large numbers of samples (i.e., macaques) relative to multivariate features. We find that integrating multiple data modes with MFs helps remove spurious connections. Finally, we used the MF to predict outcomes of perturbations at various immune nodes, including an experimentally validated B cell depletion that induced network-wide shifts without reducing vaccine protection.

Humoral correlate of vaccine-mediated protection from tuberculosis identified in humans and non-human primates

Development of an effective tuberculosis (TB) vaccine has been challenged by incomplete understanding of specific factors that provide protection against Mycobacterium tuberculosis (Mtb) and the lack of a known correlate of protection (CoP). Using a combination of samples from a vaccine showing efficacy (DarDar [NCT00052195]) and Bacille Calmette-Guerin (BCG)-immunized humans and nonhuman primates (NHP), we identify a humoral CoP that translates across species and vaccine regimens. Antibodies specific to the DarDar vaccine strain (M. obuense) sonicate (MOS) correlate with protection from the efficacy endpoint of definite TB. In humans, antibodies to MOS also scale with vaccine dose, are elicited by BCG vaccination, are observed during TB disease, and demonstrate cross-reactivity with Mtb; in NHP, MOS-specific antibodies scale with dose and serve as a CoP mediated by BCG vaccination. Collectively, this study reports a novel humoral CoP and specific antigenic targets that may be relevant to achieving vaccine-mediated protection from TB.

Understanding the development of tuberculous granulomas: insights into host protection and pathogenesis, a review in humans and animals

Granulomas, organized aggregates of immune cells which form in response to Mycobacterium tuberculosis (Mtb), are characteristic but not exclusive of tuberculosis (TB). Despite existing investigations on TB granulomas, the determinants that differentiate host-protective granulomas from granulomas that contribute to TB pathogenesis are often disputed. Thus, the goal of this narrative review is to help clarify the existing literature on such determinants. We adopt the a priori view that TB granulomas are host-protective organelles and discuss the molecular and cellular determinants that induce protective granulomas and those that promote their failure. While reports about protective TB granulomas and their failure may initially seem contradictory, it is increasingly recognized that either deficiencies or excesses of the molecular and cellular components in TB granuloma formation may be detrimental to the host. More specifically, insufficient or excessive expression/representation of the following components have been reported to skew granulomas toward the less protective phenotype: (i) epithelioid macrophages; (ii) type 1 adaptive immune response; (iii) type 2 adaptive immune response; (iv) tumor necrosis factor; (v) interleukin-12; (vi) interleukin-17; (vii) matrix metalloproteinases; (viii) hypoxia in the TB granulomas; (ix) hypoxia inducible factor-1 alpha; (x) aerobic glycolysis; (xi) indoleamine 2,3-dioxygenase activity; (xii) heme oxygenase-1 activity; (xiii) immune checkpoint; (xiv) leukotriene A4 hydrolase activity; (xv) nuclear-factor-kappa B; and (xvi) transforming growth factor-beta. Rather, more precise and timely coordinated immune responses appear essential for eradication or containment of Mtb infection. Since there are several animal models of infection with Mtb, other species within the Mtb complex, and the surrogate Mycobacterium marinum - whether natural (cattle, elephants) or experimental (zebrafish, mouse, guinea pig, rabbit, mini pig, goat, non-human primate) infections - we also compared the TB granulomatous response and other pathologic lung lesions in various animals infected with one of these mycobacteria with that of human pulmonary TB. Identifying components that dictate the formation of host-protective granulomas and the circumstances that result in their failure can enhance our understanding of the macrocosm of human TB and facilitate the development of novel remedies - whether they be direct therapeutics or indirect interventions - to efficiently eliminate Mtb infection and prevent its pathologic sequelae.

TB-vaccines: Current status & challenges

Tuberculosis continues to be among the leading causes of morbidity as well as mortality. It is appreciated that our aim of eliminating TB in the foreseeable future will not be realized until we have a new vaccine with significant efficacy among diverse populations and all age-groups. Although impressive strides have been made in more refined development of new TB vaccines based on learnings from past experiences, the substitute or a booster vaccine for the BCG vaccine is not available yet. This article puts in perspective the recent efforts in re-positioning BCG, development of newer vaccines based on novel approaches, the current TB vaccine pipeline, yet unmet challenges in vaccine development, exploring newer ideas in vaccine development and what the future holds.

Intravenous BCG vaccination in non-human primates induces superior serum antibody titers with enhanced avidity and opsonizing capacity compared to the intradermal route

A new and more effective tuberculosis (TB) vaccine is urgently needed, but development is hampered by the lack of validated immune correlates of protection. Bacillus Calmette Guérin (BCG) vaccination by the aerosol (AE) and intravenous (IV) routes has been shown to confer superior levels of protection from challenge with Mycobacterium tuberculosis (M.tb) in non-human primates (NHP) compared with standard intradermal (ID) administration. This finding offers a valuable opportunity to investigate which aspects of immunity are associated with improved control of M.tb and may represent biomarkers or correlates of protection. As TB vaccine research to date has focused largely on cellular immunity, we aimed to better characterize the poorly-understood serum antibody response to BCG administered by different routes of vaccination in NHP. We demonstrate superior M.tb-specific IgG, IgA, and IgM titers in serum following IV BCG vaccination compared to the ID or AE routes. We also observe improved capacity of IgG induced by IV BCG to opsonize the surface of mycobacteria, and report for the first time that M.tb-specific IgG from IV BCG vaccinated animals is of higher avidity compared with IgG from ID or AE BCG vaccinated animals. Notably, we identified a significant correlation between IgG avidity and measures of protection from aerosol M.tb challenge. Our findings highlight a potential role for antibodies as markers and/or mediators of the superior vaccine-induced protection IV BCG confers against TB and suggest that quality, as well as quantity, of antibodies should be considered when developing and evaluating TB vaccine candidates.

Advanced technologies for the development of infectious disease vaccines

Vaccines play a critical role in the prevention of life-threatening infectious disease. However, the development of effective vaccines against many immune-evading pathogens such as HIV has proven challenging, and existing vaccines against some diseases such as tuberculosis and malaria have limited efficacy. The historically slow rate of vaccine development and limited pan-variant immune responses also limit existing vaccine utility against rapidly emerging and mutating pathogens such as influenza and SARS-CoV-2. Additionally, reactogenic effects can contribute to vaccine hesitancy, further undermining the ability of vaccination campaigns to generate herd immunity. These limitations are fuelling the development of novel vaccine technologies to more effectively combat infectious diseases. Towards this end, advances in vaccine delivery systems, adjuvants, antigens and other technologies are paving the way for the next generation of vaccines. This Review focuses on recent advances in synthetic vaccine systems and their associated challenges, highlighting innovation in the field of nano- and nucleic acid-based vaccines.

MTBVAC induces superior antibody titers and IgG avidity compared to BCG vaccination in non-human primates

The only currently licensed vaccine against tuberculosis (TB), Bacille Calmette Guérin (BCG), is insufficient to control the epidemic. MTBVAC is a live attenuated strain of Mycobacterium tuberculosis (M.tb) and is one the most advanced TB vaccine candidates in the pipeline. It is more efficacious than BCG in preclinical models including non-human primates (NHPs), and has demonstrated safety and immunogenicity in human populations. To better understand the immune mechanisms underlying the superior efficacy conferred by MTBVAC, we characterized M.tb-specific antibody responses in NHPs vaccinated with either BCG or MTBVAC. MTBVAC vaccination induced higher titers of IgG, IgM and IgA, and higher avidity IgG compared with BCG vaccination. IgG avidity correlated with protection following M.tb challenge in the same animals, validating the association previously reported between this measure and protection in the context of intravenous BCG vaccination, suggesting that IgG avidity may represent a relevant marker or correlate of protection from TB.

Intranasal vaccination with engineered BCG expressing CCL2 induces a stronger immune barrier against Mycobacterium tuberculosis than BCG

Intradermal Mycobacterium bovis Bacillus Calmette-Guérin (BCG) vaccination is currently the only licensed strategy for preventing tuberculosis (TB). It provides limited protection against pulmonary TB. To enhance the efficacy of BCG, we developed a recombinant BCG expressing exogenous monocyte chemoattractant CC chemokine ligand 2 (CCL2) called rBCG-CCL2. Co-culturing macrophages with rBCG-CCL2 enhances their abilities in migration, phagocytosis, and effector molecule expression. In the mouse model, intranasal vaccination with rBCG-CCL2 induced greater immune cell infiltration and a more extensive innate immune response in lung compared to vaccination with parental BCG, as determined by multiparameter flow cytometry, transcriptomic analysis, and pathological assessments. Moreover, rBCG-CCL2 induced a high frequency of activated macrophages and antigen-specific T helper 1 (Th1) and Th17 T cells in lungs. The enhanced immune microenvironment responded more effectively to intravenous challenge with Mycobacterium tuberculosis (Mtb) H37Ra, leading to significant reductions in H37Ra burden and pathological damage to the lungs and spleen. Intranasal rBCG-CCL2-vaccinated mice rapidly initiated pro-inflammatory Th1 cytokine release and reduced pathological damage to the lungs and spleen during the early stage of H37Ra challenge. The finding that co-expression of CCL2 synergistically enhances the immune barrier induced by BCG provides a model for defining immune correlates and mechanisms of vaccine-elicited protection against TB.

Intravenous Bacillus Calmette-Guérin (BCG) Induces a More Potent Airway and Lung Immune Response than Intradermal BCG in Simian Immunodeficiency Virus-infected Macaques

Tuberculosis (TB), caused by Mycobacterium tuberculosis, is one of the leading causes of death due to an infectious agent. Coinfection with HIV exacerbates M. tuberculosis infection outcomes in people living with HIV. Bacillus Calmette-Guérin (BCG), the only approved TB vaccine, is effective in infants, but its efficacy in adolescents and adults is limited. In this study, we investigated the immune responses elicited by BCG administered via i.v. or intradermal (i.d.) routes in SIV-infected Mauritian cynomolgus macaques (MCM) without the confounding effects of M. tuberculosis challenge. We assessed the impact of vaccination on T cell responses in the airway, blood, and tissues (lung, thoracic lymph nodes, and spleen), as well as the expression of cytokines, cytotoxic effectors, and key transcription factors. Our results showed that i.v. BCG induces a robust and sustained immune response, including tissue-resident memory T cells in lungs, polyfunctional CD4+ and CD8αβ+ T cells expressing multiple cytokines, and CD8αβ+ T cells and NK cells expressing cytotoxic effectors in airways. We also detected higher levels of mycobacteria-specific IgG and IgM in the airways of i.v. BCG-vaccinated MCM. Although i.v. BCG vaccination resulted in an influx of tissue-resident memory T cells in lungs of MCM with controlled SIV replication, MCM with high plasma SIV RNA (>105 copies/ml) typically displayed reduced T cell responses, suggesting that uncontrolled SIV or HIV replication would have a detrimental effect on i.v. BCG-induced protection against M. tuberculosis.

Markov Field network model of multi-modal data predicts effects of immune system perturbations on intravenous BCG vaccination in macaques

Analysis of multi-modal datasets can identify multi-scale interactions underlying biological systems, but can be beset by spurious connections due to indirect impacts propagating through an unmapped biological network. For example, studies in macaques have shown that BCG vaccination by an intravenous route protects against tuberculosis, correlating with changes across various immune data modes. To eliminate spurious correlations and identify critical immune interactions in a public multi-modal dataset (systems serology, cytokines, cytometry) of vaccinated macaques, we applied Markov Fields (MF), a data-driven approach that explains vaccine efficacy and immune correlations via multivariate network paths, without requiring large numbers of samples (i.e. macaques) relative to multivariate features. Furthermore, we find that integrating multiple data modes with MFs helps to remove spurious connections. Finally, we used the MF to predict outcomes of perturbations at various immune nodes, including a B-cell depletion that induced network-wide shifts without reducing vaccine protection, which we validated experimentally.

Immune correlates of protection as a game changer in tuberculosis vaccine development

The absence of validated correlates of protection (CoPs) hampers the rational design and clinical development of new tuberculosis vaccines. In this review, we provide an overview of the potential CoPs in tuberculosis vaccine research. Major hindrances and potential opportunities are then discussed. Based on recent progress, it is reasonable to anticipate that success in the ongoing efforts to identify CoPs would be a game-changer in tuberculosis vaccine development.

The chosen few: Mycobacterium tuberculosis isolates for IMPAc-TB

The three programs that make up the Immune Mechanisms of Protection Against Mycobacterium tuberculosis Centers (IMPAc-TB) had to prioritize and select strains to be leveraged for this work. The CASCADE team based at Seattle Children's Research Institute are leveraging M.tb H37Rv, M.tb CDC1551, and M.tb SA161. The HI-IMPACT team based at Harvard T.H. Chan School of Public Health, Boston, have selected M.tb Erdman as well as a novel clinical isolate recently characterized during a longitudinal study in Peru. The PHOENIX team also based at Seattle Children's Research Institute have selected M.tb HN878 and M.tb Erdman as their isolates of choice. Here, we describe original source isolation, genomic references, key virulence characteristics, and relevant tools that make these isolates attractive for use. The global context for M.tb lineage 2 and 4 selection is reviewed including what is known about their relative abundance and acquisition of drug resistance. Host-pathogen interactions seem driven by genomic differences on each side, and these play an important role in pathogenesis and immunity. The few M.tb strains chosen for this work do not reflect the vast genomic diversity within this species. They do, however, provide specific virulence, pathology, and growth kinetics of interest to the consortium. The strains selected should not be considered as "representative" of the growing available array of M.tb isolates, but rather tools that are being used to address key outstanding questions in the field.