Assessing vaccine-mediated protection in an ultra-low dose Mycobacterium tuberculosis murine model

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.

Novel mRNA vaccines induce potent immunogenicity and afford protection against tuberculosis

Introduction

Mycobacterium tuberculosis (Mtb) is the causative agent of tuberculosis (TB), a disease with a severe global burden. The intractability of Mtb has prevented the identification of clear correlates of protection against TB and hindered the development of novel TB vaccines that are urgently required. Lipid nanoparticle (LNP)-formulated mRNA is a highly promising vaccine platform that has yet to be thoroughly applied to TB.

Methods

We selected five Mtb antigens (PPE15, ESAT6, EspC, EsxI, MetE) and evaluated their potential as LNP-formulated mRNA vaccines, both when each antigen was delivered individually, and when all five antigens were combined in a mix regimen (m-Mix).

Results

Each mRNA construct demonstrated unique cellular and humoral immunogenicity, and both m-Mix, as well as the single antigen EsxI, conferred significant protection in a murine Mtb challenge model. Whilst the potent immune responses of each mRNA were maintained when applied as a boost to BCG, there was no additional increase to the efficacy of BCG. Combination of m-Mix with a recombinant, replication-deficient chimpanzee adenovirus (ChAdOx1), in a heterologous prime-boost delivery (C-m-Mix), appeared to result in increased protection upon murine Mtb infection, than either regimen alone.

Discussion

This work warrants further investigation of LNP-formulated mRNA vaccines for TB, whilst indicating the potential of m-Mix and C-m-Mix to progress to further stages of vaccine development.

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.

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.

Heterogeneity in immune cell composition is associated with Mycobacterium tuberculosis replication at the granuloma level

The control of bacterial growth is key to the prevention and treatment of tuberculosis (TB). Granulomas represent independent foci of the host immune response that present heterogeneous capacity for control of bacterial growth. At the whole tissue level, B cells and CD4 or CD8 T cells have an established role in immune protection against TB. Immune cells interact within each granuloma response, but the impact of granuloma immune composition on bacterial replication remains unknown. Here we investigate the associations between immune cell composition, including B cell, CD4, and CD8 T cells, and the state of replicating Mycobacterium tuberculosis (Mtb) within the granuloma. A measure of ribosomal RNA synthesis, the RS ratio®, represents a proxy measure of Mtb replication at the whole tissue level. We adapted the RS ratio through use of in situ hybridization, to identify replicating and non-replicating Mtb within each designated granuloma. We applied a regression model to characterize the associations between immune cell populations and the state of Mtb replication within each respective granuloma. In the evaluation of nearly 200 granulomas, we identified heterogeneity in both immune cell composition and proportion of replicating bacteria. We found clear evidence of directional associations between immune cell composition and replicating Mtb. Controlling for vaccination status and endpoint post-infection, granulomas with lower CD4 or higher CD8 cell counts are associated with a higher percent of replicating Mtb. Conversely, changes in B cell proportions were associated with little change in Mtb replication. This study establishes heterogeneity across granulomas, demonstrating that certain immune cell types are differentially associated with control of Mtb replication. These data suggest that evaluation at the granuloma level may be imperative to identifying correlates of immune protection.

Host and pathogen genetic diversity shape vaccine-mediated protection to Mycobacterium tuberculosis

To investigate how host and pathogen diversity govern immunity against Mycobacterium tuberculosis (Mtb), we performed a large-scale screen of vaccine-mediated protection against aerosol Mtb infection using three inbred mouse strains [C57BL/6 (B6), C3HeB/FeJ (C3H), Balb/c x 129/SvJ (C129F1)] and three Mtb strains (H37Rv, CDC1551, SA161) representing two lineages and distinct virulence properties. We compared three protective modalities, all of which involve inoculation with live mycobacteria: Bacillus Calmette-Guérin (BCG), the only approved TB vaccine, delivered either subcutaneously or intravenously, and concomitant Mtb infection (CoMtb), a model of pre-existing immunity in which a low-level Mtb infection is established in the cervical lymph node following intradermal inoculation. We examined lung bacterial burdens at early (Day 28) and late (Day 98) time points after aerosol Mtb challenge and histopathology at Day 98. We observed substantial heterogeneity in the reduction of bacterial load afforded by these modalities at Day 28 across the combinations and noted a strong positive correlation between bacterial burden in unvaccinated mice and the degree of protection afforded by vaccination. Although we observed variation in the degree of reduction in bacterial burdens across the nine mouse/bacterium strain combinations, virtually all protective modalities performed similarly for a given strain-strain combination. We also noted dramatic variation in histopathology changes driven by both host and bacterial genetic backgrounds. Vaccination improved pathology scores for all infections except CDC1551. However, the most dramatic impact of vaccination on lesion development occurred for the C3H-SA161 combination, where vaccination entirely abrogated the development of the large necrotic lesions that arise in unvaccinated mice. In conclusion, we find that substantial TB heterogeneity can be recapitulated by introducing variability in both host and bacterial genetics, resulting in changes in vaccine-mediated protection as measured both by bacterial burden as well as histopathology. These differences can be harnessed in future studies to identify immune correlates of vaccine efficacy.

Protective interplay: Mycobacterium tuberculosis diminishes SARS-CoV-2 severity through innate immune priming

At the beginning of the COVID-19 pandemic those with underlying chronic lung conditions, including tuberculosis (TB), were hypothesized to be at higher risk of severe COVID-19 disease. However, there is inconclusive clinical and preclinical data to confirm the specific risk SARS-CoV-2 poses for the millions of individuals infected with Mycobacterium tuberculosis (M.tb). We and others have found that compared to singly infected mice, mice co-infected with M.tb and SARS-CoV-2 leads to reduced SARS-CoV-2 severity compared to mice infected with SARS-CoV-2 alone. Consequently, there is a large interest in identifying the molecular mechanisms responsible for the reduced SARS-CoV-2 infection severity observed in M.tb and SARS-CoV-2 co-infection. To address this, we conducted a comprehensive characterization of a co-infection model and performed mechanistic in vitro modeling to dynamically assess how the innate immune response induced by M.tb restricts viral replication. Our study has successfully identified several cytokines that induce the upregulation of anti-viral genes in lung epithelial cells, thereby providing protection prior to challenge with SARS-CoV-2. In conclusion, our study offers a comprehensive understanding of the key pathways induced by an existing bacterial infection that effectively restricts SARS-CoV-2 activity and identifies candidate therapeutic targets for SARS-CoV-2 infection.

IL-27 alters inflammatory cytokine expression and limits protective immunity against Mycobacterium tuberculosis in a neonatal BCG vaccination model

Background

Efforts to control tuberculosis (TB), caused by the pathogen Mycobacterium tuberculosis (Mtb), have been hampered by the immense variability in protection from BCG vaccination. While BCG protects young children from some forms of TB disease, long-term protection against pulmonary disease is more limited, suggesting a poor memory response. New vaccines or vaccination strategies are required to have a realistic chance of eliminating TB disease. In TB endemic areas, routine immunization occurs during the neonatal period and as such, we hypothesized that inadequate protective immunity elicited by BCG vaccination could be the result of the unique early-life immune landscape. Interleukin (IL)-27 is a heterodimeric cytokine with immune suppressive activity that is elevated in the neonatal period.

Objective

We investigated the impact of IL-27 on regulation of immune responses during neonatal BCG vaccination and protection against Mtb.

Methods

Here, we used a novel model of neonatal vaccination and adult aerosol challenge that models the human timeline of vaccine delivery and disease transmission.

Results

Overall, we observed improved control of Mtb in mice unresponsive to IL-27 (IL-27Rα-/-) that was consistent with altered expression patterns of IFN-γ and IL-17 in the lungs. The balance of these cytokines with TNF-α expression may be key to effective bacterial clearance.

Conclusions

Our findings suggest the importance of evaluating new vaccines and approaches to combat TB in the neonatal population most likely to receive them as part of global vaccination campaigns. They further indicate that temporal strategies to antagonize IL-27 during early life vaccination may improve protection.