Colony morphotype governs innate and adaptive pulmonary immune responses to Mycobacterium abscessus infection in C3HeB/FeJ mice.

Mycobacterium abscessus is an emerging pathogen that causes chronic pulmonary infection. Treatment is challenging owing in part to our incomplete understanding of M. abscessus virulence mechanisms that enable pathogen persistence, such as the differing pathogenicity of M. abscessus smooth (S) and rough (R) colony morphotype. While R M. abscessus is associated with chronic infection and worse patient outcomes, it is unknown how immune responses to S and R M. abscessus differ in an acute pulmonary infection setting. In this study, immunological outcomes of M. abscessus infection with S and R morphotypes were examined in an immune-competent C3HeB/FeJ murine model. R M. abscessus infection was associated with the rapid production of inflammatory chemokines and recruitment of activated, MHC-II+ Ly6C+ macrophages to lungs and mediastinal LN (mLN). While both S and R M. abscessus increased T helper 1 (Th1) phenotype T cells in the lung, this was markedly delayed in mice infected with S M. abscessus. However, histopathological involvement and bacterial clearance were similar regardless of colony morphotype. These results demonstrate the importance of M. abscessus colony morphotype in shaping the development of pulmonary immune responses to M. abscessus, which further informs our understanding of M. abscessus host-pathogen interactions.

Lung IL-17A-Producing CD4+ T Cells Correlate with Protection after Intrapulmonary Vaccination with Differentially Adjuvanted Tuberculosis Vaccines.

Tuberculosis (TB), caused by Mycobacterium tuberculosis, results in approximately 1.6 million deaths annually. BCG is the only TB vaccine currently in use and offers only variable protection; however, the development of more effective vaccines is hindered by a lack of defined correlates of protection (CoP) against M. tuberculosis. Pulmonary vaccine delivery is a promising strategy since it may promote lung-resident immune memory that can respond rapidly to respiratory infection. In this study, CysVac2, a subunit protein previously shown to be protective against M. tuberculosis in mouse models, was combined with either Advax® adjuvant or a mixture of alum plus MPLA and administered intratracheally into mice. Peripheral immune responses were tracked longitudinally, and lung-local immune responses were measured after challenge. Both readouts were then correlated with protection after M. tuberculosis infection. Although considered essential for the control of mycobacteria, induction of IFN-γ-expressing CD4+ T cells in the blood or lungs did not correlate with protection. Instead, CD4+ T cells in the lungs expressing IL-17A correlated with reduced bacterial burden. This study identified pulmonary IL-17A-expressing CD4+ T cells as a CoP against M. tuberculosis and suggests that mucosal immune profiles should be explored for novel CoP.

Clinical and Experimental Determination of Protection Afforded by BCG Vaccination against Infection with Non-Tuberculous Mycobacteria: A Role in Cystic Fibrosis?

Mycobacterium abscessus is a nontuberculous mycobacterium (NTM) of particular concern in individuals with obstructive lung diseases such as cystic fibrosis (CF). Treatment requires multiple drugs and is characterised by high rates of relapse; thus, new strategies to limit infection are urgently required. This study sought to determine how Bacille Calmette-Guérin (BCG) vaccination may impact NTM infection, using a murine model of Mycobacterium abscessus infection and observational data from a non-BCG vaccinated CF cohort in Sydney, Australia and a BCG-vaccinated CF cohort in Cape Town, South Africa. In mice, BCG vaccination induced multifunctional antigen-specific CD4+ T cells circulating in the blood and was protective against dissemination of bacteria to the spleen. Prior infection with M. abscessus afforded the highest level of protection against M. abscessus challenge in the lung, and immunity was characterised by a greater frequency of pulmonary cytokine-secreting CD4+ T cells compared to BCG vaccination. In the clinical CF cohorts, the overall rates of NTM sampling during a three-year period were equivalent; however, rates of NTM colonisation were significantly lower in the BCG-vaccinated (Cape Town) cohort, which was most apparent for M. abscessus. This study provides evidence that routine BCG vaccination may reduce M. abscessus colonisation in individuals with CF, which correlates with the ability of BCG to induce multifunctional CD4+ T cells recognising M. abscessus in a murine model. Further research is needed to determine the optimal strategies for limiting NTM infections in individuals with CF.

A unique cytotoxic CD4+ T cell-signature defines critical COVID-19.

Objectives: SARS-CoV-2 infection causes a spectrum of clinical disease presentation, ranging from asymptomatic to fatal. While neutralising antibody (NAb) responses correlate with protection against symptomatic and severe infection, the contribution of the T-cell response to disease resolution or progression is still unclear. As newly emerging variants of concern have the capacity to partially escape NAb responses, defining the contribution of individual T-cell subsets to disease outcome is imperative to inform the development of next-generation COVID-19 vaccines. Methods: Immunophenotyping of T-cell responses in unvaccinated individuals was performed, representing the full spectrum of COVID-19 clinical presentation. Computational and manual analyses were used to identify T-cell populations associated with distinct disease states. Results: Critical SARS-CoV-2 infection was characterised by an increase in activated and cytotoxic CD4+ lymphocytes (CTL). These CD4+ CTLs were largely absent in asymptomatic to severe disease states. In contrast, non-critical COVID-19 was associated with high frequencies of naïve T cells and lack of activation marker expression. Conclusion: Highly activated and cytotoxic CD4+ T-cell responses may contribute to cell-mediated host tissue damage and progression of COVID-19. Induction of these potentially detrimental T-cell responses should be considered when developing and implementing effective COVID-19 control strategies.

L-selectin-dependent and -independent homing of naïve lymphocytes through the lung draining lymph node support T cell response to pulmonary Mycobacterium tuberculosis infection.

Recruiting large numbers of naïve lymphocytes to lymph nodes is critical for mounting an effective adaptive immune response. While most naïve lymphocytes utilize homing molecule L-selectin to enter lymph nodes, some circulating cells can traffic to the lung-draining mediastinal lymph node (mLN) through lymphatics via the intermediate organ, lung. However, whether this alternative trafficking mechanism operates in infection and contributes to T cell priming are unknown. We report that in pulmonary Mycobacterium tuberculosis-infected mice, homing of circulating lymphocytes to the mLN is significantly less efficient than to non-draining lymph node. CD62L blockade only partially reduced the homing of naïve T lymphocytes, consistent with L-selectin-independent routing of naïve lymphocytes to the site. We further demonstrated that lymphatic vessels in infected mLN expanded significantly and inhibiting lymphangiogenesis with a vascular endothelial growth factor receptor 3 kinase inhibitor reduced the recruitment of intravenously injected naïve lymphocytes to the mLN. Finally, mycobacterium-specific T cells entering via the L-selectin-independent route were readily activated in the mLN. Our study suggests that both L-selectin-dependent and -independent pathways contribute to naïve lymphocyte entry into mLN during M. tuberculosis infection and the latter pathway may represent an important mechanism for orchestrating host defence in the lungs.

Neutrophil Conversion to a Tumor-Killing Phenotype Underpins Effective Microbial Therapy.

The inflammatory microenvironment of solid tumors creates a protumorigenic milieu that resembles chronic inflammation akin to a subverted wound healing response. Here, we investigated the effect of converting the tumor microenvironment from a chronically inflamed state to one of acute microbial inflammation by injecting microbial bioparticles directly into tumors. Intratumoral microbial bioparticle injection led to rapid and dramatic changes in the tumor immune composition, the most striking of which was a substantial increase in the presence of activated neutrophils. In situ photoconversion and intravital microscopy indicated that tumor neutrophils transiently switched from sessile producers of VEGF to highly motile neutrophils that clustered to make neutrophil-rich domains in the tumor. The neutrophil clusters remodeled tumor tissue and repressed tumor growth. Single-cell transcriptional analysis of microbe-stimulated neutrophils showed a profound shift in gene expression towards heightened activation and antimicrobial effector function. Microbe-activated neutrophils also upregulated chemokines known to regulate neutrophil and CD8+ T-cell recruitment. Microbial therapy also boosted CD8+ T-cell function and enhanced the therapeutic benefit of checkpoint inhibitor therapy in tumor-bearing mice and provided protection in a model of tumor recurrence. These data indicate that one of the major effector mechanisms of microbial therapy is the conversion of tumor neutrophils from a wound healing to an acutely activated cytotoxic phenotype, highlighting a rationale for broader deployment of microbial therapy in the treatment of solid cancers. SIGNIFICANCE: Intratumoral injection of microbial bioparticles stimulates neutrophil antitumor functions, suggesting pathways for optimizing efficacy of microbial therapies and paving the way for their broader utilization in the clinic.

Mucosal TLR2-activating protein-based vaccination induces potent pulmonary immunity and protection against SARS-CoV-2 in mice.

Current vaccines against SARS-CoV-2 substantially reduce mortality, but protection against infection is less effective. Enhancing immunity in the respiratory tract, via mucosal vaccination, may provide protection against infection and minimise viral spread. Here, we report testing of a subunit vaccine in mice, consisting of SARS-CoV-2 Spike protein with a TLR2-stimulating adjuvant (Pam2Cys), delivered to mice parenterally or mucosally. Both routes of vaccination induce substantial neutralising antibody (nAb) titres, however, mucosal vaccination uniquely generates anti-Spike IgA, increases nAb in the serum and airways, and increases lung CD4+ T-cell responses. TLR2 is expressed by respiratory epithelia and immune cells. Using TLR2 deficient chimeric mice, we determine that TLR2 expression in either compartment facilitates early innate responses to mucosal vaccination. By contrast, TLR2 on hematopoietic cells is essential for optimal lung-localised, antigen-specific responses. In K18-hACE2 mice, vaccination provides complete protection against disease and sterilising lung immunity against SARS-CoV-2, with a short-term non-specific protective effect from mucosal Pam2Cys alone. These data support mucosal vaccination as a strategy to improve protection in the respiratory tract against SARS-CoV-2 and other respiratory viruses.

Stromal structure remodeling by B lymphocytes limits T cell activation in lymph nodes of Mycobacterium tuberculosis-infected mice.

An effective adaptive immune response depends on the organized architecture of secondary lymphoid organs, including the lymph nodes (LNs). While the cellular composition and microanatomy of LNs under steady state are well defined, the impact of chronic tissue inflammation on the structure and function of draining LNs is incompletely understood. Here we showed that Mycobacterium tuberculosis infection remodeled LN architecture by increasing the number and paracortical translocation of B cells. The formation of paracortical B lymphocyte and CD35+ follicular dendritic cell clusters dispersed CCL21-producing fibroblastic reticular cells and segregated pathogen-containing myeloid cells from antigen-specific CD4+ T cells. Depletion of B cells restored the chemokine and lymphoid structure and reduced bacterial burdens in LNs of the chronically infected mice. Importantly, this remodeling process impaired activation of naive CD4+ T cells in response to mycobacterial and unrelated antigens during chronic tuberculosis infection. Our studies reveal a mechanism in the regulation of LN microanatomy during inflammation and identify B cells as a critical element limiting the T cell response to persistent intracellular infection in LNs.

Mucosal immunization with a delta-inulin adjuvanted recombinant spike vaccine elicits lung-resident immune memory and protects mice against SARS-CoV-2.

Multiple SARS-CoV-2 vaccine candidates have been approved for use and have had a major impact on the COVID-19 pandemic. There remains, however, a significant need for vaccines that are safe, easily transportable and protective against infection, as well as disease. Mucosal vaccination is favored for its ability to induce immune memory at the site of infection, making it appealing for SARS-CoV-2 vaccine strategies. In this study we performed in-depth analysis of the immune responses in mice to a subunit recombinant spike protein vaccine formulated with the delta-inulin adjuvant Advax when administered intratracheally (IT), versus intramuscular delivery (IM). Both routes produced robust neutralizing antibody titers (NAb) and generated sterilizing immunity against SARS-CoV-2. IT delivery, however, produced significantly higher systemic and lung-local NAb that resisted waning up to six months post vaccination, and only IT delivery generated inducible bronchus-associated lymphoid tissue (iBALT), a site of lymphocyte antigen presentation and proliferation. This was coupled with robust and long-lasting lung tissue-resident memory CD4+ and CD8+ T cells that were not observed in IM-vaccinated mice. This study provides a detailed view of the lung-resident cellular response to IT vaccination against SARS-CoV-2 and demonstrates the importance of delivery site selection in the development of vaccine candidates.

Calpains Released from Necrotic Tumor Cells Enhance Antigen Cross-Presentation to Activate CD8+ T Cells In Vitro.

The initiation of CD8+ T cell responses against dead cell-associated Ags is tightly regulated, facilitating adaptive immunity against pathogens and tumors while preventing autoimmunity. It is now well established that dying cells actively regulate the generation of CD8+ T cell responses via the release or exposure of damage-associated molecular patterns. However, it is unclear whether nonproteasomal proteases (activated in stressed and dying cells) can influence the availability of Ags for cross-presentation. Using a mouse model of immunogenic necrosis, we investigated the role of tumor-derived proteases in the priming of CD8+ T cells. We demonstrate that proteases released from necrotic tumor cells can degrade whole-protein Ag, generating proteolytic intermediates that are efficiently cross-presented by dendritic cells and enhance CD8+ T cell cross-priming. We identify a dominant role for calpain proteases, which are activated during necrotic cell death induced by severe heat shock. Mechanistically, proteolytic intermediates generated by tumor-derived proteases associate with necrotic tumor cell debris, which acts as a vehicle for Ag transfer that facilitates highly efficient cross-presentation in dendritic cells. Our results suggest that proteolytic systems activated in Ag donor cells during cell death may influence the availability of antigenic substrates for cross-presentation, thereby regulating the antigenicity of cell death.

Characterization of the Protective Immune Responses Conferred by Recombinant BCG Overexpressing Components of Mycobacterium tuberculosis Sec Protein Export System.

Mycobacterium bovis Bacillus Calmette-Guérin (BCG) is the only approved vaccine against tuberculosis (TB). However, its efficacy in preventing pulmonary TB in adults is limited. Despite its variable efficacy, BCG offers a number of unique and beneficial characteristics, which make it suitable as a vaccine vehicle to express recombinant molecules. In Mycobacterium tuberculosis, the general Sec pathway is an essential cellular process, and it is responsible for exporting the majority of proteins across the cytoplasmic membrane, including potent immune-protective antigens, such as members of the antigen 85 (Ag85) complex. We engineered BCG to overexpress the M. tuberculosis SecDFG proteins in order to improve the efficiency of the Sec-dependent export system and, thus, enhance the secretion of immunogenic proteins. BCGSecDFG displayed increased intracellular survival within macrophages in vitro and greater persistence in the lymphoid organs of vaccinated mice than parental BCG. In addition, vaccination with BCGSecDFG generated higher numbers of IFN-γ-secreting T cells in response to secreted mycobacterial antigens compared to BCG, particularly members of the Ag85 complex. Furthermore, vaccination with BCGSecDFG significantly reduced the bacterial load in the lungs and spleens of M. tuberculosis-infected mice, which was comparable to the protection afforded by parental BCG. Therefore, the modification of protein secretion in BCG can improve antigen-specific immunogenicity.

Virulence Mechanisms of Mycobacterium abscessus: Current Knowledge and Implications for Vaccine Design.

Mycobacterium abscessus is a member of the non-tuberculous mycobacteria (NTM) group, responsible for chronic infections in individuals with cystic fibrosis (CF) or those otherwise immunocompromised. While viewed traditionally as an opportunistic pathogen, increasing research into M. abscessus in recent years has highlighted its continued evolution into a true pathogen. This is demonstrated through an extensive collection of virulence factors (VFs) possessed by this organism which facilitate survival within the host, particularly in the harsh environment of the CF lung. These include VFs resembling those of other Mycobacteria, and non-mycobacterial VFs, both of which make a notable contribution in shaping M. abscessus interaction with the host. Mycobacterium abscessus continued acquisition of VFs is cause for concern and highlights the need for novel vaccination strategies to combat this pathogen. An effective M. abscessus vaccine must be suitably designed for target populations (i.e., individuals with CF) and incorporate current knowledge on immune correlates of protection against M. abscessus infection. Vaccination strategies must also build upon lessons learned from ongoing efforts to develop novel vaccines for other pathogens, particularly Mycobacterium tuberculosis (M. tb); decades of research into M. tb has provided insight into unconventional and innovative vaccine approaches that may be applied to M. abscessus. Continued research into M. abscessus pathogenesis will be critical for the future development of safe and effective vaccines and therapeutics to reduce global incidence of this emerging pathogen.

High-Titer Neutralizing Antibodies against the SARS-CoV-2 Delta Variant Induced by Alhydroxyquim-II-Adjuvanted Trimeric Spike Antigens.

Global control of COVID-19 will require the deployment of vaccines capable of inducing long-term protective immunity against SARS-CoV-2 variants. In this report, we describe an adjuvanted subunit candidate vaccine that affords elevated, sustained, and cross-variant SARS-CoV-2 neutralizing antibodies (NAbs) in multiple animal models. Alhydroxiquim-II is a Toll-Like Receptor (TLR) 7/8 small-molecule agonist chemisorbed on aluminum hydroxide (Alhydrogel). Vaccination with Alhydroxiquim-II combined with a stabilized, trimeric form of the SARS-CoV-2 spike protein (termed CoVac-II) resulted in high-titer NAbs in mice, with no decay in responses over an 8-month period. NAbs from sera of CoVac-II-immunized mice, horses and rabbits were broadly neutralizing against SARS-CoV-2 variants. Boosting long-term CoVac-II-immunized mice with adjuvanted spike protein from the Beta variant markedly increased levels of NAb titers against multiple SARS-CoV-2 variants; notably, high titers against the Delta variant were observed. These data strongly support the clinical assessment of Alhydroxiquim-II-adjuvanted spike proteins to protect against SARS-CoV-2 variants of concern. IMPORTANCE There is an urgent need for next-generation COVID-19 vaccines that are safe, demonstrate high protective efficacy against SARS-CoV-2 variants and can be manufactured at scale. We describe a vaccine candidate (CoVac-II) that is based on stabilized, trimeric spike antigen produced in an optimized, scalable and chemically defined production process. CoVac-II demonstrates strong and persistent immunity after vaccination of mice, and is highly immunogenic in multiple animal models, including rabbits and horses. We further show that prior immunity can be boosted using a recombinant spike antigen from the Beta variant; importantly, plasma from boosted mice effectively neutralize multiple SARS-CoV-2 variants in vitro, including Delta. The strong humoral and Th1-biased immunogenicity of CoVac-II is driven by use of Alhydroxiquim-II (AHQ-II), the first adjuvant in an authorized vaccine that acts through the dual Toll-like receptor (TLR)7 and TLR8 pathways, as part of the Covaxin vaccine. Our data suggest AHQ-II/spike protein combinations could constitute safe, affordable, and mass-manufacturable COVID-19 vaccines for global distribution.

A single dose, BCG-adjuvanted COVID-19 vaccine provides sterilising immunity against SARS-CoV-2 infection.

Global control of COVID-19 requires broadly accessible vaccines that are effective against SARS-CoV-2 variants. In this report, we exploit the immunostimulatory properties of bacille Calmette-Guérin (BCG), the existing tuberculosis vaccine, to deliver a vaccination regimen with potent SARS-CoV-2-specific protective immunity. Combination of BCG with a stabilised, trimeric form of SARS-CoV-2 spike antigen promoted rapid development of virus-specific IgG antibodies in the blood of vaccinated mice, that was further augmented by the addition of alum. This vaccine formulation, BCG:CoVac, induced high-titre SARS-CoV-2 neutralising antibodies (NAbs) and Th1-biased cytokine release by vaccine-specific T cells, which correlated with the early emergence of T follicular helper cells in local lymph nodes and heightened levels of antigen-specific plasma B cells after vaccination. Vaccination of K18-hACE2 mice with a single dose of BCG:CoVac almost completely abrogated disease after SARS-CoV-2 challenge, with minimal inflammation and no detectable virus in the lungs of infected animals. Boosting BCG:CoVac-primed mice with a heterologous vaccine further increased SARS-CoV-2-specific antibody responses, which effectively neutralised B.1.1.7 and B.1.351 SARS-CoV-2 variants of concern. These findings demonstrate the potential for BCG-based vaccination to protect against major SARS-CoV-2 variants circulating globally.

TCR Affinity Controls the Dynamics but Not the Functional Specification of the Antimycobacterial CD4+ T Cell Response.

The quality of T cell responses depends on the lymphocytes' ability to undergo clonal expansion, acquire effector functions, and traffic to the site of infection. Although TCR signal strength is thought to dominantly shape the T cell response, by using TCR transgenic CD4+ T cells with different peptide:MHC binding affinity, we reveal that TCR affinity does not control Th1 effector function acquisition or the functional output of individual effectors following mycobacterial infection in mice. Rather, TCR affinity calibrates the rate of cell division to synchronize the distinct processes of T cell proliferation, differentiation, and trafficking. By timing cell division-dependent IL-12R expression, TCR affinity controls when T cells become receptive to Th1-imprinting IL-12 signals, determining the emergence and magnitude of the Th1 effector pool. These findings reveal a distinct yet cooperative role for IL-12 and TCR binding affinity in Th1 differentiation and suggest that the temporal activation of clones with different TCR affinity is a major strategy to coordinate immune surveillance against persistent pathogens.

Advax adjuvant formulations promote protective immunity against aerosol Mycobacterium tuberculosis in the absence of deleterious inflammation and reactogenicity.

The development of safe and effective adjuvants is a critical goal of vaccine development programs. In this report, we defined the immunostimulatory profile and protective effect against aerosol Mycobacterium tuberculosis infection of vaccine formulations incorporating the semi-crystalline adjuvant δ-inulin (Advax). Advax formulated with CpG oligonucleotide and the QS-21 saponin (AdvaxCpQS) was the most effective combination, demonstrated by the capacity of CysVac2/AdvaxCpQS to significantly reduce the bacterial burden in the lungs of M. tuberculosis-infected mice. CysVac2/AdvaxCpQS protection was associated with rapid influx of neutrophils, macrophages and monocytes to the site of vaccination and the induction of antigen-specific IFN-γ+/IL-2+/TNF+ polyfunctional CD4+ T cells in the lung. When compared to the highly potent adjuvant combination of monophosphoryl lipid A and dimethyldioctadecylammonium bromide (MPL/DDA), AdvaxCpQS imparted a similar level of protective efficacy yet without the profound stimulation of inflammatory cytokines and vaccination site ulceration observed with MPL/DDA. Addition of DDA to CysVac2/AdvaxCpQS further improved the protective effect of the vaccine, which correlated with increased polyfunctional CD4+ T cells in the lung but with no increase in vaccine reactogenicity. The data demonstrate that Advax formulations can decouple protective tuberculosis immunity from reactogenicity, making them ideal candidates for human application.

Intrapulmonary vaccination with delta-inulin adjuvant stimulates non-polarised chemotactic signalling and diverse cellular interaction.

There is an urgent need for novel vaccination strategies to combat respiratory pathogens. Mucosal vaccine delivery is an attractive option as it directly targets the site of infection; however, preclinical development has been hindered by a lack of suitable mucosal adjuvants and a limited understanding of their immune effects in the lung environment. Herein, we define the early immune events following the intrapulmonary delivery of a vaccine incorporating the adjuvant delta-inulin. Analysis of the early inflammatory response showed vaccine-induced innate cell recruitment to lungs and local lymph nodes (LN) was transient and non-polarised, correlating with an increase in pulmonary chemotactic factors. Use of fluorescently labelled adjuvant revealed widespread tissue dissemination of adjuvant particles, coupled with broad cellular uptake and transit to the lung-draining LN by a range of innate immune cells. Mass cytometric analysis revealed extensive phenotypic changes in innate and adaptive cell subsets induced by vaccination; this included identification of unconventional lymphocytes such as γδ-T cells and MAIT cells that increased following vaccination and displayed an activated phenotype. This study details a comprehensive view of the immune response to intrapulmonary adjuvant administration and provides pre-clinical evidence to support delta-inulin as a suitable adjuvant for pulmonary vaccines.

Mucosal delivery of a multistage subunit vaccine promotes development of lung-resident memory T cells and affords interleukin-17-dependent protection against pulmonary tuberculosis.

The development of effective vaccines against bacterial lung infections requires the induction of protective, pathogen-specific immune responses without deleterious inflammation within the pulmonary environment. Here, we made use of a polysaccharide-adjuvanted vaccine approach to elicit resident pulmonary T cells to protect against aerosol Mycobacterium tuberculosis infection. Intratracheal administration of the multistage fusion protein CysVac2 and the delta-inulin adjuvant Advax™ (formulated with a TLR9 agonist) provided superior protection against aerosol M. tuberculosis infection in mice, compared to parenteral delivery. Surprisingly, removal of the TLR9 agonist did not impact vaccine protection despite a reduction in cytokine-secreting T cell subsets, particularly CD4+IFN-γ+IL-2+TNF+ multifunctional T cells. CysVac2/Advax-mediated protection was associated with the induction of lung-resident, antigen-specific memory CD4+ T cells that expressed IL-17 and RORγT, the master transcriptional regulator of Th17 differentiation. IL-17 was identified as a key mediator of vaccine efficacy, with blocking of IL-17 during M. tuberculosis challenge reducing phagocyte influx, suppressing priming of pathogen-specific CD4+ T cells in local lymph nodes and ablating vaccine-induced protection. These findings suggest that tuberculosis vaccines such as CysVac2/Advax that are capable of eliciting Th17 lung-resident memory T cells are promising candidates for progression to human trials.

The generation of T-cell memory to protect against tuberculosis.

Tuberculosis (TB) kills more individuals each year than any other single pathogen and a more effective vaccine is critical for the global control of the disease. Although there has been recent progress in the clinical testing of candidates, no new vaccine has been licensed for use and correlates of protective immunity in humans have not been defined. Prior Mycobacterium tuberculosis infection does not appear to confer long-term protective immunity in humans; thus mimicking the natural immune response to infection may not be a suitable approach to develop improved TB vaccines. Data from animal testing are used to progress vaccines through the "vaccine pipeline", but studies in animals have not been able to predict efficacy in humans. Furthermore, although the generation of conventional CD4+ T-cell responses are considered necessary to control infection with M. tuberculosis, these do not necessarily correlate with protection induced by candidate vaccines and other immune components may play a role, including donor unrestricted T cells, tissue-resident memory T cells and anti-M. tuberculosis antibodies. This review will summarize the current understanding of the protective immune responses following M. tuberculosis infection or vaccination, with a particular focus on vaccines that have recently entered clinical trials.

Deciphering protective immunity against tuberculosis: implications for vaccine development.

INTRODUCTION: The development of more effective tuberculosis (TB) vaccines is essential for the global control of TB. Recently, there have been major advances in the field, but an important hindrance remains the lack of correlates of protection against TB. This requires each vaccine candidate to undergo clinical efficacy trials based on data from animal protection studies, but the results from animal models do not necessarily predict efficacy in humans. AREAS COVERED: In this review we summarize our current knowledge of immune mechanisms that may contribute to protective immunity against TB following vaccination and relate these to protective efficacy in animal models and recent clinical trials. Although some initial trials did not reproduce protection against TB in humans, recent trials have demonstrated promising efficacy for three vaccine approaches. EXPERT OPINION: Although CD4+ T lymphocytes are essential for protection against TB, there is no clear correlation between conventional CD4+ or CD8+ T cell responses and protective efficacy of TB vaccines. Recent attention has focused on other immune responses, including donor unrestricted T cells, B lymphocytes, and antibodies. Prospective studies on samples from vaccinated individuals protected in recent trials will allow evaluation of these alternative immune mechanisms as potential correlates of protection.