Regulation of neutrophils by interferon-γ limits lung inflammation during tuberculosis infection

Noncanonical T cell responses are associated with protection from tuberculosis in mice and humans

While control of Mycobacterium tuberculosis (Mtb) infection is generally understood to require Th1 cells and IFNγ, infection produces a spectrum of immunological and pathological phenotypes in diverse human populations. By characterizing Mtb infection in mouse strains that model the genetic heterogeneity of an outbred population, we identified strains that control Mtb comparably to a standard IFNγ-dependent mouse model but with substantially lower lung IFNγ levels. We report that these mice have a significantly altered CD4 T cell profile that specifically lacks the terminal effector Th1 subset and that this phenotype is detectable before infection. These mice still require T cells to control bacterial burden but are less dependent on IFNγ signaling. Instead, noncanonical immune features such as Th17-like CD4 and γδT cells correlate with low bacterial burden. We find the same Th17 transcriptional programs are associated with resistance to Mtb infection in humans, implicating specific non-Th1 T cell responses as a common feature of Mtb control across species.

Using host and bacterial genetic approaches to define virulence strategies and protective immunity during Mycobacterium tuberculosis infection

Infections with Mycobacterium tuberculosis (Mtb) resulted in over one million deaths in 2024, the highest number for any infectious disease. With no vaccines that protect against pulmonary tuberculosis (TB) and the challenges associated with antibiotic therapy, there is a critical need to better understand host-Mtb interactions to help curb this major public health problem. Mtb is arguably the most successful human pathogen, and it survives in diverse environments, resulting in heterogeneous disease outcomes in patients. Five years ago, in my commentary in mSphere, I discussed how Mtb virulence strategies that sense, adapt, and evade killing in the host can be uncovered using genetic approaches. Here, I will come full circle to highlight genetic approaches that recently uncovered new mechanisms regulating protective host responses and Mtb survival tactics. The goal is to highlight a genetic framework to probe a range of unexplored Mtb phenotypes, increase our understanding of host-Mtb interactions, and identify new therapeutic targets that may help prevent TB.

Where lung cancer and tuberculosis intersect: recent advances

Lung cancer (LC) and tuberculosis (TB) represent two major global public health issues. Prior evidence has suggested a link between TB infection and an increased risk of LC. As advancements in LC treatment have led to extended survival rates for LC patients, the co-occurrence of TB and LC has grown more prevalent and poses novel clinical challenges. The intricate molecular mechanisms connecting TB and LC are closely intertwined and many issues remain to be addressed. This review focuses on resemblance between the immunosuppression in tumor and granuloma microenvironments, exploring immunometabolism, cell plasticity, inflammatory signaling pathways, microbiomics, and up-to-date information derived from spatial multi-omics between TB and LC. Furthermore, we outline immunization-related molecular mechanisms underlying these two diseases and propose future research directions. By discussing recent advances and potential targets, this review aims to establish a foundation for developing future therapeutic strategies targeting LC with concurrent TB infection.

Tuberculosis as an unconventional interferonopathy

Tuberculosis is caused by Mycobacterium tuberculosis, a bacterium that accounts for more human mortality than any other. Evidence is accumulating for the view that tuberculosis is an interferonopathy - a disease driven by type I interferons. However, how type I interferons exacerbate tuberculosis remains poorly understood. As an infection, tuberculosis is distinct from conventional interferonopathies, which are autoinflammatory diseases. Here I consider the hypothesis that type I interferons promote bacterial replication by impairing key antibacterial immune responses, including those orchestrated by interleukin-1 and interferon γ. Paradoxically, during tuberculosis, the underlying state of impaired antibacterial immunity co-exists with overt (but ineffective) inflammation. Conceiving of tuberculosis as an unconventional interferonopathy may suggest fruitful avenues for therapeutic intervention.

Type I IFN-mediated NET release promotes Mycobacterium tuberculosis replication and is associated with granuloma caseation

Neutrophils are the most abundant cell type in the airways of tuberculosis patients. Mycobacterium tuberculosis (Mtb) infection induces the release of neutrophil extracellular traps (NETs); however, the molecular regulation and impact of NET release on Mtb pathogenesis are unknown. We find that during Mtb infection in neutrophils, PAD4 citrullinates histones to decondense chromatin that gets released as NETs in a manner that can maintain neutrophil viability and promote Mtb replication. Type I interferon promotes the formation of chromatin-containing vesicles that allow NET release without compromising plasma membrane integrity. Analysis of nonhuman primate granulomas supports a model where neutrophils are exposed to type I interferon from macrophages as they migrate into the granuloma, thereby enabling the release of NETs associated with necrosis and caseation. Our data reveal NET release as a promising target to inhibit Mtb pathogenesis.

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.

Metabolically active neutrophils represent a permissive niche for Mycobacterium tuberculosis

Mycobacterium tuberculosis (Mtb)-infected neutrophils are often found in the airways of patients with active tuberculosis (TB), and excessive recruitment of neutrophils to the lung is linked to increased bacterial burden and aggravated pathology in TB. The basis for the permissiveness of neutrophils for Mtb and the ability to be pathogenic in TB has been elusive. Here, we identified metabolic and functional features of neutrophils that contribute to their permissiveness in Mtb infection. Using single-cell metabolic and transcriptional analyses, we found that neutrophils in the Mtb-infected lung displayed elevated mitochondrial metabolism, which was largely attributed to the induction of activated neutrophils with enhanced metabolic activities. The activated neutrophil subpopulation was also identified in the lung granulomas from Mtb-infected non-human primates. Functionally, activated neutrophils harbored more viable bacteria and displayed enhanced lipid uptake and accumulation. Surprisingly, we found that interferon-γ promoted the activation of lung neutrophils during Mtb infection. Lastly, perturbation of lipid uptake pathways selectively compromised Mtb survival in activated neutrophils. These findings suggest that neutrophil heterogeneity and metabolic diversity are key to their permissiveness for Mtb and that metabolic pathways in neutrophils represent potential host-directed therapeutics in TB.

Type I IFN signaling in the absence of IRGM1 promotes M. tuberculosis replication in immune cells by suppressing T cell responses

Polymorphisms in the IRGM gene are associated with susceptibility to tuberculosis in humans. A murine ortholog of Irgm, Irgm1, is also essential for controlling Mycobacterium tuberculosis (Mtb) infection in mice. Multiple processes have been associated with IRGM1 activity that could impact the host response to Mtb infection, including roles in autophagy-mediated pathogen clearance and expansion of activated T cells. However, what IRGM1-mediated pathway is necessary to control Mtb infection in vivo and the mechanistic basis for this control remains unknown. We dissected the contribution of IRGM1 to immune control of Mtb pathogenesis in vivo and found that Irgm1 deletion leads to higher levels of IRGM3-dependent type I interferon signaling. The increased type I interferon signaling precludes T cell expansion during Mtb infection. The absence of Mtb-specific T cell expansion in Irgm1-/- mice results in uncontrolled Mtb infection in neutrophils and alveolar macrophages, which directly contributes to susceptibility to infection. Together, our studies reveal that IRGM1 is required to promote T cell-mediated control of Mtb infection in neutrophils, which is essential for the survival of Mtb-infected mice. These studies also uncover new ways type I interferon signaling can impact TH1 immune responses.

Fatty acid metabolism in neutrophils promotes lung damage and bacterial replication during tuberculosis

Mycobacterium tuberculosis (Mtb) infection induces a marked influx of neutrophils into the lungs, which intensifies the severity of tuberculosis (TB). The metabolic state of neutrophils significantly influences their functional response during inflammation and interaction with bacterial pathogens. However, the effect of Mtb infection on neutrophil metabolism and its consequent role in TB pathogenesis remain unclear. In this study, we examined the contribution of glycolysis and fatty acid metabolism on neutrophil responses to Mtb HN878 infection using ex-vivo assays and murine infection models. We discover that blocking glycolysis aggravates TB pathology, whereas inhibiting fatty acid oxidation (FAO) yields protective outcomes, including reduced weight loss, immunopathology, and bacterial burden in lung. Intriguingly, FAO inhibition preferentially disrupts the recruitment of a pathogen-permissive immature neutrophil population (Ly6Glo/dim), known to accumulate during TB. Targeting carnitine palmitoyl transferase 1a (Cpt1a)-a crucial enzyme in mitochondrial β-oxidation-either through chemical or genetic methods impairs neutrophils' ability to migrate to infection sites while also enhancing their antimicrobial function. Our findings illuminate the critical influence of neutrophil immunometabolism in TB pathogenesis, suggesting that manipulating fatty acid metabolism presents a novel avenue for host-directed TB therapies by modulating neutrophil functions.

Polygenic TB control and the sequence of innate/adaptive immune responses to infection: MHC-II alleles determine the size of the S100A8/9-producing neutrophil population

Among several quantitative trait loci involved in tuberculosis (TB) control in mice, one was mapped within the chromosome 17 segment occupied by the H2 complex and another within the chromosome 3 segment comprising the S100A8/9 genes, which encode neutrophil inflammatory factor S100A8/9. Previously, we developed a panel of H2-congenic mouse strains differing by small segments of the major histocompatibility complex Class II (MHC-II) region from TB-susceptible H2j mice transferred onto the genetic background of the TB-resistant C57BL/6 (H2b) strain. Susceptible B6.I-9.3 mice differ from B6 progenitors by the alleles of their only classical MHC-II H2-Aβ gene. The goals of the present study were to: (i) comprehensively characterise the differences in TB-related phenotypes between mice of the two strains and (ii) decipher interactions between the H2-Aβ and S100A8/9 genes. Here, we describe the dynamics of TB-related phenotypes differentiating B6.I-9.3 and B6 mice (colony forming units counts, histopathology, lung immune cell infiltration and cytokine profiles). We show that disproportionally diminished CD4+ T-cell population, an enlarged S100A8/9-positive neutrophil population and higher S100A8/9 serum levels in B6.I-9.3 mice collectively form the 'susceptible' phenotype before infection. An increase in IL-17 and a decrease in intrferon-gamma production by CD4+ T-cells in these mice provide a mechanistic explanation of this phenotype. Using F2 segregation analysis, we show that the number of S100A8/9-producing neutrophils in lungs and spleens and the proportion of Th17 CD4+ T-cells in lungs are significantly lower in the presence of the MHC-II dominant 'resistant' b allele compared to the recessive 'susceptible' j/j genotype. This provides direct genetic evidence that MHC-II-regulated CD4+ T-cell landscapes determine neutrophil abundance before infection, an important pathogenic factor in TB immunity.

CD4 + T cells recruit, then engage macrophages in cognate interactions to clear Mycobacterium tuberculosis from the lungs

IFN-γ-producing CD4 + T cells are required for protection against lethal Mycobacterium tuberculosis ( Mtb ) infections. However, the ability of CD4 + T cells to suppress Mtb growth cannot be fully explained by IFN-γ or other known T cell products. In this study, we show that CD4 + T cell-derived IFN-γ promoted the recruitment of monocyte-derived macrophages (MDMs) to the lungs of Mtb -infected mice. Although the recruited MDMs became quickly and preferentially infected with Mtb , CD4 + T cells rapidly disinfected the MDMs. Clearance of Mtb from MDMs was not explained by IFN-γ, but rather by MHCII-mediated cognate interactions with CD4 + T cells. These interactions promoted MDM expression of glycolysis genes essential for Mtb control. Thus, by recruiting MDMs, CD4 + T cells initiate a cycle of bacterial phagocytosis, Mtb antigen presentation and disinfection in an attempt to clear the bacteria from the lungs.

Tuberculosis susceptibility in genetically diverse mice reveals functional diversity of neutrophils

Tuberculosis (TB) is a heterogenous disease in humans with individuals exhibiting a wide range of susceptibility. This heterogeneity is not captured by standard laboratory mouse lines. We used a new collection of 19 wild-derived inbred mouse lines collected from diverse geographic sites to identify novel phenotypes during Mycobacterium tuberculosis ( Mtb ) infection. Wild derived mice have heterogenous immune responses to infection that result in differential ability to control disease at early timepoints. Correlation analysis with multiple parameters including sex, weight, and cellular immune responses in the lungs revealed that enhanced control of infection is associated with increased numbers of CD4 T cells, CD8 T cells and B cells. Surprisingly, we did not observe strong correlations between IFN-γ production and control of infection. Although in most lines high neutrophils were associated with susceptibility, we identified a mouse line that harbors high neutrophils numbers yet controls infection. Using single-cell RNA sequencing, we identified a novel neutrophil signature associated with failure to control infection.

Reappraising the Role of T Cell-Derived IFN-γ in Restriction of Mycobacterium tuberculosis in the Murine Lung

T cells producing IFN-γ have long been considered a stalwart for immune protection against Mycobacterium tuberculosis (Mtb), but their relative importance to pulmonary immunity has been challenged by murine studies that achieved protection by adoptively transferred Mtb-specific IFN-γ-/- T cells. Using IFN-γ-/- T cell chimeric mice and adoptive transfer of IFN-γ-/- T cells into TCRβ-/-δ-/- mice, we demonstrate that control of lung Mtb burden is in fact dependent on T cell-derived IFN-γ, and, furthermore, mice selectively deficient in T cell-derived IFN-γ develop exacerbated disease compared with T cell-deficient control animals, despite equivalent lung bacterial burdens. Deficiency in T cell-derived IFN-γ skews infected and bystander monocyte-derived macrophages to an alternative M2 phenotype and promotes neutrophil and eosinophil influx. Our studies support an important role for T cell-derived IFN-γ in pulmonary immunity against tuberculosis.

Role of Type I Interferons during Mycobacterium tuberculosis and HIV Infections

Tuberculosis and AIDS remain two of the most relevant human infectious diseases. The pathogens that cause them, Mycobacterium tuberculosis (Mtb) and HIV, individually elicit an immune response that treads the line between beneficial and detrimental to the host. Co-infection further complexifies this response since the different cytokines acting on one infection might facilitate the dissemination of the other. In these responses, the role of type I interferons is often associated with antiviral mechanisms, while for bacteria such as Mtb, their importance and clinical relevance as a suitable target for manipulation are more controversial. In this article, we review the recent knowledge on how these interferons play distinct roles and sometimes have opposite consequences depending on the stage of the pathogenesis. We highlight the dichotomy between the acute and chronic infections displayed by both infections and how type I interferons contribute to an initial control of each infection individually, while their chronic induction, particularly during HIV infection, might facilitate Mtb primo-infection and progression to disease. We expect that further findings and their systematization will allow the definition of windows of opportunity for interferon manipulation according to the stage of infection, contributing to pathogen clearance and control of immunopathology.

Heterogeneity in lung macrophage control of Mycobacterium tuberculosis is modulated by T cells

Following Mycobacterium tuberculosis infection, alveolar macrophages are initially infected but ineffectively restrict bacterial replication. The distribution of M. tuberculosis among different cell types in the lung changes with the onset of T cell immunity when the dominant infected cellular niche shifts from alveolar to monocyte-derived macrophages (MDM). We hypothesize that changes in bacterial distribution among different cell types is driven by differences in T cell recognition of infected cells and their subsequent activation of antimicrobial effector mechanisms. We show that CD4 and CD8 T cells efficiently eliminate M. tuberculosis infection in alveolar macrophages, but they have less impact on suppressing infection in MDM, which may be a bacterial niche. Importantly, CD4 T cell responses enhance MDM recruitment to the lung. Thus, the outcome of infection depends on the interaction between the T cell subset and the infected cell; both contribute to the resolution and persistence of the infection.

Dual neutrophil subsets exacerbate or suppress inflammation in tuberculosis via IL-1β or PD-L1

Neutrophils can be beneficial or deleterious during tuberculosis (TB). Based on the expression of MHC-II and programmed death ligand 1 (PD-L1), we distinguished two functionally and transcriptionally distinct neutrophil subsets in the lungs of mice infected with mycobacteria. Inflammatory [MHC-II-, PD-L1lo] neutrophils produced inflammasome-dependent IL-1β in the lungs in response to virulent mycobacteria and "accelerated" deleterious inflammation, which was highly exacerbated in IFN-γR-/- mice. Regulatory [MHC-II+, PD-L1hi] neutrophils "brake" inflammation by suppressing T-cell proliferation and IFN-γ production. Such beneficial regulation, which depends on PD-L1, is controlled by IFN-γR signaling in neutrophils. The hypervirulent HN878 strain from the Beijing genotype curbed PD-L1 expression by regulatory neutrophils, abolishing the braking function and driving deleterious hyperinflammation in the lungs. These findings add a layer of complexity to the roles played by neutrophils in TB and may explain the reactivation of this disease observed in cancer patients treated with anti-PD-L1.

CD4-mediated immunity shapes neutrophil-driven tuberculous pathology

Pulmonary Mycobacterium tuberculosis (Mtb) infection results in highly heterogeneous lesions ranging from granulomas with central necrosis to those primarily comprised of alveolitis. While alveolitis has been associated with prior immunity in human post-mortem studies, the drivers of these distinct pathologic outcomes are poorly understood. Here, we show that these divergent lesion structures can be modeled in C3HeB/FeJ mice and are regulated by prior immunity. Using quantitative imaging, scRNAseq, and flow cytometry, we demonstrate that Mtb infection in the absence of prior immunity elicits dysregulated neutrophil recruitment and necrotic granulomas. In contrast, prior immunity induces rapid recruitment and activation of T cells, local macrophage activation, and diminished late neutrophil responses. Depletion studies at distinct infection stages demonstrated that neutrophils are required for early necrosis initiation and necrosis propagation at chronic stages, whereas early CD4 T cell responses prevent neutrophil feedforward circuits and necrosis. Together, these studies reveal fundamental determinants of tuberculosis lesion structure and pathogenesis, which have important implications for new strategies to prevent or treat tuberculosis.

Re-appraising the role of T-cell derived interferon gamma in restriction of Mycobacterium tuberculosis in the murine lung: T-cell derived IFNγ is required to restrict pulmonary Mtb

T cells producing interferon gamma (IFNγ) have long been considered a stalwart for immune protection against Mycobacterium tuberculosis (Mtb), but their relative importance to pulmonary immunity has been challenged by murine studies which achieved protection by adoptively transferred Mtb-specific IFNγ-/- T cells. Using IFNγ-/- T cell chimeric mice and adoptive transfer of IFNγ-/- T cells into TCRβ-/-δ-/- mice, we demonstrate that control of lung Mtb burden is in fact dependent on T cell-derived IFNγ, and furthermore, mice selectively deficient in T cell-derived IFNγ develop exacerbated disease compared to T cell-deficient controls despite equivalent lung bacterial burdens. Deficiency in T cell-derived IFNγ skews infected and bystander monocyte-derived macrophages (MDMs) to an alternative M2 phenotype, and promotes neutrophil and eosinophil influx. Our studies support an important role for T cell-derived IFNγ in pulmonary immunity against TB.

VapC12 ribonuclease toxin modulates host immune response during Mycobacterium tuberculosis infection

Mechanistic understanding of antibiotic persistence is a prerequisite in controlling the emergence of MDR cases in Tuberculosis (TB). We have reported that the cholesterol-induced activation of VapC12 ribonuclease is critical for disease persistence in TB. In this study, we observed that relative to the wild type, mice infected with ΔvapC12 induced a pro-inflammatory response, had a higher pathogen load, and responded better to the anti-TB treatment. In a high-dose infection model, all the mice infected with ΔvapC12 succumbed early to the disease. Finally, we reported that the above phenotype of ΔvapC12 was dependent on the presence of the TLR4 receptor. Overall, the data suggests that failure of a timely resolution of the early inflammation by the ΔvapC12 infected mice led to hyperinflammation, altered T-cell response and high bacterial load. In conclusion, our findings suggest the role of the VapC12 toxin in modulating the innate immune response of the host in ways that favor the long-term survival of the pathogen inside the host.

Bacteria colonization in tumor microenvironment creates a favorable niche for immunogenic chemotherapy

The tumor microenvironment (TME) presents differential selective pressure (DSP) that favors the growth of cancer cells, and monovalent therapy is often inadequate in reversing the cancer cell dominance in the TME. In this work, we introduce bacteria as a foreign species to the TME and explore combinatorial treatment strategies to alter DSP for tumor eradication. We show that cancer-selective chemotherapeutic agents and fasting can provide a strong selection pressure against tumor growth in the presence of bacteria. Moreover, we show that an immunogenic drug (oxaliplatin), but not a non-immunogenic one (5-FU), synergizes with the bacteria to activate both the innate and adaptive immunity in the TME, resulting in complete tumor remission and a sustained anti-tumor immunological memory in mice. The combination of oxaliplatin and bacteria greatly enhances the co-stimulatory and antigen-presenting molecules on antigen-presenting cells, which in turn bridge the cytotoxic T cells for cancer-cell killing. Our findings indicate that rational combination of bacterial therapy and immunogenic chemotherapy can promote anticancer immunity against the immunosuppressive TME.

Recent advances in understanding the human host immune response in tuberculous meningitis

Tuberculous meningitis (TBM), the most severe form of tuberculosis, causes death in approximately 25% cases despite antibiotic therapy, and half of survivors are left with neurological disability. Mortality and morbidity are contributed to by a dysregulated immune response, and adjunctive host-directed therapies are required to modulate this response and improve outcomes. Developing such therapies relies on improved understanding of the host immune response to TBM. The historical challenges in TBM research of limited in vivo and in vitro models have been partially overcome by recent developments in proteomics, transcriptomics, and metabolomics, and the use of these technologies in nested substudies of large clinical trials. We review the current understanding of the human immune response in TBM. We begin with M. tuberculosis entry into the central nervous system (CNS), microglial infection and blood-brain and other CNS barrier dysfunction. We then outline the innate response, including the early cytokine response, role of canonical and non-canonical inflammasomes, eicosanoids and specialised pro-resolving mediators. Next, we review the adaptive response including T cells, microRNAs and B cells, followed by the role of the glutamate-GABA neurotransmitter cycle and the tryptophan pathway. We discuss host genetic immune factors, differences between adults and children, paradoxical reaction, and the impact of HIV-1 co-infection including immune reconstitution inflammatory syndrome. Promising immunomodulatory therapies, research gaps, ongoing challenges and future paths are discussed.

Early innate cell interactions with Mycobacterium tuberculosis in protection and pathology of tuberculosis

Tuberculosis (TB) remains a significant global health challenge, claiming the lives of up to 1.5 million individuals annually. TB is caused by the human pathogen Mycobacterium tuberculosis (Mtb), which primarily infects innate immune cells in the lungs. These immune cells play a critical role in the host defense against Mtb infection, influencing the inflammatory environment in the lungs, and facilitating the development of adaptive immunity. However, Mtb exploits and manipulates innate immune cells, using them as favorable niche for replication. Unfortunately, our understanding of the early interactions between Mtb and innate effector cells remains limited. This review underscores the interactions between Mtb and various innate immune cells, such as macrophages, dendritic cells, granulocytes, NK cells, innate lymphocytes-iNKT and ILCs. In addition, the contribution of alveolar epithelial cell and endothelial cells that constitutes the mucosal barrier in TB immunity will be discussed. Gaining insights into the early cellular basis of immune reactions to Mtb infection is crucial for our understanding of Mtb resistance and disease tolerance mechanisms. We argue that a better understanding of the early host-pathogen interactions could inform on future vaccination approaches and devise intervention strategies.

Key advances in vaccine development for tuberculosis-success and challenges

Breakthrough findings in the clinical and preclinical development of tuberculosis (TB) vaccines have galvanized the field and suggest, for the first time since the development of bacille Calmette-Guérin (BCG), that a novel and protective TB vaccine is on the horizon. Here we highlight the TB vaccines that are in the development pipeline and review the basis for optimism in both the clinical and preclinical space. We describe immune signatures that could act as immunological correlates of protection (CoP) to facilitate the development and comparison of vaccines. Finally, we discuss new animal models that are expected to more faithfully model the pathology and complex immune responses observed in human populations.

Type I IFN signaling in the absence of IRGM1 promotes M. tuberculosis replication in immune cells by suppressing T cell responses

Polymorphisms in the IRGM gene are associated with susceptibility to tuberculosis in humans. A murine ortholog of Irgm, Irgm1, is also essential for controlling Mycobacterium tuberculosis (Mtb) infection in mice. Multiple processes have been associated with IRGM1 activity that could impact the host response to Mtb infection, including roles in autophagy-mediated pathogen clearance and expansion of activated T cells. However, what IRGM1-mediated pathway is necessary to control Mtb infection in vivo and the mechanistic basis for this control remains unknown. We dissected the contribution of IRGM1 to immune control of Mtb pathogenesis in vivo and found that Irgm1 deletion leads to higher levels of IRGM3-dependent type I interferon signaling. The increased type I interferon signaling precludes T cell expansion during Mtb infection. The absence of Mtb-specific T cell expansion in Irgm1-/- mice results in uncontrolled Mtb infection in neutrophils and alveolar macrophages, which directly contributes to susceptibility to infection. Together, our studies reveal that IRGM1 is required to promote T cell-mediated control of Mtb infection in neutrophils, which is essential for the survival of Mtb-infected mice. These studies also uncover new ways type I interferon signaling can impact TH1 immune responses.

Granulocytes subsets and their divergent functions in host resistance to Mycobacterium tuberculosis - a 'tipping-point' model of disease exacerbation

Granulocytes are innate immune effector cells with essential functions in host resistance to bacterial infections. I will discuss emerging evidence that during Mycobacterium tuberculosis infection, counter-intuitively, eosinophils are host-protective while neutrophils are host detrimental. Additionally, I will propose a 'tipping-point' model in which neutrophils are an integral part of a feedforward loop driving tuberculosis disease exacerbation.

Atg8ylation as a host-protective mechanism against Mycobacterium tuberculosis

Nearly two decades have passed since the first report on autophagy acting as a cell-autonomous defense against Mycobacterium tuberculosis. This helped usher a new area of research within the field of host-pathogen interactions and led to the recognition of autophagy as an immunological mechanism. Interest grew in the fundamental mechanisms of antimicrobial autophagy and in the prophylactic and therapeutic potential for tuberculosis. However, puzzling in vivo data have begun to emerge in murine models of M. tuberculosis infection. The control of infection in mice affirmed the effects of certain autophagy genes, specifically ATG5, but not of other ATGs. Recent studies with a more complete inactivation of ATG genes now show that multiple ATG genes are indeed necessary for protection against M. tuberculosis. These particular ATG genes are involved in the process of membrane atg8ylation. Atg8ylation in mammalian cells is a broad response to membrane stress, damage and remodeling of which canonical autophagy is one of the multiple downstream outputs. The current developments clarify the controversies and open new avenues for both fundamental and translational studies.

Animal models for COVID-19 and tuberculosis

Respiratory infections cause tremendous morbidity and mortality worldwide. Amongst these diseases, tuberculosis (TB), a bacterial illness caused by Mycobacterium tuberculosis which often affects the lung, and coronavirus disease 2019 (COVID-19) caused by the Severe Acute Respiratory Syndrome Coronavirus type 2 (SARS-CoV-2), stand out as major drivers of epidemics of global concern. Despite their unrelated etiology and distinct pathology, these infections affect the same vital organ and share immunopathogenesis traits and an imperative demand to model the diseases at their various progression stages and localizations. Due to the clinical spectrum and heterogeneity of both diseases experimental infections were pursued in a variety of animal models. We summarize mammalian models employed in TB and COVID-19 experimental investigations, highlighting the diversity of rodent models and species peculiarities for each infection. We discuss the utility of non-human primates for translational research and emphasize on the benefits of non-conventional experimental models such as livestock. We epitomize advances facilitated by animal models with regard to understanding disease pathophysiology and immune responses. Finally, we highlight research areas necessitating optimized models and advocate that research of pulmonary infectious diseases could benefit from cross-fertilization between studies of apparently unrelated diseases, such as TB and COVID-19.

The H2-A Class II molecule α/β-chain cis-mismatch severely affects cell surface expression, selection of conventional CD4+ T cells and protection against TB infection

Introduction

To dissect the role of the part of the H2 complex comprised of the MHC-II genes in the control of tuberculosis (TB) infection, we previously established a panel of recombinant congenic mouse strains bearing different segments of the H2 ^j haplotype on the B6 (H2 ^b) genetic background. Fine genetic mapping, gene sequencing and assessment of TB phenotypes resulted in identification of the H2-Ab gene as a major factor of TB control.

Methods

We further narrowed the MHC-II H2 ^j interval by spotting a new recombination event, sequencing newly established DNA configuration and establishing a mouse strain B6.I-103 in which j/b recombination occurred within the coding sequence of the H2-Ab gene.

Results

Unexpectedly, a novel H2-Aα ^b/Aβ^jE⁰ haplotype provided exclusively high susceptibility to TB challenge. Immunologic analysis revealed an altered CD4⁺ T-cell selection and maintenance in B6.I-103 mice, as well as seriously impaired expression of the H2-Aα^b/Aβ^j molecule on the surface of antigen presenting cells. Unlike previously reported cases of Class II malfunctioning, the defective phenotype arose not from strong structural mutations, but from regular recombination events within the MHC-II recombination hot spot region.

Discussion

Our findings provide evidence that Class II α/β-chain cis-allelic mismatches created by regular genetic recombination may severely affect immune system functioning. This issue is discussed in the context of the MHC evolution.

Autophagy prevents early proinflammatory responses and neutrophil recruitment during Mycobacterium tuberculosis infection without affecting pathogen burden in macrophages

The immune response to Mycobacterium tuberculosis infection determines tuberculosis disease outcomes, yet we have an incomplete understanding of what immune factors contribute to a protective immune response. Neutrophilic inflammation has been associated with poor disease prognosis in humans and in animal models during M. tuberculosis infection and, therefore, must be tightly regulated. ATG5 is an essential autophagy protein that is required in innate immune cells to control neutrophil-dominated inflammation and promote survival during M. tuberculosis infection; however, the mechanistic basis for how ATG5 regulates neutrophil recruitment is unknown. To interrogate what innate immune cells require ATG5 to control neutrophil recruitment during M. tuberculosis infection, we used different mouse strains that conditionally delete Atg5 in specific cell types. We found that ATG5 is required in CD11c+ cells (lung macrophages and dendritic cells) to control the production of proinflammatory cytokines and chemokines during M. tuberculosis infection, which would otherwise promote neutrophil recruitment. This role for ATG5 is autophagy dependent, but independent of mitophagy, LC3-associated phagocytosis, and inflammasome activation, which are the most well-characterized ways that autophagy proteins regulate inflammation. In addition to the increased proinflammatory cytokine production from macrophages during M. tuberculosis infection, loss of ATG5 in innate immune cells also results in an early induction of TH17 responses. Despite prior published in vitro cell culture experiments supporting a role for autophagy in controlling M. tuberculosis replication in macrophages, the effects of autophagy on inflammatory responses occur without changes in M. tuberculosis burden in macrophages. These findings reveal new roles for autophagy proteins in lung resident macrophages and dendritic cells that are required to suppress inflammatory responses that are associated with poor control of M. tuberculosis infection.

Infection with hypervirulent Mycobacterium tuberculosis triggers emergency myelopoiesis but not trained immunity

Introduction

During infection, bone marrow (BM) hematopoiesis is reprogrammed toward myeloid cell production, a mechanism named emergency myelopoiesis. In addition to replenishing myeloid cells, emergency myelopoiesis has been linked to trained immunity, a process that allows enhanced innate immune responses to secondary challenges. Although hematopoietic alterations during tuberculosis (TB) have been described and Mycobacterium tuberculosis may colonize the BM, studies using the mouse model of infection and the laboratory reference strain M. tuberculosis H37Rv have demonstrated limited emergency myelopoiesis and trained immunity.

Methods

To further address this issue, we aerosol- infected C57BL/6 mice with high doses of the hypervirulent M. tuberculosis isolate HN878 and monitored alterations to the BM. This experimental model better resembles the human blood immune signature of TB.

Results and discussion

We found increased frequencies of lineage^-Sca-1⁺cKit⁺ (LSK) cells and the granulocyte/macrophage progenitor (GMP) population. At the mature cell level, we observed an increase of monocytes and neutrophils in the blood and lung, likely reflecting the increased BM myeloid output. Monocytes or monocyte-derived macrophages recovered from the BM of M. tuberculosis HN878-infected mice did not show signs of trained immunity, suggesting an uncoupling of emergency myelopoiesis and trained immunity in the BM. Surprisingly, M. tuberculosis HN878-induced emergency myelopoiesis was not fully dependent on IFNγ, as mice lacking this cytokine and infected under the same conditions as wild-type mice still presented BM alterations. These data expand our understanding of the immune response to M. tuberculosis and raise awareness of pathogen strain-imposed differences to host responses.

Dimethyl itaconate is effective in host-directed antimicrobial responses against mycobacterial infections through multifaceted innate immune pathways

BACKGROUND

Itaconate, a crucial immunometabolite, plays a critical role in linking immune and metabolic functions to influence host defense and inflammation. Due to its polar structure, the esterified cell-permeable derivatives of itaconate are being developed to provide therapeutic opportunities in infectious and inflammatory diseases. Yet, it remains largely uncharacterized whether itaconate derivatives have potentials in promoting host-directed therapeutics (HDT) against mycobacterial infections. Here, we report dimethyl itaconate (DMI) as the promising candidate for HDT against both Mycobacterium tuberculosis (Mtb) and nontuberculous mycobacteria by orchestrating multiple innate immune programs.

RESULTS

DMI per se has low bactericidal activity against Mtb, M. bovis Bacillus Calmette-Guérin (BCG), and M. avium (Mav). However, DMI robustly activated intracellular elimination of multiple mycobacterial strains (Mtb, BCG, Mav, and even to multidrug-resistant Mtb) in macrophages and in vivo. DMI significantly suppressed the production of interleukin-6 and -10, whereas it enhanced autophagy and phagosomal maturation, during Mtb infection. DMI-mediated autophagy partly contributed to antimicrobial host defenses in macrophages. Moreover, DMI significantly downregulated the activation of signal transducer and activator of transcription 3 signaling during infection with Mtb, BCG, and Mav.

CONCLUSION

Together, DMI has potent anti-mycobacterial activities in macrophages and in vivo through promoting multifaceted ways for innate host defenses. DMI may bring light to new candidate for HDT against Mtb and nontuberculous mycobacteria, both of which infections are often intractable with antibiotic resistance.

Rapid lethality of mice lacking the phagocyte oxidase and Caspase1/11 following Mycobacterium tuberculosis infection

Immune networks that control antimicrobial and inflammatory mechanisms have overlapping regulation and functions to ensure effective host responses. Genetic interaction studies of immune pathways that compare host responses in single and combined knockout backgrounds are a useful tool to identify new mechanisms of immune control during infection. For disease caused by pulmonary Mycobacterium tuberculosis infections, which currently lacks an effective vaccine, understanding genetic interactions between protective immune pathways may identify new therapeutic targets or disease-associated genes. Previous studies suggested a direct link between the activation of NLRP3-Caspase1 inflammasome and the NADPH-dependent phagocyte oxidase complex during Mtb infection. Loss of the phagocyte oxidase complex alone resulted in increased activation of Caspase1 and IL1β production during Mtb infection, resulting in failed disease tolerance during the chronic stages of disease. To better understand this interaction, we generated mice lacking both Cybb , a key subunit of the phagocyte oxidase, and Caspase1/11 . We found that ex vivo Mtb infection of Cybb ^-/- Caspase1/11 ^-/- macrophages resulted in the expected loss of IL1β secretion but an unexpected change in other inflammatory cytokines and bacterial control. Mtb infected Cybb ^-/- Caspase1/11 ^-/- mice rapidly progressed to severe TB, succumbing within four weeks to disease characterized by high bacterial burden, increased inflammatory cytokines, and the recruitment of granulocytes that associated with Mtb in the lungs. These results uncover a key genetic interaction between the phagocyte oxidase complex and Caspase1/11 that controls protection against TB and highlight the need for a better understanding of the regulation of fundamental immune networks during Mtb infection.

Advancing personalized medicine for tuberculosis through the application of immune profiling

While early and precise diagnosis is the key to eliminating tuberculosis (TB), conventional methods using culture conversion or sputum smear microscopy have failed to meet demand. This is especially true in high-epidemic developing countries and during pandemic-associated social restrictions. Suboptimal biomarkers have restricted the improvement of TB management and eradication strategies. Therefore, the research and development of new affordable and accessible methods are required. Following the emergence of many high-throughput quantification TB studies, immunomics has the advantages of directly targeting responsive immune molecules and significantly simplifying workloads. In particular, immune profiling has been demonstrated to be a versatile tool that potentially unlocks many options for application in TB management. Herein, we review the current approaches for TB control with regard to the potentials and limitations of immunomics. Multiple directions are also proposed to hopefully unleash immunomics' potential in TB research, not least in revealing representative immune biomarkers to correctly diagnose TB. The immune profiles of patients can be valuable covariates for model-informed precision dosing-based treatment monitoring, prediction of outcome, and the optimal dose prediction of anti-TB drugs.

Mycobacterium tuberculosis Evasion of Guanylate Binding Protein-Mediated Host Defense in Mice Requires the ESX1 Secretion System

Cell-intrinsic immune mechanisms control intracellular pathogens that infect eukaryotes. The intracellular pathogen Mycobacterium tuberculosis (Mtb) evolved to withstand cell-autonomous immunity to cause persistent infections and disease. A potent inducer of cell-autonomous immunity is the lymphocyte-derived cytokine IFNγ. While the production of IFNγ by T cells is essential to protect against Mtb, it is not capable of fully eradicating Mtb infection. This suggests that Mtb evades a subset of IFNγ-mediated antimicrobial responses, yet what mechanisms Mtb resists remains unclear. The IFNγ-inducible Guanylate binding proteins (GBPs) are key host defense proteins able to control infections with intracellular pathogens. GBPs were previously shown to directly restrict Mycobacterium bovis BCG yet their role during Mtb infection has remained unknown. Here, we examine the importance of a cluster of five GBPs on mouse chromosome 3 in controlling Mycobacterial infection. While M. bovis BCG is directly restricted by GBPs, we find that the GBPs on chromosome 3 do not contribute to the control of Mtb replication or the associated host response to infection. The differential effects of GBPs during Mtb versus M. bovis BCG infection is at least partially explained by the absence of the ESX1 secretion system from M. bovis BCG, since Mtb mutants lacking the ESX1 secretion system become similarly susceptible to GBP-mediated immune defense. Therefore, this specific genetic interaction between the murine host and Mycobacteria reveals a novel function for the ESX1 virulence system in the evasion of GBP-mediated immunity.

Natural killer cells in COVID-19: from infection, to vaccination and therapy

Natural killer (NK) cells are among the most important innate immunity members, which are the first cells that fight against infected cells. The function of these cells is impaired in patients with COVID-19 and they are not able to prevent the spread of the disease or destroy the infected cells. Few studies have evaluated the effects of COVID-19 vaccines on NK cells, though it has been demonstrated that DNA vaccines and BNT162b2 can affect NK cell response. In the present paper, the effects of SARS-CoV-2 on the NK cells during infection, the effect of vaccination on NK cells, and the NK cell-based therapies were reviewed.

Lung Resident Memory T Cells Activated by Oral Vaccination Afford Comprehensive Protection against Pneumonic Yersinia pestis Infection

A growing body of evidence has shown that resident memory T (TRM) cells formed in tissue after mucosal infection or vaccination are crucial for counteracting reinfection by pathogens. However, whether lung TRM cells activated by oral immunization with Yptb1(pYA5199) play a protective role against pneumonic plague remains unclear. In this study, we demonstrated that lung CD4+ and CD8+ TRM cells significantly accumulated in the lungs of orally Yptb1(pYA5199)-vaccinated mice and dramatically expanded with elevated IL-17A, IFN-γ, and/or TNF-α production after pulmonary Yersinia pestis infection and afforded significant protection. Short-term or long-term treatment of immunized mice with FTY720 did not affect lung TRM cell formation and expansion or protection against pneumonic plague. Moreover, the intratracheal transfer of both lung CD4+ and CD8+ TRM cells conferred comprehensive protection against pneumonic plague in naive recipient mice. Lung TRM cell-mediated protection was dramatically abolished by the neutralization of both IFN-γ and IL-17A. Our findings reveal that lung TRM cells can be activated via oral Yptb1(pYA5199) vaccination, and that IL-17A and IFN-γ production play an essential role in adaptive immunity against pulmonary Y. pestis infection. This study highlights an important new target for developing an effective pneumonic plague vaccine.

Helminth species-specific effects on IFN-γ producing T cells during active and latent tuberculosis

BACKGROUND

Interferon-γ (IFN-γ) is a key cytokine inducing protective immune responses during tuberculosis (TB) infection. Helminth-induced immune responses may affect IFN-γ production by T cells, although its connection with disease severity and immune recovery during treatment is unexplored. We investigated the species-specific effect of helminths on the IFN-γ production by T cells in relation to disease severity during active and latent TB infection (LTBI).

METHODS

In this study, 69 active pulmonary TB patients (PTB), 28 with LTBI and 66 healthy controls were included. Active TB was diagnosed using GenXpert MTB/RIF while QuantiFERON test (QFT) was used for the screening of healthy community controls (CCs) and for the diagnosis of LTBI. Helminth infection was identified by routine diagnosis whereas clinical disease severity was evaluated by the TB score. Intracellular IFN-γ production of T cells in stimulated peripheral blood mononuclear cells (PBMCs) was analyzed by flow cytometry using TB antigens (PPD), the polyclonal T cell activator staphylococcal enterotoxin B (SEB), or medium as unstimulated control.

RESULTS

Helminth infected CCs and LTBI subjects showed a significant reduction of IFN-γ+ CD4+ T cells by PPD-stimulation compared to non-helminth infected control groups. The significant reduction in the frequency of IFN-γ+ T cells in both latent and active PTB patients following SEB stimulation was mostly attributed to Schistosoma mansoni infection, whereas Ascaris lumbricoides, Schistosoma mansoni, and hookworm infection contributed equally in CCs. Following anti-helminthic and anti-TB treatment for 2 months, the frequency of IFN-γ+ CD4 T cells in helminth coinfected PTB was restored to levels of helminth negative PTB before treatment. Helminth coinfected PTB patients with an intermediate and severe clinical course had reduced capacity for production of IFN-γ+ T cells compared to the corresponding non-helminth infected PTB.

CONCLUSION

We found a reduction in IFN-γ producing T cells by helminth coinfection which was restored following anti-helminthic treatment. This reduction was helminth species-dependent in an exploratory sub-analysis and correlated to increased disease severity.

IFN- Is Protective in Cytokine Release Syndrome-associated Extrapulmonary Acute Lung Injury

The mechanisms by which excessive systemic activation of adaptive T lymphocytes, as in cytokine release syndrome (CRS), leads to innate immune cell-mediated acute lung injury (ALI) or acute respiratory distress syndrome, often in the absence of any infection, remains unknown. Here, we investigated the roles of IFN-γ and IL-17A, key T-cell cytokines significantly elevated in patients with CRS, in the immunopathogenesis of CRS-induced extrapulmonary ALI. CRS was induced in wild-type (WT), IL-17A- and IFN-γ knockout (KO) human leukocyte antigen-DR3 transgenic mice with 10 μg of the superantigen, staphylococcal enterotoxin B, given intraperitoneally. Several ALI parameters, including gene expression profiling in the lungs, were studied 4, 24, or 48 hours later. Systemic T-cell activation with staphylococcal enterotoxin B resulted in robust upregulation of several chemokines, S100A8/A9, matrix metalloproteases, and other molecules implicated in tissue damage, granulocyte as well as agranulocyte adhesion, and diapedesis in the lungs as early as 4 hours, which was accompanied by subsequent neutrophil/eosinophil lung infiltration and severe ALI in IFN-γ KO mice. These pathways were significantly underexpressed in IL-17A KO mice, which manifested mildest ALI and intermediate in WT mice. Neutralization of IFN-γ worsened ALI in WT and IL-17A KO mice, whereas neutralizing IL-17A did not mitigate lung injury in IFN-γ KO mice, suggesting a dominant protective role for IFN-γ in ALI and that IL-17A is dispensable. Ruxolitinib, a Janus kinase inhibitor, increased ALI severity in WT mice. Thus, our study identified novel mechanisms of ALI in CRS and its differential modulation by IFN-γ and IL-17A.

Inflammation-mediated tissue damage in pulmonary tuberculosis and host-directed therapeutic strategies

Treatment of tuberculosis (TB) involves the administration of anti-mycobacterial drugs for several months. The emergence of drug-resistant strains of Mycobacterium tuberculosis (Mtb, the causative agent) together with increased disease severity in people with co-morbidities such as diabetes mellitus and HIV have hampered efforts to reduce case fatality. In severe disease, TB pathology is largely attributable to over-exuberant host immune responses targeted at controlling bacterial replication. Non-resolving inflammation driven by host pro-inflammatory mediators in response to high bacterial load leads to pulmonary pathology including cavitation and fibrosis. The need to improve clinical outcomes and reduce treatment times has led to a two-pronged approach involving the development of novel antimicrobials as well as host-directed therapies (HDT) that favourably modulate immune responses to Mtb. HDT strategies incorporate aspects of immune modulation aimed at downregulating non-productive inflammatory responses and augmenting antimicrobial effector mechanisms to minimise pulmonary pathology and accelerate symptom resolution. HDT in combination with existing antimycobacterial agents offers a potentially promising strategy to improve the long-term outcome for TB patients. In this review, we describe components of the host immune response that contribute to inflammation and tissue damage in pulmonary TB, including cytokines, matrix metalloproteinases, lipid mediators, and neutrophil extracellular traps. We then proceed to review HDT directed at these pathways.

More severe lung lesions in smoker patients with active pulmonary tuberculosis were associated with peripheral NK cell subsets

BACKGROUND

Both pulmonary tuberculosis (PTB) and cigarette smoke (CS) exposure may lead to lung damage. The potential impact of CS exposure on tuberculosis-associated lung damage and the disturbance of immune cells and mediators involved, need to be further elucidated.

METHODS

We firstly evaluated the chest X-ray (CXR) scores of a retrospective cohort of male patients with active PTB, followed for 6 months, and compared the scores between smoker (≥10 pack-years) and non-smoker patients. In a cross-sectional study, we measured the peripheral blood NK cell subsets and plasma inflammatory cytokines in male smoker and non-smoker patients with active PTB before anti-tuberculosis therapy, and the proportions of NK cell subsets and the levels of cytokines were analyzed for correlation with the CXR scores.

RESULTS

In the retrospective cohort, male smoker patients with active PTB showed a higher CXR score, characterized by more cavitary lesions, enlarged lymph nodes and emphysema, as compared to non-smokers. The cross-sectional study revealed that the CXR score in smoker patients was correlated inversely with the percentages of blood CD107a⁺, NKP46⁺, and TIGIT⁺ NK cells.

CONCLUSION

In patients with active PTB, CS exposure was associated with more severe lung lesions, which were correlated with peripheral NK cell subsets.

Immune cell interactions in tuberculosis

Despite having been identified as the organism that causes tuberculosis in 1882, Mycobacterium tuberculosis has managed to still evade our understanding of the protective immune response against it, defying the development of an effective vaccine. Technology and novel experimental models have revealed much new knowledge, particularly with respect to the heterogeneity of the bacillus and the host response. This review focuses on certain immunological elements that have recently yielded exciting data and highlights the importance of taking a holistic approach to understanding the interaction of M. tuberculosis with the many host cells that contribute to the development of protective immunity.

Different modalities of host cell death and their impact on Mycobacterium tuberculosis infection

Mycobacterium tuberculosis (Mtb) is the pathogen that causes tuberculosis (TB), a leading infectious disease of humans worldwide. One of the main histopathological hallmarks of TB is the formation of granulomas comprised of elaborately organized aggregates of immune cells containing the pathogen. Dissemination of Mtb from infected cells in the granulomas due to host and mycobacterial factors induces multiple cell death modalities in infected cells. Based on molecular mechanism, morphological characteristics, and signal dependency, there are two main categories of cell death: programmed and nonprogrammed. Programmed cell death (PCD), such as apoptosis and autophagy, is associated with a protective response to Mtb by keeping the bacteria encased within dead macrophages that can be readily phagocytosed by arriving in uninfected or neighboring cells. In contrast, non-PCD necrotic cell death favors the pathogen, resulting in bacterial release into the extracellular environment. Multiple types of cell death in the PCD category, including pyroptosis, necroptosis, ferroptosis, ETosis, parthanatos, and PANoptosis, may be involved in Mtb infection. Since PCD pathways are essential for host immunity to Mtb, therapeutic compounds targeting cell death signaling pathways have been experimentally tested for TB treatment. This review summarizes different modalities of Mtb-mediated host cell deaths, the molecular mechanisms underpinning host cell death during Mtb infection, and its potential implications for host immunity. In addition, targeting host cell death pathways as potential therapeutic and preventive approaches against Mtb infection is also discussed.

Leishmania major Strain-Dependent Macrophage Activation Contributes to Pathogenicity in the Absence of Lymphocytes

Infection of C57BL/6 wild-type mice with Leishmania major 5-ASKH or Friedlin strains results in relatively similar pathogenicity with self-healing lesions within weeks. Parasite clearance depends on nitric oxide production by activated macrophages in response to cytokines produced mainly by CD4⁺ Th1 cells. In contrast, C57BL/6 Rag2 knockout mice, which lack T and B lymphocytes, show distinct pathologies during infection with these strains. Despite of the similar parasite number, the 5-ASKH infection induced severe inflammation rather than the Friedlin. To determine the immunological factors behind this phenomenon, we infected C57BL/6 Rag2 knockout mice with these two strains and compared immune cell kinetics and macrophage activation status. Compared with the Friedlin strain, the 5-ASKH strain elicited increased pathology associated with the accumulation of CD11b^high, Ly6G^high neutrophils by week four and increased the expression of macrophage activation markers. We then analyzed the differentially expressed transcripts in infected bone marrow-derived macrophages by RNA sequencing. It showed upregulation of multiple inflammatory transcripts, including Toll-like receptor 1/2 (TLR1/2), CD69, and CARD14, upon 5-ASKH infection. Our findings suggest that different L. major strains can trigger distinct macrophage activation, contributing to the disease outcome observed in the absence of lymphocytes but not in the presence of lymphocytes. IMPORTANCE Disease manifestations of cutaneous leishmaniasis (CL) range from self-healing cutaneous lesions to chronic forms of the disease, depending on the infecting Leishmania sp. and host immune protection. Previous works on mouse models of CL show the distinct pathogenicity of Leishmania major strains in the absence of lymphocytes. However, the mechanisms of this pathology remain uncovered. In the trial to understand the immunological process involved in lymphocyte-independent pathology, we have found a specific induction of macrophages by different L. major strains that affect their ability to mount innate responses leading to neutrophilic pathology when lymphocytes are ablated.

Evaluation of two different vaccine platforms for immunization against melioidosis and glanders

Burkholderia pseudomallei and the closely related species, Burkholderia mallei, produce similar multifaceted diseases which range from rapidly fatal to protracted and chronic, and are a major cause of mortality in endemic regions. Besides causing natural infections, both microbes are Tier 1 potential biothreat agents. Antibiotic treatment is prolonged with variable results, hence effective vaccines are urgently needed. The purpose of our studies was to compare candidate vaccines that target both melioidosis and glanders to identify the most efficacious one(s) and define residual requirements for their transition to the non-human primate aerosol model. Studies were conducted in the C57BL/6 mouse model to evaluate the humoral and cell-mediated immune response and protective efficacy of three Burkholderia vaccine candidates against lethal aerosol challenges with B. pseudomallei K96243, B. pseudomallei MSHR5855, and B. mallei FMH. The recombinant vaccines generated significant immune responses to the vaccine antigens, and the live attenuated vaccine generated a greater immune response to OPS and the whole bacterial cells. Regardless of the candidate vaccine evaluated, the protection of mice was associated with a dampened cytokine response within the lungs after exposure to aerosolized bacteria. Despite being delivered by two different platforms and generating distinct immune responses, two experimental vaccines, a capsule conjugate + Hcp1 subunit vaccine and the live B. pseudomallei 668 ΔilvI strain, provided significant protection and were down-selected for further investigation and advanced development.

MicroRNA-20b carried by mesenchymal stem cell-derived extracellular vesicles protects alveolar epithelial type II cells from Mycobacterium tuberculosis infection in vitro

Mesenchymal stem cells (MSCs) have been largely used for their immunomodulatory and regenerative properties in the treatment of immune-based disorders and bacterial infections. This study explores the function of MSC-derived extracellular vesicles (MSC-EVs) in alveolar epithelial type II cells (AECII) against Mycobacterium tuberculosis (MTB) infection. EVs were extracted from the acquired MSCs. AECII-like MLE-15 and A549 cells were treated with MSC-EVs and then subjected to MTB infection. MSC-EVs treatment significantly prevented the increase in bacterial load, and it prevented the production of proinflammatory cytokines in cells induced by MTB infection. MicroRNA-20b (miR-20b) was upregulated in cells after MSC-EVs treatment. Artificial inhibition of miR-20b blocked the protective effects of MSC-EVs against MTB infection. A Kyoto Encyclopedia of Genes and Genomes pathway enrichment analysis was performed to analyze the key molecules involved in the immune regulation in cells mediated by miR-20b. miR-20b directly targeted nuclear factor of activated T cells 5 (NFAT5) and inactivated the Toll-Like Receptor (TLR) signaling pathway by reducing the formation of TLR2-TLR4 dimer after MTB infection. In conclusion, this study suggests that MSC-EVs carry miR-20b to inhibit NFAT5 and inactivate the TLR signaling pathway, thus mediating innate immune response and preventing AECII from MTB infection-induced damage.

Natural Killer Cells in SARS-CoV-2 Infection: Pathophysiology and Therapeutic Implications

Natural Killer (NK) cells are lymphocytes of the innate immunity that play a crucial role in the control of viral infections in the absence of a prior antigen sensitization. Indeed, they display rapid effector functions against target cells with the capability of direct cell killing and antibody-dependent cell-mediated cytotoxicity. Furthermore, NK cells are endowed with immune-modulatory functions innate and adaptive immune responses via the secretion of chemokines/cytokines and by undertaking synergic crosstalks with other innate immune cells, including monocyte/macrophages, dendritic cells and neutrophils. Recently, the Coronavirus disease 2019 (COVID-19) pandemic, caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), has spread globally. Although the specific role of NK cells in COVID-19 pathophysiology still need to be explored, mounting evidence indicates that NK cell tissue distribution and effector functions could be affected by SARS-CoV-2 infection and that a prompt NK cell response could determine a good clinical outcome in COVID-19 patients. In this review, we give a comprehensive overview of how SARS-CoV-2 infection interferes with NK cell antiviral effectiveness and their crosstalk with other innate immune cells. We also provide a detailed characterization of the specific NK cell subsets in relation to COVID-19 patient severity generated from publicly available single cell RNA sequencing datasets. Finally, we summarize the possible NK cell-based therapeutic approaches against SARS-CoV-2 infection and the ongoing clinical trials updated at the time of submission of this review. We will also discuss how a deep understanding of NK cell responses could open new possibilities for the treatment and prevention of SARS-CoV-2 infection.

Exploring the Role of Low-Density Neutrophils During Mycobacterium tuberculosis Infection

Tuberculosis (TB) is caused by infection with the bacterium Mycobacterium tuberculosis (Mtb), which primarily infects the lungs but can also cause extrapulmonary disease. Both the disease outcome and the pathology of TB are driven by the immune response mounted by the host. Infection with Mtb elicits inflammatory host responses that are necessary to control infection, but can also cause extensive tissue damage when in excess, and thus must be precisely balanced. In particular, excessive recruitment of neutrophils to the site of infection has been associated with poor control of Mtb infection, prompting investigations into the roles of neutrophils in TB disease outcomes. Recent studies have revealed that neutrophils can be divided into subpopulations that are differentially abundant in TB disease states, highlighting the potential complexities in determining the roles of neutrophils in Mtb infection. Specifically, neutrophils can be separated into normal (NDN) and low-density neutrophils (LDNs) based on their separation during density gradient centrifugation and surface marker expression. LDNs are present in higher numbers during active TB disease and increase in frequency with disease progression, although their direct contribution to TB is still unknown. In addition, the abundance of LDNs has also been associated with the severity of other lung infections, including COVID-19. In this review, we discuss recent findings regarding the roles of LDNs during lung inflammation, emphasizing their association with TB disease outcomes. This review highlights the importance of future investigations into the relationship between neutrophil diversity and TB disease severity.

Tuberculous Granuloma: Emerging Insights From Proteomics and Metabolomics

Mycobacterium tuberculosis infection, which claims hundreds of thousands of lives each year, is typically characterized by the formation of tuberculous granulomas — the histopathological hallmark of tuberculosis (TB). Our knowledge of granulomas, which comprise a biologically diverse body of pro- and anti-inflammatory cells from the host immune responses, is based mainly upon examination of lungs, in both human and animal studies, but little on their counterparts from other organs of the TB patient such as the brain. The biological heterogeneity of TB granulomas has led to their diverse, relatively uncoordinated, categorization, which is summarized here. However, there is a pressing need to elucidate more fully the phenotype of the granulomas from infected patients. Newly emerging studies at the protein (proteomics) and metabolite (metabolomics) levels have the potential to achieve this. In this review we summarize the diverse nature of TB granulomas based upon the literature, and amplify these accounts by reporting on the relatively few, emerging proteomics and metabolomics studies on TB granulomas. Metabolites (for example, trimethylamine-oxide) and proteins (such as the peptide PKAp) associated with TB granulomas, and knowledge of their localizations, help us to understand the resultant phenotype. Nevertheless, more multidisciplinary ‘omics studies, especially in human subjects, are required to contribute toward ushering in a new era of understanding of TB granulomas – both at the site of infection, and on a systemic level.

An In Vivo Model of Separate M. tuberculosis Phagocytosis by Neutrophils and Macrophages: Gene Expression Profiles in the Parasite and Disease Development in the Mouse Host

The role of neutrophils in tuberculosis infection remains less well studied compared to that of the CD4⁺ T-lymphocytes and macrophages. Thus, alterations in Mycobacterium tuberculosis transcription profile following phagocytosis by neutrophils and how these shifts differ from those caused by macrophage phagocytosis remain unknown. We developed a mouse model that allows obtaining large amounts of either neutrophils or macrophages infected in vivo with M. tuberculosis for mycobacteria isolation in quantities sufficient for the whole genome RNA sequencing and aerosol challenge of mice. Here, we present: (i) the differences in transcription profiles of mycobacteria isolated from liquid cultures, neutrophils and macrophages infected in vivo; (ii) phenotypes of infection and lung inflammation (life span, colony forming units (CFU) counts in organs, lung pathology, immune cells infiltration and cytokine production) in genetically TB-susceptible mice identically infected via respiratory tract with neutrophil-passaged (NP), macrophage-passaged (MP) and conventionally prepared (CP) mycobacteria. Two-hour residence within neutrophils caused transcriptome shifts consistent with mycobacterial transition to dormancy and diminished their capacity to attract immune cells to infected lung tissue. Mycobacterial multiplication in organs did not depend upon pre-phagocytosis, whilst survival time of infected mice was shorter in the group infected with NP bacilli. We also discuss possible reasons for these phenotypic divergences.