Cell-autonomous targeting of arabinogalactan by host immune factors inhibits mycobacterial growth

Bagcchi S.

On Oct 27, WHO released its Global Tuberculosis Report 2022. The document provides an extensive assessment of the global tuberculosis burden, based on data reported from 202 countries and territories, including over 99% of the global population and of tuberculosis cases. According to the Report, the COVID-19 pandemic is still negatively affecting tuberculosis diagnosis and care—and hence the burden of the disease—and has caused a slowdown, interruption, or reversal of the progress made up to 2019 in combating tuberculosis. An estimated 10·6 million people became ill with tuberculosis in 2021, compared with 10·1 million in 2020, and 1·6 million people died from tuberculosis in 2021 (including 187 000 individuals living with HIV), compared with 1·5 million in 2020 (including 214 000 individuals living with HIV). In addition, the incidence rate of tuberculosis increased by 3·6% in 2021 relative to 2020, suggesting a reversal from the trend of nearly 2% decrease per year during the past two decades. In 2014, the World Health Assembly adopted WHO's End TB Strategy, which counts among its aims an 80% decrease in the tuberculosis incidence by 2030, in accordance with UN Sustainable Development Goal 3, which includes ending the global tuberculosis epidemic. However, as Michel Gasana, (WHO Regional Office for Africa, Brazzaville, Congo [Brazzaville]) pointed out, “We are far from reaching the UN Sustainable Development Goal target of ending the tuberculosis epidemic by 2030.” Gasana told The Lancet Microbe, “Globally, the cumulative reduction in the tuberculosis incidence rate from 2015 to 2021 was 10%. Only the WHO African Region in 2021 reached the 2020 incidence milestone of the End TB Strategy, with a reduction of 22% since 2015.” The burden of drug-resistant tuberculosis also increased by 3% between 2020 and 2021, with 450 000 incident cases of rifampicin-resitant tuberculosis reported in 2021. Russia and other countries in eastern Europe and central Asia reported the highest proportions (>50%) of MDR or rifampicin-resistant tuberculosis among previously treated individuals. In 2021, only 161 746 people (20 000 fewer than in 2019) requiring treatment of MDR or rifampicin-resistant tuberculosis received it. During the COVID-19 pandemic, important tuberculosis services were not offered to many people who needed them. As a result, newly diagnosed tuberculosis cases decreased from 7·1 million in 2019 to 5·8 million in 2020 (a level last observed in 2012). In 2021, the number increased to 6·4 million, but remained below pre-pandemic numbers. This decrease relative to before the COVID-19 pandemic suggests an increase in undiagnosed and untreated tuberculosis cases, initially leading to increased community spread of the infection and tuberculosis deaths, followed by a rise in the number of people developing the disease. “The WHO Global Tuberculosis Report is a stark reminder that tuberculosis is now killing nearly twice as many people as COVID-19 every day,” David Dowdy (Johns Hopkins Bloomberg School of Public Health, Baltimore, MD, USA) told The Lancet Microbe. He added, “The major challenges in fighting tuberculosis are not technical. We have good diagnostic tests, good drugs, and good preventive treatment (though we can always do better). The major challenge to ending tuberculosis is a lack of global political will to mobilise resources to fight tuberculosis in the same way that we have done for diseases like COVID-19.” The 2022 Report also pointed out some gains in tackling tuberculosis. For example, in 2020, the first year of the COVID-19 pandemic, the quality of tuberculosis treatment was maintained, with an 86% success rate; in 2018–22, preventive tuberculosis treatment of individuals living with HIV has exceeded the global target of 6 million; and countries have been increasingly taking up new WHO recommendations and tools leading to improved outcomes. To improve the global tuberculosis situation, experts have recommended some measures. Gasana commented that “[there] is a decline in global spending on essential tuberculosis services from US$6 billion in 2019 to US$5·4 billion in 2021. We need to mobilise additional funds and increase tuberculosis funding to address the impact of COVID-19.” “The reported rise in notified cases calls for an invigorated response to tuberculosis control, which will require both political will on the part of governments and civil society as well as substantial financial investment in tuberculosis diagnosis and treatment throughout the world,” said Megan Murray (Harvard Medical School, Boston, MA, USA). Madhukar Pai (McGill International TB Centre, Montreal, QC, Canada) told The Lancet Microbe, “Right now, improving tuberculosis case detection is urgent. All countries have scaled up molecular testing and genetic sequencing capacity for COVID-19, and this expanded capacity could be used for tuberculosis testing. Also, huge advances have been made in digital health tools, tele-health, and home delivery of medicines. I think all of these could help with the recovery of tuberculosis services.” For the Global Tuberculosis Report 2022 see https://www.who.int/publications/i/item/9789240061729For the End TB Strategy see https://www.who.int/teams/global-tuberculosis-programme/the-end-tb-strategyFor UN Sustainable Development Goals see https://sdgs.un.org/2030agenda For the Global Tuberculosis Report 2022 see https://www.who.int/publications/i/item/9789240061729 For the End TB Strategy see https://www.who.int/teams/global-tuberculosis-programme/the-end-tb-strategy For UN Sustainable Development Goals see https://sdgs.un.org/2030agenda

Chandra P., Grigsby S.J., Philips J.A.

Mycobacterium tuberculosis, the causative agent of tuberculosis, has infected humans for millennia. M. tuberculosis is well adapted to establish infection, persist in the face of the host immune response and be transmitted to uninfected individuals. Its ability to complete this infection cycle depends on it both evading and taking advantage of host immune responses. The outcome of M. tuberculosis infection is often a state of equilibrium characterized by immunological control and bacterial persistence. Recent data have highlighted the diverse cell populations that respond to M. tuberculosis infection and the dynamic changes in the cellular and intracellular niches of M. tuberculosis during the course of infection. M. tuberculosis possesses an arsenal of protein and lipid effectors that influence macrophage functions and inflammatory responses; however, our understanding of the role that specific bacterial virulence factors play in the context of diverse cellular reservoirs and distinct infection stages is limited. In this Review, we discuss immune evasion and provocation by M. tuberculosis during its infection cycle and describe how a more detailed molecular understanding is crucial to enable the development of novel host-directed therapies, disease biomarkers and effective vaccines. In this Review, Chandra, Grigsby and Philips discuss how Mycobacterium tuberculosis evades immune-mediated clearance while capitalizing on the host inflammatory response at different phases of its life cycle. They focus on recent studies, highlight gaps in knowledge and consider how our current understanding will inform new therapies, vaccines and diagnostics.

Irvine E.B., O’Neil A., Darrah P.A., Shin S., Choudhary A., Li W., Honnen W., Mehra S., Kaushal D., Gideon H.P., Flynn J.L., Roederer M., Seder R.A., Pinter A., Fortune S., et. al.

Development of an effective tuberculosis (TB) vaccine has suffered from an incomplete understanding of the correlates of protection against Mycobacterium tuberculosis (Mtb). Intravenous (i.v.) vaccination with Bacille Calmette–Guérin (BCG) provides nearly complete protection against TB in rhesus macaques, but the antibody response it elicits remains incompletely defined. Here we show that i.v. BCG drives superior antibody responses in the plasma and the lungs of rhesus macaques compared to traditional intradermal BCG administration. While i.v. BCG broadly expands antibody titers and functions, IgM titers in the plasma and lungs of immunized macaques are among the strongest markers of reduced bacterial burden. IgM was also enriched in macaques that received protective vaccination with an attenuated strain of Mtb. Finally, an Mtb-specific IgM monoclonal antibody reduced Mtb survival in vitro. Collectively, these data highlight the potential importance of IgM responses as a marker and mediator of protection against TB. Alter and colleagues show that IgM titers in the plasma and bronchoalveolar lavage fluid represent markers of reduced Mtb burden in rhesus macaques vaccinated intravenously with Bacille Calmette–Guérin.

Bell S.L., Lopez K.L., Cox J.S., Patrick K.L., Watson R.O.

Mycobacterium tuberculosis (Mtb) infects one-quarter of the global population and causes one of the deadliest infectious diseases worldwide. Macrophages are the first line of defense against Mtb infection and are typically incredibly efficient at destroying intracellular pathogens, but Mtb has evolved to survive and replicate in this harsh environment.

Wu X., Wu Y., Zheng R., Tang F., Qin L., Lai D., Zhang L., Chen L., Yan B., Yang H., Wang Y., Li F., Zhang J., Wang F., Wang L., et. al.

Singh V., Chibale K.

Conspectus“Drug resistance is an unavoidable consequence of the use of drugs; however, the emergence of multi-drug resistance can be managed by accurate diagnosis and tailor-made regimens.”Antimicrobial resistance (AMR), is one of the most paramount health perils that has emerged in the 21st century. The global increase in drug-resistant strains of various bacterial pathogens prompted the World Health Organization (WHO) to develop a priority list of AMR pathogens. Mycobacterium tuberculosis (Mtb), an acid-fast bacillus that causes tuberculosis (TB), merits being one of the highest priority pathogens on this list since drug-resistant TB (DR-TB) accounts for ∼29% of deaths attributable to AMR. In recent years, funded collaborative efforts of researchers from academia, not-for-profit virtual R&D organizations and industry have resulted in the continuous growth of the TB drug discovery and development pipeline. This has so far led to the accelerated regulatory approval of bedaquiline and delamanid for the treatment of DR-TB. However, despite the availability of drug regimes, the current cure rate for multi-drug-resistant TB (MDR-TB) and extensively drug-resistant TB (XDR-TB) treatment regimens is 50% and 30%, respectively. It is to be noted that these regimens are administered over a long duration and have a serious side effect profile. Coupled with poor patient adherence, this has led to further acquisition of drug resistance and treatment failure. There is therefore an urgent need to develop new TB drugs with novel mechanism of actions (MoAs) and associated regimens.This Account recapitulates drug resistance in TB, existing challenges in addressing DR-TB, new drugs and regimens in development, and potential ways to treat DR-TB. We highlight our research aimed at identifying novel small molecule leads and associated targets against TB toward contributing to the global TB drug discovery and development pipeline. Our work mainly involves screening of various small molecule chemical libraries in phenotypic whole-cell based assays to identify hits for medicinal chemistry optimization, with attendant deconvolution of the MoA. We discuss the identification of small molecule chemotypes active against Mtb and subsequent structure–activity relationships (SAR) and MoA deconvolution studies. This is followed by a discussion on a chemical series identified by whole-cell cross-screening against Mtb, for which MoA deconvolution studies revealed a pathway that explained the lack of in vivo efficacy in a mouse model of TB and reiterated the importance of selecting an appropriate growth medium during phenotypic screening. We also discuss our efforts on drug repositioning toward addressing DR-TB. In the concluding section, we preview some promising future directions and the challenges inherent in advancing the drug pipeline to address DR-TB.

Iwasaki-Hozumi H., Chagan-Yasutan H., Ashino Y., Hattori T.

Galectin-9 (Gal-9) is a β-galactoside-binding lectin capable of promoting or suppressing the progression of infectious diseases. This protein is susceptible to cleavage of its linker-peptides by several proteases, and the resulting cleaved forms, N-terminal carbohydrate recognition domain (CRD) and C-terminal CRD, bind to various glycans. It has been suggested that full-length (FL)-Gal-9 and the truncated (Tr)-Gal-9s could exert different functions from one another via their different glycan-binding activities. We propose that FL-Gal-9 regulates the pathogenesis of infectious diseases, including human immunodeficiency virus (HIV) infection, HIV co-infected with opportunistic infection (HIV/OI), dengue, malaria, leptospirosis, and tuberculosis (TB). We also suggest that the blood levels of FL-Gal-9 reflect the severity of dengue, malaria, and HIV/OI, and those of Tr-Gal-9 markedly reflect the severity of HIV/OI. Recently, matrix metallopeptidase-9 (MMP-9) was suggested to be an indicator of respiratory failure from coronavirus disease 2019 (COVID-19) as well as useful for differentiating pulmonary from extrapulmonary TB. The protease cleavage of FL-Gal-9 may lead to uncontrolled hyper-immune activation, including a cytokine storm. In summary, Gal-9 has potential to reflect the disease severity for the acute and chronic infectious diseases.

Padilla S.T., Niki T., Furushima D., Bai G., Chagan-Yasutan H., Telan E.F., Tactacan-Abrenica R.J., Maeda Y., Solante R., Hattori T.

Acquired immunodeficiency syndrome (AIDS) complicated with tuberculosis (TB) is a global public issue. Due to the paucity of bacteria in AIDS/TB, blood-based biomarkers that reflect disease severity are desired. Plasma levels of matricellular proteins, such as osteopontin (OPN) and galectin-9 (Gal-9), are known to be elevated in AIDS and TB. Therefore, full-length (FL)-Gal9 and FL-OPN, and their truncated forms (Tr-Gal9, Ud-OPN), and 38 cytokines/chemokines were measured in the plasma of 24 AIDS (other than TB), 49 TB, and 33 AIDS/TB patients. Receiver-operating characteristic analysis was used to screen molecules that could distinguish either between disease and normal group, among each disease group, or between deceased patients and survivors. Selected molecules were further analyzed for significant differences. Tr-Gal9 had the highest ability to differentiate TB from AIDS or AIDS/TB, while Ud-OPN distinguished multidrug resistance (MDR)-TB from non-MDR TB, and extra-pulmonary TB from pulmonary TB. Molecules significantly elevated in deceased patients included; FL-Gal9, Tr-Gal9, interleukin (IL)-1 receptor antagonist, IL-17A and transforming growth factor-α in AIDS; IL-6, granulocyte colony-stimulating factor and monocyte chemotactic protein-1 in TB; and macrophage inflammatory protein-1β in AIDS/TB. From the sensitivity, specificity, and significant elevation, Tr-Gal9 is the best biomarker of inflammation and severity in AIDS and AIDS/TB.

Wang Z., Li C.

Autophagy is a fundamental bulk intracellular degradation and recycling process that directly eliminates intracellular microorganisms through “xenophagy” in various types of cells, especially in macrophages. Meanwhile, bacteria have evolved strategies and cellular self-defense mechanisms to prevent autophagosomal degradation and even attack the immune system of host. The lack of knowledge about the roles of autophagy in innate immunity severely limits our understanding of host defensive system and the development of farmed industry consisting of aquaculture. Increasing evidence in recent decades has shown the importance of autophagy. This review focuses on the triggering of xenophagy, targeting of invading pathogens to autophagosomes and elimination in the autophagolysosomes during pathogen infection. How the pathogen can escape from the xenophagy pathway was also discussed. Overall, we aim to reduce diseases and improve industrial production in aquaculture by providing theoretical and technical guidance on xenophagy. • The background of xenophagy in macrophage immune was summarized. • The triggering of xenophagy and elimination of invading pathogens were reviewed. • How the pathogen escape from the xenophagy pathway was also discussed.

Zhang L., Zhao Y., Gao Y., Wu L., Gao R., Zhang Q., Wang Y., Wu C., Wu F., Gurcha S.S., Veerapen N., Batt S.M., Zhao W., Qin L., Yang X., et. al.

Drug inhibition of glycosyltransferasesMycobacteria, including the species that causes tuberculosis (TB), synthesize a complex cell wall that helps to support and protect the bacterial cells. The major components of the cell wall include complex heteropolysaccharides that are synthesized in the periplasmic space. Zhanget al.determined the cryo–electron microscopy structures of two transmembrane glycosyltransferase enzyme complexes that use a lipid-anchored sugar donor to append arabinose units to the cell wall polysaccharides. They also captured the anti-TB drug ethambutol bound within these complexes and observed that it binds in a site overlapping both donor and acceptor sugars. Mapping of resistance mutants provides a structural understanding of how resistance emerges while preserving function of the enzyme and may help to guide the development of next-generation anti-TB drugs that target these enzymes.Science, this issue p.1211

Batt S.M., Minnikin D.E., Besra G.S.

Tuberculosis, caused by the pathogenic bacterium Mycobacterium tuberculosis (Mtb), is the leading cause of death from an infectious disease, with a mortality rate of over a million people per year. This pathogen's remarkable resilience and infectivity is largely due to its unique waxy cell envelope, 40% of which comprises complex lipids. Therefore, an understanding of the structure and function of the cell wall lipids is of huge indirect clinical significance. This review provides a synopsis of the cell envelope and the major lipids contained within, including structure, biosynthesis and roles in pathogenesis.

Wang Z., Li Y., Hou B., Pronobis M.I., Wang M., Wang Y., Cheng G., Weng W., Wang Y., Tang Y., Xu X., Pan R., Lin F., Wang N., Chen Z., et. al.

A massively parallel array of monoclonal antibodies enables rapid antibody development and target discovery across species.

Jia J., Bissa B., Brecht L., Allers L., Choi S.W., Gu Y., Zbinden M., Burge M.R., Timmins G., Hallows K., Behrends C., Deretic V.

AMPK is a central regulator of metabolism and autophagy. Here we show how lysosomal damage activates AMPK. This occurs via a hitherto unrecognized signal transduction system whereby cytoplasmic sentinel lectins detect membrane damage leading to ubiquitination responses. Absence of Galectin 9 (Gal9) or loss of its capacity to recognize lumenal glycans exposed during lysosomal membrane damage abrogate such ubiquitination responses. Proteomic analyses with APEX2-Gal9 have revealed global changes within the Gal9 interactome during lysosomal damage. Gal9 association with lysosomal glycoproteins increases whereas interactions with a newly identified Gal9 partner, deubiquitinase USP9X, diminishes upon lysosomal injury. In response to damage, Gal9 displaces USP9X from complexes with TAK1 and promotes K63 ubiquitination of TAK1 thus activating AMPK on damaged lysosomes. This triggers autophagy and contributes to autophagic control of membrane-damaging microbe Mycobacterium tuberculosis. Thus, galectin and ubiquitin systems converge to activate AMPK and autophagy during endomembrane homeostasis.

Lu L.L., Smith M.T., Yu K.K., Luedemann C., Suscovich T.J., Grace P.S., Cain A., Yu W.H., McKitrick T.R., Lauffenburger D., Cummings R.D., Mayanja-Kizza H., Hawn T.R., Boom W.H., Stein C.M., et. al.

Exposure to Mycobacterium tuberculosis (Mtb) results in heterogeneous clinical outcomes including primary progressive tuberculosis and latent Mtb infection (LTBI). Mtb infection is identified using the tuberculin skin test and interferon-γ (IFN-γ) release assay IGRA, and a positive result may prompt chemoprophylaxis to prevent progression to tuberculosis. In the present study, we report on a cohort of Ugandan individuals who were household contacts of patients with TB. These individuals were highly exposed to Mtb but tested negative by IFN-γ release assay and tuberculin skin test, ‘resisting’ development of classic LTBI. We show that ‘resisters’ possess IgM, class-switched IgG antibody responses and non-IFN-γ T cell responses to the Mtb-specific proteins ESAT6 and CFP10, immunologic evidence of exposure to Mtb. Compared to subjects with classic LTBI, ‘resisters’ display enhanced antibody avidity and distinct Mtb-specific IgG Fc profiles. These data reveal a distinctive adaptive immune profile among Mtb-exposed subjects, supporting an expanded definition of the host response to Mtb exposure, with implications for public health and the design of clinical trials. New immune biomarkers of exposure to tuberculosis may require a rethink of evidence of Mycobacterium tuberculosis infection and control.

Lopez A., Fleming A., Rubinsztein D.C.

Autophagy is an intracellular clearance pathway that delivers cytoplasmic contents to the lysosome for degradation. It plays a critical role in maintaining protein homeostasis and providing nutrients under conditions where the cell is starved. It also helps to remove damaged organelles and misfolded or aggregated proteins. Thus, it is not surprising that defects in this pathway are associated with a variety of pathological conditions, such as neurodegeneration, cancer and infection. Pharmacological upregulation of autophagy is considered a promising therapeutic strategy for the treatment of neurodegenerative and infectious diseases. Studies in knockout mice have demonstrated that autophagy is essential for nervous system function, and data from invertebrate and vertebrate models suggest that the efficiency of autophagic processes generally declines with age. However, much of our understanding of the intracellular regulation of autophagy comes from in vitro studies, and there is a paucity of knowledge about how this process is regulated within different tissues and during the processes of ageing and disease. Here, we review the available tools to probe these questions in vivo within vertebrate model systems. We discuss how these tools have been used to date and consider future avenues of research.