Drug resistant Mycobacterium tuberculosis strains have altered cell envelope hydrophobicity that influences infection outcomes in human macrophages

Mycobacterium tuberculosis (M.tb), the causative agent of tuberculosis (TB), is considered one of the top infectious killers in the world. In recent decades, drug resistant (DR) strains of M.tb have emerged that make TB even more difficult to treat and pose a threat to public health. M.tb has a complex cell envelope that provides protection to the bacterium from chemotherapeutic agents. Although M.tb cell envelope lipids have been studied for decades, very little is known about how their levels change in relation to drug resistance. In this study, we examined changes in the cell envelope lipids [namely, phthiocerol dimycocerosates (PDIMs)], glycolipids [phosphatidyl-myo-inositol mannosides (PIMs)], and the PIM associated lipoglycans [lipomannan (LM); mannose-capped lipoarabinomannan (ManLAM)] of 11 M.tb strains that range from drug susceptible (DS) to multi-drug resistant (MDR) to pre-extensively drug resistant (pre-XDR). We show that there was an increase in the PDIMs:PIMs ratio as drug resistance increases, and provide evidence of PDIM species only present in the DR-M.tb strains studied. Overall, the LM and ManLAM cell envelope levels did not differ between DS- and DR-M.tb strains, but ManLAM surface exposure proportionally increased with drug resistance. Evaluation of host-pathogen interactions revealed that DR-M.tb strains have decreased association with human macrophages compared to DS strains. The pre-XDR M.tb strain with the largest PDIMs:PIMs ratio had decreased uptake, but increased intracellular growth rate at early time points post-infection when compared to the DS-M.tb strain H37Rv. These findings suggest that PDIMs may play an important role in drug resistance and that this observed increase in hydrophobic cell envelope lipids on the DR-M.tb strains studied may influence M.tb-host interactions.

Selective Glycan Labeling of Mannose-Containing Glycolipids in Mycobacteria

Mycobacterium tuberculosis (Mtb) is one of history's most successful human pathogens. By subverting typical immune responses, Mtb can persist within a host until conditions become favorable for growth and proliferation. Virulence factors that enable mycobacteria to modulate host immune systems include a suite of mannose-containing glycolipids: phosphatidylinositol mannosides, lipomannan, and lipoarabinomannan (LAM). Despite their importance, tools for their covalent capture, modification, and imaging are limited. Here, we describe a chemical biology strategy to detect and visualize these glycans. Our approach, biosynthetic incorporation, is to synthesize a lipid-glycan precursor that can be incorporated at a late-stage step in glycolipid biosynthesis. We previously demonstrated selective mycobacterial arabinan modification by biosynthetic incorporation using an exogenous donor. This report reveals that biosynthetic labeling is general and selective: it allows for cell surface mannose-containing glycolipid modification without nonspecific labeling of mannosylated glycoproteins. Specifically, we employed azido-(Z,Z)-farnesyl phosphoryl-β-d-mannose probes and took advantage of the strain-promoted azide-alkyne cycloaddition to label and directly visualize the localization and dynamics of mycobacterial mannose-containing glycolipids. Our studies highlight the generality and utility of biosynthetic incorporation as the probe structure directs the selective labeling of distinct glycans. The disclosed agents allowed for direct tracking of the target immunomodulatory glycolipid dynamics in cellulo. We anticipate that these probes will facilitate investigating the diverse biological roles of these glycans.

Drug-resistant strains of Mycobacterium tuberculosis: cell envelope profiles and interactions with the host

In the past few decades, drug-resistant (DR) strains of Mycobacterium tuberculosis (M.tb), the causative agent of tuberculosis (TB), have become increasingly prevalent and pose a threat to worldwide public health. These strains range from multi (MDR) to extensively (XDR) drug-resistant, making them very difficult to treat. Further, the current and future impact of the Coronavirus Disease 2019 (COVID-19) pandemic on the development of DR-TB is still unknown. Although exhaustive studies have been conducted depicting the uniqueness of the M.tb cell envelope, little is known about how its composition changes in relation to drug resistance acquisition. This knowledge is critical to understanding the capacity of DR-M.tb strains to resist anti-TB drugs, and to inform us on the future design of anti-TB drugs to combat these difficult-to-treat strains. In this review, we discuss the complexities of the M.tb cell envelope along with recent studies investigating how M.tb structurally and biochemically changes in relation to drug resistance. Further, we will describe what is currently known about the influence of M.tb drug resistance on infection outcomes, focusing on its impact on fitness, persister-bacteria, and subclinical TB.

Collected Thoughts on Mycobacterial Lipoarabinomannan, a Cell Envelope Lipoglycan

The presence of lipoarabinomannan (LAM) in the Mycobacterium tuberculosis (Mtb) cell envelope was first reported close to 100 years ago. Since then, numerous studies have been dedicated to the isolation, purification, structural definition, and elucidation of the biological properties of Mtb LAM. In this review, we present a brief historical perspective on the discovery of Mtb LAM and the herculean efforts devoted to structurally characterizing the molecule because of its unique structural and biological features. The significance of LAM remains high to this date, mainly due to its distinct immunological properties in conjunction with its role as a biomarker for diagnostic tests due to its identification in urine, and thus can serve as a point-of-care diagnostic test for tuberculosis (TB). In recent decades, LAM has been thoroughly studied and massive amounts of information on this intriguing molecule are now available. In this review, we give the readers a historical perspective and an update on the current knowledge of LAM with information on the inherent carbohydrate composition, which is unique due to the often puzzling sugar residues that are specifically found on LAM. We then guide the readers through the complex and myriad immunological outcomes, which are strictly dependent on LAM's chemical structure. Furthermore, we present issues that remain unresolved and represent the immediate future of LAM research. Addressing the chemistry, functions, and roles of LAM will lead to innovative ways to manipulate the processes that involve this controversial and fascinating biomolecule.

Hepatitis C virus non-structural protein 4 suppresses Th1 responses by stimulating IL-10 production from monocytes

The majority of hepatitis C virus (HCV) infections become chronic, despite the presence of HCV-specific cellular and humoral immune responses. We have previously suggested that IL-10-secreting antigen-specific regulatory T cells may contribute to viral persistence, and demonstrate here that peripheral blood mononuclear cells (PBMC) from chronically HCV-infected patients secrete IL-10, but not IFN-gamma, in response to HCV nonstructural protein 4 (NS4). A neutralizing anti-IL-10 antibody restored this defective antigen-specific IFN-gamma production in vitro. Furthermore, PBMC from normal individuals secreted IL-10 in response to NS4, suggesting that cells of the innate immune system, in addition to T cells, produced IL-10 in the HCV-infected patients. Cell separation experiments revealed that the innate IL-10 was produced by blood monocytes, but not dendritic cells (DC). In addition, NS4 inhibited IL-12 production by PBMC in response to LPS and IFN-gamma, and Th1 responses to recall antigens in normal individuals. Furthermore, supernatants from NS4-stimulated monocytes inhibited LPS-induced maturation of DC and suppressed their capacity to stimulate proliferation and IFN-gamma production by allospecific T cells. Our data suggest that HCV subverts cellular immunity by inducing IL-10 and inhibiting IL-12 production by monocytes, which in turn inhibits the activation of DC that drive the differentiation of Th1 cells.

Mycobacterium tuberculosis CDC1551 Induces a More Vigorous Host Response In Vivo and In Vitro, But Is Not More Virulent Than Other Clinical Isolates

Mycobacterium tuberculosis CDC1551, a clinical isolate reported to be hypervirulent and to grow faster than other isolates, was compared with two other clinical isolates (HN60 and HN878) and two laboratory strains (H37Rv and Erdman). The initial (1–14 days) growth of CDC1551, HN60, HN878, and H37Rv was similar in the lungs of aerosol-infected mice, but growth of Erdman was slower. Thereafter, the growth rate of CDC1551 decreased relative to the other strains which continued to grow at comparable rates up to day 21. In the lungs of CDC1551-infected mice, small well-organized granulomas with high levels of TNF-α, IL-6, IL-10, IL-12, and IFN-γ mRNA were apparent sooner than in lungs of mice infected with the other strains. CDC1551-infected mice survived significantly longer. These findings were confirmed in vitro. The growth rates of H37Rv and CDC1551 in human monocytes were the same, but higher levels of TNF-α, IL-10, IL-6, and IL-12 were induced in monocytes after infection with CDC1551 or by exposure of monocytes to lipid fractions from CDC1551. CD14 expression on the surface of the monocytes was up-regulated to a greater extent by exposure to the lipids of CDC1551. Thus, CDC1551 is not more virulent than other M. tuberculosis isolates in terms of growth in vivo and in vitro, but it induces a more rapid and robust host response.

Expression of the IL-12 Receptor β1 and β2 Subunits in Human Tuberculosis

To determine whether the Th1 response in tuberculosis correlated with IL-12R expression, we measured expression of the IL-12Rβ1 and IL-12Rβ2 subunits, as well as IL-12Rβ2 mRNA expression in tuberculosis patients and healthy tuberculin reactors. In tuberculosis patients, IFN-γ production by Mycobacterium tuberculosis-stimulated PBMC was reduced, the percentages of T cells expressing IL-12Rβ1 and IL-12Rβ2 were significantly decreased, and IL-12Rβ2 mRNA expression was also markedly reduced. In contrast, in pleural fluid and lymph nodes at the site of disease in tuberculosis patients, in which IFN-γ production is enhanced, IL-12Rβ2 mRNA expression was also increased. In M. tuberculosis-stimulated peripheral blood T cells from tuberculosis patients, anti-IL-10 and anti-TGF-β enhanced IL-12Rβ1 and IL-12Rβ2 expression, and IFN-γ production. In M. tuberculosis-stimulated peripheral blood T cells from healthy tuberculin reactors, recombinant IL-10 and TGF-β reduced IL-12Rβ1 and IL-12Rβ2 expression, as well as IFN-γ production. In combination with prior studies showing increased production of TGF-β by blood monocytes from tuberculosis patients, this suggests that increased TGF-β production is the underlying abnormality that reduces IL-12Rβ1 and IL-12Rβ2 expression in tuberculosis. Our findings provide evidence that IL-12R expression correlates well with IFN-γ production in human tuberculosis, and that expression of IL-12Rβ1 and IL-12Rβ2 may play a central role in mediating a protective Th1 response.