Macrophage NFATC2 mediates angiogenic signaling during mycobacterial infection

During mycobacterial infections, pathogenic mycobacteria manipulate both host immune and stromal cells to establish and maintain a productive infection. In humans, non-human primates, and zebrafish models of infection, pathogenic mycobacteria produce and modify the specialized lipid trehalose 6,6'-dimycolate (TDM) in the bacterial cell envelope to drive host angiogenesis toward the site of forming granulomas, leading to enhanced bacterial growth. Here, we use the zebrafish-Mycobacterium marinum infection model to define the signaling basis of the host angiogenic response. Through intravital imaging and cell-restricted peptide-mediated inhibition, we identify macrophage-specific activation of NFAT signaling as essential to TDM-mediated angiogenesis in vivo. Exposure of cultured human cells to Mycobacterium tuberculosis results in robust induction of VEGFA, which is dependent on a signaling pathway downstream of host TDM detection and culminates in NFATC2 activation. As granuloma-associated angiogenesis is known to serve bacterial-beneficial roles, these findings identify potential host targets to improve tuberculosis disease outcomes.

Cyclopropane Modification of Trehalose Dimycolate Drives Granuloma Angiogenesis and Mycobacterial Growth through Vegf Signaling

Mycobacterial infection leads to the formation of characteristic immune aggregates called granulomas, a process accompanied by dramatic remodeling of the host vasculature. As granuloma angiogenesis favors the infecting mycobacteria, it may be actively promoted by bacterial determinants during infection. Using Mycobacterium marinum-infected zebrafish as a model, we identify the enzyme proximal cyclopropane synthase of alpha-mycolates (PcaA) as an important bacterial determinant of granuloma-associated angiogenesis. cis-Cyclopropanation of mycobacterial mycolic acids by pcaA drives the activation of host Vegf signaling within granuloma macrophages. Cyclopropanation of the mycobacterial cell wall glycolipid trehalose dimycolate is both required and sufficient to induce robust host angiogenesis. Inducible genetic inhibition of angiogenesis and Vegf signaling during granuloma formation results in bacterial growth deficits. Together, these data reveal a mechanism by which PcaA-mediated cis-cyclopropanation of mycolic acids promotes bacterial growth and dissemination in vivo by eliciting granuloma vascularization and suggest potential approaches for host-directed therapies.

Cyclipostins and cyclophostin analogs inhibit the antigen 85C from Mycobacterium tuberculosis both in vitro and in vivo

An increasing prevalence of cases of drug-resistant tuberculosis requires the development of more efficacious chemotherapies. We previously reported the discovery of a new class of cyclipostins and cyclophostin (CyC) analogs exhibiting potent activity against Mycobacterium tuberculosis both in vitro and in infected macrophages. Competitive labeling/enrichment assays combined with MS have identified several serine or cysteine enzymes in lipid and cell wall metabolism as putative targets of these CyC compounds. These targets included members of the antigen 85 (Ag85) complex (i.e. Ag85A, Ag85B, and Ag85C), responsible for biosynthesis of trehalose dimycolate and mycolylation of arabinogalactan. Herein, we used biochemical and structural approaches to validate the Ag85 complex as a pharmacological target of the CyC analogs. We found that CyC_7β, CyC_8β, and CyC₁₇ bind covalently to the catalytic Ser¹²⁴ residue in Ag85C; inhibit mycolyltransferase activity (i.e. the transfer of a fatty acid molecule onto trehalose); and reduce triacylglycerol synthase activity, a property previously attributed to Ag85A. Supporting these results, an X-ray structure of Ag85C in complex with CyC_8β disclosed that this inhibitor occupies Ag85C's substrate-binding pocket. Importantly, metabolic labeling of M. tuberculosis cultures revealed that the CyC compounds impair both trehalose dimycolate synthesis and mycolylation of arabinogalactan. Overall, our study provides compelling evidence that CyC analogs can inhibit the activity of the Ag85 complex in vitro and in mycobacteria, opening the door to a new strategy for inhibiting Ag85. The high-resolution crystal structure obtained will further guide the rational optimization of new CyC scaffolds with greater specificity and potency against M. tuberculosis.

Mycobacterial cord factor enhances migration of neutrophil-like HL-60 cells by prolonging AKT phosphorylation

Trehalose 6,6'-dimycolate (TDM), or cord factor, is a crucial stimulus of immune responses during Mycobacterium tuberculosis infection. Although TDM has immuno-stimulatory properties, including adjuvant activity and the ability to induce granuloma formation, the mechanisms underlying these remain unknown. We hypothesized that TDM stimulates transendothelial migration of neutrophils, which are the first immune cells to infiltrate the tissue upon infection. In this study, it was shown that TDM enhances N-formylmethionyl-leucyl-phenylalanine (fMLP)-induced chemotaxis and transendothelial movement by prolonging AKT phosphorylation in human neutrophils. TDM induced expression of macrophage-inducible C-type lectin, a receptor for TDM, and induced secretion of pro-inflammatory cytokines and chemokines in differentiated HL-60 cells. In 2- and 3-D neutrophil migration assays, TDM-stimulated neutrophils showed increased fMLP-induced chemotaxis and transendothelial migration. Interestingly, following fMLP stimulation of TDM-activated neutrophils, AKT, a crucial kinase for neutrophil polarization and chemotaxis, showed prolonged phosphorylation at serine 473. Taken together, these data suggest that TDM modulates transendothelial migration of neutrophils upon mycobacterial infection through prolonged AKT phosphorylation. AKT may therefore be a promising therapeutic target for enhancing immune responses to mycobacterial infection.

Macrophage Inducible C-Type Lectin As a Multifunctional Player in Immunity

The macrophage-inducible C-type lectin (Mincle) is an innate immune receptor on myeloid cells sensing diverse entities including pathogens and damaged cells. Mincle was first described as a receptor for the mycobacterial cell wall glycolipid, trehalose-6,6′-dimycolate, or cord factor, and the mammalian necrotic cell-derived alarmin histone deacetylase complex unit Sin3-associated protein 130. Upon engagement by its ligands, Mincle induces secretion of innate cytokines and other immune mediators modulating inflammation and immunity. Since its discovery more than 25 years ago, the understanding of Mincle’s immune function has made significant advances in recent years. In addition to mediating immune responses to infectious agents, Mincle has been linked to promote tumor progression, autoimmunity, and sterile inflammation; however, further studies are required to completely unravel the complex role of Mincle in these distinct host responses. In this review, we discuss recent findings on Mincle’s biology with an emphasis on its diverse functions in immunity.

Modulation of Trehalose Dimycolate and Immune System by Rv0774c Protein Enhanced the Intracellular Survival of Mycobacterium smegmatis in Human Macrophages Cell Line

Mycobacterium tuberculosis Rv0774c protein was reported previously to express under stress conditions. Therefore, Rv0774c gene was cloned and expressed in Mycobacterium smegmatis, a surrogate host, to determine its role in bacterial persistence and immune modulation in natural environment. The bacterial colonies expressing Rv0774c (Ms_rv0774c) were larger, smoother, more moist, and flatter than the control ones (Ms_ve). Enhanced survival of Ms_rv0774c after treatment with streptomycin was observed when compared with control. The cell envelope of Ms_rv0774c was demonstrated to have more trehalose di-mycolate (TDM) and lesser amount of mycolylmannosylphosphorylheptaprenol (Myc-PL) in comparison to control. Higher intracellular survival rate was observed for Ms_rv0774c as compared to Ms_ve in the THP-1 cells. This could be correlated to the reduction in the levels of reactive NO and iNOS expression. Infection of macrophages with Ms_rv0774c resulted in significantly increased expression of TLR2 receptor and IL-10 cytokines. However, it lowered the production of pro-inflammatory cytokines such as IL-12, TNF-α, IFN-γ, and MCP-1 in Ms_rv0774c infected macrophages in comparison to the control and could be associated with decreased phosphorylation of p38 MAPK. Though, predicted with high antigenicity index bioinformatically, extracellular in nature and accessible to host milieu, Rv0774c was not able to generate humoral response in patient samples. Overall, the present findings indicated that Rv0774c altered the morphology and streptomycin sensitivity by altering the lipid composition of M. smegmatis as well as modulated the immune response in favor of bacterial persistence.

Oral recombinant human or mouse lactoferrin reduces Mycobacterium tuberculosis TDM induced granulomatous lung pathology

Trehalose 6'6-dimycolate (TDM) is the most abundant glycolipid on the cell wall of Mycobacterium tuberculosis (MTB). TDM is capable of inducing granulomatous pathology in mouse models that resembles those induced by MTB infection. Using the acute TDM model, this work investigates the effect of recombinant human and mouse lactoferrin to reduce granulomatous pathology. C57BL/6 mice were injected intravenously with TDM at a dose of 25 μg·mouse^-1. At day 4 and 6, recombinant human or mouse lactoferrin (1 mg·(100 μL)^-1·mouse^-1) were delivered by gavage. At day 7 after TDM injection, mice were evaluated for lung pathology, cytokine production, and leukocyte populations. Mice given human or mouse lactoferrin had reduced production of IL-12p40 in their lungs. Mouse lactoferrin increased IL-6 and KC (CXCL1) in lung tissue. Increased numbers of macrophages were observed in TDM-injected mice given human or mouse lactoferrin. Granulomatous pathology, composed of mainly migrated leukocytes, was visually reduced in mice that received human or mouse lactoferrin. Quantitation of granulomatous pathology demonstrated a significant decrease in mice given human or mouse lactoferrin compared with TDM control mice. This report is the first to directly compare the immune modulatory effects of both heterologous recombinant human and homologous mouse lactoferrin on the development of TDM-induced granulomas.

Trehalose diester glycolipids are superior to the monoesters in binding to Mincle, activation of macrophages in vitro and adjuvant activity in vivo

The T-cell adjuvanticity of mycobacterial cord factor trehalose 6,6'-dimycolate (TDM) is well established. The identification of the C-type lectin Mincle on innate immune cells as the receptor for TDM and its synthetic analogue trehalose 6,6'-dibehenate (TDB) has raised interest in development of synthetic Mincle ligands as novel adjuvants. Trehalose mono- (TMXs) and diesters (TDXs) with symmetrically shortened acyl chains [denoted by X: arachidate (A), stearate (S), palmitate (P), and myristate (M)] were tested. Upon stimulation of murine macrophages, G-CSF secretion and NO production were strongly augmented by all TDXs tested, in a wide concentration range. In contrast, the TMXs triggered macrophage activation only at high concentrations. Macrophage activation by all TDXs required Mincle, but was independent of MyD88. The superior capacity of TDXs for activating macrophages was paralleled by direct binding of TDXs, but not of TMXs, to a Mincle-Fc fusion protein. Insertion of a short polyethylene glycol between the sugar and acyl chain in TDS reduced Mincle-binding and macrophage activation. Immunization of mice with cationic liposomes containing the analogues demonstrated the superior adjuvant activity of trehalose diesters. Overall, immune activation in vitro and in vivo by trehalose esters of simple fatty acids requires two acyl chains of length and involves Mincle.

Structural definition of trehalose 6-monomycolates and trehalose 6,6'-dimycolates from the pathogen Rhodococcus equi by multiple-stage linear ion-trap mass spectrometry with electrospray ionization

The cell wall of the pathogenic bacterium Rhodococcus equi (R. equi) contains abundant trehalose monomycolate (TMM) and trehalose dimycolate (TDM), the glycolipids bearing mycolic acids. Here, we describe multiple-stage (MS(n)) linear ion-trap (LIT) mass spectrometric approaches toward structural characterization of TMM and TDM desorbed as [M + Alk](+) (Alk = Na, Li) and as [M + X](-) (X = CH(3)CO(2), HCO(2)) ions by electrospray ionization (ESI). Upon MS(n) (n=2, 3, 4) on the [M + Alk](+) or the [M + X](-) adduct ions of TMM and TDM, abundant structurally informative fragment ions are readily available, permitting fast assignment of the length of the meromycolate chain and of the α-branch on the mycolyl residues. In this way, structures of TMM and TDM isolated from pathogenic R. equi strain 103 can be determined. Our results indicate that the major TMM and TDM molecules possess 6, and/or 6'-mycolyl groups that consist of mainly C14 and C16 α-branches with meromycolate branches ranging from C18 to C28, similar to the structures of the unbound mycolic acids found in the cell envelope. Up to 60 isobaric isomers varying in chain length of the α-branch and of the meromycolate backbone were observed for some of the TDM species in the mixture. This mass spectrometric approach provides a direct method that affords identification of various TMM and TDM isomers in a mixture of which the complexity of this lipid class has not been previously reported using other analytical methods.

Enzymatic hydrolysis of trehalose dimycolate releases free mycolic acids during mycobacterial growth in biofilms

Mycobacterial species, like other microbes, spontaneously form multicellular drug-tolerant biofilms when grown in vitro in detergent-free liquid media. The structure of Mycobacterium tuberculosis biofilms is formed through genetically programmed pathways and is built upon a large abundance of novel extracellular free mycolic acids (FM), although the mechanism of FM synthesis remained unclear. Here we show that the FM in Mycobacterium smegmatis biofilms is produced through the enzymatic release from constitutively present mycolyl derivatives. One of the precursors for FM is newly synthesized trehalose dimycolate (TDM), which is cleaved by a novel TDM-specific serine esterase, Msmeg_1529. Disruption of Msmeg_1529 leads to undetectable hydrolytic activity, reduced levels of FM in the mutant, and retarded biofilm growth. Furthermore, enzymatic hydrolysis of TDM remains conserved in M. tuberculosis, suggesting the presence of a TDM-specific esterase in this pathogen. Overall, this study provides the first evidence for an enzymatic release of free mycolic acids from cell envelope mycolates during mycobacterial growth.

TB research at UT-Houston--a review of cord factor: new approaches to drugs, vaccines and the pathogenesis of tuberculosis

Tuberculosis remains a major threat as drug resistance continues to increase. Pulmonary tuberculosis in adults is responsible for 80% of clinical cases and nearly 100% of transmission of infection. Unfortunately, since we have no animal models of adult type pulmonary tuberculosis, the most important type of disease remains largely out of reach of modern science and many fundamental questions remain unanswered. This paper reviews research dating back to the 1950's providing compelling evidence that cord factor (trehalose 6,6 dimycolate [TDM]) is essential for understanding tuberculosis. However, the original papers by Bloch and Noll were too far ahead of their time to have immediate impact. We can now recognize that the physical and biologic properties of cord factor are unprecedented in science, especially its ability to switch between two sets of biologic activities with changes in conformation. While TDM remains on organisms, it protects them from killing within macrophages, reduces antibiotic effectiveness and inhibits the stimulation of protective immune responses. If it comes off organisms and associates with lipid, TDM becomes a driver of tissue damage and necrosis. Studies emanating from cord factor research have produced (1) a rationale for improving vaccines, (2) an approach to new drugs that overcome natural resistance to antibiotics, (3) models of caseating granulomas that reproduce multiple manifestations of human tuberculosis. (4) evidence that TDM is a key T cell antigen in destructive lesions of tuberculosis, and (5) a new understanding of the pathology and pathogenesis of postprimary tuberculosis that can guide more informative studies of long standing mysteries of tuberculosis.