Lactate Alters Metabolism in Human Macrophages and Improves Their Ability to Kill Mycobacterium tuberculosis.

In order to mount an appropriate immune response to infection, the macrophage must alter its metabolism by increasing aerobic glycolysis and concomitantly decreasing oxidative phosphorylation; a process known as the Warburg effect. Consequently, lactate, the end-product of glycolysis, accumulates in the extracellular environment. The subsequent effect of lactate on surrounding macrophages is poorly understood. Mycobacterium tuberculosis (Mtb), the causative organism of Tuberculosis (TB), is phagocytosed by macrophages in the airways. Mtb infected macrophages upregulate aerobic glycolysis and effector functions to try to kill the bacteria. Our lab has previously shown that human macrophages produce lactate in response to infection with Mtb. Although lactate has largely been considered a waste product of aerobic glycolysis, we hypothesised that the presence of extracellular lactate would impact subsequent immunometabolic responses and modulate macrophage function. We demonstrate that the presence of exogenous lactate has an immediate effect on the cellular metabolism of resting human macrophages; causing a decrease in extracellular acidification rate (ECAR; analogous to the rate of glycolysis) and an increase in the oxygen consumption rate (OCR; analogous to oxidative phosphorylation). When lactate-treated macrophages were stimulated with Mtb or LPS, glycolysis proceeds to increase immediately upon stimulation but oxidative phosphorylation remains stable compared with untreated cells that display a decrease in OCR. This resulted in a significantly reduced ECAR/OCR ratio early in response to stimulation. Since altered metabolism is intrinsically linked to macrophage function, we examined the effect of lactate on macrophage cytokine production and ability to kill Mtb. Lactate significantly reduced the concentrations of TNF and IL-1ß produced by human macrophages in response to Mtb but did not alter IL-10 and IL-6 production. In addition, lactate significantly improved bacillary clearance in human macrophages infected with Mtb, through a mechanism that is, at least in part, mediated by promoting autophagy. These data indicate that lactate, the product of glycolysis, has a negative feedback effect on macrophages resulting in an attenuated glycolytic shift upon subsequent stimulation and reduced pro-inflammatory cytokine production. Interestingly, this pro-resolution effect of lactate is associated with increased capacity to kill Mtb.

Mycobacterium tuberculosis Limits Host Glycolysis and IL-1ß by Restriction of PFK-M via MicroRNA-21.

Increased glycolytic metabolism recently emerged as an essential process driving host defense against Mycobacterium tuberculosis (Mtb), but little is known about how this process is regulated during infection. Here, we observe repression of host glycolysis in Mtb-infected macrophages, which is dependent on sustained upregulation of anti-inflammatory microRNA-21 (miR-21) by proliferating mycobacteria. The dampening of glycolysis by miR-21 is mediated through targeting of phosphofructokinase muscle (PFK-M) isoform at the committed step of glycolysis, which facilitates bacterial growth by limiting pro-inflammatory mediators, chiefly interleukin-1ß (IL-1ß). Unlike other glycolytic genes, PFK-M expression and activity is repressed during Mtb infection through miR-21-mediated regulation, while other less-active isoenzymes dominate. Notably, interferon-γ (IFN-γ), which drives Mtb host defense, inhibits miR-21 expression, forcing an isoenzyme switch in the PFK complex, augmenting PFK-M expression and macrophage glycolysis. These findings place the targeting of PFK-M by miR-21 as a key node controlling macrophage immunometabolic function.

Selective effects of radiotherapy on viability and function of invariant natural killer T cells in vitro.

BACKGROUND AND PURPOSE: Immunotherapies involving the adoptive transfer of ex vivo expanded autologous invariant natural killer (iNKT) cells are a potential option for cancer patients and are under investigation in clinical trials. Most cancer patients receive radiotherapy at some point during their treatment. We investigated the effects of therapeutic doses of radiation on the viability and function of human primary cultures of iNKT cells in vitro. MATERIALS AND METHODS: iNKT cell lines generated from 6 healthy donors were subjected to therapeutically-relevant doses of radiation. Cell cycle arrest and cell death were assessed by flow cytometry. Double strand DNA breaks were analysed by measuring phosphorylated histone H2AX expression by fluorescence microscopy. Cytolytic degranulation, cytokine production and cytotoxicity by antigen-stimulated iNKT cells were assessed by flow cytometry. RESULTS: Radiation inhibited viability of iNKT cells in a dose-dependent manner. Radiation caused double strand DNA breaks, which were rapidly repaired, and affected the cell cycle at high doses. Moderate doses of radiation did not inhibit degranulation or cytotoxicity by iNKT cells, but induced perforin expression and inhibited proliferation and interferon-γ production by surviving iNKT cells. DISCUSSION: Exposure of iNKT cell to radiation can negatively affect their viability and function.

Cigarette Smoking Impairs the Bioenergetic Immune Response to Mycobacterium tuberculosis Infection.

Smoking is a major risk factor driving the tuberculosis epidemic, and smokers' alveolar macrophages (AM) demonstrate significant immune defects after infection. Recently, macrophage glycolytic reprogramming has emerged as crucial in the early host immune response to Mycobacterium tuberculosis (Mtb) infection. In the present study, we sought to compare baseline metabolic characteristics and the glycolytic response to infection of human AM from smokers and nonsmokers. AM were obtained at bronchoscopy, and extracellular flux analyses were performed to determine baseline metabolic characteristics compared with human monocyte-derived macrophages (MDM). Metabolic characterization of AM from smokers and nonsmokers was performed similarly. After infection with Mtb, differences in glycolytic response were measured by extracellular flux analyses and gene expression analyses and correlated with production of glycolysis-driven IL-1ß and prostaglandin E2. Similar experiments were performed in cigarette smoke extract-treated MDM as an alternative model. At baseline, human AM from nonsmokers have a significantly lower extracellular acidification rate/oxygen consumption rate ratio than MDM (P < 0.05), but they retain substantial glycolytic reserve. Compared with nonsmokers' AM, smokers' AM demonstrate reduced metabolic activity, reduced glycolytic reserve (P = 0.051), and reduced spare respiratory capacity (P < 0.01). After infection with Mtb, smokers' AM have significantly reduced glycolytic response, as measured by extracellular flux analyses (P < 0.05) and glycolytic gene expression analyses. Cigarette smoke extract-treated MDM similarly demonstrate reduced metabolic activity and reserves, as well as impaired glycolytic response to infection. Human AM demonstrate metabolic plasticity that allows glycolytic reprogramming to occur after Mtb infection. In smokers, this metabolic reserve is significantly attenuated, with consequent impairment of the glycolytic response to infection.

Phenolic Glycolipid Facilitates Mycobacterial Escape from Microbicidal Tissue-Resident Macrophages.

Mycobacterium tuberculosis (Mtb) enters the host in aerosol droplets deposited in lung alveoli, where the bacteria first encounter lung-resident alveolar macrophages. We studied the earliest mycobacterium-macrophage interactions in the optically transparent zebrafish. First-responding resident macrophages phagocytosed and eradicated infecting mycobacteria, suggesting that to establish a successful infection, mycobacteria must escape out of the initially infected resident macrophage into growth-permissive monocytes. We defined a critical role for mycobacterial membrane phenolic glycolipid (PGL) in engineering this transition. PGL activated the STING cytosolic sensing pathway in resident macrophages, inducing the production of the chemokine CCL2, which in turn recruited circulating CCR2+ monocytes toward infection. Transient fusion of infected macrophages with CCR2+ monocytes enabled bacterial transfer and subsequent dissemination, and interrupting this transfer so as to prolong mycobacterial sojourn in resident macrophages promoted clearing of infection. Human alveolar macrophages produced CCL2 in a PGL-dependent fashion following infection, arguing for the potential of PGL-blocking interventions or PGL-targeting vaccine strategies in the prevention of tuberculosis. VIDEO ABSTRACT.

Lysosomal Disorders Drive Susceptibility to Tuberculosis by Compromising Macrophage Migration.

A zebrafish genetic screen for determinants of susceptibility to Mycobacterium marinum identified a hypersusceptible mutant deficient in lysosomal cysteine cathepsins that manifests hallmarks of human lysosomal storage diseases. Under homeostatic conditions, mutant macrophages accumulate undigested lysosomal material, which disrupts endocytic recycling and impairs their migration to, and thus engulfment of, dying cells. This causes a buildup of unengulfed cell debris. During mycobacterial infection, macrophages with lysosomal storage cannot migrate toward infected macrophages undergoing apoptosis in the tuberculous granuloma. The unengulfed apoptotic macrophages undergo secondary necrosis, causing granuloma breakdown and increased mycobacterial growth. Macrophage lysosomal storage similarly impairs migration to newly infecting mycobacteria. This phenotype is recapitulated in human smokers, who are at increased risk for tuberculosis. A majority of their alveolar macrophages exhibit lysosomal accumulations of tobacco smoke particulates and do not migrate to Mycobacterium tuberculosis. The incapacitation of highly microbicidal first-responding macrophages may contribute to smokers' susceptibility to tuberculosis.

Cutting Edge: Mycobacterium tuberculosis Induces Aerobic Glycolysis in Human Alveolar Macrophages That Is Required for Control of Intracellular Bacillary Replication.

Recent advances in immunometabolism link metabolic changes in stimulated macrophages to production of IL-1ß, a crucial cytokine in the innate immune response to Mycobacterium tuberculosis. To investigate this pathway in the host response to M. tuberculosis, we performed metabolic and functional studies on human alveolar macrophages, human monocyte-derived macrophages, and murine bone marrow-derived macrophages following infection with the bacillus in vitro. M. tuberculosis infection induced a shift from oxidative phosphorylation to aerobic glycolysis in macrophages. Inhibition of this shift resulted in decreased levels of proinflammatory IL-1ß and decreased transcription of PTGS2, increased levels of anti-inflammatory IL-10, and increased intracellular bacillary survival. Blockade or absence of IL-1R negated the impact of aerobic glycolysis on intracellular bacillary survival, demonstrating that infection-induced glycolysis limits M. tuberculosis survival in macrophages through induction of IL-1ß. Drugs that manipulate host metabolism may be exploited as adjuvants for future therapeutic and vaccination strategies.

Cigarette smoking impairs human pulmonary immunity to Mycobacterium tuberculosis.

RATIONALE: Cigarette smoking is linked to important aspects of tuberculosis, such as susceptibility to infection, disease reactivation, mortality, transmission, and persistent infectiousness. The mechanistic basis for this remains poorly understood. OBJECTIVES: To compare the functional impairment seen in human alveolar macrophages (AM) from nonsmokers, smokers, and ex-smokers after infection with Mycobacterium tuberculosis (Mtb). METHODS: AM were acquired at bronchoscopy, and number and viability from smoking donors were compared with nonsmoking donors. AM were challenged in vitro with Mtb and intracellular bacterial viability was measured. Cytokine secretion was measured 24 hours postinfection by ELISA. Previously we determined the frequency of CD4(+)FoxP3(+) T cells in the presence or absence of allogeneic AM, and data were reanalyzed to separate the patient subjects according to smoking status. MEASUREMENTS AND MAIN RESULTS: There were significantly more AM from smokers compared with nonsmokers or ex-smokers (P < 0.01). AM from smokers could not control intracellular Mtb growth. Nonsmokers' AM generated significantly more tumor necrosis factor (TNF)-α, IFN-γ, and IL-1ß after Mtb infection compared with uninfected AM (P < 0.05). However, Mtb-infected AM from smokers did not secrete significantly more TNF-α, IFN-γ, and IL-1ß compared with uninfected smokers' AM. AM taken from ex-smokers also failed to secrete significantly increased TNF-α, IFN-γ, and IL-1ß after Mtb infection. Both smokers' and nonsmokers' AM induced FoxP3(+) T regulatory cell phenotype responses in allogeneic admixed T cells (>4.8 fold; P < 0.05). Even after Mtb infection, AM continued to drive this regulatory phenotype. CONCLUSIONS: In smokers, the pulmonary compartment has a number of macrophage-specific immune impairments that provide some mechanistic explanations whereby cigarette smoking renders a patient susceptible to tuberculosis infection and disease.

Bystander macrophage apoptosis after Mycobacterium tuberculosis H37Ra infection.

Human macrophages infected with Mycobacterium tuberculosis may undergo apoptosis. Macrophage apoptosis contributes to the innate immune response against M. tuberculosis by containing and limiting the growth of mycobacteria and also by depriving the bacillus of its niche cell. Apoptosis of infected macrophages is well documented; however, bystander apoptosis of uninfected macrophages has not been described in the setting of M. tuberculosis. We observed that uninfected human macrophages underwent significant bystander apoptosis 48 and 96 h after they came into contact with macrophages infected with avirulent M. tuberculosis. The bystander apoptosis was significantly greater than the background apoptosis observed in uninfected control cells cultured for the same length of time. There was no evidence of the involvement of tumor necrosis factor alpha, Fas, tumor necrosis factor-related apoptosis-inducing ligand, transforming growth factor beta, Toll-like receptor 2, or MyD88 in contact-mediated bystander apoptosis. This newly described phenomenon may further limit the spread of M. tuberculosis by eliminating the niche cells on which the bacillus relies.