Identifiers | |
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3D model (JSmol) | |
PubChem CID | |
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Properties | |
C130H250O15 | |
Molar mass | 2053.415 g·mol−1 |
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). |
Cord factor, or trehalose dimycolate (TDM), is a glycolipid molecule found in the cell wall of Mycobacterium tuberculosis and similar species. It is the primary lipid found on the exterior of M. tuberculosis cells. [1] Cord factor influences the arrangement of M. tuberculosis cells into long and slender formations, giving its name. [2] Cord factor is virulent towards mammalian cells and critical for survival of M. tuberculosis in hosts, but not outside of hosts. [3] [4] Cord factor has been observed to influence immune responses, induce the formation of granulomas, and inhibit tumor growth. [5] The antimycobacterial drug SQ109 is thought to inhibit TDM production levels and in this way disrupts its cell wall assembly. [6]
A cord factor molecule is composed of a sugar molecule, trehalose (a disaccharide), composed of two glucose molecules linked together. Trehalose is esterified to two mycolic acid residues. [7] [8] One of the two mycolic acid residues is attached to the sixth carbon of one glucose, while the other mycolic acid residue is attached to the sixth carbon of the other glucose. [7] Therefore, cord factor is also named trehalose-6,6'-dimycolate. [7] The carbon chain of the mycolic acid residues vary in length depending on the species of bacteria it is found in, but the general range is 20 to 80 carbon atoms. [3] Cord factor's amphiphilic nature leads to varying structures when many cord factor molecules are in close proximity. [3] On a hydrophobic surface, they spontaneously form a crystalline monolayer. [9] This crystalline monolayer is extremely durable and firm; it is stronger than any other amphiphile found in biology. [10] This monolayer also forms in oil-water, plastic-water, and air-water surfaces. [1] In an aqueous environment free of hydrophobic surfaces, cord factor forms a micelle. [11] Furthermore, cord factor interlocks with lipoarabinomannan (LAM), which is found on the surface of M. tuberculosis cells as well, to form an asymmetrical bilayer. [1] [12] These properties cause bacteria that produce cord factor to grow into long, intertwining filaments, giving them a rope- or cord-like appearance when stained and viewed through a microscope (hence the name). [13]
A large quantity of cord factor is found in virulent M. tuberculosis, but not in avirulent M. tuberculosis. [1] Furthermore, M. tuberculosis loses its virulence if its ability to produce cord factor molecules is compromised. [1] Consequently, when all lipids are removed from the exterior of M. tuberculosis cells, the survival of the bacteria is reduced within a host. [14] When cord factor is added back to those cells, M. tuberculosis survives at a rate similar to that of its original state. [14] Cord factor increases the virulence of tuberculosis in mice, but it has minimal effect on other infections. [1]
The function of cord factor is highly dependent on what environment it is located, and therefore its conformation. [15] This is evident as cord factor is harmful when injected with an oil solution, but not when it is with a saline solution, even in very large amounts. [15] Cord factor protects M. tuberculosis from the defenses of the host. [1] Specifically, cord factor on the surface of M. tuberculosis cells prevents fusion between phagosomal vesicles containing the M. tuberculosis cells and the lysosomes that would destroy them. [5] [16] The individual components of cord factor, the trehalose sugars and mycolic acid residues, are not able to demonstrate this activity; the cord factor molecules must be fully intact. [5] Esterase activity that targets cord factor results in the lysis of M. tuberculosis cells. [17] However, the M. tuberculosis cells must still be alive to prevent this fusion; heat-killed cells with cord factor are unable to prevent being digested. [16] This suggests an additional molecule from M. tuberculosis is required. [16] Regardless, cord factor's ability to prevent fusion is related to an increased hydration force or through steric hindrance. [5] Cord factor remains on the surface of M. tuberculosis cells until it associates with a lipid droplet, where it forms a monolayer. [15] Then, as cord factor is in a monolayer configuration, it has a different function; it becomes fatal or harmful to the host organism. [18] Macrophages can die when in contact with monolayers of cord factor, but not when cord factor is in other configurations. [1] As the monolayer surface area of cord factor increases, so does its toxicity. [19] The length of the carbon chain on cord factor has also shown to affect toxicity; a longer chain shows higher toxicity. [20] Furthermore, fibrinogen has shown to adsorb to monolayers of cord factor and act as a cofactor for its biological effects. [21]
Cord factor isolated from species of Nocardia has been shown to cause cachexia in mice. Severe muscle wasting occurred within 48 hours of the toxin being administered. [22]
Numerous responses that vary in effect result from cord factor's presence in host cells. After exposure to cord factor for 2 hours, 125 genes in the mouse genome are upregulated. [23] After 24 hours, 503 genes are upregulated, and 162 genes are downregulated. [23] The exact chemical mechanisms by which cord factor acts is not completely known. However, it is likely that the mycolic acids of cord factor must undergo a cyclopropyl modification to lead to a response from the host's immune system for initial infection. [24] Furthermore, the ester linkages in cord factor are important for its toxic effects. [25] There is evidence that cord factor is recognized by the Mincle receptor, which is found on macrophages. [26] [27] An activated Mincle receptor leads to a pathway that ultimately results in the production of several cytokines. [28] [29] These cytokines can lead to further cytokine production that promote inflammatory responses. [30] Cord factor, through the Mincle receptor, also causes the recruitment of neutrophils, which lead to pro-inflammatory cytokines as well. [31] However, there is also evidence that toll-like receptor 2 (TLR2) in conjunction with the protein MyD-88 is responsible for cytokine production rather than the Mincle receptor. [23]
Cord factor presence increases the production of the cytokines interleukin-12 (IL-12), interleukin-1 beta (IL-1β), interleukin-6 (IL-6), tumor necrosis factor (TNFα), and macrophage inflammatory protein-2 (MIP-2), which are all pro-inflammatory cytokines important for granuloma formation. [16] [28] [32] IL-12 is particularly important in the defense against M. tuberculosis; without it, M. tuberculosis spreads unhampered. [33] [34] IL-12 triggers production of more cytokines through T cells and natural killer (NK) cells, while also leading to mature Th1 cells, and thus leading to immunity. [35] Then, with IL-12 available, Th1 cells and NK cells produce interferon gamma (IFN-γ) molecules and subsequently release them. [36] The IFN-γ molecules in turn activate macrophages. [37]
When macrophages are activated by cord factor, they can arrange into granulomas around M. tuberculosis cells. [15] [38] Activated macrophages and neutrophils also cause an increase in vascular endothelial growth factor (VEGF), which is important for angiogenesis, a step in granuloma formation. [39] The granulomas can be formed either with or without T-cells, indicating that they can be foreign-body-type or hypersensitivity-type. [37] This means cord factor can stimulate a response by acting as a foreign molecule or by causing harmful reactions from the immune system if the host is already immunized. [37] Thus, cord factor can act as a nonspecific irritant or a T-cell dependent antigen. [37] Granulomas enclose M. tuberculosis cells to halt the bacteria from spreading, but they also allow the bacteria to remain in the host. [16] From there, the tissue can become damaged and the disease can transmit further with cord factor. [40] Alternatively, the activated macrophages can kill the M. tuberculosis cells through reactive nitrogen intermediates to remove the infection. [41]
Besides inducing granuloma formation, activated macrophages that result from IL-12 and IFN-γ are able to limit tumor growth. [42] Furthermore, cord factor's stimulation of TNF-α production, also known as cachectin, is also able to induce cachexia, or loss of weight, within hosts. [43] [44] Cord factor also increases NADase activity in the host, and thus it lowers NAD; enzymes that require NAD decrease in activity accordingly. [3] Cord factor is thus able to obstruct oxidative phosphorylation and the electron transport chain in mitochondrial membranes. [3] In mice, cord factor has shown to cause atrophy in the thymus through apoptosis; similarly in rabbits, atrophy of the thymus and spleen occurred. [45] [46] This atrophy occurs in conjunction with granuloma formation, and if granuloma formation is disturbed, so is the progression of atrophy. [46]
Infection by M. tuberculosis remains a serious problem in the world and knowledge of cord factor can be useful in controlling this disease. [24] For example, the glycoprotein known as lactoferrin is able to mitigate cytokine production and granuloma formation brought on by cord factor. [47] However, cord factor can serve as a useful model for all pathogenic glycolipids and therefore it can provide insight for more than just itself as a virulence factor. [11] [48] Hydrophobic beads covered with cord factor are an effective tool for such research; they are able to reproduce an organism's response to cord factor from M. tuberculosis cells. [11] [48] Cord factor beads are easily created and applied to organisms for study, and then easily recovered. [48]
It is possible to form cord factor liposomes through water emulsion; these liposomes are nontoxic and can be used to maintain a steady supply of activated macrophages. [49] Cord factor under proper control can potentially be useful in fighting cancer because IL-12 and IFN-γ are able to limit the growth of tumors. [50]
Mycobacterium tuberculosis, also known as Koch's bacillus, is a species of pathogenic bacteria in the family Mycobacteriaceae and the causative agent of tuberculosis. First discovered in 1882 by Robert Koch, M. tuberculosis has an unusual, waxy coating on its cell surface primarily due to the presence of mycolic acid. This coating makes the cells impervious to Gram staining, and as a result, M. tuberculosis can appear weakly Gram-positive. Acid-fast stains such as Ziehl–Neelsen, or fluorescent stains such as auramine are used instead to identify M. tuberculosis with a microscope. The physiology of M. tuberculosis is highly aerobic and requires high levels of oxygen. Primarily a pathogen of the mammalian respiratory system, it infects the lungs. The most frequently used diagnostic methods for tuberculosis are the tuberculin skin test, acid-fast stain, culture, and polymerase chain reaction.
Mycobacterium is a genus of over 190 species in the phylum Actinomycetota, assigned its own family, Mycobacteriaceae. This genus includes pathogens known to cause serious diseases in mammals, including tuberculosis and leprosy in humans. The Greek prefix myco- means 'fungus', alluding to this genus' mold-like colony surfaces. Since this genus has cell walls with a waxy lipid-rich outer layer that contains high concentrations of mycolic acid, acid-fast staining is used to emphasize their resistance to acids, compared to other cell types.
A granuloma is an aggregation of macrophages that forms in response to chronic inflammation. This occurs when the immune system attempts to isolate foreign substances that it is otherwise unable to eliminate. Such substances include infectious organisms including bacteria and fungi, as well as other materials such as foreign objects, keratin, and suture fragments.
Pathogen-associated molecular patterns (PAMPs) are small molecular motifs conserved within a class of microbes, but not present in the host. They are recognized by toll-like receptors (TLRs) and other pattern recognition receptors (PRRs) in both plants and animals. This allows the innate immune system to recognize pathogens and thus, protect the host from infection.
In cell biology, a phagosome is a vesicle formed around a particle engulfed by a phagocyte via phagocytosis. Professional phagocytes include macrophages, neutrophils, and dendritic cells (DCs).
Interferon gamma (IFN-γ) is a dimerized soluble cytokine that is the only member of the type II class of interferons. The existence of this interferon, which early in its history was known as immune interferon, was described by E. F. Wheelock as a product of human leukocytes stimulated with phytohemagglutinin, and by others as a product of antigen-stimulated lymphocytes. It was also shown to be produced in human lymphocytes. or tuberculin-sensitized mouse peritoneal lymphocytes challenged with Mantoux test (PPD); the resulting supernatants were shown to inhibit growth of vesicular stomatitis virus. Those reports also contained the basic observation underlying the now widely employed IFN-γ release assay used to test for tuberculosis. In humans, the IFN-γ protein is encoded by the IFNG gene.
Transforming growth factor beta (TGF-β) is a multifunctional cytokine belonging to the transforming growth factor superfamily that includes three different mammalian isoforms and many other signaling proteins. TGFB proteins are produced by all white blood cell lineages.
The innate, or nonspecific, immune system is one of the two main immunity strategies in vertebrates. The innate immune system is an alternate defense strategy and is the dominant immune system response found in plants, fungi, insects, and primitive multicellular organisms.
Mycolic acids are long fatty acids found in the cell walls of Mycobacteriales taxon, a group of bacteria that includes Mycobacterium tuberculosis, the causative agent of the disease tuberculosis. They form the major component of the cell wall of many Mycobacteriales species. Despite their name, mycolic acids have no biological link to fungi; the name arises from the filamentous appearance their presence gives Mycobacteriales under high magnification. The presence of mycolic acids in the cell wall also gives Mycobacteriales a distinct gross morphological trait known as "cording". Mycolic acids were first isolated by Stodola et al. in 1938 from an extract of M. tuberculosis.
rBCG30 is a prospective vaccine against tuberculosis created by a team headed by Marcus A. Horwitz at UCLA. It is a live vaccine, consisting of BCG genetically modified to produce abundant amounts of a 30kDa antigen that has been shown to produce a strong immune response in animals and humans. The vaccine completed a Phase I double-blind randomized controlled clinical trial that demonstrated that rBCG30 was safe and immunogenic; during nine months of follow-up, rBCG30, but not BCG, induced significantly increased Antigen 85B-specific immune responses in eight immunological assays.
Lipoarabinomannan, also called LAM, is a glycolipid, and a virulence factor associated with Mycobacterium tuberculosis, the bacteria responsible for tuberculosis. Its primary function is to inactivate macrophages and scavenge oxidative radicals.
Porphyromonas gingivalis belongs to the phylum Bacteroidota and is a nonmotile, Gram-negative, rod-shaped, anaerobic, pathogenic bacterium. It forms black colonies on blood agar.
ESAT-6 or Early Secreted Antigenic Target 6 kDa, is produced by Mycobacterium tuberculosis, it is a secretory protein and potent T cell antigen. It is used in tuberculosis diagnosis by the whole blood interferon γ test QuantiFERON-TB Gold, in conjunction with CFP-10.
Rhodococcus equi is a Gram-positive coccobacillus bacterium. The organism is commonly found in dry and dusty soil and can be important for diseases of domesticated animals. The frequency of infection can reach near 60%. R. equi is an important pathogen causing pneumonia in foals. Since 2008, R. equi has been known to infect wild boar and domestic pigs. R. equi can infect humans. At-risk groups are immunocompromised people, such as HIV-AIDS patients or transplant recipients. Rhodococcus infection in these patients resemble clinical and pathological signs of pulmonary tuberculosis. It is facultative intracellular.
Type IV hypersensitivity, in the Gell and Coombs classification of allergic reactions, often called delayed-type hypersensitivity, is a type of hypersensitivity reaction that can take a day or more to develop. Unlike the other types, it is not humoral but rather is a type of cell-mediated response. This response involves the interaction of T cells, monocytes, and macrophages.
Clostridium difficile toxin A (TcdA) is a toxin generated by Clostridioides difficile, formerly known as Clostridium difficile. It is similar to Clostridium difficile Toxin B. The toxins are the main virulence factors produced by the gram positive, anaerobic, Clostridioides difficile bacteria. The toxins function by damaging the intestinal mucosa and cause the symptoms of C. difficile infection, including pseudomembranous colitis.
Macrophage inducible Ca2+-dependent lectin receptor, (abbreviated to Mincle), is a member of the C-type lectin superfamily encoded by the gene CLEC4E. It is a pattern recognition receptor that can recognize glycolipids including mycobacterial cord factor, trehalose-6,6'-dimycolate (TDM). The mincle receptor binds a range of carbohydrate structures, predominantly containing glucose or mannose, and play an important role in recognition of bacterial glycolipids by the immune system. Upon activation by cord factor, Mincle binds the Fc receptor FcRγ and Syk. Cord factor also binds and activates the related C-type lectin MCL. Upon receptor stimulation is PKC-δ activated, which subsequently phosphorylates CARD9 that triggers recruitment of BCL10 and MALT1, leading to a CARD-CC/BCL10/MALT1 (CBM) signaling complex. This signaling complex in turn triggers downstream recruitment of TRAF6 and NF-κB activation.
Macrophage polarization is a process by which macrophages adopt different functional programs in response to the signals from their microenvironment. This ability is connected to their multiple roles in the organism: they are powerful effector cells of the innate immune system, but also important in removal of cellular debris, embryonic development and tissue repair.
Lalita Ramakrishnan is an Indian-born American microbiologist who is known for her contributions to the understanding of the biological mechanism of tuberculosis. As of 2019 she serves as a professor of Immunology and Infectious Diseases at the University of Cambridge, where she is also a Wellcome Trust Principal Research Fellow and a practicing physician. Her research is conducted at the MRC Laboratory of Molecular Biology, where she serves as the Head of the Molecular Immunity Unit of the Department of Medicine embedded at the MRC LMB. Working with Stanley Falkow at Stanford, she developed the strategy of using Mycobacterium marinum infection as a model for tuberculosis. Her work has appeared in a number of journals, including Science, Nature, and Cell. In 2018 and 2019 Ramakrishnan coauthored two influential papers in the British Medical Journal (BMJ) arguing that the widely accepted estimates of the prevalence of latent tuberculosis—estimates used as a basis for allocation of research funds—are far too high. She is married to Mark Troll, a physical chemist.
Dawn M. E. Bowdish, is a Canadian immunologist and currently a professor in the Department of Pathology and Molecular Medicine at McMaster University in Ontario, Canada. She is a Tier 2 Canada Research Chair in Aging & Immunity. She is known for several discoveries including the immunomodulatory properties of the antimicrobial peptide LL-37, how MARCO signalling complex recognizes Mycobacterium tuberculosis, age-associated inflammation and its effects on clearing pneumococcal pneumonia and how the aging gut microbiome drives age-associated inflammation.