Dancing with the Stars: Phenolic Glycolipids Partners with Macrophages
Authors: Micah D. Dunlap, Shabaana A. Khader
Journal: Cell Host & Microbe (2017)

How Mycobacterium leprae infection causes demyelination to mediate leprosy pathogenesis has been a long-standing question. In a recent Cell paper, Madigan et al., 2017Madigan C.A. Cambier C.J. Kelly-Scumpia K.M. Scumpia P.O. Cheng T.-Y. Zailaa J. Bloom B.R. Moody D.B. Smale S.T. Sagasti A. et al.Cell. 2017; 170: 973-985.e10Abstract Full Text Full Text PDF PubMed Scopus (76) Google Scholar use a zebrafish model of M. leprae infection to show that infected macrophages patrol axons to trigger mitochondrial damage and induce demyelination of nerve cells. How Mycobacterium leprae infection causes demyelination to mediate leprosy pathogenesis has been a long-standing question. In a recent Cell paper, Madigan et al., 2017Madigan C.A. Cambier C.J. Kelly-Scumpia K.M. Scumpia P.O. Cheng T.-Y. Zailaa J. Bloom B.R. Moody D.B. Smale S.T. Sagasti A. et al.Cell. 2017; 170: 973-985.e10Abstract Full Text Full Text PDF PubMed Scopus (76) Google Scholar use a zebrafish model of M. leprae infection to show that infected macrophages patrol axons to trigger mitochondrial damage and induce demyelination of nerve cells. Of the genus mycobacterium, both M. leprae (Mlep) and M. tuberculosis (Mtb) are extremely successful human pathogens. The basis for the successful prevalence of these pathogens in humans is not completely understood. Mlep infection is one of the oldest recorded infections, and leprosy still remains a public health problem. The genome sequences of Mlep exhibit an extensive collection of genes involved in lipid biosynthesis and degradation (Vissa and Brennan, 2001Vissa V.D. Brennan P.J. Genome Biol. 2001; 2: 1023.1-1023.8Crossref Google Scholar). Phenolic glycolipids (PGLs) are complex cell wall lipids produced by Mlep (and other pathogenic mycobacterial species) that are important virulence factors in disease pathogenesis (Spencer and Brennan, 2011Spencer J.S. Brennan P.J. Lepr. Rev. 2011; 82: 344-357PubMed Google Scholar). For example, PGL-1 from Mlep can bind to complement component C3 to promote invasion of host cells (Schlesinger and Horwitz, 1991Schlesinger L.S. Horwitz M.A. J. Exp. Med. 1991; 174: 1031-1038Crossref PubMed Scopus (67) Google Scholar) and can suppress pro-inflammatory cytokine responses in monocytes (Fallows et al., 2016Fallows D. Peixoto B. Kaplan G. Manca C. J. Inflamm. (Lond.). 2016; 13: 8Crossref Scopus (16) Google Scholar). Additionally, Mlep PGL-1 has been shown to bind to laminin-2 in the extracellular matrix of Schwann cells to mediate direct invasion of nerve cells (Ng et al., 2000Ng V. Zanazzi G. Timpl R. Talts J.F. Salzer J.L. Brennan P.J. Rambukkana A. Cell. 2000; 103: 511-524Abstract Full Text Full Text PDF PubMed Scopus (161) Google Scholar), which may contribute to peripheral nerve damage, a key disease feature of leprosy. Despite these known insights into how PGL-1 may modulate host responses during Mlep infections, some questions have remained in the field. If Mlep PGL-1 directly promotes infection of Schwann cells, why is neuropathy still observed in the less severe form of disease, paucibacillary leprosy? Both PGL expressing and non-expressing mycobacteria can bind to laminin-2 on Schwann cells (Marques et al., 2001Marques M.A. Ant nio V.L. Sarno E.N. Brennan P.J. Pessolani M.C. J. Med. Microbiol. 2001; 50: 23-28Crossref PubMed Scopus (9) Google Scholar). If this is the case, is a direct Mlep PGL-1 interaction with nerve cells or the involvement of other cells driving the neurotropism of Mlep? If Mlep is non-motile, how does Mlep get to the nerve cells? In this issue of Cell, Madigan et al., 2017Madigan C.A. Cambier C.J. Kelly-Scumpia K.M. Scumpia P.O. Cheng T.-Y. Zailaa J. Bloom B.R. Moody D.B. Smale S.T. Sagasti A. et al.Cell. 2017; 170: 973-985.e10Abstract Full Text Full Text PDF PubMed Scopus (76) Google Scholar shed light on these long-standing questions by developing a zebrafish model of Mlep infection. Using this model, Madigan et al., 2017Madigan C.A. Cambier C.J. Kelly-Scumpia K.M. Scumpia P.O. Cheng T.-Y. Zailaa J. Bloom B.R. Moody D.B. Smale S.T. Sagasti A. et al.Cell. 2017; 170: 973-985.e10Abstract Full Text Full Text PDF PubMed Scopus (76) Google Scholar demonstrate that Mlep PGL-1 induces nitric oxide synthase (iNOS) production and reactive nitrogen species in infected macrophages, thus creating a neurotoxic response in macrophages. Infected iNOS-expressing macrophages that patrol axons then trigger mitochondrial damage and induce demyelination of nerve cells (Figure 1). Leprosy is a debilitating disease that causes widespread demyelinating neuropathy, reflected in the crippling of hands and feet, paralysis, and often blindness in patients. Based on the number of skin lesions and extent of nerves involved, clinical manifestation can range from paucibacillary or tuberculoid disease, to multibacillary or lepromatous disease. Understanding the host-pathogen interactions of Mlep that drive disease pathogenesis has faced significant hurdles, including the absence of tractable animal models that truly reflect disease pathogenesis, the inability to culture Mlep in vitro, and the long doubling time of Mlep in existing animal models. To overcome these hurdles, Madigan et al., 2017Madigan C.A. Cambier C.J. Kelly-Scumpia K.M. Scumpia P.O. Cheng T.-Y. Zailaa J. Bloom B.R. Moody D.B. Smale S.T. Sagasti A. et al.Cell. 2017; 170: 973-985.e10Abstract Full Text Full Text PDF PubMed Scopus (76) Google Scholar have exploited the optical transparency of larval zebrafish to visualize the earliest host-pathogen interactions of Mlep. Madigan et al., 2017Madigan C.A. Cambier C.J. Kelly-Scumpia K.M. Scumpia P.O. Cheng T.-Y. Zailaa J. Bloom B.R. Moody D.B. Smale S.T. Sagasti A. et al.Cell. 2017; 170: 973-985.e10Abstract Full Text Full Text PDF PubMed Scopus (76) Google Scholar show that injecting Mlep into the caudal vein or the hindbrain ventricle of zebrafish larva resulted in accumulation of macrophages and formation of aggregates. The study by Madigan et al., 2017Madigan C.A. Cambier C.J. Kelly-Scumpia K.M. Scumpia P.O. Cheng T.-Y. Zailaa J. Bloom B.R. Moody D.B. Smale S.T. Sagasti A. et al.Cell. 2017; 170: 973-985.e10Abstract Full Text Full Text PDF PubMed Scopus (76) Google Scholar reveals that the early accumulation of macrophages upon Mlep infection is likely independent of innate immune recognition and signaling by TLR/MyD88, and macrophage accumulation is important for bacterial killing. Importantly, Madigan et al., 2017Madigan C.A. Cambier C.J. Kelly-Scumpia K.M. Scumpia P.O. Cheng T.-Y. Zailaa J. Bloom B.R. Moody D.B. Smale S.T. Sagasti A. et al.Cell. 2017; 170: 973-985.e10Abstract Full Text Full Text PDF PubMed Scopus (76) Google Scholar show that within 2 days after infection, Mlep can escape the vasculature and enter peripheral tissue, which likely involves trafficking of Mlep-infected macrophages. Using transgenic larva that express myelin basic protein reporter to label the myelination membrane of the nerve cells, Madigan et al., 2017Madigan C.A. Cambier C.J. Kelly-Scumpia K.M. Scumpia P.O. Cheng T.-Y. Zailaa J. Bloom B.R. Moody D.B. Smale S.T. Sagasti A. et al.Cell. 2017; 170: 973-985.e10Abstract Full Text Full Text PDF PubMed Scopus (76) Google Scholar demonstrate that Mlep infection resulted in formation of cellular myelin protrusions, which cluster with Mlep localization in the nerve. The Mlep-mediated induction of myelin protrusions increased over time and coincided with localization of Mlep near the nerve. Importantly, the myelin protrusions were not induced when zebrafish larvae were infected with their natural pathogen, M. marinum, suggesting that these structures were induced in response to specific Mlep pathogen determinants. Mlep PGL-1 expresses a unique trisaccharide, while M. marinum PGL contains a monosaccharide. Madigan et al., 2017Madigan C.A. Cambier C.J. Kelly-Scumpia K.M. Scumpia P.O. Cheng T.-Y. Zailaa J. Bloom B.R. Moody D.B. Smale S.T. Sagasti A. et al.Cell. 2017; 170: 973-985.e10Abstract Full Text Full Text PDF PubMed Scopus (76) Google Scholar expressed Mlep PGL-1 lipid biosynthesis genes in M. marinum and showed that PGL-1 expression in M. marinum can now cause myelin protrusions, very similar to those caused by Mlep infection of zebrafish larvae. Importantly, Madigan et al., 2017Madigan C.A. Cambier C.J. Kelly-Scumpia K.M. Scumpia P.O. Cheng T.-Y. Zailaa J. Bloom B.R. Moody D.B. Smale S.T. Sagasti A. et al.Cell. 2017; 170: 973-985.e10Abstract Full Text Full Text PDF PubMed Scopus (76) Google Scholar found that rather than direct infection of nerve cells, Mlep-infected macrophages were found adjacent to the damaged nerve. Further, upon tracking macrophages in zebrafish larvae, Madigan et al., 2017Madigan C.A. Cambier C.J. Kelly-Scumpia K.M. Scumpia P.O. Cheng T.-Y. Zailaa J. Bloom B.R. Moody D.B. Smale S.T. Sagasti A. et al.Cell. 2017; 170: 973-985.e10Abstract Full Text Full Text PDF PubMed Scopus (76) Google Scholar discovered that both uninfected and infected macrophages patrolled the axons, but only PGL-1 expressing mycobacteria induced demyelinating lesions. When macrophage-deficient zebrafish were used in similar infection models, the myelin protrusions induced by Mlep were lost, implicating a macrophage-mediated mechanism in driving demyelination of nerve cells. Madigan et al., 2017Madigan C.A. Cambier C.J. Kelly-Scumpia K.M. Scumpia P.O. Cheng T.-Y. Zailaa J. Bloom B.R. Moody D.B. Smale S.T. Sagasti A. et al.Cell. 2017; 170: 973-985.e10Abstract Full Text Full Text PDF PubMed Scopus (76) Google Scholar then show that the mechanism by which Mlep PGL-1 drives demyelination is through the induction of iNOS in macrophages and progression toward a neurotoxic macrophage population. Finally, Madigan et al., 2017Madigan C.A. Cambier C.J. Kelly-Scumpia K.M. Scumpia P.O. Cheng T.-Y. Zailaa J. Bloom B.R. Moody D.B. Smale S.T. Sagasti A. et al.Cell. 2017; 170: 973-985.e10Abstract Full Text Full Text PDF PubMed Scopus (76) Google Scholar demonstrate that PGL-1-mediated iNOS induction in the infected macrophage induces mitochondrial swelling and damage, causing myelin loss in axons. PGL expression on M. marinum in the zebrafish model can also induce the expression of chemokines such as (C-C-motif) ligand-2 (CCL2) that recruit C-C-chemokine receptor 2 (CCR2)-expressing permissive monocytes to the site of infection (Cambier et al., 2017Cambier C.J. O’Leary S.M. O’Sullivan M.P. Keane J. Ramakrishnan L. Immunity. 2017; (Published online August 23, 2017)https://doi.org/10.1016/j.immuni.2017.08.003Abstract Full Text Full Text PDF PubMed Scopus (86) Google Scholar) (Figure 1). Mlep PGL-1 also appears to use the CCR2-CCL2 axis for recruitment of permissive monocytes and formation of macrophage aggregates, which could result in perpetuation of macrophage infection, continued trafficking of infected macrophages to the nerve cells, and resulting demyelination and clinical neuropathy. By developing a tractable animal model to study host-pathogen interactions of Mlep, Madigan et al., 2017Madigan C.A. Cambier C.J. Kelly-Scumpia K.M. Scumpia P.O. Cheng T.-Y. Zailaa J. Bloom B.R. Moody D.B. Smale S.T. Sagasti A. et al.Cell. 2017; 170: 973-985.e10Abstract Full Text Full Text PDF PubMed Scopus (76) Google Scholar have opened new doors to understanding the disease pathogenesis of leprosy. Furthermore, this work puts forth an interesting paradigm by demonstrating that Mlep virulence determinants, such as PGL-1, can generate neurotoxic macrophages and that these macrophages mediate nerve cell demyelination. It is likely that during the earliest stages of infection, infected macrophages traffic Mlep to the nerve cells to mediate early stages of demyelination. During chronic infection and high bacillary load, Mlep PGL-1 can likely participate directly in invasion of nerve cells (Ng et al., 2000Ng V. Zanazzi G. Timpl R. Talts J.F. Salzer J.L. Brennan P.J. Rambukkana A. Cell. 2000; 103: 511-524Abstract Full Text Full Text PDF PubMed Scopus (161) Google Scholar) to increase neurotropism and disease progression. PGL-1 has long been considered a “star” virulence factor in leprosy pathogenesis. Thus, it is possible that PGL-1 plays several roles in leprosy pathogenesis, including dampening early host responses, binding to CR3 to enhance Mlep uptake by phagocytes, upregulation of specific chemokines that recruit permissive monocytes to enhance Mlep replication, and, as shown here, activation of macrophages into a neurotoxic effector cell that can mediate demyelination of nerve cells. Phenolic Glycolipid Facilitates Mycobacterial Escape from Microbicidal Tissue-Resident MacrophagesCambier et al.ImmunityAugust 24, 2017In BriefCambier et al. find that activation of the STING pathway in lung-resident microbicidal macrophages by the mycobacterial surface lipid PGL enables bacterial escape by inducing the recruitment of mycobacterium-permissive monocytes via the CCL2-CCR2 chemokine axis. Their findings reveal a relocation strategy that enables mycobacterial dissemination, and argue for the potential of interventions targeting PGL in the prevention of tuberculosis. Full-Text PDF Open AccessA Macrophage Response to Mycobacterium leprae Phenolic Glycolipid Initiates Nerve Damage in LeprosyMadigan et al.CellAugust 24, 2017In BriefA new model system for leprosy enables the discovery that macrophages, and not the causative bacterium M. leprae, initiate host nerve demyelination and axonal damage. Full-Text PDF Open Access