EsxH Inhibits ESCRT
Over 40 years ago, D’Arcy Hart showed that Mtb avoids delivery to lysosomes, but the molecular mechanisms that allow Mtb to elude lysosomal degradation remain poorly understood. Mtb has been reported to block MHC class II (MHC-II) antigen presentation, however the bacterial effectors and host cell targets mediating this effect are unknown. We found that the bacterial effector EsxH impairs both phagosome maturation and antigen presentation by targeting the host ESCRT machinery.
Specialized secretion systems are often used by bacterial pathogens to translocate effectors that target the host. Mtb encodes type VII secretion systems (T7SSs) that enable mycobacteria to secrete proteins across their complex cell envelope. The Esx-3 T7SS secretes EsxG and EsxH (1), which form a heterodimer (2). ESX-3 is required for the bacteria to acquire iron and zinc, essential elements for bacterial survival (1, 3). Using a high throughput, systematic strategy to identify interactions between the Mtb secretome and human proteins, we found that EsxH interacts with the human hepatocyte growth factor-regulated tyrosine kinase substrate (Hgs/Hrs), a component of the endosomal sorting complex required for transport (ESCRT). ESCRT has a well-described role in directing proteins destined for lysosomal degradation into intraluminal vesicles (ILVs) of multivesicular bodies (MVBs), ensuring degradation of the sorted cargo upon MVB-lysosome fusion. We found that ESCRT is also required to deliver Mtb to the lysosome and to restrict intracellular bacterial growth.
In addition to its intrinsic bacterial role in metal homeostasis, we showed that Mtb EsxH, in complex with EsxG, targets host ESCRT to prevent maturation of the Mtb phagosome. Thus, the ability of EsxGH to inhibit the ESCRT machinery underlies one of the central features of tuberculosis pathogenesis (Mehra et al, PLoS Pathogens, 2013). Our ongoing work has shown that ESCRT is also required for infected macrophages to present Mtb antigens and to activate effector CD4+ T cells. Correspondingly, overexpression and loss of function studies demonstrate that EsxH inhibits antigen presentation. Thus, the ability of EsxGH to inhibit ESCRT may provide a molecular explanation for how Mtb delays the initiation of an adaptive immune response and prevents effector T cells responses (Portal, C and Tufariello, J, in prep).
1- Siegrist MS, et al.(2009) Mycobacterial Esx-3 is required for mycobactin-mediated iron acquisition. Proc Natl Acad Sci U S A 106(44):18792-18797.
2- Ilghari D, et al. (2011) Solution structure of the Mycobacterium tuberculosis EsxG·EsxH complex: functional implications and comparisons with other M. tuberculosis Esx family complexes. J Biol Chem 286(34):29993-30002.
3- Serafini A, Boldrin F, Palù G, & Manganelli R (2009) Characterization of a Mycobacterium tuberculosis ESX-3 conditional mutant: essentiality and rescue by iron and zinc. J Bacteriol 191(20):6340-6344.