The tuberculous granuloma has long been considered a key host-protective immunological structure that restricts mycobacteria even under circumstances in which it fails to eradicate them. It was thought historically to be a relatively static structure and to require interactions between mycobacteria and the host adaptive immune system for its formation.
Recent advances, particularly in live imaging technology, reveal that the granuloma is a highly dynamic structure, with cells moving in and out of it and throughout the structure. These cellular dynamics resemble in some ways the movement of lymphocytes in lymph nodes, and indeed granulomas can contain tertiary lymphoid structures.
Bona fide granulomas can form in the sole context of innate immunity and, rather than being mycobacterium-restricting structures, early granulomas actually expand the infection by promoting the phagocytosis of apoptotic infected macrophages by multiple newly arriving macrophages. The bacterial secreted protein ESAT6 induces host production of matrix metalloproteinase 9 (MMP9) by epithelial cells surrounding the nascent granuloma to induce the chemotaxis of these new macrophages.
The granuloma also disseminates infection through the egress of infected macrophages to new sites. Granuloma expansion and dissemination can occur during effective antibiotic treatment through the participation of macrophages containing drug-tolerant bacteria.
The onset of adaptive immunity a few weeks after infection can thwart bacterial proliferation in the granuloma but often fails to eradicate infection. Its failure to sterilize infection is attributed to multiple complex mechanisms that result in the delayed arrival and activation of effector T cells.
An understanding of the host pathways that are exploited by mycobacteria to first expand and disseminate infection in the granuloma and to then prevent adaptive immune mechanisms from eradicating infection could provide new host-targeting therapies for tuberculosis. These might be useful not only to shorten the long treatment required for drug-sensitive tuberculosis but also to provide much-needed therapies against drug-resistant tuberculosis. Finally, this understanding might suggest entirely new ways to think about tuberculosis vaccines.
The granuloma, which is a compact aggregate of immune cells, is the hallmark structure of tuberculosis. It is historically regarded as a host-protective structure that 'walls off' the infecting mycobacteria. This Review discusses surprising new discoveries — from imaging studies coupled with genetic manipulations — that implicate the innate immune mechanisms of the tuberculous granuloma in the expansion and dissemination of infection. It also covers why the granuloma can fail to eradicate infection even after adaptive immunity develops. An understanding of the mechanisms and impact of tuberculous granuloma formation can guide the development of therapies to modulate granuloma formation. Such therapies might be effective for tuberculosis as well as for other granulomatous diseases.
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I thank K. Urdahl for discussion and critical review of the manuscript; F. Chu, J. Szumowski, D. Tobin and M. Troll for editorial comments; and F. Roca for help with figure design. I thank my students and colleagues in my research group whose development of the zebrafish model and discoveries using it over the past decade have formulated a revised view of the granuloma. In particular, I am grateful to J. M. Davis, H. Volkman, D. Beery, T. Pozos, C. Cosma, H. Clay, D. Tobin, O. Humbert and K. Takaki for the insights their research has provided. I thank D. Sherman for starting us on the exploration of RD1 in zebrafish and M. Troll for help with granuloma modelling studies. This work was supported by grants from the US National Institutes of Health, including the Director's Pioneer Award.
The author declares no competing financial interests.
A common form of cell death that frequently results from toxic injury, hypoxia or stress. Necrosis involves cell swelling, dysregulation of cell-membrane ion and water fluxes, mitochondrial swelling and the eventual release of cell contents into the interstitium. This form of cell death usually occurs together with inflammation. Recent evidence suggests that necrosis can also represent a form of programmed cell death, in which case it is often referred to as programmed necrosis or necroptosis.
Necrotic material that has a cheesy white appearance on gross examination. Areas of caseum are a hallmark feature of human tuberculous granulomas that result from a distinctive type of necrotic breakdown known as caseous necrosis. Caseous necrosis seems to be a specialized form of coagulative necrosis in which cellular degradation (rather than rapid enzymatic digestion) dominates. On microscopic examination, caseum contains fragmented cells and their amorphous debris. Caseum can be hard or soft on gross examination; hard caseum is generally bacterium poor, whereas soft caseum is often laden with bacteria.
- RD1 locus
In virulent mycobacteria, this region encodes a type VII secretion system, namely ESX-1, an important substrate of which is early secreted antigen 6 (ESAT6). ESX-1-mediated secretion and ESAT6 are essential for mycobacterial virulence and have been implicated in phagosomal escape, cytolysis and pore formation, the induction of apoptosis and the recruitment of macrophages. The deletion of RD1 is thought to be the primary mutation responsible for attenuation of the Mycobacterium bovis vaccine strain bacillus Calmette–Guérin (BCG).
- Tertiary lymphoid structures
Ectopic lymphoid aggregates that are generated during the process of chronic immune stimulation and that have the structural characteristics of secondary lymphoid organs.
A common form of cell death that is also known as intrinsic or programmed cell death. Apoptosis involves cell shrinkage, chromatin condensation in the periphery of the nucleus, cell-membrane blebbing and DNA fragmentation into multiples of ∼180 base pairs. Eventually, the cell breaks up into many membrane-bound apoptotic bodies, which are phagocytosed by neighbouring cells.
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Ramakrishnan, L. Revisiting the role of the granuloma in tuberculosis. Nat Rev Immunol 12, 352–366 (2012). https://doi.org/10.1038/nri3211
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