University of Medicine and Dentistry of New Jersey, New Jersey Medical School, Department of Medicine, Center for Emerging Pathogens, Newark, New Jersey, USA.
Address correspondence to: Padmini Salgame, UMDNJ-New Jersey Medical School, Center for Emerging Pathogens, South Orange Avenue, MSB A902, Newark, New Jersey 07101, USA. Phone: 973.972.8647; Fax: 973.972.0713; E-mail: firstname.lastname@example.org.
First published April 25, 2011 - More info
Mycobacterium tuberculosis can cause lung tissue damage to spread, but the mechanisms driving this immunopathology are poorly understood. The breakdown of lung matrix involves MMPs, which have a unique ability to degrade fibrillar collagens at neutral pH. To determine whether MMPs play a role in the immunopathology of tuberculosis (TB), we profiled MMPs and their inhibitors, the tissue inhibitor of metalloproteinases (TIMPs), in sputum and bronchoalveolar lavage fluid from patients with TB and symptomatic controls. MMP-1 concentrations were significantly increased in both HIV-negative and HIV-positive patients with TB, while TIMP concentrations were lower in HIV-negative TB patients. In primary human monocytes, M. tuberculosis infection selectively upregulated MMP1 gene expression and secretion, and Ro32-3555, a specific MMP inhibitor, suppressed M. tuberculosis–driven MMP-1 activity. Since the mouse MMP-1 ortholog is not expressed in the lung and mice infected with M. tuberculosis do not develop tissue destruction equivalent to humans, we infected transgenic mice expressing human MMP-1 with M. tuberculosis to investigate whether MMP-1 caused lung immunopathology. In the MMP-1 transgenic mice, M. tuberculosis infection increased MMP-1 expression, resulting in alveolar destruction in lung granulomas and significantly greater collagen breakdown. In summary, MMP-1 may drive tissue destruction in TB and represents a therapeutic target to limit immunopathology.
Paul Elkington, Takayuki Shiomi, Ronan Breen, Robert K. Nuttall, Cesar Augusto Ugarte-Gil, Naomi F. Walker, Luísa Saraiva, Bernadette Pedersen, Francesco Mauri, Marc Lipman, Dylan R. Edwards, Brian D. Robertson, Jeanine D’Armiento, Jon S. Friedland
Most individuals infected with Mycobacterium tuberculosis develop a latent infection, which does not progress to active tuberculosis (TB). This occurs, in part, because infected macrophages recruit immune cells to form a granuloma, isolating the bacteria and preventing its spread. In some individuals, granulomas undergo necrosis and tissue destruction occurs, releasing the bacteria and allowing the development of active disease. In this issue of the JCI, Elkington et al. provide evidence that M. tuberculosis drives the expression of MMP-1, which in turn promotes the collagen breakdown that leads to alveolar destruction in TB. These findings identify putative therapeutic targets for the prevention of TB.
An inimitable feature in the natural history of tuberculosis (TB) is the development of latent infection in individuals exposed to Mycobacterium tuberculosis. Approximately 5% of those who are latently infected eventually reactivate their latent focus and develop active TB. Infected macrophages in the latently infected host initiate a cascade of events that results in the recruitment of new monocytes from the bloodstream, providing a continuous niche for M. tuberculosis to maintain its replicative cycle. As adaptive immunity develops in the host, Th1 cells are recruited to the developing granuloma, and the IFN-γ secreted by these cells enhances the antimicrobial capacity of infected macrophages. Thus, the ordered recruitment of monocytes, T cells, B cells, NK cells, and dendritic cells to the granuloma and their positioning around infected macrophages sequesters M. tuberculosis, thereby curtailing its spread from the initial foci of infection (1). As the granulomatous response stabilizes, FoxP3+ regulatory T cells are also found in the tubercle lesion (2) and are conceivably recruited to function as gatekeepers of excessive immune response. Some of the bacteria sequestered within this immune structure reprogram their gene expression and enter the latent/persistent phase, with the ability to reactivate years or decades later. The finding that disease reactivation can be significantly reduced by treatment with isoniazid, an antibiotic that targets actively growing cells, strongly suggests that there is persistent bacterial replication within the granuloma in latent TB (3). Furthermore, the contribution of the host immune response to the maintenance of granuloma integrity and latent infection is best exemplified in immunosuppressed individuals. Loss of granuloma and the risk of reactivation are significantly increased in HIV-infected individuals and in those treated with TNF-α inhibitors (4). During reactivation, the stable granulomas progress to necrosis and caseation that promotes the release of M. tuberculosis into the airways and transmission to new hosts.
With the emergence of new molecular tools and animal models that lend themselves to intravital imaging, the detailed events underlying the structural reorganization of immune cells to form a stable granuloma or progress to a pathologic lesion as the granuloma dissolves are slowly but surely unfolding. Fascinatingly, MMPs are emerging as key players in both the creation of the granuloma and in the destruction of lung tissue.
The first MMP was discovered in 1962 by Gross and Lapiere (5), who demonstrated collagenolytic activity in amphibian tissues undergoing metamorphosis. Since that first report, 24 mammalian MMPs have been identified, with partially overlapping substrate specificities and functions (reviewed in ref. 6). MMPs are zinc-dependent proteases and consist of two conserved domains, a prodomain and a catalytic domain (6). Cooperatively, MMPs can effectively degrade all components of the extracellular matrix, including collagens, laminin, fibronectin, vitronectin, and proteoglycans. MMP activity is tightly controlled, not only by gene expression, but also by its localization in the cell, its requirement for proenzyme activation, and the concurrent expression of tissue inhibitors (6).
Although the original function ascribed to MMPs was matrix cleavage and tissue remodeling, accumulating evidence suggests that this family of enzymes also plays a major role in angiogenesis, cell motility, apoptosis, and, of particular interest here, regulating immunity, inflammation, and host defenses (reviewed in ref. 7). MMPs also activate intestinal pro-α–defensins and function as antimicrobials. Several MMPs, through proteolytic cleavage, can modulate the activity of cytokines and chemokines, including IFN-γ, IL-1β, TNF-α, CXCL8, and CCL7. Thus, MMPs can modulate chemokine gradients and regulate leukocyte recruitment to sites of inflammation (reviewed in ref. 7). The role of MMPs in lung biology is gaining attention, and several studies now show that they are important in normal lung function as well as in lung tissue destruction associated with inflammatory lung disorders, including COPD and emphysema (6).
M. tuberculosis infection induces the production of MMPs in vitro in macrophages and in vivo in murine lung tissue (8–10). Mice treated with BB-94, a broad-spectrum inhibitor of MMPs, exhibit either a delay in granuloma induction (11) or form smaller granulomas with more collagen (12), suggesting for the first time that MMPs regulate cell migration and granuloma formation after M. tuberculosis infection. Consistent with findings using inhibitors, MMP-9–deficient mice also exhibited reduced macrophage recruitment to the lungs and had smaller granulomas (13). These studies provided the basis for the emerging paradigm that MMPs remodel lung tissue and initiate granuloma formation. Subsequently, Ramakrishnan and colleagues (14) discovered the molecular mechanisms that M. tuberculosis uses to induce granuloma formation. They showed that the secreted 6-kDa early secreted antigenic target (ESAT-6) protein induced MMP-9 in epithelial cells that were in close proximity to infected macrophages. The upregulation of MMP-9 was associated with the recruitment of newer monocytes and macrophages necessary for granuloma maturation. Knockdown of MMP-9 expression with modified antisense oligonucleotides abrogated granuloma formation and limited bacterial growth, thereby demonstrating the key roles of MMPs in this process. Usurping host MMPs to ensure its long-term latent existence in the host attests to M. tuberculosis’s Machiavellian intelligence.
The hypothesis that M. tuberculosis–induced MMPs may also drive tissue destruction and subsequent transmission is suggested by studies that found a correlation between increased MMP levels and severity of disease (10, 15). Moreover, it has been observed that MMP-9 is strongly expressed in monocytes present within TB granulomas that are adjacent to caseous necrosis (16). In this issue of the JCI, Elkington, Friedland, and colleagues provide direct evidence that MMP-1, which can specifically degrade type I collagen, drives destruction of pulmonary tissue in TB (17). The authors profiled the expression of MMPs and tissue inhibitors of metalloproteinases (TIMPs) in induced sputum and bronchoalveolar lavage fluid from TB patients and healthy controls. They found that MMP-1 was significantly upregulated in TB patients while levels of TIMPs were reduced. The mouse MMP-1 ortholog is not expressed in the lung, so to further investigate the role of MMP-1 in lung immunopathology, the authors used a novel transgenic mouse that expresses human MMP-1. They found that M. tuberculosis infection of these mice led to increased MMP-1 expression in the lungs, and this was associated with increased alveolar destruction and significantly greater collagen breakdown in comparison with that of wild-type mice Thus, this study for the first time provides a direct link between MMP-1 and tissue destruction in TB.
Notably, despite extensive tissue destruction in the MMP-1 transgenic mice, granuloma architecture was maintained, and there was no increase in the bacterial burden in the lungs. The authors suggest that this may be due to the fact that necrosis of the granuloma and matrix destruction are two separate events. It is likely that caseous necrosis of granulomas followed by liquefaction and extracellular growth of bacteria occur along with matrix destruction to cause cavities and the subsequent dissemination of viable bacteria into the blood stream or bronchial tree. In the future, this might be addressed by crossing MMP-1 transgenic mice with a mouse strain like C3HeB/FeJ that develops necrotic granulomas (18) to develop a model that may closely mimic human tubercle lesions.
Rutter et al. have identified a single nucleotide polymorphism guanine insertion/deletion at –1,607 bp in the promoter of the human MMP1 gene, which results in two alternate alleles (19). The insertion of an additional guanine increases transcriptional activity due to the creation of a core binding site for a transcription factor from the Ets family (19). It would be interesting to test whether this polymorphism favors tissue destruction and enhances risk of disease progression in TB. Supporting this idea, a recent study reported that the presence of the –1607G allele was significantly higher in TB patients with advanced pulmonary fibrosis compared with that in those with minimal fibrosis (20).
Taken together, an emerging model (Figure 1) for the role of MMPs in TB is that M. tuberculosis induces tissue remodeling via induction of MMP-9 to establish itself in the host. Once M. tuberculosis is established within the granuloma, maintaining intact granulomas is beneficial to the host, since it keeps the pathogen under check. During reactivation of a latent infection, excessive MMP-1 secretion leads to matrix degradation and cavitation. Much remains to be learned regarding the spatial and temporal regulation of specific MMPs during lung remodeling in TB. Nonetheless, the work of Freidland’s group, along with other recent publications, suggest that MMPs represent viable therapeutic targets to control M. tuberculosis replication soon after infection by preventing nascent granulomas from maturing and by blocking matrix degradation and cavitary disease.
The role of MMPs in the progression of tuberculous granuloma. MMP-9 from epithelial cells initiates recruitment of new monocytes to the developing granuloma. As the granuloma matures, Th1 cells, regulatory T cells, and B cells are recruited to form a stable granuloma, and M. tuberculosis persists in a latent state. During reactivation, granulomas become caseating and necrotic, and excessive MMP-1 secretion from macrophages leads to collagen degradation and tissue destruction, which culminates in M. tuberculosis erosion into the airways.
The author’s research program is supported by NIH grants AI084822, AI069395, and AI065663.
Conflict of interest: The author has declared that no conflict of interest exists.
Reference information: J Clin Invest. 2011;121(5):1686–1688. doi:10.1172/JCI57423
See the related article at MMP-1 drives immunopathology in human tuberculosis and transgenic mice.