Pathogen-specific Treg cells expand early during mycobacterium tuberculosis infection but are later eliminated in response to Interleukin-12.

Thymically derived Foxp3⁺ regulatory T (Treg) cells have a propensity to recognize self-peptide:MHC complexes, but their ability to respond to epitope-defined foreign antigens during infectious challenge has not been demonstrated. Here we show that pulmonary infection with Mycobacterium tuberculosis (Mtb), but not Listeria monocytogenes (Lm), induced robust lymph node expansion of a highly activated population of pathogen-specific Treg cells from the pre-existing pool of thymically derived Treg cells. These antigen-specific Treg cells peaked in numbers 3 weeks after infection but subsequently underwent selective elimination driven, in part, by interleukin-12-induced intrinsic expression of the Th1-cell-promoting transcription factor T-bet. Thus, the initial Mtb-induced inflammatory response promotes pathogen-specific Treg cell proliferation, but these cells are actively culled later, probably to prevent suppression during later stages of infection. These findings have important implications for the prevention and treatment of tuberculosis and other chronic diseases in which antigen-specific Treg cells restrict immunity.

T-bet(+) Treg cells undergo abortive Th1 cell differentiation due to impaired expression of IL-12 receptor ß2.

Foxp3(+) regulatory T (Treg) cells limit inflammatory responses and maintain immune homeostasis. Although comprised of several phenotypically and functionally distinct subsets, the differentiation of specialized Treg cell populations within the periphery is poorly characterized. We demonstrate that the development of T-bet(+) Treg cells that potently inhibit T helper 1 (Th1) cell responses was dependent on the transcription factor STAT1 and occurred directly in response to interferon-γ produced by effector T cells. Additionally, delayed induction of the IL-12Rß2 receptor component after STAT1 activation helped ensure that Treg cells do not readily complete STAT4-dependent Th1 cell development and lose their ability to suppress effector T cell proliferation. Thus, we define a pathway of abortive Th1 cell development that results in the specialization of peripheral Treg cells and demonstrate that impaired expression of a single cytokine receptor helps maintain Treg cell-suppressive function in the context of inflammatory Th1 cell responses.

Macrophages, Wound Healing, and Fibrosis: Recent Insights.

PURPOSE OF REVIEW: Macrophages are central players in the immune response following tissue injury. These cells perform many functions, and the changing tissue microenvironment during injury shapes macrophage phenotype down a variety of polarized pathways. This review summarizes the current knowledge on the roles of macrophages during different stages of tissue injury, repair, and-if repair is not achieved-fibrosis. RECENT FINDINGS: Macrophages present early in inflammation are functionally distinct from those at later stages. The predominant macrophage phenotype must transition from pro-inflammatory to pro-reparative to facilitate wound healing and scar resolution. If macrophages fail to acquire a tissue-healing phenotype, dysregulated signals can be drivers of disease processes, such as sustained, exuberant inflammation-as occurs in arthropathies-and fibrosis. Comprehensive understanding of the roles of specific macrophage populations at different stages of the repair process will support the development of immune-targeted therapies for diseases such as fibrosis.

Stromelysin-2 (MMP10) Moderates Inflammation by Controlling Macrophage Activation.

Several members of the matrix metalloproteinase (MMP) family control a range of immune processes, such as leukocyte influx and chemokine activity. Stromelysin-2 (MMP10) is expressed by macrophages in numerous tissues after injury; however, little is known of its function. In this study, we report that MMP10 is expressed by macrophages in human lungs from patients with cystic fibrosis and induced in mouse macrophages in response to Pseudomonas aeruginosa infection both in vivo and by isolated resident alveolar and bone marrow-derived macrophages (BMDM). Our data indicates that macrophage MMP10 serves a beneficial function in response to acute infection. Whereas wild-type mice survived infection with minimal morbidity, 50% of Mmp10(-/-) mice died and all showed sustained weight loss (morbidity). Although bacterial clearance and neutrophil influx did not differ between genotypes, macrophage numbers were ∼3-fold greater in infected Mmp10(-/-) lungs than in wild-types. Adoptive transfer of wild-type BMDM normalized infection-induced morbidity in Mmp10(-/-) recipients to wild-type levels, demonstrating that the protective effect of MMP10 was due to its production by macrophages. Both in vivo and in cultured alveolar macrophages and BMDM, expression of several M1 macrophage markers was elevated, whereas M2 markers were reduced in Mmp10(-/-) tissue and cells. Global gene expression analysis revealed that infection-mediated transcriptional changes persisted in Mmp10(-/-) BMDM long after they were downregulated in wild-type cells. These results indicate that MMP10 serves a beneficial role in response to acute infection by moderating the proinflammatory response of resident and infiltrating macrophages.

Regulatory T-cell homeostasis: steady-state maintenance and modulation during inflammation.

Regulatory T (Treg) cells play a vital role in the prevention of autoimmunity and the maintenance of self-tolerance, but these cells also have an active role in inhibiting immune responses during viral, bacterial, and parasitic infections. Although excessive Treg activity can lead to immunodeficiency, chronic infection, and cancer, too little Treg activity results in autoimmunity and immunopathology and impairs the quality of pathogen-specific responses. Recent studies have helped define the homeostatic mechanisms that support the diverse pool of peripheral Treg cells under steady-state conditions and delineate how the abundance and function of Treg cells changes during inflammation. These findings are highly relevant for developing effective strategies to manipulate Treg cell activity to promote allograft tolerance and treat autoimmunity, chronic infection, and cancer.

Muscle paralysis induces bone marrow inflammation and predisposition to formation of giant osteoclasts.

Transient muscle paralysis engendered by a single injection of botulinum toxin A (BTxA) rapidly induces profound focal bone resorption within the medullary cavity of adjacent bones. While initially conceived as a model of mechanical disuse, osteoclastic resorption in this model is disproportionately severe compared with the modest gait defect that is created. Preliminary studies of bone marrow following muscle paralysis suggested acute upregulation of inflammatory cytokines, including TNF-α and IL-1. We therefore hypothesized that BTxA-induced muscle paralysis would rapidly alter the inflammatory microenvironment and the osteoclastic potential of bone marrow. We tested this hypothesis by defining the time course of inflammatory cell infiltration, osteoinflammatory cytokine expression, and alteration in osteoclastogenic potential in the tibia bone marrow following transient muscle paralysis of the calf muscles. Our findings identified inflammatory cell infiltration within 24 h of muscle paralysis. By 72 h, osteoclast fusion and pro-osteoclastic inflammatory gene expression were upregulated in tibia bone marrow. These alterations coincided with bone marrow becoming permissive to the formation of osteoclasts of greater size and greater nuclei numbers. Taken together, our data are consistent with the thesis that transient calf muscle paralysis induces acute inflammation within the marrow of the adjacent tibia and that these alterations are temporally consistent with a role in mediating muscle paralysis-induced bone resorption.

Filamentous Bacteriophage Produced by Pseudomonas aeruginosa Alters the Inflammatory Response and Promotes Noninvasive Infection In Vivo.

Pseudomonas aeruginosa is an important opportunistic human pathogen that lives in biofilm-like cell aggregates at sites of chronic infection, such as those that occur in the lungs of patients with cystic fibrosis and nonhealing ulcers. During growth in a biofilm, P. aeruginosa dramatically increases the production of filamentous Pf bacteriophage (Pf phage). Previous work indicated that when in vivo Pf phage production was inhibited, P. aeruginosa was less virulent. However, it is not clear how the production of abundant quantities of Pf phage similar to those produced by biofilms under in vitro conditions affects pathogenesis. Here, using a murine pneumonia model, we show that the production of biofilm-relevant amounts of Pf phage prevents the dissemination of P. aeruginosa from the lung. Furthermore, filamentous phage promoted bacterial adhesion to mucin and inhibited bacterial invasion of airway epithelial cultures, suggesting that Pf phage traps P. aeruginosa within the lung. The in vivo production of Pf phage was also associated with reduced lung injury, reduced neutrophil recruitment, and lower cytokine levels. Additionally, when producing Pf phage, P. aeruginosa was less prone to phagocytosis by macrophages than bacteria not producing Pf phage. Collectively, these data suggest that filamentous Pf phage alters the progression of the inflammatory response and promotes phenotypes typically associated with chronic infection.

Toward a Predict and Prevent Approach in Obstructive Airway Diseases.

Asthma and chronic obstructive pulmonary disease are currently diagnosed and treated after the demonstration of variable airflow limitation and symptoms. Under this framework, undiagnosed and unchecked airway inflammation is associated with recurrent acute attacks, airway remodeling, airflow limitation, adverse effects of corticosteroids, and impaired quality of life, ultimately leading to the collection of side effects termed "people remodeling." This one-size-fits-all damage control approach aims to control symptoms and treat exacerbations rather than modify the underlying disease process. The advent of highly effective therapies targeting proximal drivers of airway inflammation calls for a paradigm shift; upstream-acting therapies offer potential to alter the disease course and achieve clinical remission. We propose moving away from downstream firefighting and toward a "predict and prevent" model, measuring inflammation and providing anti-inflammatory therapy early, without waiting for further clinical deterioration. Much in the same way that high blood pressure and cholesterol are used to predict and prevent heart attacks, in asthma, elevated blood eosinophils and/or exhaled nitric oxide can be used to predict and prevent asthma attacks. We also advocate moving research further upstream by identifying patients with subclinical airway inflammation or disease who may be at risk of progressing to airflow limitation and associated morbidities and intervening early to prevent them. In summary, we call for a predict and prevent approach in obstructive airway disease.

Matrix Metalloproteinases and Leukocyte Activation.

As their name implies, matrix metalloproteinases (MMPs) are thought to degrade extracellular matrix proteins, a function that is indeed performed by some members. However, regardless of their cell source, matrix degradation is not the only function of these enzymes. Rather, individual MMPs have been shown to regulate specific immune processes, such as leukocyte influx and migration, antimicrobial activity, macrophage activation, and restoration of barrier function, typically by processing a range of nonmatrix protein substrates. Indeed, MMP expression is low under steady-state conditions but is markedly induced during inflammatory processes including infection, wound healing, and cancer. Increasing research is showing that MMPs are not just a downstream consequence of a generalized inflammatory process, but rather are critical factors in the overall regulation of the pattern, type, and duration of immune responses. This chapter outlines the role of leukocytes in tissue remodeling and describes recent progress in our understanding of how MMPs alter leukocyte activity.

CCR7 provides localized access to IL-2 and defines homeostatically distinct regulatory T cell subsets.

Immune tolerance and activation depend on precise control over the number and function of immunosuppressive Foxp3(+) regulatory T (T reg) cells, and the importance of IL-2 in maintaining tolerance and preventing autoimmunity is clear. However, the homeostatic requirement for IL-2 among specific populations of peripheral T reg cells remains poorly understood. We show that IL-2 selectively maintains a population of quiescent CD44(lo)CD62L(hi) T reg cells that gain access to paracrine IL-2 produced in the T cell zones of secondary lymphoid tissues due to their expression of the chemokine receptor CCR7. In contrast, CD44(hi)CD62L(lo)CCR7(lo) T reg cells that populate nonlymphoid tissues do not access IL-2-prevalent regions in vivo and are insensitive to IL-2 blockade; instead, their maintenance depends on continued signaling through the co-stimulatory receptor ICOS (inducible co-stimulator). Thus, we define a fundamental homeostatic subdivision in T reg cell populations based on their localization and provide an integrated framework for understanding how T reg cell abundance and function are controlled by unique signals in different tissue environments.