Published ahead of print on November 30, 2006, doi:10.1164/rccm.200606-777PP
© 2007 American Thoracic Society doi: 10.1164/rccm.200606-777PP
Targeting the Networks that Underpin Contiguous Immunity in Asthma and Chronic Obstructive Pulmonary DiseaseAcademic Units of 1 Respiratory Medicine and 2 Infectious Diseases, School of Medicine and Biomedical Sciences, University of Sheffield, Royal Hallamshire Hospital, Sheffield, United Kingdom; and 3 Allergy and Inflammation Research, Division of Infection, Inflammation, and Repair, School of Medicine, University of Southampton, Southampton General Hospital, Southampton, United Kingdom Correspondence and requests for reprints should be addressed to Professor Ian Sabroe, Ph.D., F.R.C.P, Academic Unit of Respiratory Medicine, School of Medicine and Biomedical Sciences, University of Sheffield, L Floor, Royal Hallamshire Hospital, Sheffield S10 2JF, UK. E-mail: i.sabroe{at}sheffield.ac.uk ABSTRACT Recent advances in the field of innate immunity have driven an important reappraisal of the role of these processes in airway disease. Various strands of evidence indicate that resident cells, such as macrophages and epithelial cells, have central importance in the initiation of inflammation. Macrophage activation has the potential to regulate not just typical aspects of innate immunity but also, via a variety of intricate cell–cell networks, adaptive responses and responses characterized by Th2-type cytokine production. In turn, such adaptive immune processes modify the phenotype and function of the innate immune system. Cooperative responses between monocytic cells and tissue cells are likely to be crucial to the generation of effective inflammatory responses, and a realization of the importance of these networks is providing a new way of identifying antiinflammatory therapies. Importantly, the repeated cycles of allergic and nonallergic inflammation that comprise chronic human airway disease are not necessarily well described by current terminology, and we propose and describe a concept of contiguous immunity, in which continual bidirectional cross-talk between innate and adaptive immunity describes disease processes more accurately.
Key Words: asthma • chronic obstructive pulmonary disease • innate immunity • inflammation • macrophages The last 6 years have seen a major resurgence of interest in innate immunity, driven in significant part by work in the Toll-like receptor (TLR) field. Together with the emergence of the hygiene hypothesis, this has led to a reappraisal of the pathologies of asthma and chronic obstructive pulmonary disease (COPD). As part of this process, a series of studies from the respiratory and other fields can now be seen as identifying activation signals with major roles in disease, in which cell–cell interactions are crucial. Such signals combine into cooperative networks, an understanding of which will provide new opportunities to intervene in their pathology. Moreover, a bidirectional relationship is becoming apparent in which downstream consequences, such as tissue remodeling and allergic inflammation, can in turn affect subsequent responses to further rounds of signaling initiated by exogenous events. These networks link multiple aspects of immunity, and it is becoming increasingly clear that accommodating these complexities within the classical nomenclatures of innate and adaptive responses enforces distinctions that are to some degree arbitrary. At worst, this generates a tendency to use terminology in a way that tends to fix our viewpoints of dynamic pathological processes. We suggest that the principles evident from current research are best summarized by a new term, "contiguous immunity." From early interactions with the epithelial barrier and the alveolar macrophage to communication between structural cells and infiltrating leukocytes, and the complexity of the dendritic cell (DC)–adaptive immune system interface, contiguous immunity describes not the activation of a single linear program but the summation of linear and parallel repeating subroutines, which show tendencies to alter phenotype with repetition and which are poorly modeled by many acute challenge protocols. In contiguous immunity, we observe processes with features of innate and adaptive immunity happening concurrently (temporally contiguous) in interdependent networks; in addition, we see such events occurring in close association (physically contiguous). Thus, in asthma, contiguous immunity describes the summation of processes in which environmental stimuli, such as endotoxin, infections, and allergens, cooperate to activate epithelial and macrophage defense systems, engage mast cells, activate Th2-type T cells by antigen-presenting cells, and stimulate local Th2-type responses. We contend that it is contiguous immunity that drives the summated inflammatory response, rather than single specific pathways, and we illustrate some of these networks here (Figure 1).
CONTIGUOUS IMMUNITY AND THE PATHOLOGY OF AIRWAY DISEASE Asthma and COPD are difficult to study and model, in part because of their long chronicity and the slow evolution of their pathology. In asthma, infectious and environmental stimuli present in early life (and even before birth) modulate the development of Th2-patterned adaptive immunity. Although numerous strands of evidence demonstrate continuous involvement of Th2-type T-cell–mediated processes in established disease, it is clear that many other processes contribute to pathology, and the disease probably exhibits several phenotypes (1). The Th2 axis itself may in part be dependent on natural killer (NK) T-cell activation (2), and a Th2 bias in asthma will result from signals generated by a variety of elements of the immune system. Exacerbations of asthma are commonly triggered by infections that activate the pathways of innate immunity. Environmental exposures (particularly microbial) influence disease severity throughout adult life (3), as well as during the critical early-life phase in which the Th1/2 balance is set for each individual. Such stimuli will contribute to an ongoing dialog with the adaptive immune system and modulate responses to therapy; furthermore, exploiting TLR signaling by microbial-type adjuvants represents a major potential new way to down-regulate Th2 responses (4). Importantly, therefore, infections and occupational and environmental stimuli (allergic and nonallergic) cause repeated bouts of inflammation and contiguous repetitive engagement of different components of the immune system. The resulting interactions with genotype may give rise both to distinct asthma phenotypes, and to individual variations in disease presentation and progression. COPD is no less complicated, and arguably more poorly understood. Although the activation of the innate immune system by components of cigarette smoke (which include endotoxin) is widely accepted, the links among reactive oxygen species, phagocyte proteases, control of proinflammatory gene transcription, and macrophage and neutrophil function are still far from completely established. The transformation to chronic inflammation (whether smoking continues or has ceased), with repeated rounds of acute stimulation of the innate immune system and structural lung cells by microbial and environmental agonists, together with additional input from Th1-type T cells (5), again demonstrates a process best described as contiguous immunity. THE IMPACT OF THE TLR FIELD Pattern recognition receptors that recognize pathogen-associated molecular patterns (PAMPs) are integral to the innate immune responses that provide the first line of defense against environmental agents. Several structurally discrete groups are known, including TLRs, C-type lectins, cytosolic nucleotide-binding oligomerization domain proteins (NODs), and scavenger receptors. Of these, the best characterized are the TLRs. Studies in Drosophila first identified Toll as a receptor important in antifungal immunity: subsequent work in the mouse and humans revealed a homologous family of mammalian proteins that enable responses to a range of agonists principally associated with infectious stimuli, and potentially with tissue damage (6). The identification of these molecules has led to a reappraisal of how we respond to commensals and pathogens, and the mechanisms by which such stimuli tune and activate the immune system. TLRs divide into two main groups of receptors: those principally concerned with responding to bacteria, protozoa, and fungi (TLRs 1, 2, 4, 5, 6, and 9) and those facilitating antiviral immunity (TLRs 3, 7, and 8), although there is overlap, with some receptors involved in the detection of multiple families of pathogens (e.g., TLR9, which recognizes foreign DNA). Beyond the scope of this piece, a series of reviews have appraised the TLR field from the pulmonary viewpoint (e.g., Reference 4). INITIATION OF INFLAMMATION: THE ROLES OF INTERCELLULAR NETWORKS Both in vitro and in vivo studies demonstrate that inflammation is the product of cellular interactions in which potentially very small numbers of critical cells provide the first response to each round of proinflammatory stimuli, and subsequently direct the fate of many cells and tissues. The sentinel cells of the airways include alveolar macrophages, mast cells, and the epithelium. As the cells that form the interface with the environment, epithelial cells play a primary role in innate defense, forming a physical barrier whose surface is protected by mucus, which provides nonspecific protection by trapping inhaled particles and allowing clearance via the mucociliary escalator. This mucus also contains a range of antimicrobial molecules, including defensins, cathelicidins, collectins, lysozyme, and lactoferrin, which not only protect against respiratory pathogens but also contribute to the influx of inflammatory cells and activation of adaptive immunity. In addition to the humoral protection provided by the epithelium, it forms a highly selective physical barrier where intercellular tight junctions effectively restrict transepithelial movement of particulates and even of hydrophilic molecules of a molecular weight greater than approximately 2 kDa. Disruption of the epithelial barrier can allow host exposure to environmental particles, resulting in release of cytokines and activation of an immune inflammatory response. Alveolar macrophages also survey the airway while the epithelium is intact (discussed below), and studies in the gut have also shown that mucosal DCs sample the external environment while maintaining barrier integrity by forming tight junctions with the epithelial cells (7). This suggests that physical interactions between DCs and epithelial cells can provide important signals for immune homeostasis. Responses to environmental stimuli are mediated in large part by TLRs (4, 6), which are crucial to responses to LPS (TLR4), and mediate responses to other stimuli such as tissue damage signals (TLR2 and TLR4), and specific bacterial proteins such as pneumolysin (TLR4), bacterial lipoproteins (TLR2), and bacterial DNA (TLR9). The alveolar macrophage would appear to have a pivotal role in initial responses to inhaled LPS (8) and infections such as those caused by the pneumococcus (9), but tissue cells are also important, because expression of TLR4 by tissues may be required for the bronchoconstrictive actions of LPS, whereas alveolar macrophage TLR4 is necessary for leukocyte recruitment (10).
In vitro models have also provided valuable insights. When monocytes are cocultured with tissue cells and stimulated with LPS, there is a profound induction of cytokines from tissue cells, such as airway smooth muscle, which are detectable at surprisingly low monocyte:tissue cell ratios (11). There is strong evidence that cooperative responses to LPS are heavily dependent on monocyte-derived IL-1
Not all activators of the innate immune compartment are effective stimuli of monocytes, however, as exemplified by double-stranded RNAs (often generated during viral replication), which typically activate tissue cells and some macrophage populations. Synthetic double-stranded RNAs are less efficacious activators of peripheral blood mononuclear cells (12), and their ability to activate alveolar macrophages is restricted by an absence of IFN-
Importantly, then, it is possible to define, in vitro and in vivo, early inflammatory networks activated by stimuli of the immune system. Although antagonism of IL-1 signaling can attenuate airway hyperresponsiveness after allergen challenge or ozone exposure (23, 24), data from different approaches do not always generate complementary results, as illustrated by experiments showing no role for IL-1 HOW INFLAMMATION MIGHT BE PERPETUATED: COMPLEX ENVIRONMENTS AND INVOLVEMENT OF TH2-TYPE RESPONSES Although the pathways described above are involved in the initiation of inflammation, persistent or recurrent exposures drive repeated or continuous cycles of activation. In contiguous immunity, the line between initiation and perpetuation of inflammation becomes blurred with bidirectional signals that modulate outputs. It is therefore helpful to consider the further networks that spiral out from, and feed back to, events and cells we would consider to be front line; and also to note that inflammatory cycles become substantially moderated with repetition. The interaction between innate and adaptive responses is in part genetically regulated, as evidenced by the putative M1/M2 bias of monocyte/macrophage responses (25). Abnormal phenotypes of cytokine generation and responsiveness to inflammatory insults of airway epithelium and smooth muscle in asthma can result from many underlying genetic factors influencing not just the macrophage but also epithelium or smooth muscle, and chronic inflammation or epigenetic mechanisms may also favor altered tissue phenotypes (18, 26, 27). Changes in the proportions of tissues and cells available to respond to stimuli, as well as their location within the airways, will further influence responses to repeated insults (5, 15, 26–28). Epithelial disruption in asthma, and the changes in epithelial phenotype toward squamous metaplasia in COPD, will contribute toward altered inflammatory responses. The potential for defective barrier function also to be a primary lesion contributing to the development of allergic disease is indicated by the finding that polymorphisms in the filaggrin gene have been linked with both atopic dermatitis and asthma (29). How, then, is signaling bridged to Th2-type immunity in asthma? The reader will be familiar with the established links dependent on DC activation and their presentation of antigen to T cells, with the development of a Th2-type memory response. LPS concentrations influence atopic sensitization in a dose-dependent fashion: low amounts of bacterial products, such as LPS, are actually required for the efficient development of a Th2-type T-cell response (30), even while the hygiene hypothesis proposes protection from atopy in early life by stimuli potentially including higher amounts of LPS. Considerable research has demonstrated the enormous importance of the Th2 cytokine axis to the pathology of asthma, but it is clear that the Th2-type T cell is only a component of the Th2-type network. Recent studies have implicated NK T cells as important sources of Th2-type cytokines in asthma (2), and NK cells may also play significant roles in allergic inflammation (31). Once again, activation of the monocyte/macrophage is relevant, as these cells can influence NK cell activation, via production of a more complex cytokine mix, including IL-1, interferons, and IL-12 (32, 33). In addition, the role of the mast cell is likely to be significant. Although mast cells are well known for their ability to secrete Th2-type cytokines in response to allergen, their responses are also regulated by viral and bacterial stimuli activating TLRs (e.g., Reference 34), although the spectrum of TLR expression on different mast cell tissue phenotypes from humans and mice is likely to vary. Mast cells provide a dispersed airway surveillance and response system (35), with the potential to alter their tissue distribution in established disease, as exemplified by their selective recruitment to airway smooth muscle in asthma (26). The concept of contiguous immunity encompasses scenarios in which repeated rounds of inflammation initiated by infections and environmental stimuli cause activation of innate immunity and also continuously input into activation of the adaptive response. In asthma, Th2-type immunity is likely to be supported not just by repeated rounds of antigenic stimulation but by chronic activation of mast cells and cells such as NK T cells, perhaps helping to explain why allergen avoidance is often a disappointing therapeutic option for an allergic disease. Stimuli of the innate immune system may also directly regulate proliferation of T cells (36, 37), B cells (38), and the function of T-regulatory cells (39). Of note, the effects of a lifelong exposure to varied amounts of inhaled immune activators on the phenotype of disease are extremely difficult to model in a short-lived species such as the mouse, on which much of our understanding of Th2 inflammation is based. Established inflammation may remain at least partially under the control of sentinel cells, such as the monocyte/macrophage, because regulation of eosinophil (40) and neutrophil (41) survival in response to LPS has been shown to be largely monocyte dependent. Such networks potentially use different cytokines to those that are implicated in the activation of tissue cells, because LPS-mediated, monocyte-dependent neutrophil survival is IL-1 independent (41). Like descending into a Mandelbrot fractal where patterns repeat on a smaller and smaller scale, each recruited cell type may modify the tissue into which it is drawn, as exemplified by the ability of neutrophils to activate epithelial cells (42), influence Th1-type T-cell inflammatory responses (43), or induce apoptosis in airway smooth muscle (44). In addition, neutrophils may secrete factors modulating macrophage function (45), and although neutrophil death by apoptosis is typically held to be injury limiting, where neutrophils die by necrosis, or die after pathogen encounter, they may markedly enhance tissue damage and generate a further proinflammatory signal from tissue phagocytes (46). It is clear that established Th2 inflammation can exert a dramatic influence on responses to repeated rounds of inflammation, through effects on immune cell recruitment, airway remodeling, and alterations to the phenotype of lung tissue cells. The many actions of Th2 cytokines such as IL-13 have been extensively documented in a series of excellent reviews (e.g., Reference 47) and are therefore not reprised at this point, but have the potential to contribute significantly to the processes of contiguous immunity described here. The contiguous nature of immunity is further emphasized by the impact of Th2-type immunity on responses to infection. Allergic airway inflammation can inhibit pulmonary host defense by inhibiting epithelial antimicrobial peptide production (48). Asthma increases susceptibility to invasive pneumococcal disease (49), and the capacity of a macrophage-dependent response to limit this outcome in vivo (9) is potentially less with a Th2-biased immune response (50). Modified responses to pathogens are in turn likely to contribute to the temporal evolution of inflammatory subsystems and hence disease phenotypes. TARGETING CONTIGUOUS IMMUNITY AND NETWORKED INFLAMMATION Can we use the concept of contiguous inflammation to influence rational drug design? In asthma, an appreciation of the contribution of complex networks to a Th2-phenotype disease may provide new ways of down-regulating these processes. For both asthma and COPD, an awareness of these networks, and an understanding of their temporal plasticity, might have implications for targeting. First, it is reasonable to assume that targeting early signaling events during repeated rounds of activation might be feasible: for example, targeting IL-1 or other upstream mediators may be an effective strategy in acute exacerbations of airway disease, particularly in those driven by infections, to limit amplification of tissue inflammation. Second, more accurate mapping of the networks initiated in vivo and in vitro might generate some surprising findings with respect to mediators occupying key positions in inflammation. Identification of the clusters of subsystems activated below the early-response networks, and the discernment of how such subsystems evolve toward chronic self-perpetuation, might do much to generate new drug targets. Consideration also needs to be given to the use of multiple targets. Where activation of a critical subsystem can be driven by more than one route, we may need to intervene at the level of two or more of the drivers of the targeted process. For example, a combination of corticosteroids to down-regulate inflammatory cytokine production, and a chemokine receptor antagonist to inhibit monocyte recruitment, might over the course of several months profoundly alter the lung microenvironment and dramatically decrease the initiation of inflammatory signals and consequent inflammation. The limitations of relatively short-term animal models of predominantly allergic inflammation make predicting the outcome of specific therapies in humans relatively challenging, because such models, valuable though they are, cannot hope to recapitulate the full phenotype(s) of human disease nor the full complexity of the networks of contiguous immunity. This has been clearly recognized by others, and more prolonged in vivo challenge models are now being exploited, although the compression of years of human disease into weeks or months of murine pathogenesis is still a significant challenge. It may be that a combination of in vitro and in vivo approaches using a series of models breaking down contiguous immunity into specific components will overcome these limitations and identify key recurring players. We are doing much to address this already, but there remains considerable room for a broadening of approaches beyond those focusing on the Th2-type T cell and the DC, important though these cells undoubtedly are. In conclusion, a resurgent interest in innate immunity has developed new opportunities to target inflammatory airway disease. Many strands of evidence identify cells such as monocytes/macrophages and epithelial cells at the apex of inflammatory cascades central to responses to microbial agonists. In cooperation with epithelial-based defenses against viral infections, intricate networks rapidly create effective immunity, and are amenable to scientific exploration and therapeutic targeting. As these networks combine with activation induced by allergen, and are repeatedly activated over time, complex patterns of inflammation develop that move beyond simple categorization into innate and adaptive responses; for such systems, contiguous immunity may be a better description of the pathological processes involved. FOOTNOTES Supported by an MRC Senior Clinical Fellowship (No. G116/170; I.S.), and by a Wellcome Trust Senior Clinical Fellowship (No. 076945; D.H.D.). Originally Published in Press as DOI: 10.1164/rccm.200606-777PP on November 30, 2006 Conflict of Interest Statement: I.S. received support from GlaxoSmithKline (GSK) and AstraZeneca (AZ) for conference attendance, and has received lecture fees from GSK, Boehringer-Ingelheim (BI) and AZ. He has also received an unrestricted research fellowship from GSK that did not contribute to the funding of this work. L.C.P. does not have a financial relationship with a commercial entity that has an interest in the subject of this manuscript. D.H.D. has received support from GSK, Gilead, Roche, Abbott, and BI for conference attendance and has received lecture fees from GSK. D.E.D. received research grants from AZ in 1999 and from Novartis and Aventis in 2003. She currently consults for, and holds share in, Synairgen and holds patents for the use of interferons to treat exacerbations of asthma and COPD. S.K.D. does not have a financial relationship with a commercial entity that has an interest in the subject of this manuscript. M.K.B.W. has received a research grant for GSK, relating to a multicenter asthma genetics study. She has received support from BI for conference attendance and lecture fees from AZ. Received in original form June 12, 2006; accepted in final form November 28, 2006 REFERENCES
This article has been cited by other articles:
|
||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
in cocultures of monocytes and airway smooth muscle (
induction by rhinovirus or LPS in asthmatic macrophages was also deficient and correlated with exacerbation severity. 
