© 2003 American Thoracic Society DOI: 10.1165/rcmb.F265
CCR3A Key to Mast Cell Phenotypic and Functional Diversity?Pulmonary Research Group, Department of Medicine, University of Alberta, Edmonton, Alberta, Canada Address correspondence to: Dean Befus, Ph.D., Pulmonary Research Group, Room 574 HMRC, University of Alberta, Edmonton, AB, T6G 2S2 Canada. E-mail: dean.befus{at}ualberta.ca
Abbreviations: airway hyperresponsiveness, AHR • immunoglobulin E, IgE • interleukin, IL • lipopolysaccharide, LPS • mast cells, MC • ovalbumin, OVA • Toll-like receptor, TLR
In this issue, Price and coworkers report that expression of the chemokine receptor CCR3 by human mast cells (MC) is predominantly intracellular, and that cross-linking of Fc  RI selectively mobilizes CCR3 to the cell surface (1). This distinguishes MC from other cells such as eosinophils and Th2 type lymphocytes where CCR3 expression does not appear to have an intracellular store, and is restricted to the cell surface (2, 3). The ability of MC to mobilize CCR3 to their surface following activation, and the subsequent co-stimulatory role of eotaxin in enhancing interleukin (IL)-13 release, indicates that chemokines and their receptors may play a unique role in regulating MC function. Mobile pools of chemokine receptors, influenced by environmental factors and which control selective MC responses, may be important determinants of site-specific functional heterogeneity in MC. Below we discuss how these and other recent observations suggest CCR3 receptors play multiple roles in MC, including trafficking of progenitors, selective programming of MC phenotype and effector function. The implications of CCR3 control of MC involvement in chronic inflammation and airway hyperresponsiveness (AHR) are also discussed. Chemokines play a fundamental role in directing infiltration of inflammatory cells into tissue. These effects are mediated by interactions with specific receptors that belong to the seven transmembrane G-protein–coupled family, and increasing evidence suggests that their function is not restricted to chemotaxis. Chemokines induce B cell antibody class switching (4), and are implicated in the maturation and activation of a number of inflammatory cell types (5–9). There are four subclasses of chemokines (CC, CXC, C, and CXC3C), based on location of the first two cysteines in their sequence. Although most chemokine receptors bind more than one chemokine, CC receptors bind only CC chemokines, and CXC receptors bind only CXC chemokines.
CCR3 is a CC chemokine receptor expressed on eosinophils, MC, basophils, and a subset of human TH2-like T lymphocytes (10, 11). CCR3 is the only known receptor for eotaxin, but also recognizes other chemokines such as RANTES, macrophage inflammatory protein-1
MC are ubiquitously distributed throughout the body. They are heterogeneous and exhibit site-specific adaptations induced by microenvironmental signals that lead to selective expression of potential MC characteristics (17, 18). This flexibility in phenotype has important functional implications and allows the MC to adapt to organ- or tissue-specific roles. MC express high-affinity receptors for immunoglobulin (Ig)E that become occupied by IgE antibodies. Re-exposure to sensitizing allergens leads to release of several potent mediators stored in cytoplasmic granules, production and release of prostaglandins and leukotrienes, and de novo synthesis of cytokines. These events lead to classical allergic reactions with associated cascades of inflammatory events and symptoms. Although MC represent important sources of inflammatory mediators that promote acute allergic responses and early phase inflammation in asthma, it is becoming evident that MC contribute to late-phase asthmatic inflammation, AHR, and chronic tissue changes associated with immune responses (19, 20). For MC to take part in chronic inflammatory responses, mechanisms must exist to regulate not only tissue localization, proliferation, and viability of MC precursors, but also terminal differentiation, survival, and functional characteristics of mature MC. These mechanisms regulate MC numbers within normal connective tissues, while allowing for an increased supply of MC precursors and differentiation of mature cells within intraepithelial compartments during inflammatory responses. The exact nature of these control mechanisms is not clear. However, there is growing interest in the role of chemokines and their receptors in the determination of MC tissue localization and functional heterogeneity. In particular, CCR3 and its ligands are emerging as important components of these regulatory systems.
Unlike basophils or eosinophils, MC do not leave the bone marrow as mature cells but as committed progenitors that lack secretory granules and functions characteristic of MC (21). Upon reaching peripheral tissues, committed progenitors differentiate under the influence of microenvironmental factors to morphologically and functionally mature MC. Transit of human MC precursors to various sites within tissues may be regulated by their expression of multiple chemokine receptors and availability of respective ligands in the microenvironment. The transition from precursor to mature MC is accompanied by a change in chemokine receptor profile. Initially human MC progenitors express CXCR2, CCR3, CXCR4, and CCR5. Upon differentiation to mature cells, only CCR3 is maintained (22), an observation that may reflect the importance of CCR3 and its ligands in allergic and other inflammatory responses. With expression of eotaxin and other chemokines, epithelial cells provide a stimulus for movement of MC progenitors toward mucosal surfaces. There is marked heterogeneity among MC from different species and in different tissue sites within the same species (17, 18, 23, 24). In rodents, staining properties distinguish mucosal from connective tissue MC (25). In humans, however, histochemical subtype appears less related to tissue site. Human MC can also be subtyped based on the presence of serine proteases (tryptase and chymase). Cells that contain tryptase alone are designated MCT, whereas cells that contain both tryptase and chymase are designated MCTC and are predominantly associated with connective tissue. The two phenotypes are also heterogeneous with respect to content of certain cytokines. IL-4 is expressed preferentially by the MCTC. In contrast, IL-5 and IL-6 are restricted almost exclusively to the MCT subset (26). In addition to phenotypic differences in the MC subtypes, there is also functional heterogeneity that suggests the cells modify their characteristics on an organ- and/or tissue-specific basis (18, 23). Romagnani and colleagues reported that CCR3 expression is limited to MCTC in gut, lung, and skin tissue (22). Consequently CCR3-positive MC appear more abundant in the skin dermis and intestinal submucosa than in intestinal mucosa and lung interstitium. The migration and persistence of the MCTC subset into normal and inflamed tissue may depend to a large extent on the expression of CCR3. Co-culture of human umbilical cord blood cells with fibroblasts results in the development of mature MC that are predominantly of the MCTC phenotype (27), suggesting that fibroblasts not only facilitate differentiation of MC but also program for the MCTC phenotype. Fibroblasts are major sources of eotaxin (28, 29) and, in mouse embryos eotaxin acts synergistically with stem cell factor to induce the differentiation of MCTC cells from progenitors (30). Eotaxin is also constitutively expressed in tissues such as intestine, skin, and mammary gland, where the vast majority of MC are of MCTC (31). Evidence therefore points to a role for eotaxin, acting through CCR3, in the selective development of the MCTC phenotype. This has implications for the pathophysiology of allergic diseases, as the MCTC subtype provides a source of IL-4 and IL-13 in subjects with asthma (26). IL-13 can induce AHR and mucus hypersecretion, whereas IL-4 has diverse roles in the development of airway inflammation (32, 33). Interestingly, IL-4 and IL-13 also increase expression of eotaxin by lung fibroblasts and smooth muscle cells (34, 35). This, in addition to promoting eosinophilic airway inflammation, may further promote the development and activation of MCTC. The numbers of MC change during immunologic or inflammatory responses. However, little is known about the homing mechanisms of MC progenitors and the regulation of their hyperplasia at sites of allergic mucosal inflammation. Eotaxin is highly expressed in the epithelium and submucosa of bronchial biopsies from patients with atopic asthma (12–14). The demonstration of CCR3 on T-lymphocytes, basophils, and MC progenitors suggests that this chemokine may recruit several blood-borne cells that participate in allergic inflammation. Human MC expressing CCR3 receptor migrate ex vivo in response to eotaxin (22), and the C-C cytokine RANTES induces MC hyperplasia when administered in vivo (36). However, a study using CCR3-deficient mice showed that although eosinophil recruitment was impaired, there was an increase in baseline tracheal intraepithelial MC that became even more marked following allergen challenge (37). Such an increase in cell number was not observed when comparing jejunal intraepithelial MC hyperplasia of wild-type and CCR3-deficient mice in response to helminth infection (38). Therefore, although eotaxin acting through the CCR3 may determine the MCTC phenotype, the role of CCR3 and its ligands in regulating migration of MC into inflamed tissue is more complex than in the eosinophil, where it is an absolute requirement for recruitment of the cell (37).
It is interesting that the majority of MC in the bronchial smooth muscle of individuals with asthma are MCTC, and that their presence distinguishes asthma from eosinophilic bronchitis, a condition characterized by eosinophillia in the absence of AHR (39). Airway smooth muscle is a source of eotaxin and other meditors that could provide the correct microenvironment for the differentiation, activation, and survival of MC (40). Therefore, CCR3-mediated differentiation of MCTC in smooth muscle of individuals with asthma may play a role in AHR characteristic of this disease. The use of CCR3-deficient mice has yielded important information regarding the potential role for this receptor in regulating multiple aspects of MC behavior, including their potential involvement in AHR (37, 38). Wild-type mice sensitized intraperitoneally with ovalbumin (OVA) and subsequently challenged with aerosolized antigen develop eosinophilic inflammation and AHR to inhaled methacholine characteristic of asthma. Interestingly, in CCR3-deficient mice the recruitment of eosinophils to the lung and bronchoalveolar lavage fluid is significantly diminished, whereas AHR is enhanced (37). This observation is not as contradictory as it might appear, as a causal relationship between eosinophilic inflammation and AHR is far from established. Indeed, AHR may be induced by at least two independent pathways, one involving an eosinophil-dependent process and another that requires activation of MC (41, 42). MC numbers in the airway epithelium of CCR3-deficient mice, sensitized intraperitoneally with OVA, are significantly increased compared with wild-type mice (37). It therefore appears that sensitization and allergen challenge of CCR3-deficient mouse airways induces both differentiation and subsequent activation of intraepithelial MC to release mediators that enhance airway responsiveness to methacholine. This explanation appears counterintuitive, given that eotaxin has chemoattractant effects on MC (22) and that increased expression of eotaxin in the airways of patients with asthma correlates with enhanced AHR (14). Price and coworkers (1) suggest that during allergen exposure, CCR3 facilitates emigration of MC from the intraepithelial compartment of the lung rather than recruitment. In this model, MC progenitors move toward mucosal surfaces of the lung, where they differentiate into mature intraepithelial MC and in the absence of CCR3 are retained. It is also possible that in MC precursors lacking CCR3, alternative chemoattractant receptors, including CXCR2, CXCR4, and CCR5, influence the localization of these cells to the airway. Notably, the function of CCR3 on MC appears be organ-specific. The increase in MC numbers seen in allergen-challenged CCR3-deficient mice is restricted to the airways with no evidence for increased MC in the small intestine, skin, or spleen (37). Similarly, in contrast to the increase in tracheal intraepithelial MC, there is no difference in intraepithelial jejunal MC number between CCR3-deficient and wild-type mice in response to helminth infection (38). In the absence of intraepithelial MC, CCR3-deficient mice are protected from allergen-induced AHR. Wild-type mice epicutaneously sensitized to OVA and then challenged with aerosolized allergen exhibit eosinophilic airway inflammation and AHR similar to that seen in intraperitoneally sensitized animals (43). However, there is no increase in tracheal MC numbers in either wild-type or CCR3-deficient animals following antigen challenge. As might be expected, recruitment of eosinophils to the lung parenchyma and airways is severely impaired in CCR3-deficient mice. However, in contrast to intraperitoneally sensitized animals, these mice fail to develop AHR. In addition to supporting the role of MC in enhancing AHR in CCR3-deficient animals, these observations also highlight the fact that mechanisms involved in the development of AHR differ with different sensitization protocols. Clearly, the intraperitoneal sensitization described by Humbles and colleagues (37) leads to development of AHR that is MC-dependent, whereas epicutaneously sensitized mice developed AHR that is mediated by eosinophils. The potential for these two independent mechanisms to develop AHR has been well documented (41, 42).
Eotaxin potentiates the IgE-dependent generation of IL-4 by human basophils (44), whereas RANTES, a C-C chemokine, can induce histamine release from human basophils (45). In cultured human MC, eotaxin induces a rise in intracellular calcium (46), and although eotaxin alone does not cause release of histamine from sensitized MC, stimulation of cells with the chemokine following Fc RI cross-linking causes a significant increase in release of IL-13 (1). Taken together, these observations indicate that CCR3 has a co-stimulatory effect on FcRI signaling. Thus, chemokine receptors on MC have a role in determining the activation state and site-specific functional heterogeneity of the MC.
The intracellular distribution of chemokine receptors appears to be unique to MC, and indicates that the role of these receptors on the MC is different from that on eosinophils. Intracellular CCR3 is transported to the cell surface following Fc
It is clear that CCR3 may not only influence development, targeting, and phenotypic characteristics of MC, but also the extent of effector responses induced by immunologic stimuli. These recent developments suggest that chemokine receptors form part of the exquisite control mechanisms that make MC an adaptable component of the body's immunologic arsenal.
One of the most interesting advances in MC biology has been the observation that they can selectively release different mediators. For example Toll-like receptors (TLR) are critical for responses to a variety of bacterial, viral, and fungal products. MC express TLR2 that is activated by peptidoglycan from gram-positive bacteria and TLR4 that is involved in responses to lipopolysaccharide (LPS), the major constituent of the outer membrane of gram-negative bacteria (47). Activation of either receptor releases a range of cytokines from MC. However, only TLR2 activation concomitantly releases ß-hexosaminidase, a marker of MC degranulation (47, 48). Therefore, in response to infection, TLR4 activates the MC to release cytokines through a mechanism that does not involve classical degranulation. Furthermore, the profile of cytokine release induced by TLR2 and -4 differs. TLR2 activation releases tumor necrosis factor, IL-4, IL-5, IL-6, and IL-13, but not IL-1ß. In contrast, TLR4 induces the release of IL-1ß, tumor necrosis factor, IL-6, and IL-13, but not IL-4 or -5 (47). Thus certain receptors activate mechanisms that exist for the independent release of cytokines and allow the MC response to be tailored to specific physiologic or pathophysiologic conditions. Because eotaxin upregulates antigen-induced release of IL-13 without altering ß-hexosaminidase or IL-5 secretion, CCR3 may be one such receptor. The ability of CCR3 to induce MC to release cytokines other than IL-13 must be studied.
To fully realize the potential of CCR3-targeted therapy in allergic disease, there must be greater understanding of the mechanisms involved in mobilization of CCR3 to the cell surface and the interaction of the receptor with Fc The ability of CCR3 ligands, other than eotaxin, to modulate MC activation must also be addressed. Differential effects of the multiple ligands for CCR3 could expand the ability to "fine tune" MC effector function. Heterodimerization of the receptor is another mechanism through which greater flexibility could be achieved. Some, if not all, chemokine receptors initiate their ligand-induced signaling cascades by receptor dimerization (53, 54). As predicted by the extensive sequence identity among some chemokine receptors, they can also form heterodimers (53, 55). Heterodimerization is an efficient way to increase specificity and sensitivity, because assembly of different combinations of receptors generates variability in receptor subtypes and increases capacity for a variable response. The concept of chemokine receptor heterodimerization has important functional consequences, including increased sensitivity of some responses or the initiation of signaling events not triggered by individual chemokines (55). Investigation of potential heterodimer formation involving CCR3 in the MC could reveal hitherto unrecognized functions of the receptor.
It is likely that the ability of CCR3 to modulate MC responses is not restricted to Fc Received in original form February 20, 2003
|
||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Top
Introduction
MC
, and monocyte chemotactic proteins-2, -3, and -4, which bind to additional receptors. Recent studies in human asthma have made CCR3 an attractive target for therapeutic intervention (