The association between airway smooth muscle and asthma has been acknowledged for nearly a hundred years, in that excessive contraction of the muscle was known to form the basis of the exaggerated airway narrowing which is an integral part of an asthma "attack". With the recent advances made in understanding the biology of the airway smooth muscle cell, its role as simply a contractile element has expanded into that of a cell which can also modulate the immune response, affect airway remodelling, and contribute significantly to the pathology of airways disease. The airway smooth muscle cell is the target for many therapeutic regimens, and has been shown to respond both in vivo and in vitro to anti-inflammatory agents such as corticosteroids, and to bronchodilators such as β₂-agonists. Given that increased airway smooth muscle mass is a consistent finding in the asthmatic remodelling process, this cell has become the focus of research for many groups world wide.

It is interesting however to speculate on the function of the airway smooth muscle in the non- diseased airway. It appears to serve little purpose either in terms of playing a role in inhalation or exhalation, assisting the propulsion of mucus, optimising ventilation/perfusion relationships or protecting the peripheral lung from noxious particles/allergens. Mitzner has therefore suggested that the airway smooth muscle is vestigial and likens it to the appendix—assuming importance only in the diseased state.1

The importance of the smooth muscle in asthma has recently been emphasised by the investigation of the use of thermoplasty—i.e. the delivery of thermal energy via a bronchoscope to target the airway smooth muscle. Initial experiments in dogs revealed that the ensuing reduction in smooth muscle mass resulted in an increase in airway size whether the airway was contracted or relaxed2 and a reduction in airway hyperresponsiveness.3 Preliminary studies in non asthmatic humans have established feasibility and safety.4 Recently, in a study of 16 subjects with mild to moderate asthma5 thermoplasty was well tolerated and produced improvements in airway hyperresponsiveness which were maintained for 2 years. Whether thermoplasty will become a widely accepted therapeutic intervention for remains to be seen.

The nature of the essential abnormality which causes contraction of airway smooth muscle in an asthmatic subject in response to stimuli as diverse as allergens, occupational triggers, cold air etc while these have no effect in a nonasthmatic subject, remains unknown. Attempts to prove that the muscle is "supercontractile" have met with little success, in that the opportunities to conduct contractile studies on asthma-derived muscle are rare and the results of the few studies that do exist vary from demonstrating increased contractility to decreased contractility to decreased relaxation to no clear differences.6-10

Several theories proposing a contractile dysfunction have been proposed. The difference in response of asthmatics and nonasthmatics to deep inspiration (DI) following bronchoconstriction has provided a stimulus for much discussion.11 DI in nonasthmatic patients produces prolonged bronchodilatation, whereas this effect is diminished in patients with asthma. The reasons for this difference are not known but could be the result of stiffer airways in asthmatic patients, or perhaps an increased propensity for the asthmatic airways to renarrow after dilatation.

The contraction of airway smooth muscle is dependent on activation of actin and myosin crossbridges and the phosphorylation of myosin light chain by myosin light chain kinase. The degree to which myosin light chain is phosphorylated can determine the velocity of shortening and maximum contraction and there is a possibility that this may be altered by the coexistence of allergy or sensitisation. Recently Ma et al.12 have attempted to show that a phenomenon that exists in canine isolated airway smooth muscle cells viz the presence of supercontractile muscle cells13 occurs in humans. They found that single cells isolated from asthmatic subjects contracted to a greater extent and more quickly than muscle cells from non asthmatic subjects. Moreover there was increased messenger RNA for myosin light chain kinase in the asthmatic cells, which they hypothesise explains their findings. More studies such as these are needed to confirm that asthmatic muscle is intrinsically hypercontractile.

The link between asthma and allergy is well established but the role of the airway smooth muscle cell in allergic events is controversial. Almost 20 years ago it was noted that muscle tissue derived from atopic nonasthmatic subjects exhibited different in vitro contractile properties compared with muscle from non atopic controls.14 Mast cell derived tryptase potentiated contractile responses in tissue derived from atopic but nonasthmatic subjects but not that obtained from nonatopic subjects, and the mechanisms appeared to be calcium dependent.15 In addition the incubation of human airway smooth muscle rings in serum from highly atopic asthmatic patients—so-called passive sensitisation- altered contractile and relaxation responses to a wide range of agonists.16, 17 The atopic serum contained high concentrations of IgE. Incubation of bronchial rings in either high concentrations of IgE or atopic serum increased the number of IgE bearing cells which were identified as largely mast cells.18 In addition a contractile response to anti IgE could be elicited in both these situations. All these findings would lead to a logical conclusion that IgE itself was the agent responsible for the changes in contractile and relaxant properties. Moreover human airway smooth muscle cells have been shown to constitutively express Fc epsilon R1.19 However Watson et al. demonstrated convincingly that IgE was not the factor responsible for the increased responsiveness induced in vitro by passive sensitisation.20 Tunon de Lara has speculated that specific IgG may play a contributory role, as could the presence of inflammatory cell derived proteases and cytokines such as stem cell factor and tryptase.21 It is of interest that the anti-human IgE antibody (omalizumab) has been demonstrated to have beneficial effects as an add on therapy in several clinical studies but it is unlikely that these can be related to effects on the airway smooth muscle cell.

Airway smooth muscle cells are a key component of the inflammatory milieu, and have the ability to synthesise a vast repertoire of cytokines, chemokines and other inflammatory mediators, such as RANTES, eotaxin, IL-1β, IL-5, IL-6, IL-8, IL-11, MCP-1, MCP-2, MCP-3, GM-CSF, IFN-β, leukaemia inhibitory factor, and prostanoids such as prostaglandin E₂ (PGE₂).22 Many of these factors are produced in response to stimulation with proinflammatory cytokines such as TNF-α, INF-γ, and IL-1β, and since these factors are found to be elevated in the airways of asthmatic patients, in vitro stimulation is likely to reflect the inflammatory cascade present within the asthmatic airway. In addition many cytokine and chemokine receptors are present upon the surface of these cells, suggesting that both autocrine and paracrine signaling control airway smooth muscle function in vivo (see Fig. 1).

Antigen presentation is function which is generally considered to be restricted to so called professional antigen presenting (APC) cells (macrophages, dendritic cells and B lymphocytes) of the immune system. Given that airway smooth muscle cells can express MHC class II,23 and the co-stimulatory molecules CD80 and CD86,24 it is surprising that they appear unable to act as antigen presenting cells. However as alveolar macrophages from only asthmatic subjects are able to function as APC,25 it remains to be determined if the same is true of asthmatic muscle cells. Airway smooth muscle cells do however possess the ability to function as a cell of the innate immune system since they express, and respond to ligation of, several Toll-like receptors.26

Eosinophils bind to smooth muscle cells, adhesion is modulated by TNF-α and is also mediated through ICAM-1 and VCAM-1.27 Mast cells are found to localise within smooth muscle bundles in vivo, however, the mechanism by which mast cells are recruited and adhere to smooth muscle cells is not known. It is hypothesised that the elevated cytokine and chemokine production observed in asthma, in combination with smooth muscle derived stem cell factor (SCF)28 could provide a mechanism by which mast cell numbers within smooth muscle bundles are elevated in asthmatic subjects. Recently Brightling et al. have reported that the chemokine CXCL10 (IP-10) is expressed preferentially in the muscle of asthmatic patients and ex vivo asthma derived smooth muscle cells. Together with the fact that the chemokine receptor CXCR3 was preferentially expressed on mast cells located in the muscle, this constitutes a mechanism for the attraction of the mast cells to the muscle.29 The chemokine fractalkine or CX3CL1 which is also produced by airway smooth muscle cells also may contribute to mast cell recruitment, especially when the mast cell are stimulated with vasoactive intestinal polypeptide (VIP).30 Mast cell adhesion to smooth muscle cells has recently been shown to be mediated in part by tumour suppressor in lung cancer-1 (TSLC1), which is preferentially expressed upon the surface of human lung mast cells, but not by ICAM-1, VCAM-1, CD18, and the alpha4 and beta1 integrins.31 Adhesion of Mast cells to smooth muscle cells induces the release of eotaxin which is dependant upon p38 MAPK.32 Mast cells located within the airway bundles of asthmatic subjects produce IL-4 and IL-13.33 IL-4 and IL-13 are T helper 2 cytokines that are thought to contribute to the pathogenesis of asthma, and, furthermore, have been shown to have direct effects upon airway smooth muscle cells, such as the induction of eotaxin34 and the reduction of beta-adrenergic responsiveness by IL-13.35 In addition, in vivo degranulation of mast cells correlates to asthma severity, and the increased degranulation observed in cartilaginous versus membranous bronchioles suggests an inhaled stimulus is activating these cells.36

Smooth muscle cells express several cell surface receptors capable of activating and binding other immune cells. ICAM-1 and vascular cell adhesion molecule (VCAM) -1 are constitutively expressed upon the cell surface and are inducible by pro-inflammatory cytokines such as TNF-α.37 ICAM-1, in addition to being the cellular receptor for the major groups of rhinoviruses, and VCAM-1 can be used by activated T cells to mediate adhesion. CD44, which is the cellular receptor for hyaluronan, is also constitutively expressed upon the surface of smooth muscle cells and T cells. It is hypothesized that hyaluronan which is bound to CD44 upon the surface of smooth muscle cells can act as a binding site for T cell CD44.37 Of particular importance in the context of asthma, adhesion of T cells to smooth muscle cells induces smooth muscle proliferation.37 CD40, a member of the TNF receptor family, is expressed upon the surface of airway smooth muscle cells. CD40 binds to a specific ligand (CD40 L), which is expressed upon the surface of T cells. Direct activation of smooth muscle CD40 increased intracellular calcium and IL-6 protein secretion,38 further indicating the possible importance of T cell airway smooth muscle interaction. Further evidence for the possible interactions between smooth muscle cells and T cells was provided in a recent report by Burgess et al.. Smooth muscle cells express OX40 ligand, and furthermore, activation of OX40 ligand increased the IL-6 release from airway smooth muscle cells. As OX40 is expressed upon the surface of T cells activated through T cell receptor stimulation, this report provides further evidence for the possible interaction between T cells and smooth muscle cells in orchestrating the immune response.39 The potential interactions of smooth muscle cells and inflammatory cells are shown in Figure 2.

Airway remodelling is now recognised as a characteristic feature of severe persistent asthma and includes increased smooth muscle mass, increased thickening of the basement membrane, increased new vessel formation and epithelial and mucous cell dysplasia. Initial observations indicated that the increase in smooth muscle was due to cellular hyperplasia, since a 3 fold increase in nuclei was found.40 A more recent report which examined the three dimensional structure of the airways in asthma found that both hyperplasia and hypertrophy occur in the large and small airways respectively.41 The milieu of cytokines, chemokines and matrix proteins produced by the smooth muscle are thought to provide the conditions necessary for the increased smooth muscle mass seen in the asthmatic lung. The ability of cytokines to stimulate proliferation of airway smooth muscle cells may to some extent depend upon both the autocrine and paracrine production of PGE₂, which inhibits DNA synthesis.42 Autocrine production of PGE₂ is reduced in asthmatic smooth muscle cells,43 providing a potential mechanism by which accelerated reproduction of smooth muscle cells may occur. Production of other autocrine mediators such as INF-β also have been shown to reduce proliferation,44 highlighting the importance of the effect of autocrine inflammatory mediators upon the function of the airway smooth muscle cell.

In addition, intrinsic differences in the smooth muscle cells between asthmatics and non-asthmatics now have been discovered.45, 46 In vitro airway smooth muscle cells from asthmatic subjects proliferate at a faster rate than non-asthmatic cells,45 produce greater amounts of proinflammatory cytokines, both constitutively and induced, and respond differently to common respiratory drugs. In addition the transcription factor CCAAT/enhancer binding protein α (C/EBPα) is deficient in airway smooth muscle cells from asthmatic subjects.46 There are six members of the C/EBP transcription factor family, which not only regulate cytokine expression, but are involved in cell cycle regulation and cellular differentiation.47 C/EBPα inhibits proliferation of many cell types,47 and in airway smooth muscle cells derived from non-asthmatic subjects neutralisation of C/EBPα using antisense oligonucleotides reversed the anti proliferative effects of glucocorticoids. Furthermore the reintroduction of C/EBPα into smooth muscle cells derived from asthmatic subjects enabled the anit-proliferative effects of steroids to be observed within these cells. Since C/EBPα is deficient in airway smooth muscle cells from asthmatic patients, this provides a potential mechanism by which increased proliferation occurs (see Fig. 3).

The airway smooth muscle cell is surrounded by an extracellular matrix (ECM) bed. Which consists of matrix proteins, such as collagen I, II, IV and V, fibronectin, decorin, chondroitin sulfate, elastin, perlecan, laminin β(1), γ(1), β(2), α(1) chains, thrombospondin and versican,48 matrix degrading enzymes MMP 1, 2 and 9, as well as tissue inhibitors of metalloproteinases (TIMP)-149 and TIMP-2.50 The interaction between the ECM and the airway smooth muscle cell can regulate both ECM composition and airway smooth muscle cell function. The ECM is very dynamic, with a turnover rate of 10—15% per day. Degradation of the ECM releases inflammatory modulators and growth factors that can feed back on the smooth muscle cell, and in addition the smooth muscle cell can respond to ECM components through integrins. Smooth muscle cell proliferation has been shown to be dependant upon autocrine production of MMP-2,51 and furthermore smooth muscle proliferation in response to stimulation with PDGF BB or α-thrombin is inhibited by laminin whereas fibronectin and collagen I increase cell division (see Fig. 4).52

In vivo differences are observed in the composition of the ECM in asthma, and in vitro airway smooth muscle cells from asthmatic subjects produce greater perlecan and collagen I and decreased collagen IV, laminin α1, and chondroitin sulfate in comparison to non-asthmatic cells.53 In addition the signalling pathways leading to the deposition of extracellular matrix proteins from airway smooth muscle cells may be different in asthmatic and nonasthmatic patients.54 Interestingly the relationship between the airway smooth muscle cell, matrix protein deposition and cytokines such as vascular endothelial growth factor and connective tissue growth factor may have implications for the development of the angiogenesis which constitutes a component of the remodelling process.

The foremost contributor to morbidity, mortality and health care costs associated with asthma are exacerbations, of which at least 70% are associated with viral infection.

Viruses are pathogens of the lower respiratory tract, however much controversy surrounds the ability for airway smooth muscle to become infected with virus in vivo. Importantly, in situ hybridisation has demonstrated that both submucosal55 and smooth muscle cells,56 can be infected with viruses in-vivo. Rhinovirus is the most commonly encountered viral infection producing exacerbations in adult asthmatics and infection of airway smooth muscle cells has been demonstrated in vitro.57 We as well as others have shown that rhinovirus induced the release of proinflammatory cytokines from airway smooth muscle cells, thereby providing a potential mechanism by which rhinovirus induced exacerbations occur.58 There is clinical evidence to support a decreased response to beta adrenoceptor agonists during a respiratory virus infection.59 This has led to the hypothesis that viral infection may cause dysfunction in the beta adrenoceptor on the smooth muscle and some preliminary evidence exists in support of this,60 in addition we have preliminary evidence that rhinovirus induces the release of PGE₂ from airway smooth muscle cells, thereby providing a mechanism by which dysfunction in the beta adrenoceptor occurs (see Fig. 5).61 Other proposed mechanisms of virus-induced exacerbations include downregulation of the M₂ muscarinic on vagal nerve endings62 and modulation of the endothelin system.63 Conversely, Billington et al., 2001 suggest that rhinovirus induces sensitisation of the adenylyl cyclase pathway, leading the authors to speculate that the increased production of cAMP is a defensive mechanism designed to promote airway relaxation upon infection with rhinovirus.64

The airway smooth muscle cell has many roles in the airway, from maintaining airway tone to regulating ECM composition and the local inflammatory and growth factor milieu. These cells are the therapeutic targets for drugs used to treat asthma and other airway disorders. Since they are intrinsically different in asthmatic patients they are likely to be intimately involved in the remodelling process and the progression of the asthmatic phenotype. Their role in airway disease is only beginning to be understood, and in the future the interaction of allergens and lower respiratory tract infections with the smooth muscle cell may yield valuable insights into the pathogenesis of asthma and other respiratory diseases.

REFERENCES

1
Mitzner W. Airway smooth muscle: the appendix of the lung. Am. J. Respir. Crit. Care Med. 2004; 169: 787-790.

2
Brown RH, Wizeman W, Danek C, Mitzner W. Effect of bronchial thermoplasty on airway distensibility. Eur. Respir. J. 2005; 26: 277-282.
 

3
Danek CJ, Lombard CM, Dungworth DL et al. Reduction in airway hyperresponsiveness to methacholine by the application of RF energy in dog. J. Appl. Physiol. 2004; 97: 1946-1953.
 

4
Miller JD, Cox G, Vincic L, Lombard CM, Loomas BE, Danek CJ. A prospective feasibility study of bronchial thermoplasty in the human airway. Chest 2005; 127: 1999-2006.

5
Cox G, Miller JD, McWilliams A, Fitzerald JM, Lam S. Bronchial thermoplasty for asthma. Am. J. Respir. Crit. Care Med. 2006; 173: 965-969.

6
Seow CY, Schellenberg RR, Pare PD. Structural and functional changes in the airway smooth muscle of asthmatic subjects. Am. J. Respir. Crit. Care Med. 1998; 158: S179-S186.
 

7
Whicker SD, Armour CL, Black JL. Responsiveness of bronchial smooth muscle from asthmatic patients to relaxant and contractile agonists. Pulm. Pharmacol. 1988; 1: 25-31.
 

8
Bai TR. Abnormalities in airway smooth muscle in fatal asthma. A comparison between trachea and bronchus. Am. Rev. Respir. Dis. 1991; 143: 441-443.
 

9
Bai TR. Abnormalities in airway smooth muscle in fatal asthma. Am. Rev. Respir. Dis. 1990; 141: 552-557.

10
Goldie RG, Spina D, Henry PJ, Lulich KM, Paterson JW. In vitro responsiveness of human asthmatic bronchus to carbachol, histamine, beta-adrenoceptor agonists and theophylline. Br. J. Clin. Pharmacol. 1986; 22: 669-676.
 

11
Fish JE, Ankin MG, Kelly JF, Peterman VI. Regulation of bronchomotor tone by lung inflation in asthmatic and nonasthmatic subjects. J. Appl. Physiol. 1981; 50: 1079-1086.
 

12
Ma X, Cheng Z, Kong H et al. Changes in biophysical and biochemical properties of single bronchial smooth muscle cells from asthmatic subjects. Am. J. Physiol. Lung Cell Mol. Physiol. 2002; 283: L1181-L1189.
 

13
Halayko AJ, Camoretti-Mercado B, Forsythe SM et al. Divergent differentiation paths in airway smooth muscle culture: induction of functionally contractile myocytes. Am. J. Physiol. 1999; 276: L197-L206.
 

14
Black JL, Marthan R, Armour CL, Johnson PR. Sensitization alters contractile responses and calcium influx in human airway smooth muscle. J. Allergy Clin. Immunol. 1989; 84: 440-447.
 

15
Johnson PR, Ammit AJ, Carlin SM, Armour CL, Caughey GH, Black JL. Mast cell tryptase potentiates histamine-induced contraction in human sensitized bronchus. Eur. Respir. J. 1997; 10: 38-43.
 

16
Villanove X, Marthan R, Tunon de et al. Sensitization decreases relaxation in human isolated airways. Am. Rev. Respir. Dis. 1993; 148: 107-112.
 

17
Mitchell RW, Rabe KF, Magnussen H, Leff AR. Passive sensitization of human airways induces myogenic contractile responses in vitro. J. Appl. Physiol. 1997; 83: 1276-1281.
 

18
Berger P, Walls AF, Marthan R, Tunon-de-Lara JM. Immunoglobulin E-induced passive sensitization of human airways: an immunohistochemical study. Am. J. Respir. Crit. Care Med. 1998; 157: 610-616.
 

19
Gounni AS, Wellemans V, Yang J et al. Human airway smooth muscle cells express the high affinity receptor for IgE Fc epsilon RI: a critical role of Fc epsilon RI in human airway smooth muscle cell function. J. Immunol. 2005; 175: 2613-2621.
 

20
Watson N, Ruhlmann E, Magnussen H, Rabe KF. Histamine hypersensitivity induced by passive sensitization of human bronchus: effect of serum IgE depletion. Clin. Exp. Allergy 1998; 28: 679-685.
 

21
Tunon de Lara JM. Sensitization of human airways: what is the role of immunoglobulin-E? Clin. Exp. Allergy 1998; 28: 660-663.

22
Pascual RM, Awsare BK, Farber SA, Panettieri RA, Peters SP, Penn RB. Regulation of phospholipase A2 by interleulin-1 in human airway smooth muscle. Chest 2003; 123: 433S-434S.

23
Lazaar AL, Reitz HE, Panettieri RA Jr, Peters SP, Pure E. Antigen receptor-stimulated peripheral blood and bronchoalveolar lavage-derived T cells induce MHC class II and ICAM-1 expression on human airway smooth muscle. Am. J. Respir. Cell Mol. Biol. 1997; 16: 38-45.
 

24
Hakonarson H, Kim C, Whelan R, Campbell D, Grunstein MM. Bi-directional activation between human airway smooth muscle cells and T lymphocytes: role in induction of altered airway responsiveness. J. Immunol. 2001; 166: 293-303.
 

25
Balbo P, Silvestri M, Rossi GA, Crimi E, Burastero SE. Differential role of CD80 and CD86 on alveolar macrophages in the presentation of allergen to T lymphocytes in asthma. Clin. Exp. Allergy 2001; 31: 625-636.
 

26
Morris GE, Whyte MK, Martin GF, Jose PJ, Dower SK, Sabroe I. Agonists of Toll-like Receptors 2 and 4 Activate Airway Smooth Muscle via Mononuclear Leukocytes. Am. J. Respir. Crit. Care Med. 2005; 171: 814-822.

27
Hughes JM, Arthur CA, Baracho S et al. Human eosinophil-airway smooth muscle cell interactions. Mediators Inflamm. 2000; 9: 93-99.
 

28
Kassel O, Schmidlin F, Duvernelle C, Gasser B, Massard G, Frossard N. Human bronchial smooth muscle cells in culture produce stem cell factor. Eur. Respir. J. 1999; 13: 951-954.
 

29
Brightling CE, Ammit AJ, Kaur D et al. The CXCL10/CXCR3 axis mediates human lung mast cell migration to asthmatic airway smooth muscle. Am. J. Respir. Crit. Care Med. 2005; 171: 1103-1108.

30
El-Shazly A, Berger P, Girodet PO et al. Fraktalkine produced by airway smooth muscle cells contributes to mast cell recruitment in asthma. J. Immunol. 2006; 176: 1860-1868.
 

31
Yang W, Kaur D, Okayama Y et al. Human lung mast cells adhere to human airway smooth muscle, in part, via tumor suppressor in lung cancer-1. J. Immunol. 2006; 176: 1238-1243.
 

32
Liu L, Yang J, Huang Y. Human Airway Smooth Muscle Cells Express Eotaxin in Response to Signaling following Mast Cell Contact. Respiration 2006; 73: 227-235.
 

33
Brightling CE, Symon FA, Holgate ST, Wardlaw AJ, Pavord ID, Bradding P. Interleukin-4 and -13 expression is co-localized to mast cells within the airway smooth muscle in asthma. Clin. Exp. Allergy 2003; 33: 1711-1716.
 

34
Moore PE, Church TL, Chism DD, Panettieri RA Jr., Shore SA. IL-13 and IL-4 cause eotaxin release in human airway smooth muscle cells: a role for ERK. Am. J. Physiol. Lung Cell Mol. Physiol. 2002; 282: L847-L853.
 

35
Laporte JC, Moore PE, Baraldo S et al. Direct effects of interleukin-13 on signaling pathways for physiological responses in cultured human airway smooth muscle cells. Am. J. Respir. Crit. Care Med. 2001; 164: 141-148.
 

36
Carroll NG, Mutavdzic S, James AL. Distribution and degranulation of airway mast cells in normal and asthmatic subjects. Eur. Respir. J. 2002; 19: 879-885.
 

37
Lazaar AL, Albelda SM, Pilewski JM, Brennan B, Pure E, Panettieri RA. T lymphocytes adhere to airway smooth muscle cells via integrins and CD44 and induce smooth muscle cell DNA synthesis. J. Exp. Med. 1994; 180: 807-816.
 

38
Lazaar AL, Amrani Y, Hsu J et al. CD40-mediated signal transduction in human airway smooth muscle. J. Immunol. 1998; 161: 3120-3127.
 

39
Burgess JK, Carlin S, Pack RA et al. Detection and characterization of OX40 ligand expression in human airway smooth muscle cells: a possible role in asthma? J. Allergy Clin. Immunol. 2004; 113: 683-689.
 

40
Hossain S. Quantitative measurement of bronchial muscle in men with asthma. Am. Rev. Respir. Dis. 1973; 107: 99-109.
 

41
Ebina M, Takahashi T, Chiba T, Motomiya M. Cellular hypertrophy and hyperplasia of airway smooth muscles underlying bronchial asthma. A 3-D morphometric study. Am. Rev. Respir. Dis. 1993; 148: 720-726.
 

42
Belvisi MG, Saunders M, Yacoub M, Mitchell JA. Expression of cyclo-oxygenase-2 in human airway smooth muscle is associated with profound reductions in cell growth. Br. J. Pharmacol. 1998; 125: 1102-1108.
 

43
Chambers LS, Black JL, Ge Q et al. PAR-2 activation, PGE2, and COX-2 in human asthmatic and nonasthmatic airway smooth muscle cells. Am. J. Physiol. Lung Cell Mol. Physiol. 2003; 285: L619-L627.

44
Tliba O, Tliba S, Da HC et al. Tumor necrosis factor alpha modulates airway smooth muscle function via the autocrine action of interferon beta. J. Biol. Chem. 2003; 278: 50615-50623.
 

45
Johnson PR, Roth M, Tamm M et al. Airway smooth muscle cell proliferation is increased in asthma. Am. J. Respir. Crit. Care Med. 2001; 164: 474-477.
 

46
Roth M, Johnson PR, Borger P et al. Dysfunctional interaction of C/EBPalpha and the glucocorticoid receptor in asthmatic bronchial smooth-muscle cells. N. Engl. J. Med. 2004; 351: 560-574.
 

47
Borger P, Black JL, Roth M. Asthma and the CCAAT-enhancer binding proteins: a holistic view on airway inflammation and remodeling. J. Allergy Clin. Immunol. 2002; 110: 841-846.
 

48
Johnson PR, Black JL, Carlin S, Ge Q, Underwood PA. The production of extracellular matrix proteins by human passively sensitized airway smooth-muscle cells in culture: the effect of beclomethasone. Am. J. Respir. Crit. Care Med. 2000; 162: 2145-2151.
 

49
Black JL, Burgess JK, Johnson PR. Airway smooth muscle—its relationship to the extracellular matrix. Respir. Physiol. Neurobiol. 2003; 137: 339-346.
 

50
Elshaw SR, Henderson N, Knox AJ, Watson SA, Buttle DJ, Johnson SR. Matrix metalloproteinase expression and activity in human airway smooth muscle cells. Br. J. Pharmacol. 2004; 142: 1318-1324.
 

51
Johnson S, Knox A. Autocrine production of matrix metalloproteinase-2 is required for human airway smooth muscle proliferation. Am. J. Physiol. 1999; 277: L1109-L1117.
 

52
Hirst SJ, Twort CH, Lee TH. Differential effects of extracellular matrix proteins on human airway smooth muscle cell proliferation and phenotype. Am. J. Respir. Cell Mol. Biol. 2000; 23: 335-344.
 

53
Johnson PR, Burgess JK, Underwood PA et al. Extracellular matrix proteins modulate asthmatic airway smooth muscle cell proliferation via an autocrine mechanism. J. Allergy Clin. Immunol. 2004; 113: 690-696.
 

54
Johnson PR, Burgess JK, Ge Q et al. Connective tissue growth factor induces extracellular matrix in asthmatic airway smooth muscle. Am. J. Respir. Crit. Care Med. 2006; 173: 32-41.

55
Papadopoulos NG, Bates PJ, Bardin PG et al. Rhinoviruses infect the lower airways. J. Infect. Dis. 2000; 181: 1875-1884.
 

56
Morbini P, Arbustini E. In situ characterization of human cytomegalovirus infection of bronchiolar cells in human transplanted lung. Virchows Arch. 2001; 438: 558-566.
 

57
Grunstein MM, Hakonarson H, Maskeri N, Chuang S. Autocrine cytokine signaling mediates effects of rhinovirus on airway responsiveness. Am. J. Physiol. Lung Cell Mol. Physiol. 2000; 278: L1146-L1153.
 

58
Hakonarson H, Carter C, Maskeri N, Hodinka R, Grunstein MM. Rhinovirus-mediated changes in airway smooth muscle responsiveness: induced autocrine role of interleukin-1beta. Am. J. Physiol. 1999; 277: L13-L21.

59
Reddel H, Ware S, Marks G, Salome C, Jenkins C, Woolcock A. Differences between asthma exacerbations and poor asthma control. Lancet 1999; 353: 364-369.
 

60
Hakonarson H, Maskeri N, Carter C, Hodinka RL, Campbell D, Grunstein MM. Mechanism of rhinovirus-induced changes in airway smooth muscle responsiveness. J. Clin. Invest. 1998; 102: 1732-1741.
 

61
Penn RB, Panettieri RA Jr., Benovic JL. Mechanisms of acute desensitization of the beta2AR-adenylyl cyclase pathway in human airway smooth muscle. Am. J. Respir. Cell Mol. Biol. 1998; 19: 338-348.
 

62
Jacoby DB, Fryer AD. Interaction of viral infections with muscarinic receptors. Clin. Exp. Allergy 1999; 29: (Suppl 2) 59-64.
 

63
Fernandes LB, D'Aprile AC, Self GJ, Harnett GB, Goldie RG. The Impact of Respiratory Syncytial Virus Infection on Endothelin Receptor Function and Release in Sheep Bronchial Explants. J. Cardiovasc. Pharmacol. 2004; 44: S202-S206.
 

64
Billington CK, Pascual RM, Hawkins ML, Penn RB, Hall IP. Interleukin-1beta and rhinovirus sensitize adenylyl cyclase in human airway smooth-muscle cells. Am. J. Respir. Cell Mol. Biol. 2001; 24: 633-639.