The ROAD to a focused view of airway smooth muscle and inflammatory cells in asthmatic sensitization: a tribute to Newman Stephens

The view of the role of airway smooth muscle (ASM) in asthma and other disorders of airway hyperresponsiveness has been one of extremes. In the 1980s its role was deemed so central that a unifying concept was created entitled ‘reversible obstructive airway disease’, or ROAD, to indicate that whatever the combinations of aetiologies of airway narrowing (ASM spasm, dysplasia of nerves, epithelium, mucus-secreting cells, fibrocytes, decreased parenchymal elastic recoil, chronic inflammation), if there was a component of drug-induced reversibility or sudden narrowing, then the common pathophysiological factor and the key treatable target was ASM. Airway inflammation was not ignored, but viewed as a path by which cells could secrete spasmogens to excite ASM contraction. As of the 1990s this view changed radically. Whicker et al. (1988) determined in ASM from asthmatics that without inflammation isometric contractile responses were not augmented by graded increases in agonists, which argued against native hypercontractility. Animal models of asthmatic inflammation suggested CD4+ lymphocyte-directed inflammation drove airway narrowing and hyperresponsiveness. Mouse models provided some evidence of inducible ASM hyperresponsiveness, but evidence of bronchial ASM participation was limited, in part because mouse bronchial ASM is sparse. This inflammatory story was confirmed in lavage and biopsy studies of asthmatics. A reprieve for a role for ASM resulted from studies of airway remodelling, a process by which ASM and other cells proliferated with dysfunctional growth and repair. Hyperplastic ASM could contribute to airway hyperresponsiveness even if the ASM myocytes were hypocontractile. ASM genes were determined to contribute a range of inflammatory products to promote remodelling. Nonetheless airway hyperresponsiveness continued to be viewed as arising from chronic stimulation by robust, complex Th-2 lymphocyte-directed inflammation. The solitary voice of Newman Stephens (Fig. ​(Fig.1)1) argued for a role for increased ASM shortening in asthma that could be strikingly independent of persistent inflammatory stimulation. He asserted that isometric contraction of ASM was a flawed surrogate for ASM-directed airway narrowing. The basis of this observation was the outcome of an increased airway resistance compared to normal and hyperresponsiveness to methacholine in mongrel dogs that were sensitized to ragweed (Becker et al. 1989). In this model developed by M. W. Kepron, there was no evidence of chronic airway inflammation or remodelling and no change in isometric contractile responses to agonists. Instead there was an increase in maximal shortening velocity and shortening capacity. The principal biochemical correlate was increased content of myosin light chain kinase (MLCK), an enzyme known to catalyse the phosphorylation of 20 kDa myosin light chain and promote load-dependent combinations of shortening and/or active force via actin–myosin cross-bridge cycling (Jiang et al. 1992). These observations have been validated in other models including human airway specimens studied in Dr Stephens’ laboratory. A study of mild asthmatics showed that in biopsied ASM mRNA expression of MLCK was increased compared to normal (Ma et al. 2002). Taken together this suggested that early asthmatic-type sensitization with intermittent augmentation could produce in human ASM a predisposition to exaggerated narrowing without an obvious alteration in isometric force generation and with a notable absence of chronic cellular inflammation. Figure 1 Professor Newman Stephens as mentor The seven-amino-acid-inserted myosin heavy chains (MHCs) in ASM were determined to have twice the ATPase activity and also twice the velocity of motility in an in vitro assay (Kelley et al. 1993) and were further linked to asthma (Leguillette et al. 2009). Thus, their potential contribution when augmented to increasing ASM shortening appeared potentially greater than that of MLCK. Since T lymphocytes appeared in animal models and many human studies to have a central role in directing complex asthmatic inflammation, the question arose whether they could stimulate ASM directly to contribute to inflammation, remodelling and hyperresponsiveness. An initial investigation (Lazaar et al. 1994) demonstrated binding and signalling molecules on ASM, such as ICAM-1, VCAM-1 and CD44, whose activation was associated with ASM proliferation in response to stimulation. A subsequent study (Al Heialy et al. 2013) demonstrated a role for upregulated epidermal growth factor on ASM that responded with a proliferative response to CD4+ T cells. Thus the stage was set for determining whether ASM activation by activated T cell binding could also affect ASM contractility. The first challenge became how to determine whether these T cells could augment ASM expression of the relevant contractile proteins associated with increased shortening through direct cell to cell communication. The next challenge would be to determine whether or not this interaction could be modelled to demonstrate chronic upregulation. In the this issue of The Journal of Physiology, Matusovsky et al. have provided important insights into the first challenge. They determined that de-epithelialized tracheal rings bearing ASM co-cultured for 24 h with activated CD4+ T-lymphocytes from ovalbumin-sensitized Brown–Norway rats produced an increase in fast isoform MHC protein expression in ASM that was associated with an increase of maximal shortening velocity as well as increased MLCK content and phosphorylation of myosin light chain at 24 h. These findings lend tantalizing evidence for this targeted T cell–ASM cell interaction resulting in increased ASM shortening velocity. The enthusiasm for the relevance of this finding to human asthma is damped somewhat by the failure to see increased protein expression persist through 48 h. Given the history of the ragweed-sensitized dog, the failure to obtain a persistent expression signal is not surprising. Successful sensitization of dogs to produce increased chronic protein expression in the ASM of key contractile proteins required bimonthly injections starting at 6 months. The key contribution of the model was to demonstrate that ASM hyperresponsiveness did not depend on the presence of chronic airway inflammation and remodelling. This model and subsequent human studies predicted what was needed, so that when novel candidate molecules and technologies were developed, the necessary next steps, ably studied by Lauzon and colleagues, were importantly outlined. Thus the role of ASM cellular research in asthma is coming full circle – from overly dominant in models that ignored inflammation, to a relatively passive passenger model that overemphasized complex inflammation and ignored ASM, to a more reasoned role of ASM as a semi-independent contributor to airway inflammation, remodelling and hyperresponsiveness that underscores the need for asthma-related ASM research and the development of novel ASM-targeted asthma therapeutics. One such ASM-targeted therapy, bronchial thermoplasty, is the first airway structural cell treatment approved for the management of severe asthma. The vision of Newman Stephens has allowed reason and focus to recentre these discussions and prevented the dismissal of the importance of ASM in asthma.