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• Unravelling the Pathogenesis of Asthma
• Respiratory Virus Infection as Asthma and COPD Exacerbation
• Translating Insights from Asthma to SARS-CoV-2-induced COVID-19
• Battling the scourge of air pollution
• Epithelial Injury and Aberrant Repair Drives Airway Wall Remodelling in Asthma

Unravelling the Pathogenesis of Asthma

With over 300m patients worldwide and increasing, asthma is the commonest chronic lung disease affecting all age groups and varying in severity from mild intermittent to life-threatening. In the 1970s, at the start of Stephen Holgate’s research career, asthma was regarded largely as a disorder of intermittent bronchospasm relieved with inhaled bronchodilators. However, their overuse led to increased asthma mortality. To gain insight into why asthmatic airways experience variable airflow obstruction, he documented the importance of allergen driven IgE-dependent inflammation (subsequently designated Type-2) involving activation of human airway mucosal mast cells and T helper cells with recruitment of eosinophils and basophils, whose mediators and cytokines led to non-specific bronchial hyperresponsiveness (BHR) with airflow obstruction (1), and all being responsive to inhaled corticosteroids (ICS) (2). As a founder Expert Panel Member of the newly formed in 1993, Holgate’s research contributed to asthma being redefined as an inflammatory disorder requiring ICS for disease control and, in 1995, this formed the basis of the first which further to target and the .

Building on inflammation, the Southampton Team were the first to identify defective epithelial barrier function(s) (3) as a constitutive feature of asthma. In being upstream of the mucosal Type-2 response, a dysfunctional epithelium provides a for how environmental stressors, such as allergens, viruses and air pollutants, initiate and exacerbate asthma (e.g. through oxidant stress and enhanced production of alarmins to trigger Type-2 inflammation) as well as driving airway wall remodelling in chronic and severe disease. Recognition of the critical importance of the epithelium in orchestrating different asthma phenotypes in response to environmental stressors has opened a new era of precision/stratified asthma management in which biologics (e.g., Interferon [IFN]-b, anti-alarmins [; , , and ADAM33 ] are now being effectively to epithelial and mesenchymal pathways responsible for initiating, exacerbating and perpetuating asthma across the lifecourse.

Respiratory Virus Infection as Asthma and COPD Exacerbation

After developing the first PCR-based method to detect human rhinoviruses (HRVs) in airway secretions, Holgate’s team showed that in both children and adults, virus infection accounted for most asthma exacerbations, the majority being HRVs (4). Using controlled nasal HRV-16 infection in volunteers, they showed that in asthmatic but not in normal subjects, HRVs preferentially colonised the lower airways epithelium with amplification of the Type-2 inflammatory response, thereby replicating the asthma exacerbation (5). Using asthmatic primary epithelial cells in monolayer culture, they then found that infection with HRVs was the consequence of impaired epithelial cell triggering of anti-viral interferon (IFN)-b production (6). Since this abnormality involved an initial Toll-like receptor (TLR) 3 step that recognises viral dsRNA, the second amplification step (involving the common IFN receptor [IFNAR]) inducing hundreds of protective IFN-stimulated genes [ISGs]), remained intact. Thus, addition of exogenous IFN-b to the asthmatic epithelium was able to fully restore ISG-mediated anti-viral protection (7). To exploit this discovery, in 2003 Holgate and two Southampton colleagues formed with the aim of bringing these advances to patients. In a RCT they showed that inhaled human IFN-b1a in asthma not only accelerated viral clearance with multi-fold increases in ISG biomarkers in airway secretions but also attenuated asthma exacerbation (8). Because the airways in COPD patients also proved more vulnerable to respiratory viruses, again due to , was similarly restored with exogenous IFN-b1a. of inhaled IFN-b1a in exacerbating COPD patients with confirmed respiratory viral infection had to be paused in early 2020 because of COVID-19. However, has revealed significant improvements in lung function, reduced serum C-reactive protein and increased viral clearance. In addition  to  impaired IFN-b in response to viral dsRNA, and airway epithelial cells were biased towards higher TSLP production to direct a Type-2 inflammatory response as they had shown in response to viral infection (5).

Translating Insights from Asthma to SARS-CoV-2-induced COVID-19

SARS-CoV-2 has evolved mechanisms to evade the first step of the IFN-b response in addition to host genetic and acquired factors limiting Type 1 IFN production. Thus, in 2020 Holgate’s team investigated daily treatment with inhaled IFN-b1a vs placebo once daily for 14 days in hospitalised patients with COVID-19. Not only was inhaled IFN-b1a well tolerated but by day 28 there was significant clinical improvement, accelerated recovery to no limitation of activity and reduced breathlessness (8). While a follow-on RCT () did not meet primary endpoints due to progressive improvements in overall standard of care since their Phase II trial, in the 30% of COVID-19 patients with impaired lung function, stratification revealed that inhaled IFN-b1a significantly reduced progression to severe disease or death by 70%. At 60-90 days, those trial patients who had received active treatment also experienced significantly reduced symptoms of long-COVID. Since inhaled IFN-b1a is (and other respiratory viruses), it is now being to prevent progression to severe disease and death in , including COVID-19. With the emergence of a pandemic, the GCSA asked Holgate to chair Academy of Medical Sciences Expert Advisory Groups (EAGs) to advise on NHS and social care planning as the COVID-19 pandemic evolved: 2020 - ; 2021 - , and 2022 - . These Reports were commended by SAGE and equivalent bodies in the Devolved Administrations and to central and local governments.

Battling the scourge of air pollution

Starting with the in 1993, accumulating epidemiological studies provided new evidence that air pollution - especially NO₂ O₃, and particulate matter (PM₁₀, PM_2.5) from fossil fuel combustion - was becoming one of the greatest environmental risks to human health (9). However, establishing causality was limited by lack of human toxicology. In 1998, Holgate’s team was among the first to address this gap in humans using controlled exposures, bronchial biopsy and lavage to track lung responses to air pollutants. Beginning with ozone (O₃) (a secondary photochemical pollutant ) exposure for 2 hours caused immediate reflex bronchoconstriction followed by neutrophilic airways inflammation driven by the chemokines, CXCL-1 and -8 (10). Short term inhalation of NO₂ also induced neutrophilic airways inflammation similarly driven by epithelial-derived CXCL-8, whereas repeated NO₂ exposures induced a pro-asthmatic Type-2 inflammatory profile accompanied by upregulation of Intercellular Adhesion Molecule-1 (CD54) to promote leukocyte influx (11). Induction of Type-2 mechanisms () helps explain the role of , while CD54, as the receptor for the major class of HRV, provides a mechanism for enhanced viral exacerbation observed in asthmatic children exposed to high personal levels of NO₂ (12). In the case of particles, there were indirect inflammatory effects on human airways. Two hours exposure to diesel exhaust particles (DEPs) or concentrated ambient PM_2.5 provoked neutrophilic airway inflammation (13) from to the airway epithelium, of the epidermal growth factor receptor (EGFR) and subsequent . Oxidant stress is an shared by all three pollutants with the epithelium exhibiting greater sensitivity in asthma (14). This research informed the and the later , both of which Holgate was a principal contributor, and used to support the EC's (‎CAFÉ)‎ programme and the . The clear need for greater awareness of the major health impacts of air pollution also led Holgate and Robert Maynard to initiate , the first comprehensive international textbook on this subject.

With increasing health concerns over the “new” air pollutants, Holgate was asked to chair the Department of Health (DH) Advisory Group on Air Pollution Episodes ( 1989-92) which published reports on SO₂, O₃, NO₂, PM and air pollutant mixtures. Appreciating the importance of chronic exposure to ambient air pollutants on health, he was then asked to chair a new advisory body, the DH Committee on the Medical Effects of Air Pollutants (, Chair 1992-2001, Member to 2008) which produced e.g. Asthma (1995), Non-Biological Particles (1995), Quantification of Health Effects (1998), Indoor Air Pollutants (2001), Cardiovascular Disease and Air Pollution (2006). In 2011, he chaired the COMEAP subgroup which produced the currently used UK Daily Air Quality Index () to inform the public about prevailing air pollution and is currently advising DEFRA on its update. The importance of providing evidence-informed knowledge to the public and other key stakeholders on issues relating to air pollution enabled Holgate to lead an influential 2016 RCP/RCPCH Report - highlighting the 40,000 annual deaths attributed to UK air pollution with a cost of £20 billion, followed in 2020 by - . Both Reports are proving highly effective in bringing air pollution to the forefront of policy development and were used to provide health evidence for new air quality research, the (2019-25, £42.5m) for which Holgate is .

Based on his asthma and air pollution research expertise, Holgate provided a large part of the medical and scientific evidence on Ella Adoo-Kissi-Debrah, a 9-year-old London girl who died of asthma in 2013 after 28 hospital admissions over 30 months. This identified air pollution as a likely cause leading the Attorney General to overturn the original 2013 inquest. In a landmark 2^nd inquest on December 16^th 2020 (including his own research on NO₂, DEPs and PM_2.5) played a pivotal role in the Coroner’s conclusion that to the induction and repeated exacerbation of Ella’s asthma, culminating in her death. Ella was the first person in the UK (and possibly worldwide) to have air pollution listed as the cause of death on their death certificate. Subsequent responses to a , issued by the Coroner on April 20^th 2021, are proving to be a powerful influence for of the insidious health dangers of air pollution and are .

Epithelial Injury and Aberrant Repair Drives Airway Wall Remodelling in Asthma

Direct access to airway tissue and cells enabled Holgate’s team recognised the importance of airway epithelial disruption and tissue remodelling by subepithelial myofibroblasts (1, 15) as new features of severe and chronic asthma. Using cultured primary airway epithelial cells, an abnormal asthmatic epithelium responds to injury, not only by enhancing inflammatory pathways (via oxidant stress and releasing alarmins), but also by initiating a chronic wound-like repair response with secretion of epidermal (EGF, HB-EGF, amphiregulin), fibrogenic (TGF-b₁, basic-FGF), angiogenic (VEGF), and neurotrophic (NGF family) growth factors indicating activation of the (EMTU) (16). In being unable to repair seamlessly, the asthmatic epithelium loses its barrier function(s) (3) to render it especially sensitive to the environmental determinants of asthma (17). In this state, even in the absence of inflammation, physical distortion of the asthmatic epithelium from repeated bronchoconstriction alone proved sufficient to invoke airway remodelling via the EMTU (18) and supported the for treating moderate-severe disease. Holgate’s Group also showed that in asthma was important in reducing the IFN-b response to viral infection thereby connecting impaired innate immunity with dysfunctional epithelial repair.

With asthma’s high heritability, the Southampton group uncovered the importance of genetic drivers of remodelling in asthma. Using positional cloning (and before GWAS), they identified the first novel asthma susceptibility gene, A Disintegrin Metalloprotease 33 (ADAM33) on chromosome 20p13 (19). ADAM33 encodes a 120 kD proteolytic enzyme selectively expressed in airway fibroblasts, myofibroblasts and smooth muscle and whose expression increases in . Association of ADAM33 polymorphism with asthma has been replicated in >100 populations worldwide and also in , with disease variants being associated with bronchial hyperresponsiveness, reduced lung function in childhood and its decline over time indicating a key role for this protease in the natural history of asthma and airway remodelling. Membrane-associated full-length ADAM33 protein is upregulated in proportion to disease severity and cleaved to release a soluble 55kD enzymatic fragment which, in initiating airway remodelling (20) and a , makes ADAM33 an attractive therapeutic target for chronic severe asthma e.g., through inhaled which effectively silence ADAM33.