Allergen-induced asthma

Division of Respirology, Critical Care and Sleep Medicine, Royal University Hospital, Saskatoon, Saskatchewan Correspondence: Dr Donald W Cockcroft, Division of Respirology, Critical Care and Sleep Medicine, Royal University Hospital, 103 Hospital Drive, Ellis Hall, Room 551, Saskatoon, Saskatchewan S7N 0W8. Telephone 306-844-1140, fax 306-844-1532, e-mail don.cockcroft@usask.ca Inhalant allergen exposure in atopic individuals is now recognized to be the single most important cause of asthma. In the current article, the history of our understanding of allergen-induced asthma is reviewed. This includes a focus on the remarkable achievements of Dr Frederick E Hargreave – known as ‘Freddy’ to friends and colleagues – and his many successful fellows. The present article was presented as the Canadian Thoracic Society Honourary Lecture at CHEST 2013, held in Chicago, Illinois (USA).

and was able to induce both rhinitis and asthma in himself by sniffing ragweed. In 1882, Henry Hyde Salter (5) reported on asthma precipitated by exposure to cats and rabbits, and later on to horses and dogs. In 1873, Charles Blackley, in his monograph entitled Catarrhus AEstivus (6), identified grass pollen as the (or at least one) cause of rhinitis, conjunctivitis and asthma; all of his clinical experiments were performed on himself because he claimed he was unable to convince any other sufferers to participate in his trials. The idea that pollen contained a toxin was popular (7) and led to the development of pollen inoculation (allergen injection therapy), which proved to be effective despite the incorrect rationale (8). In 1921, passive transfer of sensitivity using the serum of a highly fish-allergic individual pointed to a serum factor as being a cause of the syndrome (9). The terms 'reagin' (or reaginic antibody) and 'atopy' were coined in 1923 (10). It was not until 1967 that immunoglobulin E (Ig) E was identified as the nature of the reaginic antibody (11).

Allergen CHAllenge History
Clinical allergen challenges began with Charles Blackley (6). Throughout the first half of the 20th century, there were scattered reports of allergen challenge being performed to test the clinical relevance of various allergens. These reports all focused on the immediate response, the so-called 'early asthmatic response' (EAR), which develops shortly after exposure. Expiratory flow rates had not yet been introduced; the response to allergen was measured fairly crudely by such things as symptoms, heavy breathing and chest auscultation. When objective measurements of lung function were made, it was only the vital capacity that was measured and it was not uncommon to see reports of 35% to 50% reduction in vital capacity; this represents a very marked degree of bronchoconstriction. Tiffeneau (12) introduced forced expiratory volume in 1 s (FEV 1 ) in 1947 and suggested that this would be a useful measurement to study the effect of drugs (bronchoconstrictors, bronchodilators) and allergens in asthma. It was only in the late 19th century that specific allergens, pollen, animal antigens and, later, house dust mite, were identified to cause upper and lower airway disease. Early allergen challenge studies, crudely monitored before measurement of forced expiratory volume in 1 s became widespread in the 1950s, focused on the immediate effects but noted in passing prolonged and/or recurrent asthma symptoms. The late asthmatic response, recurrent bronchoconstriction after spontaneous resolution of the early responses occurring 3 h to 8 h or more postchallenge, has been identified and well characterized over the past 50 years. The associated allergeninduced airway hyper-responsiveness (1977) and allergen-induced airway inflammation (1985) indicate that these late sequelae are important in the mechanism of allergen-induced asthma. Allergens are now recognized to be the most important cause of asthma. A standardized allergen inhalation challenge model has been developed and is proving to be a valuable research tool in the investigation of asthma pathophysiology and of potential new pharmacological agents for the treatment of asthma.
The 'late asthmatic response' (LAR), which we now recognize as a recurrence of bronchoconstriction developing between 3 h and ≥8 h after allergen challenge and after spontaneous resolution of the EAR (13), was a term first coined in 1951 by Herxheimer (14). A review of previous challenge study reports shows that Blackley described a classic late asthmatic response due to his own overdose of grass pollen, and that many of the early allergen challenge reports noted that recurrent or prolonged symptoms were sometimes induced. Herxheimer himself was still monitoring his challenges with vital capacity up to 50% decline, and suspected that many of the late responses were actually a continuous response with interval transient improvement after isoproterenol had been administered to treat the early response (14). Ten years later, there was an excellent description of mite-induced dual asthmatic responses (DAR) (15). In the late 1960s and early 1970s, Jack Pepys (16,17) and Dick Orie (18,19), working independently in the United Kingdom and the Netherlands, respectively, characterized the DAR to inhaled allergens. There was an ongoing controversy as to whether the late response was caused by IgG precipitins (Pepys) or not (Orie). The 'pro' argument related to the timing of the response similar to that seen in extrinsic allergic alveolitis (EAA), and the fact that many of Pepys' subjects were challenged with Aspergillus to which precipitins were present (20). The 'con' argument, related to observations that the LAR was obstructive (not restrictive), was not associated with systemic symptoms such as seen in EAA, and that precipitins were not present to the common aeroallergens house dust mite, pollen or animal danders (21). The IgE dependence of late responses was shown by Jerry Dolovich and Freddy Hargreave in the skin in 1973 (22), and confirmed in the airway in 1986 (23) (both studies using polyclonal anti-human IgE) and for the residual skeptics in 1997 when LARs were shown to be inhibited exceptionally well by the monoclonal anti-IgE omalizumab (24). The first classical graphic description of a DAR with an uninterrupted time axis appears to be from Freddy Hargreave's ragweed challenge asthma article in 1974 (25).

Allergen-inDuCeD inCreAse in AirWAy responsiveness
Measurement of airway hyper-responsiveness (AHR) was pioneered by Tiffeneau in the 1940s (26) and had become quite popular in the 1960s in Europe. Challenge protocols were published in North America in the late 1970s (27). AHR was regarded as a classic feature of asthma. There was no appreciation of the possibility that AHR had the potential to vary over time to any degree and with any speed. Roger Altounyan, whose first two claims to fame were the synthesis of sodium cromoglycate and nedocromil (28), and as a role model along with his siblings for the Swallows in Arthur Ransome's iconic adolescent series Swallows and Amazons (29) did recognize seasonal AHR. His findings are buried in his 1964 article, in which he made the observation that AHR to histamine was stable except in grass pollen asthmatics who demonstrated marked seasonal increases in AHR (30). It was Dick Ruffin and I, working in Freddy Hargreave's laboratory who, in 1977, documented increased airway responsiveness to both histamine and methacholine 7 h and 30 h following allergen challenges (31). The changes were sometimes marked (up to 10-fold) and lasted for up to seven to 10 days, and appeared to be associated with allergen-induced LARs. Follow-up studies in Freddy's laboratory documented a definite correlation of allergen-induced AHR, both in magnitude and duration (32), with the LAR and also documented that individuals demonstrating this in the laboratory were those who were more likely to develop seasonal AHR during ragweed pollen season (33). This previously unrecognized (and somewhat unexpected) observation marked an important change in our thinking about allergeninduced asthma.

Allergen-inDuCeD AirWAy inflAmmAtion
Early investigators had speculated that the LAR was likely associated with more than just bronchoconstriction and postulated the existence or coexistence of inflammation. De Monchy's classic bronchoscopy study in 1985 documented marked airway eosinophilia 7 h after allergen challenge in individuals with a DAR (34). This was not seen 7 h after allergen challenge in normal individuals or nonresponding asthmatics. Relatively minor airway eosinophilia was seen 7 h after allergen challenge in subjects with an isolated EAR and 3 h after allergen challenge in subjects with a DAR. While investigations were generally performed in different individuals (n=5 per group), there were two subjects who participated in the 3 h and 7 h DAR bronchoscopies.
Longitudinal studies pre-and post-allergen in Freddy Hargreave's laboratory using induced sputum analysis documented significant increases in airway eosinophils and airway metachromatic cells (mast cells, basophils) after allergen challenge (35). Collectively, the late sequelae following allergen challenge including the LAR, increased AHR and increased airway inflammation, appeared to be linked to one another and, unlike the allergen-induced EAR, are mimickers of clinical asthma. Over this time period, our ideas have changed. Allergens were once recognized as one of many causes of bronchoconstriction in asthmatics, whereas they are now recognized as an important and major cause (the most common cause) of asthma. It has been attractive to speculate that airway inflammation is the cause of late sequelae; however, investigations with an antiinterleukin 5 monoclonal antibody appeared to divorce the late response and increased responsiveness from at least eosinophilic inflammation (36). The non-cellular aspect of inflammation may be important; however, the precise mechanism(s) of these late sequelae remain incompletely understood.

tHe Allergen CHAllenge moDel
Allergen challenge is a research tool exclusively for the study of asthma pathophysiology and for investigation of novel treatments. The standardized method involves capturing the EAR, the LAR, and the changes in both AHR and inflammation (37). The pharmacology with current asthma drugs is as follows. Inhaled beta-agonists inhibit the EAR but not the LAR (38), inhaled corticosteroids (ICS) inhibit the LAR but not the EAR (38), while cromones (16,19,38) and leukotriene receptor antagonists (LTRAs) (39) inhibit both. ICS given 2 h after the challenge will inhibit the LAR (40). Phosphodiesterase inhibitors (41)(42)(43) and inhaled anticholinergic bronchodilators (44) have minimal effects. Regular use of beta-agonists enhance all aspects of the allergen response (45,46).
We hypothesize that the allergen challenge model primarily examining the late sequelae should be a valuable tool for investigation of new, potentially effective asthma therapies. This is based on allergen challenge responses to known effective drugs (particularly ICS, LTRAs and cromones), the minimal effectiveness of phosphodiesterase inhibitors and the lack of effectiveness of beta 2 -agonists and anticholinergics. A clinically ineffective ICS was shown to be ineffective in the allergen challenge model when comparing a known effective ICS (47).
Many of the molecules have demonstrated significant and promising effects. The most effective molecules we have studied to this point would be an ultra-long-acting beta-agonist effect on the magnitude and duration of protection against methacholine-induced bronchoconstriction (51), and the ability of IgE molecules to inhibit the EAR. To date, the best LAR inhibitor we have seen is an anti-interleukin 13 molecule (52). The group continues to collaborate on the investigation of new and exciting molecules.

summAry
Our understanding of causes and mechanisms of allergen-induced asthma has increased exponentially over the past few decades. It is now appreciated that allergen exposure in sensitized individuals is a genuine cause of asthma and is probably the most common etiological factor in the disease we know as asthma. A standardized allergen challenge model addressing the late sequelae (LAR, induced AHR and induced inflammation) has proven valuable in investigating asthma pathogenesis, pathophysiology and pharmacology. Much of this increased understanding can be attributed to the pioneering work of Freddy Hargreave, his colleague Jerry Dolovich and Freddy's numerous former fellows.

ACKnoWleDgements:
The author thanks Jacquie Bramley for assisting in the preparation of the manuscript.