The present findings involving human airways indicate that established allergic eosinophilic inflammation are reduced by therapeutic steroid intervention already five days after institution of treatment. We further demonstrate a degree of selectivity in the steroid action in that tissue CCL5 expression, but not CCL11, is reduced by budesonide along with its attenuating effects on tissue eosinophilia. In view of the established dogma [2
] it was most surprising that apoptotic eosinophils are not detected in the airway tissues at ongoing, spontaneous or drug-induced, resolution of the allergic airway inflammation. However, the present data obtained in a validated model of human allergic rhinitis agree with previous observations in vivo
on early resolution of eosinophilic inflammation in steroid-treated allergic animals.
Except for observations in animal models [8
] there was little prior information regarding early resolution of allergic eosinophilic airways disease in vivo
. This study thus produced new data on the action of airway corticosteroids on eosinophilic inflammation in human airway tissues in vivo
. Exit of inflammatory cells and proteins into the airway lumen may be a mode of elimination of these players from the inflamed tissue [9
]. During resolution, increased inflammatory indices on the mucosal surface may thus be associated with corresponding reductions of the same indices in the diseased airway tissue. This complicates the interpretation of lavage fluid data in this study. Hence, the present focus was on airway tissue indices. Since the present model has a demonstrated consistency at repeated studies of symptom development and treatment effects of allergic rhinitis [19
], we resorted to one biopsy occasion only and parallel group analyses. This was also done to avoid too much discomfort by the biopsy procedures and prevent dropouts. An additional consideration was that the institution of treatment had to be optimal to pick up early phase resolution effects and at the same time allow for a slow onset of action of the steroid treatment. This is a field where helpful precedents are scarce. As it turned out in this study, the tissue indices of eosinophilic inflammation clearly remained present in the placebo-treated subjects whereas biopsies obtained from the steroid-treated group exhibited signs of a resolution that was speeded up compared to the placebo group of patients. At allergen exposure, individuals with allergic asthma or allergic rhinitis may loose epithelial cells and the ensuing epithelial restitution processes involve increased cell proliferation [24
]. It can further be expected that ongoing inflammatory processes will increase a general occurrence of cell apoptosis in the affected airway tissue. The present finding of a reduced number of apoptotic cells in airway tissues receiving therapeutic steroids thus tallies with an overall anti-inflammatory efficacy of institution of the steroid treatment [7
It has been widely assumed that apoptosis of airway tissue eosinophils would be increased by corticosteroid treatment [2
]. Inducement of eosinophil apoptosis (followed by a postulated efficient engulfment of the apoptotic eosinophils) has thus been advocated as a major pharmacological mechanism to bring about resolution of airway eosinophilic inflammation [2
]. By contrast, animal in vivo
data have scarcely supported a role of apoptosis in the pharmacology of eosinophil elimination [6
]. Thus, similar to the present findings, apoptotic eosinophils have not been unequivocally detected in steroid-treated allergic airway tissues in vivo
. Such negative data are strengthened by the concomitant demonstration, as in this study, that other types of apoptotic cells are observed in the allergic tissues. The present observations are remarkable because we examined a phase where the likelihood of eosinophil apoptosis should be the greatest both in spontaneous and drug-induced resolution. Our data may explain the scarcity of publicised reports on eosinophil apoptosis in airway biopsies including the lack of confirmation of a reported insignificant trend at steroid-induced increased eosinophil apoptosis in asthma [4
]. Furthermore, the present observations question the support that the current dogma is getting by reference to reports on occurrence of apoptotic eosinophils in the lumen of diseased airways [4
]. Indeed, airway lumen findings in this case have little relevance because apoptotic cells in the airway tissue cannot migrate and can, therefore, not be revealed in airway lumen samples. It is at present unclear if apoptosis of tissue eosinophils is a desirable drug action since poor engulfment of these cells may aggravate airway allergic inflammation [25
]. A poor engulfment in vivo
would further mean that apoptotic eosinophils, when they occur in the tissue, should be readily detected. Hence, we suggest that the present negative observations reflect true rarity of eosinophil apoptosis in spontaneously resolving as well as in corticosteroid-treated human nasal allergic airway tissues.
The present negative data on occurrence of apoptotic eosinophils in the airway tissue at resolution of inflammation support the possibility that airway tissue eosinophils can be swiftly and non-injuriously eliminated through egression into the airway lumen [9
]. In the lower airways, egressed cells would be mixed with plasma exudates and airway secretions and removed by the mucociliary escalator. Removal of inflammatory cells on the airway surface would be even more functional in the nasal passages since final elimination, through swallowing or blowing of the nose, is prompt and relatively uncomplicated. Animal data suggest that corticosteroid treatment does not prevent but may permit the egression of eosinophils into the airway lumen [8
]. In the present study, it was considered impractical to collect all nasal outputs during several days and nights to try and calculate egression of eosinophils during the post challenge period. However, the finding that subepithelial eosinophilia tended to be reduced more than the epithelial eosinophilia is compatible with the steroid treatment permitting traffic of eosinophils towards the lumen in airways with resolving eosinophilia.
The reduced eosinophilia in the corticosteroid group would in part reflect reduced recruitment of these cells during the period of steroid treatment. Although many locally present eosinophil chemoattractants might contribute, CCL5 and CCL11 have been pointed out as two major chemokines involved in recruiting circulating eosinophils to allergic airway tissues [26
]. The present demonstration of a reduced expression of CCL5, occurring simultaneously to the reduced tissue eosinophilia, suggests that CCL5, with its proposed roles in eosinophil and lymphocyte recruitment, can be of special importance as pharmacological target. It is also possible that disappearance of CCL5 from the tissue could have contributed to migration of eosinophils into the airway lumen [27
]. Currently entertained molecular actions of the anti-inflammatory steroids suggest that these drugs exert non-selective inhibition of the generation of inflammatory chemokines [26
]. Hence, the mechanism behind the particular in vivo
anti-CCL5 action of corticosteroids in mouse [8
] and human (this study) allergic airways remains mechanistically challenging. Other selective actions of steroids have been demonstrated previously including observations that steroid treatment, despite its potent anti-inflammatory effects, may spare leukocyte and microvascular innate responses mobilised in relation to epithelial repair [28
] and microbial defence [29
]. The present inhibition of CCL5 and the association between increase in airway CCL5 and deterioration of asthma at stopping steroid treatment [17
] support the view that this protein may have a central role in maintaining inflammatory processes in rhinitis and asthma.
We conclude that institution of nasal steroid treatment in subjects with established symptoms of allergic rhinitis reduces tissue indices of allergic eosinophilic inflammation within five days. Our data support a role of inhibition of CCL5-dependent cell recruitment, but challenge the dogma that eosinophil apoptosis is involved in corticosteroid-induced resolution of allergic airway inflammation.