Asthma is a chronic airways disease characterised by recurrent episodes of wheezing and variable airflow obstruction, which is usually reversible spontaneously or with treatment, and with airway hyperresponsiveness, and airway inflammation. Allergen inhalation by sensitized, atopic asthmatics enhances airway hyperresponsiveness and inflammation, providing a useful model to study allergic asthma.

The aim of this thesis was to characterise the allergen-induced changes in bone marrow eosinophilA»asophil (Eo/B) progenitors, their ability to migrate out of the bone marrow and to determine whether these cells accumulate in the airways to participate in the allergen-induced airway inflammatory response. In addition, this thesis attempted to investigate the importance of pro- and anti-inflammatory cytokines and chemokines which contribute to the migration and differentiation of these cells. Bone marrow aspirates, blood and sputum samples were obtained from subjects at various time points both before and following allergen inhalation challenge.

In comparison to isolated early responders, who develop no allergen-induced late asthmatic responses or airway hyperresponsiveness, hone marrow Eo/B progenitor numbers were increased in dual responders, who develop marked allergen-induced airway eosinophilia and airway hyperresponsiveness. Increases in IL-3-responsive progenitors were detected as early as 5 hours post-allergen, and IL-5-responsive progenitors at 12 and 24 hours post-allergen in dual responders only. Bone marrow, blood and sputum IL-5 protein levels increased at 12 and 24 hours in dual responders only and these increases correlated with increases in IL-5-responsive progenitors. Additionally, bone marrow IFN-γ levels increased in dual responders at 48 hours coinciding with decreases in bone marrow E o/B progenitors. Expression of the receptor for eotaxin (CCR3) was detected on primitive (CD34 immunopositive cells) and eosinophil-lineage committed progenitors (CD34⁺IL-5Rα⁺ cells) by flow cytometry and confirmed by co-localization of CCR3 messenger RNA to CD34⁺ cells using in-situ hybridization. When pre-allergen was compared to 24hours post-allergen levels, significant increases in bone marrow CD34⁺CCR3⁺ cells were detected in dual responders, who also developed a significant sputum and blood eosinophilia and increased methacholine airway responsiveness. In contrast, a significant attenuation of bone marrow CD34⁺CCR3⁺ cells was observed in isolated early responders. In a dose dependent fashion eotaxin, but not IL-5, stimulated CD34⁺ progenitor cell migration in vitro.

Allergen-induced dual responders also developed a significant decrease in bone marrow SDF-1 a 24 hours post-allergen while developing a significant increase in circulating eotaxin and CXCR4^ cells. These findings indicate that the bone marrow actively participates in the development and persistence of allergen-induced airway inflammation and the participation is regulated by IL-3, IL-5 and eotaxin and down-regulation of SDF-1α for differentiation and migration.

Eo/B progenitors also migrate into the airway as part of the allergen-induced inflammatory response. In comparison to diluent control, sputum CD34⁺ cells increased at 7 hours following allergen inhalation, an effect which was sustained at 24 hours in the dual responder group only, associated with sustained increases in sputum CD34⁺IL-5Rα⁺ cells, eosinophils and IL-5 protein levels. These findings indicate that CD34⁺ cells increase in the airways post-allergen inhalation and have the potential to respond to IL-5, contributing to allergen-induced asthma.

This thesis has demonstrated time-dependent changes in cytokine levels in airway, blood and bone marrow coordinating the activation, recruitment and differentiation of eosinophil/basophil progenitors from the bone marrow to the airways. Furthennore, the activation and trafficking of eosinophil progenitors is more pronounced in asthmatics developing a late inflammatory airway response with associated airway hyperresponsiveness and pronounced eosinophilia. Eosinophil/basophil progenitors, potentially regulated by inflammatory cytokines appear to play an important role in the inflammatory process associated with allergic asthma.