The role of bone marrow inflammatory cell progenitors in allergen-induced asthma

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 eosinophil/basophil (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, bone 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 EoB 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 24 hours 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. (Abstract shortened by UMI.)