Local increase in the number of mast cells and expression of nerve growth factor in the bronchus of asthmatic patients after repeated inhalation of allergen at low-dose

Background Repeated inhalation of allergen at low-dose induces an increase in bronchial hyper-responsiveness, without any associated symptom. The conc omitant events in the bronchus have not been described. Objective We have studied the dynamic number of mast cells in the airways o f patients with mild asthma before and after repeated inhalation of allerge n at low-dose and the expression of nerve growth factor (NGF), which is rep orted to promote growth and survival of mast cells. Methods Twelve patients with mild asthma to cat allergen were enrolled at r andom in a blind placebo-controlled study, and submitted to repeated low-do se allergen exposure (1/5 of the provocative dose). Mast cells were immunol ocalized using an antibody against mast cell tryptase. NGF and its high aff inity receptor, TrkA, were immunolocalized using anti-NGF and anti-TrkA ant ibodies, respectively. NGF mRNA was quantified by competitive polymerase ch ain reaction (PCR) after reverse transcription of total RNA extracted from bronchial biopsy. NGF protein levels were measured by ELISA in bronchoalveo lar lavage (BAL) fluid. Results Bronchial mast cell number was increased significantly after allerg en exposure as compared with before. NGF expression in the bronchus was imm unolocalized mainly to epithelial cells, but also to fibroblasts, blood ves sels, and a few infiltrated cells. NGF mRNA levels in bronchial biopsies we re increased significantly after allergen exposure. The high affinity recep tor for NGF, TrkA, was immunolocalized to the infiltrated mast cell membran e. Conclusion Our study shows that the increase in the number of mast cells an d in the expression of NGF induced by allergen exposure in the bronchus of asthmatic patients is occurring before the onset of symptoms. In addition, our finding of the presence of the TrkA receptor on the membrane of the inf iltrated mast cell in situ brings evidence of the mast cell as a target cel l for the growth factor activity of NGF in the airways in asthma.