Dissecting the Molecular Pathology of Asthma through the Use of Genetic Mouse Models: Role of α₄ and β₂ Integrins.

Asthma is a complex syndrome with well described pathology. However, animal studies and clinical studies in humans continue to provide conflicting data on the contribution of local cells comprising the “respiratory membrane”( airway epithelial, endothelial and smooth muscle cells, fibroblasts etc) vs cells recruited from circulation. Although anti α₄ monoclonal antibody treatment ameliorates the clinical syndrome, i.v. vs local administration provided disparate results. To get further insight, we induced asthma phenotype in genetic mouse models deficient in α₄ or β₂ integrin and compared the results to WT mice. On day 22, 24 hrs post 4^th ovalbumin(OVA) challenge, accumulation of cells in BAL(bronchoalveolar lavage), lung parenchyma(LP) and peripheral blood(PB),airway hyperreactivity (AHR) to methacholine(Mch) and Th2 cytokines were measured.Compared to significant cell accumulation in OVA treated WT, BAL of both α₄−/− and β₂−/− mice showed very few cells (Table). In contrast to BAL, increased number of cells were observed in LP in β₂−/− mice and less so in α₄−/− compared to control.However the increase in β₂−/− LP was 18-fold (post OVA vs untreated) compared to 67-fold in WT and 12-fold in α₄−/−.Cells in β₂−/−LP represented only 3.8% of total circulating leukocytes vs 19.7% in WT and 1.6% in α₄−/−.BAL had increased levels of Th2 cytokines IL-4 (11.3±4.6 pg/ml) and IL-5 (97.8±36 pg/ml,n=5/group) in OVA treated WT, whereas BAL from α₄−/− and β₂−/− mice had undetectable levels of these cytokines. AHR to Mch was measured by whole body plethysmography and expressed as P_enh (% of air). In WT at 40 mg/ml Mch AHR increased from 250±27 to 650±70 after OVA treatment while in α₄−/− levels remained similar to untreated (240±25 to 270±20 post OVA challenge) (n=8/group). Additional evaluation by histochemistry showed nearly total abrogation of cell accumulation around blood vessels (H^&E stain) and drastic reduction in mucus secretion and airway goblet cell hyperplasia (Alcian blue stain) in k/o vs WT OVA treated mice. Our data suggest that:(1) a direct correlation exists between presence of inflammatory cells and Th2 cytokines in BAL with AHR, (2) moderate accumulation of α₄−/− cells in lung and considerably more of β₂−/− cells in lung post OVA challenge is not accompanied by a concurrent migration to BAL and does not contribute to AHR,(3) intra-tissue migration of cells (lymphocytes,eosinophils and macrophages) from LP to BAL and/or their activation requires presence of either α₄ or β₂ integrins and as such provides a novel insight into the mechanism of activation of cells in the respiratory membrane.

Number of leukocytes in BAL, lung and blood before and after OVA