The purpose of this study is to investigate whether AHR to mannitol is a suitable marker of response to MEDI9929, but also to better understand the anti-inflammatory effects of MEDI9929 in the lungs, including whether a reduced AHR to mannitol following treatment with MEDI9929 is related to a reduction in chymase/CPA-3 positive mast cells. The investigators hypothesize that MEDI9929 will decrease the response to mannitol (measured as an increase in PD15) after 12 weeks of treatment as compared to placebo. It is further hypothesized that the number of chymase/CPA-3 positive mast cells in airway epithelium and submucosa will be reduced after 12 weeks of treatment with MEDI9929 in subjects with AHR to mannitol.