ac‐ cepted for publication February 01, 2016; available online without sub‐ scription through the open access option. ©AlphaMed Press 1066‐5099/2016/$30.00/0 This article has been accepted for publication and undergone full peer review but has not been through the copyediting, typeset‐ ting, pagination and proofreading process which may lead to differ‐ ences between this version and the Version of Record. Please cite this article as 1,2,3,4 Key words. house dust mite asthma  mesenchymal stem cells  M2 macro‐ phage  airway hyper‐responsiveness  phagocytosis  airway smooth muscle contraction. ABSTRACT Mesenchymal stem cell (MSC) immunosuppressive functions make them attractive candidates for anti‐inflammatory therapy in allergic asthma. However the mechanisms by which they ensure therapeutic effects remain to be elucidated. In an acute mouse model of house dust mite (Der f)‐ induced asthma, one i.v. MSC injection was sufficient to normalize and stabilize lung function in Der f‐sensitized mice as compared to control mice. MSC injection decreased in vivo airway responsiveness and de‐ creased ex vivo carbachol‐induced bronchial contraction, maintaining bronchial expression of the inhibitory type 2 muscarinic receptor. To eval‐ uate in vivo MSC survival, MSCs were labelled with PKH26 fluorescent marker prior to i.v. injection, and 1 to 10 days later total lungs were digest‐ ed to obtain single‐cell suspensions. 91.5 ± 2.3% and 86.6 ± 6.3% of the recovered PKH26 + lung cells expressed specific macrophage markers in control and Der f mice respectively, suggesting that macrophages had phagocyted in vivo the injected MSCs. Interestingly, only PKH26 + macro‐ phages expressed M2 phenotype, while the innate PKH26 ‐ macrophages expressed M1 phenotype. Finally, the remaining 0.5% PKH26 + MSCs ex‐ pressed 10 to 100 fold more COX‐2 than before injection, suggesting in vivo MSC phenotype modification. Together, the results of this study indicate that MSCs attenuate asthma by being phagocyted by lung macrophages, which in turn acquire a M2 suppressive phenotype. STEM CELLS 2016; 00:000–000 SIGNIFICANCE STATEMENT In a model of asthma, injected mesenchymal stem cells (MSCs) are in vivo phagocyted by lung macrophages in the next 24 hours following i.v. injec‐ tion. Lung macrophages that have phagocyted MSCs, in turn, acquire an im‐ munosuppressive phenotype, responsible for MSC anti‐inflammatory in vivo efficacy.