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 decreased ex vivo carbachol-induced bronchial contraction, maintaining bronchial expression of the inhibitory type 2 muscarinic receptor. To evaluate in vivo MSC survival, MSCs were labeled with PKH26 fluorescent marker prior to i.v. injection, and 1 to 10 days later total lungs were digested to obtain single-cell suspensions. 91.5 6 2.3% and 86.6 6 6.3% of the recovered PKH26 1 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 1 macrophages expressed M2 phenotype, while the innate PKH26 2 macrophages expressed M1 phenotype. Finally, the remaining 0.5% PKH26 1 MSCs expressed 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.

Atopic dermatitis (AD), allergic rhinitis (AR), chronic rhinosinusitis with (CRSwNP) or without (CRSsNP) nasal polyps and asthma share a complex interplay between a genetic background, a polarized immune response and the environment. Their increasing prevalence in urban areas in comparison with rural areas further highlights the role of environmental factors in their development. 1 Interestingly, they can succeed themselves or coexist at the same time in a single individual throughout life. For example, atopic march describes the