Bone marrow-derived mesenchymal stromal cells (BMSCs) mitigate inflammation in mouse models of acute lung injury. However, specific mechanisms of BMSC actions on CD4 T lymphocyte-mediated inflammation in vivo remain poorly understood. Limited data suggests promotion of Th2 phenotype in models of Th1-mediated diseases. However, whether this might alleviate or worsen Th2-mediated diseases such as allergic asthma is unknown. To ascertain the effects of systemic administration of BMSCs in a mouse model of Th2-mediated allergic airways inflammation, ovalbumin (OVA)-induced allergic airways inflammation was induced in wild-type C57BL/6 and BALB/c mice as well as in interferon-γ (IFNγ) receptor null mice. Effects of systemic administration during antigen sensitization of either syngeneic or allogeneic BMSC on airways hyperreactivity, lung inflammation, antigen-specific CD4 T lymphocytes, and serum immunoglobulins were assessed. Both syngeneic and allogeneic BMSCs inhibited airways hyperreactivity and lung inflammation through a mechanism partly dependent on IFNγ. However, contrary to existing data, BMSCs did not affect antigen-specific CD4 T lymphocyte proliferation but rather promoted Th1 phenotype in vivo as assessed by both OVA-specific CD4 T lymphocyte cytokine production and OVA-specific circulating immunoglobulins. BMSCs treated to prevent release of soluble mediators and a control cell population of primary dermal skin fibroblasts only partly mimicked the BMSC effects and in some cases worsened inflammation. In conclusion, BMSCs inhibit Th2-mediated allergic airways inflammation by influencing antigen-specific CD4 T lymphocyte differentiation. Promotion of a Th1 phenotype in antigen-specific CD4 T lymphocytes by BMSCs is sufficient to inhibit Th2-mediated allergic airways inflammation through an IFNγ-dependent process. © AlphaMed Press.