We previously reported in an ovalbumin-induced model of allergic asthma that Fms-like tyrosine kinase 3 ligand (Flt3-L) reversed airway hyperresponsiveness (AHR) and airway inflammation, and increased the number of regulatory CD11c(high)CD8 alpha(high)CD11b(low) dendritic cells in the lung. In this study, we investigated the effect of Flt3-L in a clinically relevant aeroallergen-induced asthma on the phenotypic expression of lung T cells. Balb/c mice were sensitized and challenged with cockroach antigen (CRA), and AHR to methacholine was established. These mice received three intraperitoneal injections of anti-CD25 antibody (PC61; 250 microg) and Flt3-L (3 microg) daily for 10 days. Cytokines and Ig levels in the serum were measured and differential bronchoalveolar lavage fluid (BALF) cell counts were examined. Flt3-L reversed AHR to methacholine to the control level. Flt3-L significantly decreased levels of BALF IL-5, IFN-gamma, eosinophilia and substantially increased IL-10 and the number of CD4(+)CD25(+) Forkhead winged helix transcription factor box P3 (Foxp3(+)) IL-10(+) T cells in the lung. Administration of PC61 antibody blocked the effect of Flt3-L and substantially increased AHR, eosinophilia, and BALF IL-5 and IFN-gamma levels, and decreased BALF IL-10 levels and the number of CD4(+)CD25(+)Foxp3(+)IL-10(+) T cells. Flt3-L significantly decreased CD62-L, but increased inducible costimulatory molecule and Foxp3 mRNA expression in the CD4(+)CD25(+) T cells isolated from lungs of Flt3-L-treated, CRA-sensitized mice compared to CRA-sensitized mice without Flt3-L treatment and PBS control group. Flt3-L significantly inhibited the effect of CRA sensitization and challenge to increase GATA3 expression in lung CD4(+)CD25(+) T cells. Collectively, these data suggest that the therapeutic effect of Flt3-L is mediated by increased density of naturally occurring CD4(+)CD25(+)Foxp3(+)IL-10(+)ICOS(+) T-regulatory cells in the lung. Flt3-L could be a therapeutic strategy for the management and prevention of allergic asthma.