We have demonstrated that Valpha24(+)Vbeta11(+) invariant (Valpha24(+)i) NKT cells from patients with allergic asthma express CCR9 at high frequency. CCR9 ligand CCL25 induces chemotaxis of asthmatic Valpha24(+)i NKT cells but not the normal cells. A large number of CCR9-positive Valpha24(+)i NKT cells are found in asthmatic bronchi mucosa, where high levels of Th2 cytokines are detected. Asthmatic Valpha24(+)i NKT cells, themselves Th1 biased, induce CD3(+) T cells into an expression of Th2 cytokines (IL-4 and IL-13) in cell-cell contact manner in vitro. CD226 are overexpressed on asthmatic Valpha24(+)i NKT cells. CCL25/CCR9 ligation causes directly phosphorylation of CD226, indicating that CCL25/CCR9 signals can cross-talk with CD226 signals to activate Valpha24(+)i NKT cells. Prestimulation with immobilized CD226 mAb does not change ability of asthmatic Valpha24(+)i NKT cells to induce Th2-cytokine production, whereas soluble CD226 mAb or short hairpin RNA of CD226 inhibits Valpha24(+)i NKT cells to induce Th2-cytokine production by CD3(+) T cells, indicating that CD226 engagement is necessary for Valpha24(+)i NKT cells to induce Th2 bias of CD3(+) T cells. Our results are providing with direct evidence that aberration of CCR9 expression on asthmatic Valpha24(+)i NKT cells. CCL25 is first time shown promoting the recruitment of CCR9-expressing Valpha24(+)i NKT cells into the lung to promote other T cells to produce Th2 cytokines to establish and develop allergic asthma. Our findings provide evidence that abnormal asthmatic Valpha24(+)i NKT cells induce systemically and locally a Th2 bias in T cells that is at least partially critical for the pathogenesis of allergic asthma.