To investigate the role of HLA-DQ molecules and/or CD4+ T cells in the pathogenesis of allergic asthma, we generated HLA-DQ6 and HLA-DQ8 transgenic mice lacking endogenous class II (Aβ null) and CD4 genes and challenged them intranasally with short ragweed allergenic extract (SRW). We found that DQ6/CD4 null mice developed a strong eosinophilic infiltration into the bronchoalveolar lavage and lung tissue, while DQ8/CD4 null mice were normal. However, neither cytokines nor eosinophil peroxidase in the bronchoalveolar lavage of DQ6/CD4 null mice was found. In addition, the airway reactivity to methacholine was elevated moderately in DQ6/CD4 null mice compared with the high response in DQ/CD4+ counterparts and was only partially augmented by CD4+ T cell transfer. The DQ6/CD4 null mice showed Th1/Th2-type cytokines and SRW-specific Abs in the immune sera in contrast to a direct Th2 response observed in DQ6/CD4+ mice. The proliferative response of spleen mononuclear cells and peribronchial lymph node cells demonstrated that the response to SRW in DQ6/CD4 null mice was mediated by HLA-DQ-restricted CD4− CD8− NK1. 1− T cells. FACS analysis of PBMC and spleen mononuclear cells demonstrated an expansion of double-negative (DN) CD4− CD8− TCRαβ+ T cells in SRW-treated DQ6/CD4 null mice. These cells produced IL-4, IL-5, IL-13, and IFN-γ when stimulated with immobilized anti-CD3. IL-5 ELISPOT assay revealed that DN T cells were the cellular origin of IL-5 in allergen-challenged DQ6/CD4 null mice. Our study shows a role for HLA-DQ-restricted CD4+ and DN T cells in the allergic response.