Toll-like receptor 9, which is expressed on the surface of antigen presenting cells and which was recently identified in the cytoplasmic follicle, recognizes bacterial CpG oligodeoxynucleotides (ODNs), resulting in the induction of a potent immune response. However, in our previous study, we found that TLR9 potentially recognizes not only CpG ODN but also non-CpG ODN such as AT ODN. Therefore, in the present study, to investigate this possibility, we elucidated the effects of AT ODN on T-H-1, T-H-2 type cytokine induction via TLR9 by real-time quantitative PCR analysis and ELISA of the swine TLR9 transfectant. The results demonstrated that the T-H-1 type cytokines such as interleukin (IL)-12p70 and interferon (IFN)-gamma were strongly induced by AT ODN compared to the unexposed controls, while T-H-2 type cytokines were not induced. These results indicate that the AT ODN can augment the T-H-1 immune response, which plays an important role in prevention of allergic responses. Moreover, the swine TLR9 transfectant demonstrated its usefulness for evaluation of immunostimulation by bacterial DNA through the detection of T-H-1, T-H-2 type cytokine induction via TLR9 signaling. (C) 2004 Elsevier Inc. All rights reserved.