Our data demonstrate that intranasal immunization of Bac-RSV/G vaccine induces complete protection from RSV infection, which is associated with enhanced Th1 and Th17 responses in the absence of significant Th2 responses. Although the role of Th1 or CTL responses to RSV clearance is well delineated (34) , the role of Th17-mediated responses have not been fully elucidated in host defense against RSV. Th17 cells, which are characterized by massive production of IL-17, are thought to contribute to autoimmune diseases but also play a crucial role in host defense against extracellular bacteria (35, 36) . In addition, Th17-associated effector molecules seem to be necessary and sufficient for protection against several viral pathogens. For example, recent studies have reported that Tc17 cells are potently cytolytic against vaccinia virus-infected cells (37, 38) . In these studies, both IL-17-producing CD4＋ and CD8＋ T cells are necessary to confer pro-tection against viral infections, exhibiting cytotoxic activity for clearance of vaccinia virus-infected cells. Mice immunized with a recombinant vaccinia virus expressing the G protein of RSV (vvG) exhibit pulmonary eosinophilia induced by Th2 type cytokines after challenge RSV infection (6) . Th2 type cytokines are thought to promote eosinophil infiltration to the lungs of RSV-infected mice and may contribute to immunopathology associated with vaccine-enhanced diseases. Interestingly, we showed that the levels of Th2-type cytokines, such as IL-4, IL-5, IL-10, or IL-13, were very low in the lungs of Bac-RSV/G-immune mice following RSV challenge, not significantly different to those in the PBS control group. Several evidences suggest that IL-17 negatively regulates Th2 cytokine production (39, 40) . Thus, the induction of Th1/Th17 responses by Bac-RSV/G vaccination may have attenuated the development of Th2 responses. Furthermore, IL-17 may play certain roles in immune defense in the lungs, including augmentation of mucosal immunity by delivery of IgA and IgM into the airway lumen (41) . Though the exact mechanism by which the Th17 responses are induced by Bac-RSV/G vaccination is not clear, the G-specific Th17 response may be associated with clearance of RSV from the lungs. Further studies will be needed to elucidate the exact role of Th17 immunity in RSV infection.