Compeling evidence has now demonstrated that IL-17-producing CD4 cells (Th17) are a major contributor to autoimmune pathogenesis, whereas CD4 CD25 T regulatory cells (Treg) play a major role in suppression of autoimmunity. Differentiation of proinflammatory Th17 and immunosuppressive Treg from naive CD4 cells is reciprocally related and contingent upon the cytokine environment. We and others have reported that in vivo administration of pertussis toxin (PTx) reduces the number and function of mouse Treg. In this study, we have shown that supernatants from PTx-treated mouse splenic cells, which contained IL-6 and other proinflammatory cytokines, but not PTx itself, overcame the inhibition of proliferation seen in cocultures of Treg and CD4CD25 T effector cells. This stimulatory effect could be mimicked by individual inflammatory cytokines such as IL-1β, IL-6, and TNF-α. The combination of these cytokines synergistically stimulated the proliferation of CD4CD25 T effector cells despite the presence of Treg with a concomitant reduction in the percentage of FoxP3 cells and generation of IL-17-expresstag cells. PTx generated Th17 cells, while inhibiting the differentiation of FoxP cells, from naive CD4 cells when cocultured with bone marrow-derived dendritic cells from wild-type mice, but not from IL-6 mice. In vivo treatment with PTx induced IL-17-secreting cells in wild-type mice, but not in IL-6 mice. Thus, in addition to inhibiting the development of Treg, the immunoadjuvant activity of PTx can be attributable to the generation of IL-6-dependent IL-17-producing CD4 cells.