Th1-response plays a crucial role in determining pathogenesis of organ-specific autoimmune diseases. It is believed that both IL-12 and INF-a are initiators to regulate Th1-response. In our experimental autoimmune uveitis (EAU) model, both Lewis and Fischer 344 rats share the same MHC class II molecules, while Lewis rat is EAU susceptible and Fischer 344 rat is EAU resistant. However, under the same condition of immunization, if pertussis toxin (PTX) was injected intraperitoneally as an additional adjuvant, Fischer 344 rat can develop EAU. In this study we investigate which mechanisms are involved in the induction of EAU in CFA + R16 + PTX treated (CRP- treated) Fischer 344 rats. In vivo and in vitro data demonstrated that Th1-cytokine, IFN- mRNA expression was significantly increased in disease target tissue-eyes and in draining lymph node cells of CRP-treated Fischer 344 rat. When IL-12 and IFN-a mRNA expression were compared in the experimental groups, only IFN-a mRNA expression was associated with EAU development. To distinguish the sources of IFN-a producing cells, it was observed that IFN-a expression was mainly produced by macrophages. It was further confirmed that normal macrophage from Fischer 344 rat was able to produce significant IFN-a in the presence of PTX. The data strongly suggested that IFN-a might be involved in initiating Th1-cell differentiation and in turn contribute to the induction of EAU. High IFN-a expression induced by PTX may represent a novel pathway to initiate Th1 response in Fischer 344 rat. Th1-response plays a crucial role in determining pathogenesis of organ-specific autoimmune diseases. It is believed that both both IL-12 and INF-a are initiators to regulate Th1-response. In our experimental autoimmune uveitis (EAU) model, both Lewis and Fischer 344 rats share the same MHC class II molecules, while Lewis rat is EAU susceptible and Fischer 344 rat is EAU resistant. However, under the same condition of immunization, if pertussis toxin (PTX) was injected intraperitoneally as an additional adjuvant, Fischer 344 rat can develop EAU. In this study we investigate which mechanisms are involved in the induction of EAU in CFA + R16 + PTX treated (CRP-treated) Fischer 344 rats. In vivo and in vitro data demonstrated that Th1-cytokine, IFN- mRNA expression was significantly increased in disease target tissue-eyes and in draining lymph node cells of CRP-treated Fischer 344 rat. When IL-12 and IFN-a mRNA expression were compared in the experimental groups, only IFN-a mRNA expression wa s associated with EAU development. To distinguish the sources of IFN-a producing cells, it was observed that IFN-a expression was mainly produced by macrophages. It was further confirmed that normal macrophage from Fischer 344 rat was able to produce significant IFN-a in the presence of PTX. The data strongly suggested that IFN-a might be involved in initiating Th1-cell differentiation and in turn contribute to the induction of EAU. High IFN-a expression induced by PTX may represent a novel pathway to Th1 response in Fischer 344 rat.