Aims: In this study, we investigate the role of reactive oxygen species (ROS) and reactive nitrogen species (RNS) in autoimmune diseases. We focus on oxidative regulation at the interaction between antigen-presenting cells (APCs) and T cells, and consequent effect of ROS and RNS on type II collagen (CII)-induced arthritis (CIA) model in mice. Results: Mice deficient in ROS and peroxide, due to a mutation in Ncf1 gene, develop an exaggerated CIA and a stronger T cell response to CII. In contrast, nitric oxide synthase inhibitor NG-nitro-L-arginine methyl ester (L-NAME) was found to protect against CIA. The most pronounced protective effect was observed when L-NAME treatment started immediately after CII immunization. Ten days after immunization, the CII-reactive T cell-proliferative response was greater in Ncf1-mutant mice that were treated with L-NAME. T cells from L-NAME-treated mice, primed with CII, showed lower interleukin-2 secretion in response to CII in vitro. Moreover, inhibition of RNS production resulted in dysregulation of NOS1 (neuronal) expression in CII-reactive T cells. Innovation and Conclusion: The results support that deficiency of a paracrine factor as ROS and peroxide released by APC leads to pronounced activation of T cells and enhanced arthritis. An intrinsic factor might be RNS produced by NOS1, which likely enhanced T cell activation in an autocrine manner.Keywords: NCF1; NOS1; arthritis; autoimmunity; reactive nitrogen species; reactive oxygen species.