IκBα is an inhibitor of the transcriptional factor NF-κB, and it is an essential component of the signaling pathways that lead to expression of inflammatory molecules. These include cytokines and costimulatory molecules associated with antigen presentation in an inflammatory immune response. In this study, we report that antigen-presenting cells exposed to TGF-β induce peripheral tolerance by increasing IκBα expression. Exposure of antigen presenting cells (APCs) to TGF-β is known to impair their ability to secrete IL-12, and such impairment correlated with reduced NF-κB activity as indicated by significantly reduced nuclear levels of p50, an essential subunit of NF-κB for IL-12 transcription. Blockade of increased nuclear IκBα in APCs by expression of small interfering RNA molecules (siRNAs) targeting IκBα transcripts prevented IL-12 impairment and the decline in nuclear p50 levels. Furthermore, such IκBα blockade also interfered with the tolerogenic property of TGF-β-exposed APCs. However, increased expression of IκBα in APCs, independent of TGF-β exposure, reduced nuclear p50 levels and permitted tolerance induction by APCs. Thus, our findings attribute a direct and significant role to IκBα in the tolerogenic potential of APCs. Increased IκBα expression in APCs may therefore offer a therapeutic approach to achieve antigen-specific immunomodulation.—Ghafoori, P., Yoshimura, T., Turpie, B., Masli, S. Increased IκBα expression is essential for the tolerogenic property of TGF-β-exposed APCs.