A chimeric protein vaccine composed of the cholera toxin B subunit fused to proinsulin (CTB-INS) is shown to suppress type 1 diabetes (T1D) onset in NOD mice and upregulate the biosynthesis of the tryptophan catabolic enzyme indoleamine 2,3-dioxygenase (IDO1) in DCs. This evidence concerns the gene IDO1 and type 1 diabetes mellitus.