Benzothiazole derivatives target oligoadenylate synthetase and enhance RSV vaccine efficacy as adjuvants

2',5' oligoadenylate (2-5A) activates RNase L to degrade intracellular RNA and inhibit viral proliferation. The resulting nucleotide fragments bind RIG-I-like receptors to initiate type I interferon (IFN-I) signaling, enhancing maturation of antigen processing cells to trigger adaptive immunity. Moreover, 2-5A is an immune mediator to initiate neighboring cells for antiviral immunity. However, due to its short half-life in biological systems hinders its application in vaccine. Herein, we present a series of benzothiazole derivatives with modification at 2, 3, and 6 positions, that exhibit enhancing antigen immunogenicity. Among them, 3-benzyl-6-hydroxy-2-methylbenzo[d]thiazol-3-ium bromide and 3-benzyl-6-methoxy-2-methylbenzo[d]thiazol-3-ium bromide, named as CG and DG, significantly promoted dendritic cell maturation and elevated IgG titers when combination with model antigen OVA. By transcriptome sequencing analysis and knock out experiments, they activated 2-5A synthetase (OAS) to increase 2-5A production, and further was corroborated by in vitro OAS activity assay. When combined with pre-F antigen from respiratory syncytial virus (RSV), CG and DG significantly increased populations of memory B and T cells and enhanced neutralizing antibody titers. Thus, this study developed two small non-nucleotide agonists, CG and DG, that may active the OAS-2-5A-RIG-I innate sensing axis. Their stable chemical structure and clear molecular target OAS render them as promising adjuvant for technology translation.