Identification of a novel CpG DNA class and motif that optimally stimulate B cell and plasmacytoid dendritic cell functions

Recent reports have identified two major classes of CpG motif‐containing oligodeoxynucleotide immunostimulatory sequences (ISS): uniformly modified phosphorothioate (PS) oligodeoxyribonucleotides (ODNs), which initiate B cell functions but poorly activate dendritic cells (DCs) to make interferon (IFN)‐α, and chimeric PS/phosphodiester (PO) ODNs containing runs of six contiguous guanosines, which induce very high levels of plasmacytoid DC (PDC)‐derived IFN‐α but poorly stimulate B cells. We have generated the first reported ISS, C274, which exhibits very potent effects on all human immune cells known to recognize ISS. C274 is a potent inducer of IFN‐γ/IFN‐α from peripheral blood mononuclear cells and exhibits accelerated kinetics of activity compared with standard ISS. This ODN also effectively stimulates B cells to proliferate, secrete cytokines, and express costimulatory antigens. In addition, C274 specifically activates PDCs to undergo maturation and secrete cytokines, including very high levels of IFN‐α. Sequence variation studies based on C274 were used to identify the general motif requirements for this novel and distinct class of ISS. In contrast, chimeric PO/PS CpG‐containing ODNs with polyguanosine sequences exert a differential pattern of ISS activity compared with C274, perhaps in part as a result of their greatly different structural nature. This pattern is composed of high IFN‐α/IFN‐γ induction and low DC maturation in the absence of B cell stimulation. In conclusion, we have generated a novel class of ISS that transcends the limitations ascribed to classes described previously in that it provides excellent stimulation of B cells and simultaneously activates PDCs to differentiate and secrete large amounts of type I IFN.