Adjuvant saponins have been identified and purified from an aqueous extract of the bark of the South American tree, Quillaja saponaria Molina. Among the 22 saponin peaks which were separable, the more predominant purified saponins have been identified as QS-7, QS-17, QS-18, and QS-21, also known as QA-7, QA-17, QA-18, and QA-21, respectively. These saponins have been substantially purified by various methods including high pressure liquid chromatography (“HPLC”), low pressure liquid silica chromatography, and hydrophilic interactive chromatography (“HILIC”). The substantially pure saponins have been found to be useful as immune adjuvants for enhancing immune responses in individuals. (Kensil, et al., U.S. Pat. No. 5,057,540; Kensil, et al., J. Immunol. 148:2357 (1991); Marciani, et al., Vaccine 9:89 (1991).)
Recently, oligonucleotides containing the unmethylated cytosine-guanine (“CpG”) dinucleotide in a particular sequence context or motif have been shown to be potent stimulators of several types of immune cells in vitro. (Weiner, et al., Proc. Natl. Acad. Sci. 94:10833 (1997).) An immunostimulatory oligonucleotide comprising an unmethylated CpG motif is an dinucleotide within the oligonucleotide that consistently triggers an immunostimulatory response and release of cytokines. CpG motifs can stimulate monocytes, macrophages, and dendritic cells that can produce several cytokines, including the T helper 1 (“Th 1”) cytokine interleukin (“IL”) 12. (Carson, et al., J. Exp. Med. 186:1621 (1997).) This effect causes the induction of IFN-γ secretion by natural killer cells, which in turn, activates macrophages and enhances immunoglobulin isotype switching to IgG2a, a hallmark of T helper cell immunity and differentiation. (Chu, et al., J. Exp. Med. 186:1623 (1997).) Klinman, et al., have shown that a DNA motif consisting of an unmethylated CpG dinucleotide flanked by two 5′ purines (GpA or ApA) and two 3′ pyrimidines (TpC or TpT) optimally stimulated B cells to produce IL-6 and IL-12 and stimulated CD4+ T cells to produce IL-6 and IFN-γ both in vitro and in vivo. (Klinman, et al., Proc. Natl. Acad. Sci., 93:2879 (1996).) Davis, et al., the contents of which are incorporated herein by reference, discovered that nucleic acids containing at least one unmethylated CpG dinucleotide may affect the immune response of a subject (Davis, et al., WO 98/40100, PCT/US98/04703).