Oligonucleotide sequences known as CpG ODNs are potent TLR9 agonists. The literature shows that formulation of CpG ODNs elicits more potent immune responses than the formulation or the adjuvant molecule alone. Given the importance of formulation on adjuvant biological activity and toxicity, formulation parameters should be well characterized and rationally designed. An understanding of the biophysical phenomena related to adjuvant interactions with formulation components can be obtained through complementary analytical techniques. Microeletrophoresis, UV spectrophotometry, and HPLC are used to characterize the association of CpG ODNs with aluminum hydroxide and oil-in-water emulsion formulations.
It was also shown that CpG ODN formulated with aluminum hydroxide or an oil-in-water emulsion induced significantly higher antibody titers than either component alone in a rabbit model with various antigens or a piglet model with swine streptococcic septicemia killed vaccine.4,5 In addition, total antibody levels and CTL activity induced in mice by a hepatitis B surface antigen formulated with CpG ODN and aluminum hydroxide were significantly higher than CpG or alum alone.6 Finally, encapsulation of CpG ODNs in polymer microparticles has been shown to increase antibody titers to a meningitis antigen in mice compared to non-encapsulated CpG.7
Given that the combination of adjuvant molecules such as CpG ODNs with particulate formulations translates into significantly more potent immune responses or reduced toxicity, it is important to optimize the formulation and presentation of the adjuvant to the immune system. Improving adjuvant formulation design requires an understanding of the basic biophysical principles underlying adjuvant association with the particulate formulation. This topic will be discussed using several specific examples of the formulation of CpG ODNs with particulate formulations commonly employed as vaccine adjuvants, namely aluminum hydroxide gel and oil-in-water emulsion. A particular emphasis has been placed on practical aspects of material properties and characterization methodology.