CpG oligonucleotides as adjuvant in therapeutic vaccines against parasitic infections

Abstract

Immunostimulatory CpG DNA is recognized by cells of the innate immune system through Toll-like receptor 9 (TLR9). Synthetic CpG oligonucleotides (CpG-ODN) belong to the most potent vaccine adjuvants known today. This is due to their capacity not only to stimulate cells of the innate immune system but also to trigger effectively specific T- and B-lymphocyte responses. This unique quality seems to be superior in the induction of long-term effects and immunological memory. In addition to prophylactic vaccination, we showed that mice already infected with Toxoplasma gondii could be therapeutically vaccinated by the combined use of CpG-ODN and a bradyzoite antigen (BAG1 protein subunit). This treatment was effective against the infection and resulted in a long-term survival of the mice and reduced parasite burden in the brain. Different routes of CpG-ODN vaccine application including intranasal, oral or intraperitoneal delivery were compared with the classical subcutaneous application in two established experimental infection models of murine leishmaniasis and toxoplasmosis. Comparable effects were demonstrated for these modes of inoculation except for the oral uptake of uncoupled CpG-ODN, which resulted in a complete failure of treatment. Detailed studies were performed to optimize the time point of CpG-ODN application. The best results were obtained when the ODN were given within a few days around the infection, in contrast to former trials showing a time window of several weeks for significant oligonucleotide effects in non-infectious models. As CPG-ODN is a synthetic compound, it is not only available in high purity and reproducible quality, but can also be produced with different backbone modifications and sequence modifications. Combination of these possibilities resulted in new molecules that were highly effective as adjuvant in parasite infection models. Finally, our studies revealed not only that bacterial DNA is an effective vaccine adjuvants, but also that Leishmania DNA itself had immunostimulatory properties which are counteracted by a yet undefined inhibitory principle from living Leishmania.