Cationic liposomes extend the immunostimulatory effect of CpG oligodeoxynucleotide against burkholderia pseudomallei infection in BALB/c mice

Melioidosis is a severe disease caused by the Gram-negative bacterium Burkholderia pseudomallei. Previously we showed that pretreatment of mice with CpG oligodeoxynucleotide (CpG ODN) 2 to 10 days prior to B. pseudomallei challenge conferred as high as 90% protection, but this window of protection was rather short. In the present study, we therefore aimed to prolong this protective window and to gain further insight into the mechanisms underlying the protection induced by CpG ODN against B. pseudomallei infection. It was found that the CpG ODN incorporated with cationic liposomes (DOTAP) but not zwitterionic liposomes (DOPC) provided complete protection against bacterial challenge. Although marked elevation of gamma interferon (IFN-γ) was found in the infected animals 2 days postinfection, it was significantly lowered by the DOTAP-plus-CpG ODN pretreatment. When appropriately activated, the phagocytic index and oxidative burst responses of neutrophils appeared not to be elevated. However, macrophages from stimulated mice showed higher levels of nitric oxide production and exhibited higher levels of antimicrobial activities, judging from lower numbers of viable intracellular bacteria. Taken together, our results demonstrate that DOTAP can enhance the protective window period of CpG ODN to at least 30 days and provide 100% protection against B. pseudomallei infection. The protective effect of DOTAP plus CpG ODN could provide an alternative approach to preventing this lethal infection, for which no vaccine is yet available.