Optimization of codon usage of plasmid DNA vaccine is required for the effective MHC class I-restricted T cell responses against an intracellular bacterium.

In an attempt to study codon usage effects of DNA vaccines on the induction of MHC class I-restricted T cell responses against an intracellular bacterium, Listeria monocytogenes, we designed two plasmid DNA vaccines encoding an H-2Kd-restricted epitope of listeriolysin O (LLO) of L. monocytogenes, LLO 91-99. One DNA vaccine, p91wt, carries the wild-type DNA sequence encoding LLO 91-99, and the other one, p91mam, possesses the altered DNA sequence in which the codon usage was optimized for murine system. Our read-through analyses with LLO 91-99/luciferase fusion genes confirmed that the optimized 91mam DNA sequence showed extremely higher translation efficiency than the wild-type sequence in murine cells. Consistent with this, i.m. injections of p91mam, but not of p91wt, into BALB/c mice were capable of inducing specific CTL- and IFN-gamma-producing CD8+ T cells able to confer partial protection against listerial challenge. Taken together, these observations suggest that optimization of codon should be taken into consideration in the construction of DNA vaccines against nonviral pathogens.