The role of CpG in DNA vaccines

One of the most exciting developments in the field of vaccine research in recent years has been DNA vaccines, with which immune responses are induced subsequent to the in vivo expression of antigen from directly introduced plasmid DNA. Strong immune responses have been demonstrated in a number of animal models against many viral, bacterial and parasitic pathogens, and several human clinical trials have been undertaken. The strong and long-lasting antigen-specific humoral (antibodies) and cell-mediated (T help, other cytokine functions and cytotoxic T cells) immune responses induced by DNA vaccines appear to be due to the sustained in vivo expression of antigen, efficient antigen presentation and the presence of stimulatory CpG motifs. These features are desirable for the development of prophylactic vaccines against numerous infectious agents. Furthermore, the strong cellular responses are also very desirable for the development of therapeutic DNA vaccines to treat chronic viral infections or cancer. Efforts are now focusing on understanding the mechanisms for the induction of these immune responses, which in turn should aid in the optimization of DNA vaccines. This review will focus on the role of CpG motifs in DNA vaccines.