A series of recent studies, including an article in this issue of the European Journal of Immunology, have demonstrated that the administration of CpG‐rich oligodeoxynucleotides (CpG‐ODN) in experimental settings may lead to the activation of the immunosuppressive pathway of tryptophan catabolism, depending on several factors, including the route of CpG‐ODN administration. These studies call attention to the need for a careful evaluation of the modalities of inclusion of CpG‐ODN in vaccines for human use. At the same time, these studies may offer novel opportunities for use of CpG‐ODN as immunosuppressive agents and may also lead to an improved understanding of the cellular events mediated by Toll‐like receptor 9 signaling.

See accompanying article: http://dx.doi.org/10.1002/eji.200535602