Natural products and the search for novel vaccine adjuvants

Vaccines that protect against intracellular infections such as malaria, Leishmania and Chlamydia require strong cellular responses based on CD4(+) T cells and CD8(+) T cells in addition to antibodies. Such cell-mediated responses can be potentiated with adjuvants. However, very few adjuvants have been licensed for use in humans; thus there is an urgent need for the discovery of new non-toxic adjuvants in order to produce more efficacious vaccines. Until recently, the mechanisms of how adjuvants worked remained largely unknown, but, it is becoming clearer that many function via host germline-encoded pattern recognition receptors (PRRs) expressed by most immune and non-immune cells. Most PRRs sense infection and transmit a series of signals that ultimately lead to the development of immunity. PRR mediated signalling can be harnessed to search for new vaccine adjuvants. Dendritic cells (DCs) express many PRRs and are remarkably effective at directing T cell immunity. Natural products (NPs) have been the basis of many drugs and are a rich source of immune activators and/or regulators of the immune response. Here we review PRRs in the context of NPs and propose the use of DCs as biological probes to help identify novel immune type molecules and adjuvants within collections of NPs.