Researchers developing a cancer vaccine based on dendritic cells have substantially improved the vaccine's efficacy using the cytokine interleukin-2 (IL-2), suggesting that future cancer therapies might rely on exploiting the natural regulatory pathways of the immune system. The team, whose results appear in PNAS (96, 2268–2273, 1999), had previously developed a technique in which host dendritic cells are exposed to tumor cell lysates, then re-introduced into the host. Because dendritic cells present the antigens to T cells to initiate an immune response against the tumor, the scientists reasoned that IL-2—a T cell growth promoter that has already been approved for clinical use—might help the vaccine produce a more robust response. Tests of the new approach in mice showed that the combined vaccine can render the animals immune to lethal tumor challenge and cause the regression of micrometastases in mice with established tumors. According to James Mulé, a professor in the Department of Surgery at the University of Michigan and the senior author on the new study, a phase I trial on the dendritic cell vaccine has shown that "small numbers of dendritic cells can prime patients to react strongly to that antigen when presented by dendritic cells," and that the cells have low toxicity. Phase II trials on the combined vaccine are slated to begin in the next few months.