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Breast tumor size increases when DCs and T cells team up (red).
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On page 1037, Aspord et al. find that immune cells are swayed to the dark side by cancerous forces. Instead of priming a killing response, dendritic cells (DCs) that infiltrate breast tumors actually help the tumor grow.
The group previously found that human breast tumors often include a large number of mature dendritic cells that form clusters with CD4+ T cells. This association suggested that immunity was at work in the tumor, but it also hinted at an oddity: mature DCs are normally found in lymph nodes, not the affected tissue.
To figure out what these DCs were doing in the tumors, Aspord and colleagues studied humanized mice that were grafted with human breast cancer cell lines. In these mice, the DCs recruited CD4+ T cells to the tumors but did not prime an antitumor immune response. Instead, the DCs instigated the T cells to secrete IL-13 and thereby promote tumor growth.
IL-13 helped tumors by activating a signaling pathway that has been implicated in metastasis. But why the DCs initiated this tumor-friendly response is still unclear.
The findings might help explain why DC-based vaccine therapies sometimes fail in clinical trials. The team now plans to test IL-13 antagonists in breast cancer patients in combination with candidate DC vaccines. The hope is that the antagonists will prevent the vaccine-induced antitumor immunity from being subverted into a cancer-promoting response. 
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