Overcoming immune tolerance is a significant challenge for cancer vaccines. In HER2/neu transgenic (neu-N) mice, a neu-targeted granulocyte macrophage colony stimulating facor (GM-CSF) secreting vaccine is ineffective due to tolerance to the proto-oncogene neu. However, we have previously reported that Paclitaxel given in immune modulating doses one day prior to vaccination can enhance the response of neu-specific T cells that are capable of curing neu-expressing tumors in tolerized neu-N mice. Paclitaxel has also been shown to mimic the activating effects of LPS on macrophages in vitro via Toll Like Receptor-4 (TLR-4). Since our GM-CSF-secreting tumor vaccine works by recruiting and activating dendritic cells (DCs), the most potent professional antigen presenting cell (APC), it is possible that Paclitaxel also enhances DC maturation via TLR-4. To study this, we cultured DCs from murine bone marrow using complete medium supplemented with GM-CSF and Paclitaxel. LPS was added on day 6 to stimulate DC maturation. DC maturation was then analyzed by flow cytometry to assess the expression levels of various DC maturation and functional markers. Our results show that, in vitro, Paclitaxel-treated DCs undergo enhanced maturation and are more responsive to LPS activation than GM-CSF treated DCs controls. Furthermore, Paclitaxel-induced maturation is completely inhibited when DCs are pre-treated with an anti-TLR-4 antibody during the first 12 hours of in vitro culture. Studies in neu-N mice are are on-going to examine the role of the TLR-4 pathway in Paclitaxel-mediated DC maturation in vivo. These studies suggest that activation through an innate immune signaling pathway has the potential to enhance vaccine induced adaptive T cell responses. These findings have implications for the future design of vaccine therapies for the treatment of cancer.

[Proc Amer Assoc Cancer Res, Volume 47, 2006]