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Antibodies against cancer-relevant targets can recruit Fc receptor-bearing myeloid and natural killer cells to mediate antibody dependent cellular cytotoxicity (ADCC). Toll-like receptors are potent activators of the innate immune system and generate signals leading to the initiation of an adaptive immune response that can be utilized for therapeutic purposes. We tested the efficacy of E6020, a TLR4 agonist, administered in combination with the clinically approved anti-HER2 antibody trastuzumab. A four-week course of therapy with trastuzumab and E6020 protected against the outgrowth of syngeneic D5 melanoma cells transfected with the human tumor antigen HER2 in immunodeficient C57BL/6 SCID mice, in immunocompetent wild-type C57BL/6 mice as well as in human HER2-tolerant C57BL/6 hmHER2 transgenic mice. In each tested strain of mice, the addition of E6020 to trastuzumab significantly increased the proportion of animals that survived initial tumor challenge compared to trastuzumab treatment alone (83% vs. 33% in C57BL/6 hmHER2 transgenic mice; p < 0.05). To determine if apparently cured mice had been effectively immunized against human HER2, the transgenic mice were inoculated subcutaneously with 103 D5-HER2 cells and then received a 4-week course of trastuzumab+E6020, or trastuzumab alone. Animals that survived the primary tumor challenge were followed for at least 120 days and then rechallenged with 5 × 103 D5-HER2 or unmodified D5 cells and followed for a minimum of 60 additional days. Mice that had been initially treated with the combination of trastuzumab and E6020 demonstrated superior protection (22/30; 73%) to those treated with trastuzumab alone (16/36; 44%; p < 0.05) against rechallenge with D5-HER2, but no protection was seen against D5 challenge. Both CD4 and CD8 T-cells were required for full protection against rechallenge. These findings suggest that trastuzumab therapy can induce a host-protective adaptive immune response. Moreover, combined treatment with trastuzumab and a TLR4 agonist enhances both the direct anti-tumor effects of trastuzumab and a host-protective human HER2-directed adaptive immune response. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr LB-324.