Augmenting the potency of breast cancer vaccines by modulating immune tolerance and targeting the tumor microenvironment
Due to the complexity of tumors and their microenvironment, we hypothesized that strategies which target multiple components of the tumor may be more successful against cancer than single agents. The HER-2/neu (neu) mouse model is ideal for investigating this hypothesis for several reasons. First, the proto-oncogene neu is overexpressed in 30% of breast cancers and correlates with poor survival. Second, in neu transgenic (neu-N) mice, which are derived from wild-type FVB mice to overexpress rat neu in mammary tissue, both spontaneous tumor development and peripheral immune tolerance to neu are similar to that found in breast cancer patients. Neu-specific GM-CSF-secreting vaccines induce curative immune responses in non-tolerized FVB mice, but are ineffective in tolerized neu-N mice. The first strategy used to enhance responses in neu-N mice was immune modulation with low-dose chemotherapy. Up to 30% of vaccinated neu-N mice receiving Cyclophosphamide (CY) and Doxorubicin (DOX) rejected tumors and developed high avidity, CD8 + T cells specific for RNEU420-429, the immunodominant epitope of neu. CY uncovered these cells by relieving immune suppression by CD4 +CD25+ regulatory T cells (Tregs). The second approach aimed to inhibit cyclooxygenase-2 (COX2), which can provide a tumor growth-promoting environment. However, even when COX2 inhibition with celecoxib was combined with distinct vaccine strategies, no significant anti-tumor effects were observed. The third strategy targeted tumor-associated vasculature by inhibiting the vascular endothelial growth factor receptor-2 (VEGF-R2) with DC101. DC101 decreased angiogenesis, increased tumor cell apoptosis, and raised serum levels of VEGF, an immunosuppressive cytokine, without inhibiting immune responses. In FVB mice, DC101 alone and with vaccination induced tumor-specific CD8 + T cell-mediated immune responses, leading to tumor rejection and memory responses. In neu-N mice, DC101 only augmented antitumor responses when vaccinated mice also received CY-chemotherapy to inhibit Treg activity. These data revealed a novel mechanism for DC101, demonstrating improved tumor-specific CD8+ T cell responses in the absence of immune tolerance, possibly via enhanced antigen cross-presentation. Collectively, these data identify immune modulation with low-dose chemotherapy and anti-angiogenic therapy with DC101 as effective treatments to combine with neu-specific vaccination in the HER-2/neu mouse model, and support the development of multi-targeted treatments for clinical translation.