Background Bexarotene and 9cUAB30 are oral retinoid X receptor (RXR) agonists which inhibit proliferation in breast cancer. We previously demonstrated that bexarotene enhanced the efficacy of a plasmid HER2-IGFBP2-IGF1R vaccine to prevent breast cancer in the TgMMTV-neu model. In humans, RXRα was expressed in 24.9±13% of macrophages, 38.6±14% of plasmacytic dendritic cells, and 33.1±16% of monocytic dendritic cells. We therefore evaluated whether RXR agonists could enhance antigen-specific immunogenicity and enhance prevention in two transgenic mouse mammary tumor models.

Methods and Results We vaccinated FVB mice with 150 µg HER2-IGFBP2-IGF1R plasmid vaccine and 5ug GMCSF adjuvant every 2 weeks for four doses and saw significantly increased antigen-specific IFN-g T cells as compared to empty vector vaccination. Treatment for five days before each vaccine of 30 mg/kg bexarotene or 200 mg/kg 9cUAB30 significantly increased antigen-specific IFN-g immune responses to the vaccine (sequential). We therefore evaluated the ability of sequential RXR therapy to augment prevention with the vaccine in spontaneous tumor development in an indolent (TgMMTV-neu n=15) and an aggressive (C3(1)Tag n=9) transgenic mouse mammary tumor model. Unfortunately, while tumors were delayed with addition of RXR agonists to the vaccine, there was no prevention in either model. There were increased systemic type 1 (Tbet+) T cells with the addition of either RXR agonist (for TgMMTV-neu bexarotene p=0.05 and 9cUAB30 p=0.001 compared to empty vector and for C3(1)Tag 9cUAB30 p=0.01 compared to empty vector) but not Th2 T cells. Vaccine or RXR agonists alone did not increase Tbet+ T cells. Similarly, addition of RXR agonists to the vaccine increased antigen-specific polyfunctional T cells in both the TgMMTV-neu (for CD4 p=0.04 with bexarotene and p=0.03 with 9cUAB30 and for CD8 p=0.0002 with bexarotene and p=0.02 with 9cUAB30) and C3(1)Tag (for CD4 p=0.01 with bexarotene and p=0.07 with 9cUAB30 and for CD8 p=0.05 with bexarotene and p=0.05 with 9cUAB30). Compared to the tumors of mice treated with vaccine alone, the tumors of mice with the vaccine with either RXR agonist showed an increased number of CD8+ T cells and reduced FOXP3+ T cells and PD1+ cells by immunohistochemistry. There was no change in the level of CD4+ T cells. There was no evidence by IHC of antigen expression loss with sequential RXR agonist with the vaccine.

Conclusions The RXR agonists have an immunostimulatory role with plasmid cancer vaccines, but further modification of the immune environment may be needed for prevention vaccines.