Data from Dendritic Cell–Activating Vaccine Adjuvants Differ in the Ability to Elicit Antitumor Immunity Due to an Adjuvant-Specific Induction of Immunosuppressive Cells

Abstract

<div>Abstract<p><b>Purpose:</b> We questioned whether the vaccine adjuvant combination of TLR-7 ligand agonist, imiquimod, with granulocyte macrophage colony-stimulating factor (GM-CSF) would result in enhanced dendritic cell recruitment and activation with increased antigen-specific immunity as compared with either adjuvant used alone.</p><p><b>Experimental Design:</b> The adjuvant effects of GM-CSF and imiquimod were studied in ovalbumin (OVA) and MMTVneu transgenic mice using peptide-based vaccines. Type I immunity, serum cytokines, myeloid-derived suppressive cells (MDSC), and regulatory T cells (Treg) levels were examined.</p><p><b>Results:</b> Both GM-CSF and imiquimod equally induced local accumulation and activation of dendritic cells. Both adjuvants effectively enhanced OVA-specific T-cell responses. We further evaluated the antitumor efficacy of adjuvant GM-CSF and imiquimod immunizing against murine insulin-like growth factor–binding protein-2 (IGFBP-2), a nonmutated oncoprotein overexpressed in the tumors of MMTVneu transgenic mice. Tumor growth was significantly inhibited in the mice receiving IGFBP-2 peptides with GM-CSF (<i>P</i> = 0.000), but not in imiquimod vaccine-treated groups (<i>P</i> = 0.141). Moreover, the addition of imiquimod to GM-CSF negated the antitumor activity of the vaccine when GM-CSF was used as the sole adjuvant. While GM-CSF stimulated significant levels of antigen-specific T-helper cell (T<sub>H</sub>)1, imiquimod induced elevated serum interleukin (IL)-10. Both MDSC and Tregs were increased in the imiquimod-treated but not GM-CSF–treated groups (<i>P</i> = 0.000 and 0.006, respectively). Depleting MDSC and Treg in animals immunized with imiquimod and IGFBP-2 peptides restored antitumor activity to the levels observed with vaccination using GM-CSF as the sole adjuvant.</p><p><b>Conclusion:</b> Adjuvants may induce regulatory responses in the context of a self-antigen vaccine. Adjuvant triggered immunosuppression may limit vaccine efficacy and should be evaluated in preclinical models especially when contemplating combination approaches. <i>Clin Cancer Res; 18(11); 3122–31. ©2012 AACR</i>.</p></div>