Data from Peptide Vaccination Breaks Tolerance to HER-2/<i>neu</i> by Generating Vaccine-Specific FasL<sup>+</sup> CD4<sup>+</sup> T Cells: First Evidence for Intratumor Apoptotic Regulatory T Cells

BALB/c mice transgenic (Tg) for the transforming rat neu oncogene (BALB-neuT) are genetically predestined to develop mammary carcinogenesis in a process similar to that in humans. We crossed HLA-A2.1/HLA-DR1 (A2.1/DR1) Tg mice with BALB-neuT mice to generate A2.1/DR1 × BALB-neuT triple Tg (A2.1/DR1 × neuT⁺) mice, which represent an improvement over BALB-neuT mice for evaluating vaccination regimens to overcome tolerance against HER-2/neu. A vaccine formulation strategy, consisting of synthetic peptides from the rat HER-2/neu oncogene combined with granulocyte macrophage colony-stimulating factor, was highly effective in preventing the growth of established transplantable tumors in male A2.1/DR1 × neuT⁺ mice. Vaccination with HER-2(435–443) (p435) CTL peptide alone induced weak antitumor responses, which were characterized by increased numbers of regulatory T cells (Treg) and low numbers of vaccine-specific CD8⁺ CTL and helper T cells (Th). The administration of p435 plus HER-2(776–790) (p776; helper peptide) reversed this situation, inducing functionally active, peptide-specific CTL and Th. There was a striking change in the intratumoral balance of Tregs (decrease) and vaccine-specific Th (increase) that directly correlated with tumor rejection. Intratumoral administration of anti-FasL antibody promoted tumor growth. The decrease in Tregs (Fas⁺) was due to apoptosis induced by cell contact with Fas ligand⁺ (L)⁺ Th. Mice vaccinated with p435 plus p776 exhibited long-lasting antitumor immunity. Our vaccine regimen also significantly delayed the outgrowth of mammary carcinomas in female A2.1/DR1 × neuT⁺ animals. We provide a mechanism to overcome tolerance against HER-2/neu, which proposes a combined vaccination with two (Th and CTL) HER-2 peptides against HER-2/neu–expressing tumors. Cancer Res; 70(7); 2686–96