Evaluation of chimeric B-cell epitope vaccines of HER-2: Application to cancer patients

Overexpression of Human Epidermal growth factor Receptor-2 (HER-2) causes cellular transformation in experimental models. About 30% of carcinoma patients manifest HER-2 overexpression, which is associated with poor clinical prognosis and resistance to conventional forms of cancer therapy. Amplified expression of HER-2 activates host immune response during early stages of cancer, however the natural immune responses are too low to control tumor progression. Passive therapy with HER-2 specific monoclonal antibodies produced objective responses in about 20% of receptor positive patients. Active specific immunotherapy offers the possibility for generating sustained antitumor responses and is potentially more beneficial than passive therapy. The main goal of this study was to develop a vaccine strategy that can elicit an effective immune response in an outbred population capable of inhibiting the growth of HER-2 associated cancer. This goal was addressed by an incremental approach. First synthetic peptides corresponding to the Bell epitopes of HER 2 were evaluated for their ability to produce high levels of antibodies reactive with native receptor. Second, the antitumor activity of the peptide antibodies were tested on cancer cell proliferation in culture and on tumors induced in mice. Third, a combination of selected B-cell epitope peptides was evaluated for synergistic growth inhibition. Fourth, the potential of supplemental immunization with cytokines to enhance the immunogenicity and antitumor activity of the peptide epitopes was assessed. In addition, the molecular mechanisms of peptide antibody-mediated tumor growth inhibition were investigated. Attempts were made to stabilize the conformation of the B-cell epitopes to elicit high affinity antibodies. A total of nine HER-2 B-cell epitopes were synthesized as chimeras with a promiscuous T-helper epitope derived from Measles Virus Fusion (MVF) protein. Following initial evaluation of four chimeric peptides, one candidate, MVF HER-2 [628–647] was identified that selectively inhibited growth of cancer cells in vitro and prevented spontaneous tumor development in HER-2/neu transgenic mice. Three of the five cancer patients immunized with MVF HER-2 [628–647] elicited antibodies capable of killing breast cancer cells in culture. Further analyses lead to the identification of two more epitopes, 316–339 and 485–503, antibodies against which inhibited tumor cell proliferation. Immunization with a combination of 316–339 and 628–647 elicited antibodies with the best growth inhibitory activities. Supplemental immunization with interleukin-12 elicited significantly higher levels of antibodies to both single and a combination of chimeric peptides in addition to inducing antibody subtypes (e.g. IgG2a) that have been described to kill tumor cells more efficiently. Significant reduction of lung metastases was observed in mice vaccinated with peptide 316–339 alone or a combination of peptides, 316–339 and 628–647 plus IL-12. Based on these results, the combination of two peptides, 316–339 and 628–647 has been approved for testing in human cancer patients. Antibodies induced by peptide vaccines inhibit tumor growth by multiple mechanisms including down modulation of cell surface receptor, release of interferon-γ and lysis of tumor cells in presence of mononuclear cells.