Peptide-based, but not whole protein, vaccines elicit immunity to HER-2/neu, oncogenic self-protein.

HER-2/neu, an overexpressed oncogenic protein, has been proposed as a human cancer vaccine target. HER-2/neu is a "self" protein, however, and methods of vaccine strategies that would be effective in immunizing patients to a "self" tumor Ag have not been established. Many of the tumor Ags defined in humans are nonmutated self proteins, e.g., MAGE, and overcoming tolerance may be key in the generation of effective anti-tumor immunity. One theory states that tolerance to self proteins is directed only to dominant epitopes of proteins and not to every portion of the protein. Accordingly, tolerance can be circumvented by immunization to peptide fragments, but not whole protein. The studies outlined here demonstrate rat neu-specific immunity could be elicited in rats by vaccination with immunogenic rat neu peptides, but not by immunization with the intact protein. A rat model was used since rat neu protein is 89% homologous to human HER-2/neu protein and has a similar tissue distribution and level of expression. Rats were immunized with groups of peptides derived from the amino acid sequence of the intracellular domain or extracellular domain of rat neu protein and both groups developed CD4+ T cell immunity and Ab immunity to rat neu peptides and protein. Animals immunized in a similar fashion with intact purified rat neu protein did not develop Ab or T cell immunity to rat neu. Furthermore, rats that developed neu-specific immunity showed no histopathologic evidence of autoimmunity directed against organs expressing basal levels of rat neu protein. These studies suggest an immunization strategy that might be effective in human cancer vaccines targeting self tumor Ag.