- Highly metastatic clones of malignant murine tumors are characterized by low immunogenicity and reduced MHC Class I expression. Metastatic lesions of human tumors are similarly impaired in HLA expression. Gene modification using plasmid and retroviral vectors carrying cDNAs for MHC Class I, γIFN, IL-2 or IL-6 increases immunogenicity of lung carcinoma (3LL) and melanoma (BI6) cells. Vaccines based on irradiated gene modified cells and their combinations were used in diseased animals and achieved certain cure rates. CTL recognize peptide sequences of defined length presented in the groove of MHC class I. TAA peptides presented by H-2K b were purified from 3LL carcinoma and proven to be mutants of a peptide from the gap junction protein connexin 37 and normal peptides of an aberrantly expressed β globin gene. Structural as pects and therapeutic efficacy of peptide vaccines will be discussed.