Immunogenicity of H-2K(b)-low affinity, high affinity, and covalently-bound peptides in anti-tumor vaccination

CTL induction by immunization with synthetic peptide epitopes has been show n to inhibit tumor growth and its metastatic spread. Ex vivo pulsing of pep tides on MHC class I-bearing cells such as RMA-S cells or professional APCs elicits an effective CTL response. Since the stability of the MHC-peptide complex is strongly correlated with the overall immunogenecity, we compared the effect of immunization with low affinity, high affinity, and irreversi bly bound MHC peptides in the context of immunotherapy of metastasis. MUT1, a tumor-associated antigen peptide that was isolated from 3LL Lewis lung c arcinoma, is a low H-2K(b) binder. MUTI was modified into a high binder by changing positions 3, 5, and 8 to the favorable anchor residues. In additio n, we introduced a photo-active chemical moiety, which can bind irreversibl y to MHC upon illumination. These peptides, loaded onto RMA-S, were used to immunize mice against the 3LL tumor. Vaccination via the covalent conjugat ion of the low binder peptide was found to increase the CTL response measur ed against MUT1 loaded cells and against H-2K(b) transfected D122 cells rel ative to the native MUTI peptide. However, the photo cross-linking of the h igh affinity peptide to the MHC did not significantly improve the induction of specific CTL. The level of CTL activity was elevated to the same extent by either cross-linking the peptide to the MHC or by modifying it into a h igh-binder peptide. The protective capacity of all the peptide-based vaccin es against D122 metastatic spread to the lungs was found to be comparable. These results indicate that augmentation of the affinity of a TAA peptide t o the RMA-S surface MHC molecules, by conversion to a high-affinity mimotop e or by photo-conjugation, can significantly enhance the immune response. T here seems to be, however, a ceiling beyond which increase in the peptide-b inding affinity does not lead to a corresponding enhancement of the overall immunogenicity of the peptide. (C) 1999 Elsevier Science B.V. All rights r eserved.