Fine mapping of an H-2Kk restricted cytotoxic T lymphocyte epitope in SV40 T antigen by using in-frame deletion mutants and a synthetic peptide.

The CTL response to SV40 in C3H/HeJ mice is directed against the tumor (T) Ag and is H-2Kk restricted. CTL specific for both the amino terminus (residues 1-271) and the carboxyl terminus (residues 512-708) of the T Ag molecule have been detected, and we have previously cloned CTL of both specificities. In this paper we show that the panel of 10 CTL clones specific for the C-terminal region includes clones specific for three different epitopes, termed C1, C2, and C3. Epitopes C1 and C2 are conserved in the T Ag of the related papova viruses BK and SA12, and only epitopes C2 and C3 are present on SV40 transformed targets bearing the Kk mutant Kkml. Epitopes C1 and C2 were mapped to residues 563-576 by using in-frame deletion mutants of SV40 T antigen, and all clones specific for these two epitopes can lyse Kk bearing target cells in the presence of a synthetic peptide comprising residues 559-576. Kk and Kkml differ at residue 152, which is located in the Ag-binding pocket. Because epitopes C1 and C2 can be formed by the same antigenic peptide, but epitope C1 is not present on SV40 transformed Kkml cells, epitopes C1 and C2 must differ in the contribution made by residue 152 of the MHC class I molecule. These data show that CTL epitopes on transformed cells can be made up of Ag fragments, and strengthen the idea that this is a general phenomenon for both class I and class II restricted T cell epitopes.