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Patients with pT1 rectal tumours represent a suitable group for local treatment because of the acceptability of the procedure and the low recurrence rate.

Currently, no method allows direct and quantitative comparison of MHC-presented peptides in pairs of samples, such as transfected and untransfected, tumorous and normal or infected and uninfected tissues or cell lines. Here we introduce two approaches that use isotopically labeled reagents to quantify by mass spectrometry the ratio of peptides from each source. The first method involves acetylation and is both fast and simple. However, higher peptide recoveries and a finer sensitivity are achieved by the second method, which combines guanidination and nicotinylation, because the charge state of peptides can be maintained. Using differential acetylation, we identified a beta catenin-derived peptide in solid colon carcinoma overpresented on human leucocyte antigen-A (HLA-A)(*)6801. Guanidination/nicotinylation was applied to keratin 18-transfected cells and resulted in the characterization of the peptide RLASYLDRV (HLA-A(*)0201), exclusively presented on the transfectant. Thus, we demonstrate methods that enable a pairwise quantitative comparison leading to the identification of overpresented MHC ligands.

In selected cases, local therapy of rectal carcinoma avoids high morbidity and mortality of the classical operation. Quality of life will be improved, especially if an artificial anus can be avoided. In case of recurrence, the chance of a secondary procedure for cure is not to be underestimated.