|Beschreibung:||Part I . MHC peptides derived from tumor associated antigens (TAA) can serve as the basis for development of immunotherapeutics for treatment of human malignancies. Recently, we identified novel HLA-A*0201 (HLA-A2) restricted peptides recovered from soluble HLA (sHLA) molecules secreted by sHLA-transfected human tumor cell lines. Here, four candidate peptides eluted from sHLA-A2 were selected on the basis of their precursor proteins being TAA. Peptide p1028 (GLIEKNIEL), derived from DNA methyl transferase I (DNMT-1), which is overexpressed in various human tumors, showed the highest affinity to HLA-A2 and was relatively abundant in the sHLA/peptide complexes of all transfected breast, ovarian and prostate cancer cell lines. Peptide p1028 was immunogenic in vitro and p1028 effector CTL were capable of lysing tumor cells. p1028 CTL were peptide-specific and HLA-A2-restricted. Since DNMT-1 is a widely expressed tumor-associated enzyme, the novel DNMT-1-derived and HLA-A2 restricted peptide GLIEKNIEL identified here might find use as a therapeutic cancer vaccine. Part II. CD28 and CTLA-4 are the critical costimulatory receptors that predominantly determine the outcome of T cell stimulation, with CD28 promoting positive costimulation and CTLA-4 inducing inhibitory signals. However, the exact mechanism of CTLA-4’s inhibitory function remains elusive. Here, we investigated CD8+ T cell regulation by CTLA-4 and the influence of CTLA-4 upregulation on CD28 expression using CD8+ CTLA-4+ T cell transfectants. Upregulation of CTLA-4 induced abrogation of IL-2 production as well as inhibition of the TcR proximal tyrosine kinases ZAP-70, p56lck and p59fyn, a phenotype of anergic T cells. In addition, CTLA-4 upregulation promoted the internalization and proteolytic degradation of CD28, leading to decreased CD28 surface expression. These data suggest that apart from the established competition of CD28 and CTLA-4 for their ligands CD80 and CD86, inhibition of T cells by CTLA-4 might be explained by significant reduction of CD28 on the cell surface, which might impede T cell response to stimulation. Our data provide a previously unrecognized mechanism for T cell regulation by CTLA-4.