The identification of an agonist peptide (YLSGADLNL, designated CAP1-6D) to an immunodominant cytotoxic T-lymphocyte (CTL) epitope (designated CAP1) of human carcinoembryonic antigen (CEA) has previously been reported. The agonist peptide harbors a single amino acid substitution at a non-MHC anchor residue and is proposed to exert its effects at the level of the T-cell receptor (TCR). The type and magnitude of cytokines produced by CAP1-reactive CTL upon stimulation with the agonist peptide, CAP1-6D, were compared to those obtained upon stimulation with the cognate CAP1 peptide. In addition, early events in the TCR signaling pathway were examined for differences in tyrosine phosphorylation. Upon stimulation with the agonist peptide CAP1-6D, several different CEA-specific CTL lines exhibited a marked shift in the peptide dose response, which resulted in as much as a 1,000-fold increase in the levels of GM-CSF and gamma-IFN produced as compared with the use of the CAP1 peptide. However, levels of IL-4 and IL-10, which are associated with anti-inflammatory effects, were very low or non-existent. The cytokine profile of CAP1- and CAP1-6D-specific CTL is consistent with a Tc1-type CTL. Consistent with these findings, CEA-specific CTL showed increased tyrosine phosphorylation of TCR signaling proteins ZAP-70 and TCR zeta chains in response to both peptides. However, when CAP1-6D was compared with the wild-type peptide, the increase in ZAP-70 phosphorylation was greater than the increase in zeta phosphorylation. CTL generated with the CAP1-6D agonist were shown capable of lysis of human carcinoma cells expressing native CEA. The ability to upregulate the production of GM-CSF, gamma-IFN, TNFalpha and IL-2 with the agonist peptide, as compared with CAP1, may help in initiating or sustaining anti-tumor immune responses and thus potentially prove to be useful in the treatment of CEA-positive tumors.
Copyright 2000 Wiley-Liss, Inc.