MOLECULAR MIMICRY OF CARCINOEMBRYONIC ANTIGEN BY PEPTIDES DERIVED FROM THE STRUCTURE OF AN ANTIIDIOTYPE ANTIBODY

Our goal was to use carcinoembryonic antigen (CEA) as a target for imm unotherapy in CEA-positive cancer patients who are all immune tolerant to the native antigen, We isolated and characterized an anti-idiotype monoclonal antibody 3H1, which mimics a distinct and specific epitope of the M-r 180,000 CEA and can be used as a surrogate for CEA, In Pha se Ib clinical trials in a group of 23 advanced colorectal cancer pati ents, 3H1 induced both humoral and cellular anti-3H1 responses, as wel l as anti-CEA immunity, To study the cellular immunity invoked by 3H1 at the molecular level, me have cloned and sequenced the cDNAs encodin g the variable heavy and light chains of 3H1 and deduced the amino aci d sequences of the encoded proteins, To identify any crossreactive pep tides of 3H1 and CEA, we compared the amino acid sequences of 3H1 with those of CEA and found several regions of homology in 3H1 heavy and l ight chain variable domains, as wed as in the framework regions. To se arch for potential cross-reactive T-cell epitopes, a number of peptide s were synthesized based on 3H1/CEA homology and were used as stimulan ts in cell proliferation assays, using peripheral blood mononuclear ce lls from a group of 3H1-immunized CEA-positive cancer patients in the adjuvant setting, Two partially homologous peptides, designated LCD-2 (from 3H1) and CEA-B (from CEA), mere identified in 10 of 21 adjuvant patients by strong proliferation responses (stimulation index, 3-50-fo ld), which were extensively studied in five of these individuals over an extended period of time (12-24 months), We saw no correlation with the MHC class I haplotype of the patients, Analysis of the subtype of the responding T cells demonstrated that primarily CD4(+) T cells were stimulated by both 3H1 and 3H1-derived peptides, Interleukin 2, inter leukin 4, and IFN-gamma were assayed in the culture medium of peripher al blood mononuclear cells stimulated with 3H1, CEA, and LCD-2 to dete rmine the T-cell helper subset induced by these stimulants, The in vit ro responses were mainly associated with secretion of IFN-gamma, which suggested that the induced T cells mere most likely CD4(+) Th1 type, Future studies will include the design of second-generation LCD-2 and CEA peptides to further enhance antigenicity, to characterize the resp onding T-cell populations more fully, and to test refined peptides for immunogenicity.