CD4(+) T cells from healthy subjects and colon cancer patients recognize a carcinoembryonic antigen-specific immunodominant epitope.

The carcinoembryonic antigen (CEA) is an attractive target for immunotherapeutic purposes because of its expression profile, its role in tumor progression, and its immunogenicity. However, CEA belongs to the CD66 immunoglobulin super-gene family that comprises highly homologous molecules expressed on leukocytes, making CEA a potential autoantigen expressed on hematopoietic cells. We used a MHC class II epitope prediction algorithm (TEPITOPE) to select 11 sequence segments of CEA that could form promiscuous CD4(+) T-cell epitopes and used synthetic peptides corresponding to the predicted sequences to propagate in vitro CD4(+) T cells from healthy donors and colon cancer patients. CD4(+) T cells from all subjects strongly recognized the sequence segment (LWWVNNQSLPVSP), repeated at residues 177-189 and 355-367. Importantly, we demonstrated that this highly immunodominant region contains a naturally processed epitope(s). Cross-recognition experiments with peptide analogues present on the CD66 homologous proteins showed that CEA(177-189/355-367)-specific CD4(+) T cells did not recognize the analogues, demonstrating that recognition of the immunodominant epitope is CEA specific. These data suggest that the repertoire of CEA(177-189/355-367)-specific CD4(+) T cells might have been shaped by a selective process to exclude CD4(+) T cells specific for CD66 homologues expressed on leukocyte, while preserving the CEA-specific repertoire. The features of strong immunogenicity and immunodominance in the absence of potential induction of autoimmunity make the identified CEA epitope of particular interest for the development of antitumor vaccines.

Identification of novel subdominant epitopes on the carcinoembryonic antigen recognized by CD4+ T cells of lung cancer patients.

The carcinoembryonic Ag (CEA) is an attractive target for immunotherapy because of its expression profile and role in tumor progression. To verify the existence of spontaneous anti-CEA CD4+ T cells in lung cancer patients, we first identified CEA sequences forming naturally processed epitopes, and then used the identified epitopes to test their recognition by CD4+ T cells from the patients. We had previously identified CEA(177-189/355-367) as an immunodominant epitope recognized by CD4+ T cells in association with several HLA-DR alleles. In this study, we identified four additional subdominant CEA sequences (CEA(99-111), CEA(425-437), CEA(568-582), and CEA(666-678)), recognized in association with one or more HLA-DR alleles. Peptide-specific CD4+ T cells produced proinflammatory cytokines when challenged with the native protein and CEA-expressing tumor cells, thus demonstrating that the identified CEA sequences contain naturally processed epitopes. However, CEA is expressed in the thymus and belongs to the CD66 family that comprises highly homologous molecules expressed on hemopoietic cells, raising concerns about tolerance interfering with the in vivo development of anti-CEA immunity. We thus tested the spontaneous reactivity to the identified epitopes of peripheral blood CD4+ T lymphocytes from eight early-stage lung cancer patients bearing CEA-positive tumors. We found GM-CSF- and IFN-gamma-producing CD4+ T cells in two patients. Our data indicate that CD4+ immune responses against CEA develop in neoplastic patients, suggesting that tolerance toward CEA or cross-reactive CD66 homologous molecules might be either not absolute or be overcome in the neoplastic disease.

Identification of immunodominant regions among promiscuous HLA-DR-restricted CD4+ T-cell epitopes on the tumor antigen MAGE-3.

The molecular characterization of the CD4(+) T-cell epitope repertoire on human tumor antigens would contribute to both clinical investigation and cancer immunotherapy. In particular, the identification of promiscuous epitopes would be beneficial to a large number of patients with neoplastic diseases regardless of their HLA-DR type. MAGE-3 is a tumor-specific antigen widely expressed in solid and hematologic malignancies; therefore, is an excellent candidate antigen. We used a major histocompatability complex (MHC) class II epitope prediction algorithm, the TEPITOPE software, to predict 11 sequence segments of MAGE-3 that could form promiscuous CD4(+) T-cell epitopes. In binding assays, the synthetic peptides corresponding to the 11 predicted sequences bound at least 3 different HLA-DR alleles. Nine of the 11 peptides induced proliferation of CD4(+) T cells from both healthy subjects and melanoma patients. Four immunodominant regions (residues 111-125, 146-160, 191-205, and 281-295), containing naturally processed epitopes, were recognized by most of the donors, in association with 3 to 4 different HLA-DR alleles, thus covering up to 94% of the alleles expressed in whites. On the contrary, the other promiscuous regions (residues 161-175 and 171-185) contained epitopes not naturally processed in vitro. The immunodominant epitopes identified will be useful in the design of peptide-based cancer vaccines and in the study of the functional state of tumor-specific CD4(+) T cells in patients bearing tumors expressing MAGE-3.