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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.

CTLs were generated in vitro from two healthy donors and one melanoma patient by stimulation of CD8+ T cells with autologous dendritic cells pulsed with natural melanoma peptides (NMPs), obtained by acid treatment of HLA-matched melanoma cells. CTLs showed MHC class I-restricted melanoma-specific cytolytic activity. Importantly, CTLs from the patient, induced with NMPs obtained from an allogeneic HLA-A-matched melanoma, killed the autologous tumor. COS-7 cells cotransfected with the cDNA of 13 melanoma antigens and the HLA-A1-restricting allele did not induce cytokines release from NMP-specific CTLs, suggesting that they recognize unidentified shared melanoma antigens and that they may be valuable for identification of new tumor antigens. These results strongly support the use of autologous and/or allogeneic NMP-pulsed dendritic cells as cancer vaccines in patients whose neoplasms do not express or have lost expression of known tumor antigens.

In this study we used TEPITOPE, a new epitope prediction software, to identify sequence segments on the MAGE-3 protein with promiscuous binding to histocompatibility leukocyte antigen (HLA)-DR molecules. Synthetic peptides corresponding to the identified sequences were synthesized and used to propagate CD4(+) T cells from the blood of a healthy donor. CD4(+) T cells strongly recognized MAGE-3281-295 and, to a lesser extent, MAGE-3141-155 and MAGE-3146-160. Moreover, CD4(+) T cells proliferated in the presence of recombinant MAGE-3 after processing and presentation by autologous antigen presenting cells, demonstrating that the MAGE-3 epitopes recognized are naturally processed. CD4(+) T cells, mostly of the T helper 1 type, showed specific lytic activity against HLA-DR11/MAGE-3-positive melanoma cells. Cold target inhibition experiments demonstrated indeed that the CD4(+) T cells recognized MAGE-3281-295 in association with HLA-DR11 on melanoma cells. This is the first evidence that a tumor-specific shared antigen forms CD4(+) T cell epitopes. Furthermore, we validated the use of algorithms for the prediction of promiscuous CD4(+) T cell epitopes, thus opening the possibility of wide application to other tumor-associated antigens. These results have direct implications for cancer immunotherapy in the design of peptide-based vaccines with tumor-specific CD4(+) T cell epitopes.

Neoplastic cells express tumor-associated antigens, but tumor rejection seldom occurs in vivo. The absence of an effective immune response may be explained by the inability of tumor cells to deliver co-stimulatory signals. Indeed, transfection of either B7-1 or B7-2 co-stimulatory molecules into mouse tumor cells enhances antitumor immune responses. In this study, we stably transfected human melanoma cells with the cDNA encoding the B7-2 molecule to evaluate in vitro: (i) the induction of anti-melanoma cytotoxic T lymphocytes (CTL) by stimulation of CD8+ T cells, purified from healthy donors and a melanoma patient, with B7-2 transfected allogeneic HLA-matched melanoma cells; (ii) the tumor specificity and the HLA restriction of the induced CTL; and (iii) the feasibility to propagate long-term antimelanoma CTL lines. We found that B7-2 transfected, but not untransfected or mock-transfected, melanoma cells activated MHC-class I-restricted, melanoma-specific CD8+ CTL from healthy donors. More importantly, CD8+ tumor-associated lymphocytes, purified from a tumor-invaded lymph node of a melanoma patient and stimulated with B7-2-transfected melanoma cells, acquired a strong reactivity toward the autologous tumor. CTL lines with specific cytolytic activity could be propagated in long-term culture. These results indicate that: (i) the expression of the B7-2 molecule into human melanoma cells makes them immunogenic and able to act as antigen-presenting cells and (ii) purified CD8+ cells, stimulated with B7-2+ allogeneic HLA-matched melanoma cells, preferentially recognize melanoma-specific rather than allogeneic antigens. This study may have clinical implications for passive and/or active immunotherapy in melanoma patients.

We describe a subset of peripheral CD14+ cells, coexpressing the CD34 progenitor marker and able to migrate across endothelial cell monolayers. On culture with granulocyte-macrophage-CSF, this population differentiated into dendritic cells expressing CD83, CD80, HLA-DR(bright), CD86, and CD54. These dendritic cells were immunostimulatory, in that they induced proliferation of allogenic and tetanus toxoid-specific T lymphocytes. The CD14+ CD34+ population expressed higher levels of platelet endothelial cell adhesion molecule-1 (PECAM-1) and alpha4beta1 integrin than the CD14+ CD34- counterpart, being dull positive for other integrins. Using stably transfected PECAM-1+, VCAM-1+, or ICAM-1+ cells, we found that PECAM-1 and, to a lesser extent, VCAM-1, could support transmigration of CD14+ CD34+ cells, whereas the alphaL-ICAM-1 interaction was involved in cell adhesion. PECAM-1-driven transmigration was conceivably dependent on a haptotactic gradient, as it was reduced by 80% across NIH3T3 cells transfected with the PECAM-1-delta cyto deletion mutant. This mutant lacks the cytoplasmic tail and displays a reduced tendency to localize at the intercellular junctions, thus failing to form a molecular junctional gradient. Once differentiated, dendritic cells derived from CD14+ CD34+ precursors retained their transendothelial migratory capability, using both PECAM-1 and ICAM-1 for transmigration. We suggest that a subset of CD14+ CD34+ circulating leukocytes can localize to peripheral tissues and differentiate into functional dendritic cells, thus representing a functional reservoir of potential APC. PECAM-1, constitutively expressed on vascular endothelium, is likely to play a relevant role in the egress of this population from the bloodstream.