論文

Tumor reactive T cells can be induced from peripheral blood or tumor infiltrating lymphocytes of patients with melanoma. Based on the analysis of the previous immunotherapies, T cells appeared to be involved in vivo tumor regression. Genes encoding human melanoma antigens and their epitope peptides recognized by T cells, have been identified using cDNA expression cloning techniques. Mechanisms for generation of T cell epitopes on growing melanoma cells have been identified. A variety of Phase I clinical studies using MART- 1 and gp100 antigens are being performed in the Surgery Branch, National Cancer Institute, NIH, and some patients have responded to these immunotherapy protocols.

To isolate melanoma Ags recognized by T cells, cDNA libraries made from melanoma cell lines were screened with four CTLs derived from tumor infiltrating lymphocytes (TIL) that were able to recognize melanoma cells in a HLA-A1, -A2, or -A3 restricted manner. Although cDNAs encoding the previously identified melanoma Ags, tyrosinase and gp100, were isolated, these TIL were found to recognize previously unidentified peptides. An HLA- A1-restricted CTL, TIL1388, was found to recognize a tyrosinase peptide (SSDYVIPIGTY), and an HLA-A3-restricted CTL, TIL1351, recognized a gp100 peptide (LIYRRRLMK). CTL clones isolated from the HLA-A2-restricted TIL1383 recognized a gp100 peptide (RLMKQDFSV). HLA-A2-restricted CTL, TIL1200, recognized a gp100 peptide (RLPRIFCSC). Replacement of either cysteine residue with α-amino butyric acid in the gp100 peptide, RLPRIFCSC, enhanced CTL recognition, suggesting that the peptide epitope naturally presented on the tumor cell surface may contain reduced cysteine residues. Oxidation of these cysteines might have occurred during the course of the synthesis or pulsing of the peptide in culture. These modifications may have important implications for the development of efficient peptide-based vaccines. These newly identified peptide epitopes can extend the ability to perform immunotherapy using synthetic peptides to a broader population of patients, especially those expressing HLA-A1 or HLA-A3 for whom only a few melanoma epitopes have previously been identified.

Tyrosinase-related protein 2 (TRP2) is a melanosomal enzyme expressed in most mammalian melanocytes and melanomas. This protein has been identified as a melanoma antigen recognized by tumor reactive CTLs derived from tumor infiltrating lymphocytes in the context of HLA-A31 and HLA-A33. The frequencies of these HLA-A alleles among melanoma patients in the United States is low (~6% for HLA-A31 and ~2% for HLA-A33) compared with that of HLA-A*0201 (~46%). Therefore, to extend significantly the use of TRP2- based immunotherapies for the treatment of patients with melanoma, we searched for new HLA-A*0201-restricted epitopes from this protein by screening TRP2-derived peptides for the induction of melanoma-reactive CTL. Fifty-one peptides were selected from TRP2 based on a permissive HLA-A*0201 binding motif, and the 21 peptides with the highest experimentally determined binding affinities were used to stimulate peripheral blood lymphocytes from HLA-A*0201+ melanoma patients in vitro. One peptide, TRP2(180188) (SVYDFFVWL), induced CTLs from three of four patients that specifically recognized peptide-pulsed T2 cells, COS-7 cells expressing HLA-A*0201 and TRP2, and HLA-A2+ TRP2+ melanomas. TRP2(180188) is identical to a previously identified TRP2 epitope recognized by murine melanoma-reactive CTLs in the context of H-2Kb. These results suggest that TRP2 may be useful for the development of murine tumor immunotherapy models and for the treatment of melanoma patients who are diverse in HLA expression.

T-cells play an important role in in vivo tumor rejection in many animal tumor models and in human melanoma. Many human tumor antigens recognized by autologous T-cells have now been identified. These are found to be nonmutated and mutated peptides derived from various self proteins as well as viral proteins. A variety of mechanisms involved in generating these T-cell epitopes on growing cancers have also been identified. However, the role of these identified antigens remains to be evaluated. Passive or active immunotherapies using these identified tumor antigens are being conducted in many institutions. The results obtained from these clinical trials may give us better insight into the role of T-cell responses to each antigen in tumor rejection as well as the development of new antigen-specific immunotherapies for patients with cancer.

Pigmentation of our skin, hair and eyes is essential for photoprotection, embryological development, detoxification end protective/cosmetic coloration. A number of proteins important to the production of melanin within melanosomes have now been identified including enzymatic and structural proteins encoded at the murine albino, brown, pinkeyed-dilution, MART1, siaty end silver loci. Interestingly, many of those melanosomal proteins (including epitopes derived from tyrosinase, TRP1/gp75, silver/gp100 end MART1/melan-A) function in vivo as targets of humoral and cellular autoimmune responses directed specifically against normal or transformed melanocytes. These findings have provided new impetus to research on immune responses to melanoma end, perhaps more importantly, examining why they are insufficient to provide protection against tumour growth end what type of immune therapy can be designed to correct that. The melanosome must now be considered beyond its function in pigmentation, end assumes the role of a valuable source for specific immune targets for malignant melenoma.

Twenty-eight primary and 29 metastatic melanoma lesions and 18 pigmented nevi lesions were analyzed by using the immunoperoxidase reaction with anti- MART-1 and anti-gp100 monoclonal antibodies (mAbs). The MART-1 was expressed in 28, 29, and 18, and gp100 was expressed in 27, 28, and eight of these lesions, respectively. Intensity and percentage of stained cells with anti- MART-1 mAb were stronger and higher than those with anti-gp100 mAb. MART-1 was expressed homogeneously in primary melanoma and pigmented nevi, whereas it was heterogeneously expressed in metastatic melanoma lesions. The level of expression of MART-1 in primary melanoma lesions did not correlate with any clinicopathologic parameters. These results suggest that anti-MART-1 mAb is a useful tool for immunohistochemical analysis of melanocytic lesions and also is useful for patient's selection and monitoring of antigen-loss variants in clinical trials with the MART-1-based immunotherapy.

Recent studies have characterized a number of the Ags that are recognized by melanoma-reactive T cells. Although the majority of tumor Ags appear to represent nonmutated gene products, a variety of epitopes have been shown to arise from either mutated or alternatively processed transcripts. Here, we report that the screening of a cDNA library with a HLA-A24-restricted melanoma-reactive T cell cloid derived from tumor infiltrating lymphocytes resulted in the isolation of a variant of the gp100 gene that had retained the entire fourth intron of this gene, termed gp100-in4. The gp100-in4 transcript could be detected by reverse transcriptase-PCR but could not be detected in Northern blots conducted with melanoma RNA, indicating that it represents a relatively rare transcript. Read-through of this transcript into the region corresponding to the fourth intron gave rise to an additional 35 amino acids not found in the normal gp100 glycoprotein, and a peptide within this region conforming to the HLA-A24 consensus motif (VYFFLPDHL) was shown to be recognized by the T cell cloid. The sequence of the intron was identical with that of a previously isolated genomic gp100 clone, and T cells that recognized the gp100-in4 gene product were found to recognize HLA-A24-matched allogeneic melanoma cell lines and melanocytes, demonstrating that this represents a nonmutated epitooe. These results further extend the types of Ags that can be recognized by melanoma-reactive T cells to aberrant transcripts of melanosomal genes.

Several epitopes in the human melanoma antigens recognized by HLA-A2- restricted CTLs have a relatively low MHC-binding affinity and as a result may be expressed at very low densities on the cell surface, indicating that these epitopes may not be efficient immunogens. To express these epitopes at higher densities on the surface of antigen-presenting cells and therefore improve their immunogenicity, a DNA construct in which a cDNA fragment encoding the melanoma epitope MART-127-35 or gp100280-288 was inserted between sequences encoding the leader and the HLA-A* 0201 protein. Cells transfected with these epitope-HLA fusion constructs were recognized by HLA-A2-restricted melanoma-reactive CTLs specific for the MART-1 or gp100 epitope. In addition, tumor-reactive CTLs could be induced from PBMCs of patients with metastatic melanoma by in vitro stimulation with HMY-C1R B- cell lines expressing the MART-1 or gp100 epitope-HLA-A* 0201 fusion protein. These epitope-HLA fusion constructs may be useful for the development of immunotherapies for patients with melanoma.

Recombinant human MART-1 protein was produced by bacterial and baculoviral-insect cell expression systems. By immunization with bacterial MBP-MART-1 fusion prorein or MBP cleaved MART-1 protein, a rabbit polyclonal and two murine monoclonal antibodies specific for MART-1 were produced. These antibodies specifically detected MART-1 in immuno-precipitation, Western blotting, flow cytometric assays and in immunohistochemical analysis of tissue sections. They also stained cytoplasmic components in melanocytes and most melanoma cells in frozen or paraffin embedded tissue sections, indicating that these antibodies may be useful for the diagnosis of melanoma. One of the monoclonal antibodies M2-7 C10 recognized only human MART-1, but the other monoclonal antibody M2-9 E3 recognized both human and murine MART-1. The size of the human MART-1 molecule detected by SDS-PAGE with these antibodies was approximately 18 kDa, suggesting possible posttranslational modifications in the MART-1 protein. Subcellular fractionation studies suggested that MART-1 was present in melanosomes and endoplasmic reticulum, although known melanogenic enzymatic activities were not detected in the MART-1 protein. These reagents may be useful for biological studies on melanocytes and melanoma cells as well as for the development and monitoring of immunotherapy for patients with melanoma.

The gp100 melanoma-associated tumor Ag was selected as a model system to study the diversity of human antitumor cytotoxic T cell responses. First, peptides corresponding to dominant gp100 HLA-A2.1-restricted CTL epitopes were tested using lymphocytes from normal volunteers and an in vitro priming protocol that uses peptide-pulsed dendritic cells as APCs and IL-7 and IL-10 as immune-enhancing cytokines. High CTL activity toward both peptide-pulsed target cells and gp100+ melanoma cells was obtained with four out of five peptides tested. Second, HLA-A2.1-binding peptides from gp100 that do no appear to represent CTL epitopes in melanoma patients were also tested for their capacity to induce CTL using the in vitro priming protocol. Three of six peptides tested induced CTL in lymphocytes from normal volunteers. One of these peptides was also immunogenic for lymphocytes derived from a melanoma patient in remission. Because these three CTL epitopes were not recognized in the natural immune response in melanoma patients but do appear as immunogens when peptides are used to induce the T cell response, they may be considered as typical "subdominant" epitopes. The results are discussed in the context of the usefulness of this approach to detail the immunologic potential of a given tumor-associated Ag and its relevance for the design of effective immune-based therapies.

MART-1 and gp100 melanoma associated antigens (MAA) are expressed by cells of the melanocytic lineage and are recognized by the majority of HLA- A2 restricted tumor-infiltrating lymphocytes. Heterogeneity of expression of MAA in tumor deposits may affect the natural history or response to therapy of patients with melanoma. In this study, we evaluated the expression of these MAA with a new monoclonal antibody (mAb) directed against MART-1 (M2- 7C10) and the commercially available HMB45 mAb directed against gp100. Expression was tested in vitro by intracellular fluorescence analysis and in vivo by immunophenotyping of tissue specimens. Nine melanoma cell lines and 25 tissue specimens from metastatic melanoma were analyzed. One cell line did not express MART-1 or gp100. The expression of both antigens was more heterogeneous and significantly reduced (p < 0.01) in melanoma cell lines compared with melanocytes, suggesting progressive loss of expression of MAA by neoplastic cells. None of the nonmelanoma cancer lines tested stained for MART-1 or gp100. Analysis of melanoma lesions by immunohistochemistry showed significant heterogeneity of expression of both MART-1 and gp100 MAA either as a percentage of cells expressing MAA or as intensity of expression. Ten of 25 frozen sections expressed MART-1 in <50% of the cells. In 6 of 25 lesions, immunoreactivity for MART-1 was totally absent. Fine needle aspiration of metastatic lesions seemed to yield information accurately about amount and heterogeneity of expression of MAA in tumor lesions in vivo. Heterogeneity of expression of MAA may be one of several mechanisms leading to tumor escape from immune recognition, and pretreatment evaluation of tumor lesion for expression of these antigens may help in selecting patients best suited to antigen-specific vaccine therapies.