Mining the melanosome for tumor vaccine targets: P.polypeptide is a novel tumor-associated antigen.

To identify novel, tumor-specific target antigens for vaccine development, we studied immune responses to P.polypeptide, an M(r) 110,000 integral melanosomal membrane protein associated with the Prader-Willi syndrome. Together with expressed sequence tag (EST) and serial analyses of gene expression (SAGE) library analyses, reverse transcription-PCR and Northern blotting verified that P.polypeptide expression was limited to melanoma and melanocytes. A single dominant epitope corresponding to positions 427-435 (IMLCLIAAV) was identified using allele-specific epitope forecasting combined with work in HLA-A*0201/K(b) transgenic mice. This epitope was then used to generate de novo human P.polypeptide-specific CD8+ T cells capable of recognizing P.polypeptide expressing human tumor cell lines in an HLA-A*0201-restricted fashion. Thus, P.polypeptide may be valuable in the creation of novel therapeutic anticancer vaccines.

Retrovirally transduced human dendritic cells can generate T cells recognizing multiple MHC class I and class II epitopes from the melanoma antigen glycoprotein 100.

Involvement of tumor-Ag specific CD4(+) and CD8(+) T cells could be critical in the generation of an effective immunotherapy for cancer. In an attempt to optimize the T cell response against defined tumor Ags, we previously developed a method allowing transgene expression in human dendritic cells (DCs) using retroviral vectors. One advantage of using gene-modified DCs is the potential ability to generate CD8(+) T cells against multiple class I-restricted epitopes within the Ag, thereby eliciting a broad antitumor immune response. To test this, we generated tumor-reactive CD8(+) T cells with DCs transduced with the melanoma Ag gp100, for which a number of HLA-A2-restricted epitopes have been described. Using gp100-transduced DCs, we were indeed able to raise T cells recognizing three distinct HLA-A2 epitopes within the Ag, gp100(154-162), gp100(209-217), and gp100(280-288). We next tested the ability of transduced DCs to raise class II-restricted CD4(+) T cells. Interestingly, stimulation with gp100-transduced DCs resulted in the generation of CD4(+) T cells specific for a novel HLA-DRbeta1*0701-restricted epitope of gp100. The minimal determinant of this epitope was defined as gp100(174-190) (TGRAMLGTHTMEVTVYH). These observations suggest that retrovirally transduced DCs have the capacity to present multiple MHC class I- and class II-restricted peptides derived from a tumor Ag, thereby eliciting a robust immune response against that Ag.

N-linked carbohydrates in tyrosinase are required for its recognition by human MHC class II-restricted CD4(+) T cells.

Glycosylation of mammalian proteins is known to influence their intracellular trafficking, half life, and susceptibility to enzymatic degradation. Rare instances of natural T cell epitopes dependent upon glycosylation for recognition have been described. We report here on human CD4(+) T lymphocyte cultures and clones from two melanoma patients that recognize the melanoma-associated Ag tyrosinase in the context of HLA-DR4 and -DR8. These T cells recognize tyrosinase, normally a heavily glycosylated molecule, when expressed constitutively in melanoma cells or in COS-7 transfectants pulsed as lysates onto autologous APC. However, these T cells fail to recognize tyrosinase expressed in bacteria, nor do they react with overlapping peptides covering full-length tyrosinase, suggesting a critical role for glycosylation in the processing and / or composition of the stimulatory epitopes. The requirement for glycosylation was demonstrated by the failure of tyrosinase-specific CD4(+) T cells to recognize tyrosinase synthesized in the presence of glycosylation inhibitors, or deglycosylated enzymatically. Site-directed mutagenesis of each of seven potential N-glycosylation sites showed that four sites were required to generate forms of tyrosinase that could be recognized by individual T cell clones. These data indicate that certain carbohydrate moieties are required for processing the tyrosinase peptides recognized by CD4(+) T cells. Post-translational modifications of human tumor-associated proteins such as tyrosinase could be a critical factor for the development of antitumor immune responses.

Melanoma-Reactive CD8+ T cells recognize a novel tumor antigen expressed in a wide variety of tumor types.

An autologous melanoma cell line selected for loss of expression of the immunodominant MART-1 and gp100 antigens was initially used to carry out a mixed lymphocyte tumor culture (MLTC) in a patient who expressed the human leukocyte antigen (HLA)-AI and HLA-A2 class I major histocompatibility complex alleles. Ten clones identified from this MLTC seemed to recognize melanoma in an HLA-A1-restricted manner but failed to recognize a panel of previously described melanoma antigens. The screening of an autologous melanoma cDNA library with one HLA-Al-restricted melanoma-reactive T-cell clone resulted in the isolation of a cDNA clone called AIM-2 (antigen isolated from immunoselected melanoma-2). The AIM-2 transcript seemed to have retained an intronic sequence based on its alignment with genomic sequences as well as expressed sequence tags. This transcript was not readily detected after Northern blot analysis of melanoma mRNA, indicating that only low levels of this product may be expressed in tumor cells. Quantitative reverse transcriptase-polymerase chain reaction analysis, however, demonstrated a correlation between T-cell recognition and expression in HLA-A1-expressing tumor cell lines. A peptide that was encoded within a short open reading frame of 23 amino acids and conformed to the HLA-A1 binding motif RSDSGQQARY was found to represent the T-cell epitope. The AIM-2-reactive T-cell clone recognized a number of neuroectodermal tumors as well as breast, ovarian, and colon carcinomas that expressed HLA-A1, indicating that this represents a widely expressed tumor antigen. Thus, AIM-2 may represent a potential target for the development of vaccines in patients bearing tumors of a variety of histologies.

CD4(+) T cell recognition of MHC class II-restricted epitopes from NY-ESO-1 presented by a prevalent HLA DP4 allele: association with NY-ESO-1 antibody production.

NY-ESO-1 is a tumor-specific shared antigen with distinctive immunogenicity. Both CD8(+) T cells and class-switched Ab responses have been detected from patients with cancer. In this study, a CD4(+) T cell line was generated from peripheral blood mononuclear cells of a melanoma patient and was shown to recognize NY-ESO-1 peptides presented by HLA-DP4, a dominant MHC class II allele expressed in 43--70% of Caucasians. The ESO p157--170 peptide containing the core region of DP4-restricted T cell epitope was present in a number of tumor cell lines tested and found to be recognized by both CD4(+) T cells as well as HLA-A2-restricted CD8(+) T cells. Thus, the ESO p157--170 epitope represents a potential candidate for cancer vaccines aimed at generating both CD4(+) and CD8(+) T cell responses. More importantly, 16 of 17 melanoma patients who developed Ab against NY-ESO-1 were found to be HLA-DP4-positive. CD4(+) T cells specific for the NY-ESO-1 epitopes were generated from 5 of 6 melanoma patients with NY-ESO-1 Ab. In contrast, no specific DP4-restricted T cells were generated from two patients without detectable NY-ESO-1 Ab. These results suggested that NY-ESO-1-specific DP4-restricted CD4(+) T cells were closely associated with NY-ESO-1 Ab observed in melanoma patients and might play an important role in providing help for activating B cells for NY-ESO-1-specific Ab production.

Use of an in vitro immunoselected tumor line to identify shared melanoma antigens recognized by HLA-A*0201-restricted T cells.

An immunoselected melanoma cell line that had lost expression of the dominant melanoma antigens MART-1 and gp100 was generated in an attempt to identify previously unknown tumor antigens. After repeated stimulation with the autologous immunoselected tumor line, a number of HLA-A*0201-restricted T-cell clones were established from the peripheral blood of a single melanoma patient. One T-cell clone (C-22) recognized 14 of 16 HLA-A2+ melanoma cell lines, as well as HLA-A2+ melanocytes but recognized neither HLA-A2+ fibroblasts nor autologous B cells. Screening of an autologous cDNA library resulted in the isolation of a transcript identical to an entry in the expressed sequence tag database. Northern blot analysis revealed that this gene was expressed in most melanoma cell lines and melanocytes but not in normal tissues. The peptide epitope (AMF-GREFCYA) recognized by clone C-22 was identified based on studies of the recognition of truncated cDNAs and the use of the consensus HLAA*0201 binding motif. A second T-cell clone (C-29) was found to recognize a new tyrosinase-related protein 2 epitope (455-463; YAIDLPVSV) in an HLA-A*0201-restricted manner. Together, these results provide additional targets that can be used for the development of immunotherapeutic protocols in HLA-A2+ melanoma patients and demonstrate the utility of immunoselected tumor lines for the identification of new melanoma antigens.

Identification of a MHC class II-restricted human gp100 epitope using DR4-IE transgenic mice.

CD4+ T cells play a central role in the induction and persistence of CD8+ T cells in several models of autoimmune and infectious disease. To improve the efficacy of a synthetic peptide vaccine based on the self-Ag, gp100, we sought to provide Ag-specific T cell help. To identify a gp100 epitope restricted by the MHC class II allele with the highest prevalence in patients with malignant melanoma (HLA-DRB1*0401), we immunized mice transgenic for a chimeric human-mouse class II molecule (DR4-IE) with recombinant human gp100 protein. We then searched for the induction of CD4+ T cell reactivity using candidate epitopes predicted to bind to DRB1*0401 by a computer-assisted algorithm. Of the 21 peptides forecasted to bind most avidly, murine CD4+ T cells recognized the epitope (human gp10044-59, WNRQLYPEWTEAQRLD) that was predicted to bind best. Interestingly, the mouse helper T cells also recognized human melanoma cells expressing DRB1*0401. To evaluate whether human CD4+ T cells could be generated from the peripheral blood of patients with melanoma, we used the synthetic peptide h-gp10044-59 to sensitize lymphocytes ex vivo. Resultant human CD4+ T cells specifically recognized melanoma, as measured by tumor cytolysis and the specific release of cytokines and chemokines. HLA class II transgenic mice may be useful in the identification of helper epitopes derived from Ags of potentially great clinical utility.

Recognition of shared melanoma antigens in association with major HLA-A alleles by tumor infiltrating T lymphocytes from 123 patients with melanoma.

A total of 123 tumor-infiltrating T lymphocyte (TIL) cultures established from patients with HLA-A1, -A2, -A3, -A24, or -A31 metastatic melanoma in the Surgery Branch, National Cancer Institute, were screened for recognition of shared melanoma antigens including five melanosomal proteins (tyrosinase, MART-1/melan-A, gp100, TRP1, TRP2) as well as peptides derived from MAGE-1 and MAGE-3. Examination of the specificity of these T cells indicated that 16% of HLA-A1 TIL, 57% of HLA-A2 TIL, 7% of HLA-A3 TIL, 13% of HLA-A24 TIL, and 27% of HLA-A31 TIL recognized shared melanoma antigens restricted by major histocompatibility complex class I. Melanosomal proteins were frequently recognized by these TIL, and MART-1(27-35), gp100(154-162), gp100(209-217), and gp100(280-288) represent highly immunogenic epitopes that were recognized by a high percentage of HLA-A2 restricted melanoma reactive TIL. Recognition of gp100 by HLA-A2 restricted TIL significantly correlated with clinical response to adoptive immunotherapy with TIL in 21 HLA-A2 melanoma patients (p = 0.024). Four HLA-A1, two HLA-A2, two HLA-A3, one HLA-A24, and two HLA-A31 restricted shared antigen-specific TIL did not recognize the previously identified antigens tested in this study, and may be useful for the identification of new melanoma antigens. The observation that TILs isolated from patients with metastatic melanoma recognized melanosomal proteins in the context of predominant HLA-A alleles implies that it may be possible to develop immunotherapies for patients with melanoma expressing diverse HLA types.

Recombinant virus vaccination against "self" antigens using anchor-fixed immunogens.

To study the induction of anti-"self" CD8+ T-cell reactivity against the tumor antigen gp100, we used a mouse transgenic for a chimeric HLA-A*0201/H-2 Kb molecule (A2/Kb). We immunized the mice with a recombinant vaccinia virus encoding a form of gp100 that had been modified at position 210 (from a threonine to a methionine) to increase epitope binding to the restricting class I molecule. Immunogens containing the "anchor-fixed" modification elicited anti-self CD8+ T cells specific for the wild-type gp100(209-217) peptide pulsed onto target cells. More important, these cells specifically recognized the naturally presented epitope on the surface of an A2/Kb-expressing murine melanoma, B16. These data indicate that anchor-fixing epitopes could enhance the function of recombinant virus-based immunogens.

Changes in the fine specificity of gp100(209-217)-reactive T cells in patients following vaccination with a peptide modified at an HLA-A2.1 anchor residue.

In a recent clinical trial, HLA-A2+ melanoma patients were vaccinated with a peptide derived from the melanoma Ag gp100, which had been modified at the second position (g9-209 2M) to enhance MHC binding affinity. Vaccination led to a significant increase in lymphocyte precursors in 10 of 11 patients but did not result in objective cancer responses. We observed that some postvaccination PBMC cultures were less reactive with tumor cells than they were with g9-209 peptide-pulsed T2 cells. In contrast, g9-209-reactive tumor-infiltrating lymphocyte cultures generally reacted equally with tumor cells and g9-209 peptide-pulsed T2 cells. To investigate this difference in T cell reactivity, T cell cloids derived from the PBMC of three patients vaccinated with g9-209 2M were compared with T cell cloids isolated from g9-209-reactive TIL cultures. All of the T cell cloids obtained from TIL reacted with HLA-A2+, gp100+ melanoma cell lines as well as with g9-209 and g9-209 2M peptide-pulsed targets. In contrast, only 3 of 20 PBMC-derived T cell cloids reacted with melanoma cell lines in addition to g9-209 and to g9-209 2M peptide-pulsed targets. Twelve of twenty PBMC-derived cloids reacted with g9-209 and g9-209 2M peptide-pulsed targets but not with melanoma cell lines. And 5 of 20 PBMC-derived cloids recognized only the g9-209 2M-modified peptide-pulsed targets. These results suggest that immunizing patients with the modified peptide affected the T cell repertoire by expanding an array of T cells with different fine specificities, only some of which recognized melanoma cells.

MHC class I-restricted recognition of a melanoma antigen by a human CD4+ tumor infiltrating lymphocyte.

It is generally considered that MHC class I-restricted antigens are recognized by CD8+ T cells, whereas MHC class II-restricted antigens are recognized by CD4+ T cells. In the present study, we report an MHC class I-restricted CD4+ T cell isolated from the tumor infiltrating lymphocytes (TILs) of a patient with metastatic melanoma. TIL 1383 I recognized HLA-A2+ melanoma cell lines but not autologous transformed B cells or fibroblasts. The antigen recognized by TIL 1383 I was tyrosinase, and the epitope was the 368-376 peptide. Antibody blocking assays confirmed that TIL 1383 I was MHC class I restricted, and the CD4 and CD8 coreceptors did not contribute significantly to antigen recognition. TIL 1383 I was weakly cytolytic and secreted cytokines in a pattern consistent with it being a Th1 cell. The avidity of TIL 1383 I for peptide pulsed targets is 10-100-fold lower than most melanoma-reactive CD8+ T cell clones. These CD4+ T cells may represent a relatively rare population of T cells that express a T-cell receptor capable of cross-reacting with an MHC class I/peptide complex with sufficient affinity to allow triggering in the absence of the CD4 coreceptor.

Identification of new melanoma epitopes on melanosomal proteins recognized by tumor infiltrating T lymphocytes restricted by HLA-A1, -A2, and -A3 alleles.

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 alpha-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.

Development of a retrovirus-based complementary DNA expression system for the cloning of tumor antigens.

A new retroviral system has been developed for the generation of a cDNA library and the functional cloning of tumor antigens. These retroviral vectors contain a cytomegalovirus promoter in the 5' long terminal repeat, an extended packaging signal for rapid production of high-titer retroviral particles, and many convenient cloning sites for cDNA library construction. The vesicular stomatitis virus G protein has been used to generate pseudotype retroviral particles to enable efficient viral infection. Using this system, viral titers in the range of 10(6) colony-forming units/ml could be generated routinely, and a high transduction efficiency in human primary cells, including fibroblasts, was achieved. In addition, a new procedure has been devised for screening a retrovirus-based cDNA library without a functional selection. The utility of this system was demonstrated by constructing a retrovirus-based cDNA library and re-isolating the NY-ESO-1 tumor antigen from a cDNA library using an antigen-specific CTL. This approach can facilitate the identification of novel tumor antigens recognized by T cells without knowledge of MHC class I restriction elements and is generally applicable for the isolation of any gene as long as a biological assay is available.

The use of melanosomal proteins in the immunotherapy of melanoma.

Clinical observations in the interleukin (IL) 2-based immunotherapies suggest that T cells play a central role in the rejection of melanoma. Using cDNA expression cloning, we have isolated genes encoding melanoma antigens recognized by tumor-infiltrating T lymphocytes. These antigens are categorized as (a) melanocyte-specific melanosomal proteins (MART-1/melan A, gp100, tyrosinase, TRP-1, and TRP-2), (b) tumor-specific mutated proteins (beta-catenin), and (c) others (p15). A variety of mechanisms has been identified for the generation of T cell epitopes on tumor cells. Some of the HLA-A2 binding epitopes from the melanosomal antigens appear to be subdominant self-determinants with relatively low major histocompatibility complex binding affinity. The effectiveness of adoptive transfer into patients of cytotoxic T lymphocytes recognizing the melanosomal antigens, the significant correlation between vitiligo development and clinical response in patients receiving IL-2-based immunotherapies, and the sporadic tumor regressions observed in some patients following immunization with the MART-1 or gp100 peptides in incomplete Freund's adjuvant or recombinant viruses expressing the MART-1 antigen suggest that these epitopes may represent tumor rejection antigens. Phase I immunization trials using peptides or recombinant viruses containing genes encoding the melanosomal antigens MART-1 or gp100, with or without co-administration of cytokines such as IL-2, IL-12, or granulocyte-macrophage colony-stimulating factor, are being conducted in the Surgery Branch of the National Cancer Institute. These studies may demonstrate the feasibility of using melanosomal proteins for the immunotherapy of patients with melanoma.

The intronic region of an incompletely spliced gp100 gene transcript encodes an epitope recognized by melanoma-reactive tumor-infiltrating lymphocytes.

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 epitope. These results further extend the types of Ags that can be recognized by melanoma-reactive T cells to aberrant transcripts of melanosomal genes.

Production of recombinant MART-1 proteins and specific antiMART-1 polyclonal and monoclonal antibodies: use in the characterization of the human melanoma antigen MART-1.

Recombinant human MART-1 protein was produced by bacterial and baculoviral-insect cell expression systems. By immunization with bacterial MBP-MART-1 fusion protein 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.

Identification of tyrosinase-related protein 2 as a tumor rejection antigen for the B16 melanoma.

Recently, major advances have been made in the identification of antigens from human melanoma which are recognized by T cells. In spite of this, little is known about the optimal ways to use these antigens to treat patients with cancer. Progress in this area is likely to require accurate preclinical animal models, but the availability of such models has lagged behind developments in human tumor immunology. Whereas many of the identified human melanoma antigens are normal tissue differentiation proteins, analogous murine tumor antigens have not yet been identified. In this paper we identify a normal tissue differentiation antigen, tyrosinase-related protein 2 (TRP-2), expressed by the murine B16 melanoma which was found by screening a cDNA library from B16 with tumor-reactive cytotoxic T lymphocytes (CTL). A peptide conforming to the predicted MHC class I H2-Kb binding motif, TRP-2181-188, was identified as the major reactive epitope within TRP-2 recognized by these anti-B16 CTLs. By site-directed mutagenesis, it was shown that alteration of this epitope eliminated recognition of TRP-2. It was further demonstrated that a CTL line raised from splenocytes by repeated stimulation in vitro with this peptide could recognize B16 tumor and was therapeutic against 3-d-old established pulmonary metastases. The use of TRP-2 in a preclinical model of tumor immunotherapy may be helpful in suggesting optimal vaccination strategies for cancer therapy in patients.

Induction of melanoma reactive T cells by stimulator cells expressing melanoma epitope-major histocompatibility complex class I fusion proteins.

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-1(27-35) or gp100(280-288) 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.

Human tumor antigens recognized by T cells.

The ability of tumor-reactive T cells to mediate in vivo tumor regression has been demonstrated in murine tumor models and by the clinical responses to adoptive immunotherapy with tumor-infiltrating lymphocytes isolated from human melanomas. Investigations carried out in the past several years have resulted in the isolation of a number of the genes encoding antigens recognized by melanoma-reactive T cells. The ability of these products to serve as tumor regression antigens has now begun to be evaluated in clinical vaccine trials.