Expression of gp100 in melanoma metastases resected before or after treatment with IFN alpha and IL-2

Unraveling the Wide Spectrum of Melanoma Biomarkers

The use of biomarkers in medicine has become essential in clinical practice in order to help with diagnosis, prognostication and prediction of treatment response. Since Alexander Breslow’s original report on “melanoma and prognostic values of thickness”, providing the first biomarker for melanoma, many promising new biomarkers have followed. These include serum markers, such as lactate dehydrogenase and S100 calcium-binding protein B. However, as our understanding of the DNA mutational profile progresses, new gene targets and proteins have been identified. These include point mutations, such as mutations of the BRAF gene and tumour suppressor gene tP53. At present, only a small number of the available biomarkers are being utilised, but this may soon change as more studies are published. The aim of this article is to provide a comprehensive review of melanoma biomarkers and their utility for current and, potentially, future clinical practice.

Tumor Microenvironment: What have we Learned Studying the Immune Response in this Puzzling Battlefield?

Recent developments hallmark the progress in the understanding of tumor immunology and related therapeutic strategies. The administration of interleukin-2 (IL-2) to patients with cancer has shown that immune manipulation can mediate the regression of established cancers. The identification of the genes encoding cancer antigens and the development of means for effectively immunizing against these antigens has opened new avenues for the development of active immunization of patients with cancer. However, an efficient immune response against tumor comprises an intricate molecular network still poorly understood. Only when the code governing immune responsiveness of cancer will be deciphered, new therapeutic strategies could be designed to fit biologically defined mechanisms of immune rejection of cancer.

In this review, we propose that the mechanisms regulating tumor rejection in response to vaccination will be more efficiently identified by following the evolution of treatment induced events within the tumor microenvironment taking advantage of recently developed technological tools. As a model, we will discuss the observed immune response to tumor antigen -specific immunization and its relationship with the systemic administration of IL-2.

Melanoma biomarkers: Vox clamantis in deserto (Review)

Detecting malignant melanoma at an early stage, monitoring therapy, predicting recurrence and identifying patients at risk for metastasis continue to be a challenging and demanding objective. The last two decades have witnessed innovations in the field of melanoma biomarkers. However, global agreement concerning monitoring and early detection has yet to be reached. This is a review of the current literature regarding melanoma biomarkers including demographic, clinical, pathological and molecular biomarkers that are produced by melanoma or non-melanoma cells. A number of these biomarkers demonstrate promising results as possible methods for early detection, predicting recurrence and monitoring therapy. Other biomarkers appear to be promising for identifying patients at risk for metastasis. We reviewed the most pertinent information in the field thus far and how this knowledge can impact, or not, the management of melanoma patients prognostically and therapeutically.

Mechanisms of immune evasion by tumors

In the past decade, basic studies in animal models have begun to elucidate the physiological barriers which impede a successful antitumor immune response. These barriers operate at a number of levels, and involve the tumor, the tumor microenvironment and various components of the innate and adaptive immune systems. In this review, we discuss the multiple mechanisms by which tumors evade an immune response, with an emphasis on clinically relevant strategies to overcome these inhibitory checkpoints.

Melanoma patient staging: histopathological versus molecular evaluation of the sentinel node

Lymphatic mapping and sentinel lymphadenectomy provide a minimally invasive means of directly determining the status of the regional lymph nodes in all patients who have a primary melanoma >1 mm thick but no clinical evidence of nodal involvement. Since the histological status of the sentinel node (SN) has been shown to be the most important prognostic factor in primary melanoma patients, the World Health Organization has recently recommended that sentinel lymphadenectomy should become the new standard of care for primary melanoma patients. This paper reviews the literature with regards to developments in and the current status of SN evaluation. Developments in the histopathological versus molecular detection of melanoma nodal metastases are reviewed, with specific emphasis on the strengths, limitations and clinical significance of these techniques. Molecular evaluation of the SN offers several advantages over standard histopathological analysis. These include an improved sensitivity, the cost-effective use of multiple markers for the improvement of detection rate and prognosis, as well as being less labour-intensive and costly. Moreover, molecular analysis has the potential to allow estimation of tumour burden. We review the potential causes of technical false-negative and false-positive reverse transcription-polymerase chain reaction (RT-PCR) results and how these could be eliminated by a systematic approach consisting of (i) careful and systematic assay design, which would include efficient tissue homogenization, choice of reagents and molecular markers, primer design and the use of one-stage versus two-stage PCR; (ii) careful optimization of the RT-PCR parameters (in particular the PCR cycle number) through the use of appropriate control tissues; and (iii) aiming for high assay reproducibility and lastly by applying the necessary positive and negative controls with each batch of samples. We also review the significant improvement in patient prognosis and management that has been made possible by the development of sentinel lymphadenectomy and histopathological evaluation of the SN, and compare the clinical (predictive) value of histopathological analysis with that of RT-PCR. Although RT-PCR is able to detect additional, clinically significant SN metastases that are missed by routine histopathology, its current limitation is that it overestimates the number of patients who have clinically significant melanoma metastases. Therefore, we suggest and discuss appropriate steps that need to be taken in order to minimize these false-positives and make this molecular tool more acceptable for routine clinical use.

Dissecting tumor responsiveness to immunotherapy: the experience of peptide-based melanoma vaccines

Recent years have witnessed important breakthroughs in our understanding of tumor immunology. A variety of immunotherapeutic strategies has shown that immune manipulation can induce the regression of established cancer in humans. The identification of the genes encoding tumor-associated antigens (TAA) and the development of means for immunizing against these antigens have opened new avenues for the development of an effective anticancer immunotherapy. However, an efficient immune response against tumor requires an intricate cross-talk between cancer and immune system cells, which is still poorly understood. Only when the molecular basis underlying tumor susceptibility to an immune response is deciphered could new therapeutic strategies be designed to fit biologically defined mechanisms of cancer immune rejection. In this article, we address some of the critical issues that have been identified in cancer immunotherapy, in part from our own studies on immune therapies in melanoma patients treated with peptide-based vaccination regimens. This is not meant to be a comprehensive overview of the immunological phenomena accompanying cancer patient vaccination but rather emphasizes some emergent findings, puzzling controversies and unanswered questions that characterize this complex field of oncology. In addition to reviewing the main immunological concepts underlying peptide-based vaccination, we also review the available data regarding naturally occurring and therapeutically induced anticancer immune response, both at the peripheral and intratumoral level. The hypothesized role of innate immunity in predetermining tumor responsiveness to immunotherapeutic manipulation is also discussed.

Does the immune system see tumors as foreign or self?

Given the vast number of genetic and epigenetic changes associated with carcinogenesis, it is clear that tumors express many neoantigens. A central question in cancer immunology is whether recognition of tumor antigens by the immune system leads to activation (i.e., surveillance) or tolerance. Paradoxically, while strong evidence exists that specific immune surveillance systems operate at early stages of tumorigenesis, established tumors primarily induce immune tolerance. A unifying hypothesis posits that the fundamental processes of cancer progression, namely tissue invasion and metastasis, are inherently proinflammatory and thus activating for innate and adaptive antitumor immunity. To elude immune surveillance, tumors must develop mechanisms that block the elaboration and sensing of proinflammatory danger signals, thereby shifting the balance from activation to tolerance induction. Elucidation of these mechanisms provides new strategies for cancer immunotherapy.

Quantitative nested real-time RT-PCR specific for tyrosinase transcripts to quantitate minimal residual disease

BACKGROUND

Quantification of tumor markers expressed by occult-circulating tumor cells may be of prognostic value in a variety of neoplasms and disease stages. Here, we report on a quantitative nested real-time polymerase chain reaction (PCR) assay to quantify rare transcripts using the Light Cycler system. Tyrosinase mRNA, only expressed by melanocytes and melanoma cells, was used as the model tumor marker.

METHODS

For the establishment and sensitivity testing of this novel real-time PCR assay, 10 ml EDTA blood from healthy volunteers was spiked with 10(1)-10(5) MKR cells (the melanoma cell line). Following RNA extraction and cDNA synthesis, nested and single round PCRs were performed. Subsequently, 35 blood samples of 22 melanoma patients were analyzed.

RESULTS

Nested PCRs provided quantitative data with a quantitative range of five orders of magnitude of tyrosinase mRNA equivalent to 1-10(4) MKR cells/ml of blood. Nested PCR with 35 pre-amplification cycles displayed the quantitative data for nine tyrosinase transcript-positive samples out of 35 blood samples, whereas nested PCR with 20 pre-amplification cycles or single round PCR were less sensitive.

CONCLUSIONS

These experiments indicate that nested PCR, which is much more sensitive than single round PCR, is a useful tool even for the quantification of rare transcripts-a result with important implications for the study of minimal residual disease (MRD) in melanoma as well as in other malignancies.

Short-term kinetics of tumor antigen expression in response to vaccination

The melanoma patient's immune response to tumor has been extensively studied. Yet, the frequently observed coexistence of tumor-associated Ag (TAA)-specific T cells with their target cells in vivo remains unexplained. Loss of TAA expression might contribute to this paradox. We studied TAA expression in metastases by obtaining fine-needle aspirations from 52 tumor lesions in 30 patients with melanoma before and soon after immunotherapy. Limitations due to low amounts of starting material were overcome with a high fidelity antisense RNA amplification method. TAA expression was measured by quantitative real-time PCR of anti-sense RNA. Decrease in gp100/Pmel-17 TAA preceded tumor disappearance in several instances and could be best explained by immune selection because most patients had received gp100/Pmel-17-specific vaccination. Conversely, immune selection was absent in nonregressing lesions. These observations suggest that vaccination, when successful, triggers a broad inflammatory reaction that can lead to tumor destruction despite immune selection. Additionally, lack of clinical response might be attributed to lack of this initiating event rather than immune escape. This study provides an insight into the natural history of tumors and defines a strategy for the characterization of gene expression in tumors during therapy.

Minimal residual disease in melanoma

A number of specific genes encoding for melanosomal proteins are selectively expressed in melanocytes and melanomas. For detection of circulating melanoma cells, the expression of the tyrosinase gene is most widely used. Several cohorts of melanoma patients from single institutions have been analyzed by various research groups for the presence of circulating melanoma cells in all stages of disease. The percentage of patients with evidence of occult tumor dissemination has been correlated with stage of disease in several, but not all, reports. Two prospective analyses suggest that the PCR result is of prognostic value in melanoma. Several laboratories have found PCR evidence for circulating melanoma cells in the vast majority of untreated patients with stage IV disease, although other groups have reported much lower frequencies. Taken together, there is a wide range of results. Methodological differences likely account for this discrepancy. With the availability of true quantitative RT-PCR systems, such as the Light Cycler system, accurate quantification of tyrosinase transcripts over a range of 1-10,000 tumor cells per ml of blood is possible. Quantitative RT-PCR systems also dramatically improve quality control, since exact quantitation of housekeeping gene mRNA facilitates determination of sample quality.

T-cell-directed cancer vaccines: mechanisms of immune escape and immune tolerance

Recent clinical trials using vaccines directed toward tumour-associated antigens (TA) have shown the increasing capacity of vaccines to cause immunologic responses. In fact, strongly reactive TA-specific cytolytic T-lymphocytes and tumour-infiltrating lymphocytes (TIL) can be identified and expanded ex vivo from patients with metastatic melanoma vaccinated with melanoma-associated antigens. Paradoxically, this strong immunological response does not correlate with clinical tumour regression. Proposed mechanisms responsible for this glaring inconsistency are numerous and varied; systemic immunosuppressive as well as local mechanistic factors are implicated. In this review we will critically evaluate the possible mechanisms that allow tumours to escape immune destruction and be tolerated by the immune system. In addition, strategies that may allow further insight into the biology of tumour rejection are discussed, in the hope of deepening the understanding of this phenomenon and enhancing its therapeutic potential.

Nested quantitative real time PCR for detection of occult tumor cells

Quantification of tumor mRNA markers expressed by occult circulating tumor cells may be of prognostic value in a variety of neoplasms and disease stages. We therefore developed a novel real time nested polymerase chain reaction (PCR) assay to quantify rare transcripts using the light cycler system. Tyrosinase mRNA expressed by melanocytes and melanoma cells was used as a model. Ten-milliliter samples of ethylenediaminetetra-acetate (EDTA) blood from healthy volunteers were spiked with 10-10(4) RVH melanoma cells. Following RNA extraction and cDNA synthesis, a nested PCR with specific primers was performed. Resonance energy transfer between fluorescently labeled hybridization probes specific for the amplified portion of the tyrosinase transcript allowed continuous monitoring of the second round of PCR. The number of preamplification cycles in the first round was varied between 20 and 35. Our results show that nested PCR revealed quantitative data, regardless of the number of cycles used in the first round. The range of real time data can span four logs of target mRNA concentrations (1-10(3) tumor cells/ml of blood) and the x-intercepts of the log phases were always consistent with the amount of transcripts in the sample. These experiments indicate that nested PCR, which is much more sensitive than single-round PCR, is a useful tool for the quantification of rare transcripts--a result with important implications for the study of minimal residual disease. The assay is currently being adapted for other tumor types including leukemias as well as other solid tumors.

A panel immunoblot using co-incubated monoclonal antibodies for identification of melanoma cells

Antigen expression in melanoma is heterogeneous. Immunophenotyping using a panel of monoclonal antibodies may facilitate immunotherapy. An immunoblot procedure was developed to detect antigens in melanoma cells. Numerous monoclonal antibodies were tested to determine if (1) antigens were detected after transfer to membranes, (2) single bands or discrete multiple bands were obtained, (3) co-incubation of multiple monoclonal antibodies had no interference, and (4) banding patterns were non-overlapping. Antigens were selected based upon their association with melanoma and the availability of respective monoclonal antibodies. Antigens were melanoma antigen recognized by T-cells (MART-1), tyrosinase, tyrosinase-related protein 1 (TRP-1), S100, vimentin, glycoprotein 130 (gp130), a carcinoembryonic antigen (CEA)-like marker, KBA-62 and NKI-C3. Actin positive controls could be assessed simultaneously. Test samples were separated by polyacrylamide gel electrophoresis in a 4-15% polyacrylamide gradient, transferred to polyvinylidine fluoride membrane, blotted using a Fast-Blot apparatus (Pierce), and developed using diaminobenzidine/metal. Melanoma cell lines were immunophenotyped using this panel immunoblot, and were compared to a standard control and to non-melanoma cells. Up to four antigens could be detected simultaneously in a single lane of the immunoblot, using a single test sample of greater than 100000 cells.

Threshold levels of gene expression of the melanoma antigen gp100 correlate with tumor cell recognition by cytotoxic T lymphocytes

The level of expression of melanoma antigens (MA) may modulate the host immunologic response. Thus, the accurate measurement of MA expression may allow proper patient selection for antigen-specific therapies and yield important information for the evaluation of clinical results. In this study, we measured the absolute levels of MA messenger ribonucleic acid (mRNA) in tumor cell lines utilizing real-time quantitative reverse transcriptase-polymerase chain reaction (qRT-PCR). mRNA levels of MART-1, gp100, tyrosinase, TRP-1 and TRP-2 melanoma differentiation antigens and MAGE-1, MAGE-3 and ESO-1 cancer testis (CT) antigens were compared in 24 early-passage (<5 passages in culture) and 12 archival melanoma cell lines. MA mRNA expression was extremely variable among cell lines, occasionally reaching levels comparable to ribosomal RNA (rRNA). gp100 and MART-1 mRNA levels correlated with protein expression measurement obtained by FACS analysis. More significantly, a threshold of gp100 mRNA expression required for T-cell stimulation and target-cell killing was identified. This threshold level corresponded to approximately 500 mRNA copies per 10(8) copies of rRNA. Our results suggest that the measurements of MA mRNA levels may yield useful information relevant to the interpretation of clinical outcome during antigen-specific treatments.

A tumor-infiltrating lymphocyte from a melanoma metastasis with decreased expression of melanoma differentiation antigens recognizes MAGE-12

Twenty separate tumor infiltrating lymphocyte (TIL) bulk cultures and a tumor cell line were originated simultaneously from a fine needle aspiration biopsy of a metastasis in a patient with melanoma (F001) previously immunized with the HLA-A*0201-associated gp100:209-217(210 M) peptide. None of the TIL recognized gp100. However, 12 recognized autologous (F001-MEL) and allogeneic melanoma cells expressing the HLA haplotype A*0201, B*0702, Cw*0702. Further characterization of F001-MEL demonstrated loss of gp100/PMel17, severely decreased expression of other melanoma differentiation Ags and retained expression of tumor-specific Ags. Transfection of HLA class I alleles into B*0702/Cw*0702-negative melanoma cell lines identified HLA-Cw*0702 as the restriction element for F001-TIL. A cDNA library from F001-MEL was used to transfect IFN-alpha-stimulated 293 human embryonal kidney (293-HEK) cells expressing HLA-Cw*0702. A 100-gene pool was identified that induced recognition of 293-HEK cells by F001-TIL. Subsequent cloning of the pool identified a cDNA sequence homologous, except for one amino acid (aa 187 D-->A), to MAGE-12. Among 25 peptide sequences from MAGE-12 with the HLA-Cw*0702 binding motif, MAGE-12:170-178 (VRIGHLYIL) induced IFN-gamma release by F001-TIL when pulsed on F001-EBV-B cells at concentrations as low as 10 pg/ml. Peptide sequences from MAGE-1, 2, 3, 4a, and 6 aligned to MAGE-12:170-178 were not recognized by F001-TIL. In summary a TIL recognizing a MAGE protein was developed from an HLA-A*0201 expressing tumor with strongly reduced expression of melanoma differentiation Ags. Persisting tumor-specific Ag expression maintained tumor immune competence suggesting that tumor-specific Ags/melanoma differentiation Ags may complement each other in the context of melanoma Ag-specific vaccination.

Heterogeneity in expression of human leukocyte antigens and melanoma-associated antigens in advanced melanoma

The study of tumor immunology has led to many innovative therapeutic strategies for the treatment of melanoma. The strategies are primarily dependent on melanoma-associated antigen peptide vaccination or T-cell-based therapy. These immunotherapies are totally reliant on proper copresentation of human leukocyte antigen class I molecules in sufficient quantity and the presence and availability of melanoma-associated antigenic peptides. Altered expression of either HLA class I molecules or melanoma antigens is known to occur. These defects lead to altered manufacture and copresentation of HLA class I molecules with melanoma-associated antigens to T-cells. Defects in any one combination can lead to loss of recognition of melanoma cells and their subsequent destruction by cytotoxic T-lymphocytes. Thus, these immunotherapy strategies can be thwarted by defects or heterogeneity of expression of human leukocyte antigen class I or of melanoma-associated antigens.

Expansion of tumor-T cell pairs from fine needle aspirates of melanoma metastases

Lymphocytes expanded from excised specimens can be used to characterize intratumoral T cell responses. These analyses, however, are limited to one time point in the natural history of the removed tumor. The expansion of autologous tumor cells and tumor-infiltrating lymphocytes (TIL) from fine needle aspirates (FNA) of tumors potentially allows a dynamic evaluation of T cell responses within the same lesion at moments relevant to the disease course or response to therapy. Fourteen TIL cultures and 8 tumor cell lines were generated from 18 FNA (12 patients). Five of six TIL that could be tested against autologous tumor demonstrated specific reactivity. Two additional TIL for which no autologous tumor was available demonstrated recognition of HLA-matched melanoma cell lines. Serial FNA of the same lesions were performed in five HLA-A*0201 patients vaccinated with the emulsified melanoma Ag (MA) epitopes: MART-1:27-35; tyrosinase:368-376(370D); gp100:280-288(288V); and gp100:209-217 (210M). FNA material was separately cultured for a short time in IL-2 (300 IU/ml) after stimulation with irradiated autologous PBMC pulsed with each peptide or FluM1:58-66 (1 micromol/ml). No peptide-specific TIL could be expanded from prevaccination FNA. However, after vaccination, TIL specific for gp100:280(g280), gp100:209 (g209), and MART-1:27-35 (MART-1)-related epitopes were identified in three, three, and two patients, respectively. No Flu reactivity could be elicited in TIL, whereas it was consistently present in parallel PBMC cultures. This excluded PBMC contamination of the FNA material. This analysis suggests the feasibility of TIL expansion from minimal FNA material and localization of vaccine-specific T cells at the tumor site.

Development and characterization of melanoma cell lines established by fine-needle aspiration biopsy: advances in the monitoring of patients with metastatic melanoma

The establishment of melanoma cell lines from fine-needle aspiration biopsies (FNAB) has allowed for an enhanced understanding of the complex interactions that occur between T cells and tumor cells. The technique of FNAB offers the advantage of providing a sequential analysis of the same tumor nodules throughout treatment. The expression of melanoma antigens (MAs) was assessed in fresh melanoma FNAB samples and from tumor cell lines derived from these samples using several different approaches. Cytospin preparations of freshly isolated tumor cell explants were analyzed by immunocytochemistry (ICC), while the daughter cell line was analyzed by fluorescent activated cell sorting (FACS) analysis, and semiquantitative and quantitative reverse transcriptase-polymerase chain reaction (RT-PCR, qRT-PCR). As assessed by these methods, the level of MA expression by the original tumor cell explants correlated with the expression in established in vitro cell lines. Molecular analysis of the established cell lines utilizing PCR technology improved the sensitivity of detection of MA expression. Thus FNAB of melanoma is an efficient and effective method of tissue procurement, capable of generating, sequentially and from the same lesion, fresh tumor cells, tumor infiltrating lymphocytes (TIL), and long-term melanoma cell lines.

Natural variation of the expression of HLA and endogenous antigen modulates CTL recognition in an in vitro melanoma model

Increasing attention has been devoted to elucidating the mechanism of lost or decreased expression of MHC or melanoma-associated antigens (MAAs), which may lead to tumor escape from immune recognition. Loss of expression of HLA class I or MAA has, as an undisputed consequence, loss of recognition by HLA class I-restricted cytotoxic T cells (CTLs). However, the relevance of down-regulation remains in question in terms of frequency of occurrence. Moreover the functional significance of epitope down-regulation, defining the relationship between MHC/epitope density and CTL interactions, is a matter of controversy, particularly with regard to whether the noted variability of expression of MHC/epitope occurs within a range likely to affect target recognition by CTLs. In this study, bulk metastatic melanoma cell lines originated from 25 HLA-A*0201 patients were analyzed for expression of HLA-A2 and MAAs. HLA-A2 expression was heterogeneous and correlated with lysis by CTLs. Sensitivity to lysis was also independently affected by the amount of ligand available for binding at concentrations of 0.001 to 1 mM. Natural expression of MAA was variable, independent from the expression of HLA-A*0201, and a significant co-factor determining recognition of melanoma targets. Thus, the naturally occurring variation in the expression of MAA and/or HLA documented by our in vitro results modulates recognition of melanoma targets and may (i) partially explain CTL-target interactions in vitro and (ii) elucidate potential mechanisms for progressive escape of tumor cells from immune recognition in vivo.

New prognostic factors in melanoma: mRNA tumour markers

Circulating tumour cells in the peripheral blood may be important for haematogenous spread of malignant disease. Monitoring these cells may therefore be of prognostic value. Reverse transcriptase-polymerase chain reaction (RT-PCR)-based assays to detect occult neoplastic cells offer the highest sensitivity for the study of tumour dissemination and minimal residual disease. This review summarises technical considerations and clinical investigations in melanoma patients of various disease stages. The clinical data are promising, but to clearly define the clinical usefulness of messenger RNA (mRNA) tumour markers, methodological issues must be resolved and the clinical value must be assessed prospectively in sufficiently large patient cohorts.

Heterogeneous expression of melanoma-associated antigens and HLA-A2 in metastatic melanoma in vivo

MART-1/MelanA and Pmel17/gp100 are melanoma-associated antigens (MAAs) that can be recognized by tumor-infiltrating lymphocytes (TILs) capable of mediating successful adoptive therapy in vivo. Analysis of melanoma cell lines in vitro has demonstrated that heterogeneous antigen expression in the context of class I MHC is a significant co-factor in determining the recognition of melanoma targets by cytotoxic lymphocytes (CTLs). In this study, 217 specimens from 103 patients with metastatic melanoma were examined for the expression of MART-1/MelanA (monoclonal antibody [MAb] M27C10) and Pmel17/gp100 (HMB45 MAb) by immuno-histochemistry. Marked heterogeneity in the expression of both MAAs was confirmed by analysis of the percentage of positively staining tumor cells or the average intensity of tumor staining. We also noted heterogeneity of expression among multiple lesions taken from different anatomic sites within a patient. A dissociation was noted in the detection of MART-1 and gp100 in some lesions, with gp100 being undetectable in 24% of the lesions and MART-1 being undetectable in 11%. In several cases, loss of one MAA was not associated with loss of the other MAA, suggesting that MART-1 can represent a useful additional marker for the diagnosis of melanoma in gp100 (HMB45)-negative lesions. Of the 217 specimens, 155 were obtained from HLA-A*0201 patients, of which 6% were negative for HLA-A2, 8% were negative for MART-1/MelanA and 21% were negative for Pmel17/gp100. The potential significance of our findings is illustrated by a case study in which a patient with melanoma experienced rapid tumor progression in association with loss of either MAA or HLA expression in several lesions.