Identification of HLA-A2-restricted CTL epitope encoded by the MAGE-n gene of human hepatocellular carcinoma

BACKGROUND: Identification of the cytotoxic T lymphocytes (CTL) restricted epitopes of tumor antigens opens up possibilities of developing a new cancer vaccine. For the MAGE-n has been demonstrated closely associated with hepatocellular carcinoma (HCC) and HLA-A2.1 is found in over 50% of HCC patients in China, we aim at identifying MAGE-n-encoded peptide presented by HLA-A2.1. MATERIALS: A HLA-A2.1-restricted CTL epitope was identified by using an improved "reverse immunology" strategy: (a) computer-based epitope prediction from the amino acid sequence of MAGE-n antigen; (b) peptide-binding assay to determine the affinity of the predicted peptide with HLA-A2.1 molecule; (c) stimulation of primary T-cell response against the predicted peptides in vitro; and (d) testing of the induced CTLs toward HCC cells expressing MAGE-n antigen and HLA-A2.1. RESULTS: Of the five tested peptides, effectors induced by a peptide of MAGE-n at residue position 159-167(QLVFGIEVV) lysed HCC cells expressing both MAGE-n and HLA-A2.1. Our results indicated that peptide QLVFGIEVV was a new HLA-A2.1-restricted CTL epitope capable of inducing MAGE-n specific CTLs in vitro. CONCLUSIONS: Identification of the MAGE-n /HLA-A2.1 peptide QLVFGIEVV may facilitate peptide-based specific immunotherapy for HCC. The combination of epitope prediction, epitope reconstruction method and immunological methods can improve the efficiency and accuracy of CTL epitope studies.     

Identification of new HLA-A*0201-restricted cytotoxic T lymphocyte epitopes from neuritin

Identification of cytotoxic T lymphocyte (CTL) epitopes from additional tumor antigens is essential for the development of specific immunotherapy of malignant tumors. Neuritin, a recently discovered antigen overexpressed in astrocytoma, is considered to be a promising target for biological therapy. In the present study, we predicted and identified HLA-A2-restricted CTL epitopes from neuritin by using the following four-step procedure: (1) computer-based epitope prediction from the amino acid sequence of neuritin; (2) peptide-binding assay to determine the affinity of the predicted peptide with HLA-A2.1 molecule; (3) stimulation of primary T cell response against the predicted peptides in vitro; and (4) testing of the induced CTLs toward target cells expressing neuritin and HLA-A2.1. The results demonstrated that effectors induced by peptides of neuritin containing residues 13–21, 121–129 and 4–12 could specifically-secrete interferon-γ and lyse target cells. Our results indicate that these peptides are new HLA-A2.1-restricted CTL epitopes, and may serve as valuable tools for astrocytoma immunotherapy.     

Inducible Hsp70 as target of anticancer immunotherapy: Identification of HLA-A*0201-restricted epitopes

The design of a broad application tumor vaccine requires the identification of tumor antigens expressed in a majority of tumors of various origins. We questioned whether the major stress-inducible heat shock protein Hsp70 (also known as Hsp72), a protein frequently overexpressed in human tumors of various histological origins, but not in most physiological normal tissues, constitutes a tumor antigen. We selected the p391 and p393 peptides from the sequence of the human inducible Hsp70 that had a high affinity for HLA-A*0201. These peptides were able to trigger a CTL response in vivo in HLA-A*0201-transgenic HHD mice and in vitro in HLA-A*0201+ healthy donors. p391- and p393-specific human and murine CTL recognized human tumor cells overexpressing Hsp70 in a HLA-A*0201-restricted manner. Tetramer analysis of TILs showed that these Hsp70 epitopes are targets of an immune response in many HLA-A*0201+ breast cancer patients. Hsp70 is a tumor antigen and the Hsp70-derived peptides p391 and p393 could be used to raise a cytotoxic response against tumors of various origins.     

EphA2 as target of anticancer immunotherapy: identification of HLA-A*0201-restricted epitopes

EphA2 (Eck) is a tyrosine kinase receptor that is overexpressed in several human cancers such as breast, colon, lung, prostate, gastric carcinoma, and metastatic melanoma but not in nonmalignant counterparts. To validate EphA2 as a tumor antigen recognized by CD8+ T lymphocytes, we used reverse immunology approach to identify HLA-A*0201-restricted epitopes. Peptides bearing the HLA-A*0201-specific anchor motifs were analyzed for their capacity to bind and stabilize the HLA-A*0201 molecules. Two peptides, EphA2(58) and EphA2(550), with a high affinity for HLA-A*0201 were selected. Both peptides were immunogenic in the HLA-A*0201-transgenic HHD mice. Interestingly, peptide-specific murine CTLs cell lines responded to COS-7 cells coexpressing HLA-A*0201 and EphA2 and to EphA2-positive human tumor cells of various origin (renal cell, lung, and colon carcinoma and sarcoma). This demonstrates that EphA2(58) and EphA2(550) are naturally processed from endogenous EphA2. In addition, EphA2(58) and EphA2(550) stimulated specific CD8(+) T cells from healthy donor peripheral blood mononuclear cells. These T cells recognized EphA2-positive human tumor cells in an HLA-A*0201-restricted manner. Interestingly, EphA2-specific CD8+ T cells were detected in the peripheral blood mononuclear cells of prostate cancer patients. These results show for the first time that EphA2 is a tumor rejection antigen and lead us to propose EphA2(58) and EphA2(550) peptides for a broad-spectrum-tumor immunotherapy.     

人黑色素瘤抗原MAGE-3 肿瘤疫苗的构建及其诱导HLA-A *0201 转基因小鼠CTL活性的观察

 目的 通过基因工程的方法获得人黑色素瘤抗原MAGE-3的DNA和蛋白疫苗，并观察其对HLA-A＊0201转基因小鼠的CTL活性的影响。方法 用RT-PCR的方法，从人黑色素瘤细胞株A375中扩增MAGE-3的cDNA片段，分别将其插入到pcDNA3．1／v5-His真核扣pET32a原核表达载体。将pcDNA-MAGE-3转染B16肿瘤细胞观察其是否能在真核细胞中表达；将pET32a-MAGE-3转化大肠杆菌BL21，经IPTG诱导，SDS-PAGE观察融合蛋白表达情况。并用镍柱亲和层析的方法纯化融合蛋白。然后，采用DNA疫苗初次免疫和蛋白疫苗加强的策略免疫HL-A＊0201转基因小鼠，LDH方法检测CTL活性。结果 酶切结果证实，成功地构建了pcDNA-MAGE-3真核表达载体和pET32a-MAGE-3原核表达载体，并在真核细胞B16和原核细胞大肠杆菌中获稳定表达，原核表达产物为融合蛋白且分布于包涵体。用DNA疫苗进行初次免疫，镍柱纯化的融合蛋白加强免疫后，可成功地诱导HL-A＊0201转基因小鼠CTL活性。结论 成功地获得了MAGE-3肿瘤抗原，为进一步完善MAGE-3肿瘤疫苗打下了基础。     

Identification of HLA-A2- or HLA-A24-restricted CTL epitopes possibly useful for glypican-3-specific immunotherapy of hepatocellular carcinoma.

PURPOSE AND EXPERIMENTAL DESIGN: We previously reported that glypican-3 (GPC3) was overexpressed, specifically in hepatocellular carcinoma (HCC) and melanoma in humans, and it was useful as a novel tumor marker. We also reported that the preimmunization of BALB/c mice with dendritic cells pulsed with the H-2K(d)-restricted mouse GPC3(298-306) (EYILSLEEL) peptide prevented the growth of tumor-expressing mouse GPC3. Because of similarities in the peptide binding motifs between H-2K(d) and HLA-A24 (A*2402), the GPC3(298-306) peptide therefore seemed to be useful for the immunotherapy of HLA-A24+ patients with HCC and melanoma. In this report, we investigated whether the GPC3(298-306) peptide could induce GPC3-reactive CTLs from the peripheral blood mononuclear cells (PBMC) of HLA-A24 (A*2402)+ HCC patients. In addition, we used HLA-A2.1 (HHD) transgenic mice to identify the HLA-A2 (A*0201)-restricted GPC3 epitopes to expand the applications of GPC3-based immunotherapy to the HLA-A2+ HCC patients. RESULTS: We found that the GPC3(144-152) (FVGEFFTDV) peptide could induce peptide-reactive CTLs in HLA-A2.1 (HHD) transgenic mice without inducing autoimmunity. In five out of eight HLA-A2+ GPC3+ HCC patients, the GPC3(144-152) peptide-reactive CTLs were generated from PBMCs by in vitro stimulation with the peptide and the GPC3(298-306) peptide-reactive CTLs were also generated from PBMCs in four of six HLA-A24+ GPC3+ HCC patients. The inoculation of these CTLs reduced the human HCC tumor mass implanted into nonobese diabetic/severe combined immunodeficiency mice. CONCLUSION: Our study raises the possibility that these GPC3 peptides may therefore be applicable to cancer immunotherapy for a large number of HCC patients.     

Recognition of HLA-A2-restricted mammaglobin-A-derived epitopes

A breast cancer-associated antigen, mammaglobin-A, is specifically expressed in 80% of primary breast tumors. The definition of immune responses against this highly expressed breast cancer-specific antigen should be of great value in the development of new therapeutic strategies for breast cancer. Thus, the purpose of this study was to identify HLA-A2-restricted mammaglobin-A-derived epitopes recognized by CD8+ cytotoxic T lymphocytes (CTL). We identified seven mammaglobin-A-derived candidate epitopes that bind the HLA-A2 molecule (Mam-A2.1-7) by means of a HLA class I-peptide binding computer algorithm from the Bioinformatics & Molecular Analysis Section of the National Institutes of Health. Subsequently, we determined that CD8+ CTLs from breast cancer patients reacted to the Mam-A2.1 (83–92, LIYDSSLCDL), Mam-A2.2 (2–10, KLLMVLMLA), Mam-A2.3 (4–12, LMVLMLAAL), Mam-A2.4 (66–74, FLNQTDETL), and Mam-A2.7 (32–40, TINPQVSKT) epitopes using an IFN-c ELISPOT assay. Interestingly, healthy individuals also showed high reactivity to the Mam-A2.2 epitope. Two CD8+ CTL lines generated in vitro against TAP-deficient T2 cells loaded with the candidate epitopes showed significant cytotoxic activity against the Mam-A2.1-4 epitopes. These CD8+CTL lines recognized a HLA-A2+breast cancer cell line expressing the Mam-A2.1 epitope. In addition, DNA vaccination of HLA-A2+/human CD8+ double-transgenic mice with a DNA construct encoding the Mam-A2.1 epitope and the HLA-A2 molecule induced a significant expansion of epitope-specific CD8+ CTLs that recognize the same HLA- A2+/Mam-A2.1+ breast cancer cell line. In conclusion, these results demonstrate the immunotherapeutic potential of mammaglobin-A for the treatment and prevention of breast cancer.     

Identification of three non-VNTR MUC1-derived HLA-A*0201-restricted T-cell epitopes that induce protective anti-tumor immunity in HLA-A2/K(b)-transgenic mice

The human epithelial mucin MUC1 is over-expressed in more than 90% of carcinomas of the breast, ovary, and pancreas as well as in some other tumours, making it a potential target for tumour immunotherapy. We have identified several MUC1-derived peptides mapping outside the variable number tandem repeat region that comply with the peptide-binding motif for HLA-A*0201 and that become processed into stable major histocompatibility complex-peptide complexes as assessed by in vitro assays. In A2/K(b) transgenic mice, 3 peptides, namely MUC(79-87) (TLAPATEPA), MUC(167-175) (ALGSTAPPV) and MUC(264-272) (FLSFHISNL) elicit peptide-specific cytotoxic T lymphocyte (CTL) immunity, which protects these mice against a challenge with MUC1, A2/K(b)-expressing tumour cells. These peptides therefore represent naturally processed MUC1-derived CTL epitopes that could be used as components in peptide-based vaccines and for the analysis of anti-MUC1 CTL responses in A*0201-positive patients with MUC1-expressing tumours.     

Identification of HLA-A*0201-restricted cytotoxic T lymphocyte epitope from proliferating cell nuclear antigen

Peptide-based immunotherapy strategies appear promising as an approach to successfully induce an antitumor immune response and prolong survival in patients with various cancers. Protein antigens and their specific epitopes are formulation targets for anti-tumor vaccines. Bioinformatical approaches to predict major histocompatibility complex binding peptides can facilitate the resource-consuming effort of T cell epitope identification. Proliferating cell nuclear antigen including Ki-67 and PCNA, associated with the proliferation process of the cell, seems to be an attractive new target for tumor-specific immunotherapy. In this study, we predicted seven HLA-A*0201-restricted CTL candidate epitope of Ki-67 and eight epitope of PCNA by computer algorithm SYFPEITHI, BIMAS, and IEDB_ANN. Subsequently, biological functions of these peptides were tested by experiments in vitro. We found Ki-67_(280–288) (LQGETQLLV) had the strongest binding-affinity with HLA-A*0201. Further study revealed that Ki-67_(280–288) increased the frequency of IFN-γ-producing T cells compared to a negative peptide. Because Ki-67 was broadly expressed in most advanced malignant tumors, indicating a potential anti-tumor application in the future.     

Identification of HLA-A*0201-restricted cytotoxic T lymphocyte epitope from TRAG-3 antigen.

PURPOSE: Identification of tumor antigen and subsequent identification of T-cell epitope from these antigens make specific immunotherapy for malignant tumor applicable. Because TRAG-3 antigen is expressed in most melanomas and 54% of non-small cell lung carcinomas and HLA-A2.1-expressing individuals cover >50% in the population of China, we aim at identifying TRAG-3-encoded peptide presented by HLA-A2.1. EXPERIMENTAL DESIGN: In our study, a HLA-A2.1-restricted CTL epitope was identified by using the following four-step procedure: (a) computer-based epitope prediction from the amino acid sequence of TRAG-3 antigen; (b) peptide-binding assay to determine the affinity of the predicted peptide with HLA-A2.1 molecule; (c) stimulation of primary T-cell response against the predicted peptides in vitro; and (d) testing of the induced CTLs toward LB373-MEL cells expressing TRAG-3 antigen and HLA-A2.1. RESULTS: Of the four tested peptides, effectors induced by a peptide of TRAG-3 at residue position 58-66 lysed LB373-MEL cells expressing both TRAG-3 and HLA-A2.1. Our results indicate that peptide TRAG-3(58 approximately 66) (ILLRDAGLV) is a new HLA-A2.1-restricted CTL epitope capable of inducing TRAG-3 specific CTLs in vitro. CONCLUSIONS: Because TRAG-3 is a cancer/testis antigen expressed in most melanomas and half of non-small cell lung carcinomas, identification of the TRAG-3/HLA-A2.1 peptide ILLRDAGLV may facilitate peptide-based specific immunotherapy for various histological tumors.     

Identification of a new HLA-A*0201-restricted cryptic epitope from CYP1B1

Cytochrome P450 1B1 (CYP1B1) was recently shown to be a candidate tumor antigen broadly expressed in solid and hematologic malignancies. Nevertheless, use of such self-antigens as targets for immune intervention can be limited because of loss of high-avidity T cells during negative selection in the thymus. Recent data suggest that targeting of cryptic epitopes may represent a way to circumvent such self-tolerance and induce efficient antitumor CTL responses. Here, we present the identification and characterization of a novel, cryptic HLA-A*0201-binding peptide from CYP1B1. The nanomer CYP246 was identified by epitope deduction using algorithms to predict HLA-A*0201-binding peptides. CYP246 is characterized by strong initial HLA-A*0201 binding but a short MHC/peptide binding half-life. Expansion of high-avidity CTL was readily possible using autologous CD40-activated B cells from normal donors and cancer patients as antigen-presenting cells, suggesting that an intact T-cell repertoire can be expanded for this epitope. Lysis of CYP1B1-expressing, HLA-A*0201+ tumor cell lines and primary tumor cells confirmed that sufficient levels of CYP246 are presented by tumor cells for effector CTL killing. These findings indicate that CYP246 is a candidate cryptic epitope for immune interventions in which tumor CYP1B1 is targeted.     

Identification of HLA-A2-restricted CTL epitopes of a novel tumour-associated antigen, KIF20A, overexpressed in pancreatic cancer

Background:

Identification of tumour-associated antigens (TAAs) that induce cytotoxic T lymphocytes (CTLs) specific to cancer cells is critical for the development of anticancer immunotherapy. In this study, we aimed at identifying a novel TAA of pancreatic cancer for immunotherapy.

Methods:

On the basis of the genome-wide cDNA microarray analysis, we focused on KIF20A (also known as RAB6KIFL/MKlp2) as a candidate TAA in pancreatic cancer cells. The HLA-A2 (A*02:01)-restricted CTL epitopes of KIF20A were identified using HLA-A2 transgenic mice (Tgm) and the peptides were examined to check whether they could generate human CTLs exhibiting cytotoxic responses against KIF20A⁺, HLA-A2⁺ tumour cells in vitro.

Results:

KIF20A was overexpressed in pancreatic cancer and in some other malignancies, but not in their non-cancerous counterparts and many normal adult tissues. We found that KIF20A-2 (p12–20, LLSDDDVVV), KIF20A-8 (p809–817, CIAEQYHTV), and KIF20A-28 (p284–293, AQPDTAPLPV) peptides could induce HLA-A2-restricted CTLs in HLA-A2 Tgm without causing autoimmunity. Peptide-reactive human CTLs were generated from peripheral blood mononuclear cells of HLA-A2⁺ healthy donors by in vitro stimulation with the three peptides, and those CTLs successfully exhibited cytotoxic responses to cancer cells expressing both KIF20A and HLA-A2.

Conclusion:

KIF20A is a novel promising candidate for anticancer immunotherapeutic target for pancreatic cancers.     

Identification of HLA-A2-restricted epitopes of the tumor-associated antigen MUC2 recognized by human cytotoxic T cells

Previous studies have shown that self-antigens overexpressed in malignant tissue can provide a basis for a tumor-specific immune response. The mucin MUC2 is strongly overexpressed in all mucinous tumors of colon, breast, ovary and pancreas. In the corresponding normal tissue it is either not expressed (breast, ovary, pancreas) or it is expressed at considerably lower levels than in the mucinous tumors (colon). We therefore investigated whether the MUC2 molecule comprises HLA-A2-binding epitopes recognized by human cytotoxic T cells. Four MUC2 peptides with high affinity and stable binding to HLA-A2 were identified. Those peptides and additionally 3 peptides with moderate binding to HLA-A2 were loaded onto dendritic cells, which were used for stimulation of autologous T cells from healthy donors. Two MUC2 peptides, which belonged to the group of stable binders, induced specific cytotoxic T-cell lines. Target cells loaded with these peptides were strongly lysed in a concentration-dependent and HLA-A2-restricted manner. Our data show that the tumor-associated mucin MUC2 has potential as a target antigen for cytotoxic T cells in patients with mucinous carcinomas. Int. J. Cancer 75:688–693, 1998.© 1998 Wiley-Liss, Inc.     

卵巢癌相关抗原TM4SF1 HLA-A2限制性CTL表位预测及鉴定

目的 预测并筛选免疫反应性较强的四跨膜蛋白超家族成员1（transmembrane 4 L6 family member 1,TM4SF1） 人类白细胞抗原A2（human leukocyte antigen A2,HLA-A2）限制性T淋巴细胞（cytotoxic lymphocyte,CTL）表位肽。方法 联合4种软件（BIMAS、SYFPEITHI、IEDB、PROPRED I）对TM4SF1的 HLA-A2限制性CTL表位进行预测,并采用预培养法酶联免疫斑点法（enzyme-linked immunospot assy,ELISOPT）、直接法ELISOPT对所测得表位肽的免疫反应性进行鉴定。结果 4种不同软件共预测出10条表位多肽,对其中4条（P1、P2、P8、P10）进行免疫反应性验证。多肽活化CTL能力结果显示,预培养法ELISPOT产生斑点形成细胞（spvt forming cell,SFC）的净值T明显高于直接法ELISPOT：［（阳性对照肽：322±8 vs 169±22,P<0.05）、（多肽P1：114±10 vs 39±7,P<0.05）、（多肽 P10：156±31 vs 52±8,P<0.05）］。单个活化CTL细胞分泌INF-γ因子的水平检测结果显示,预培养法 ELISPOT 产生斑点的平均大小明显大于直接法 ELISPOT［（阳性对照肽：21.91±2.45 vs 13.80±1.76,P<0.05）、（多肽P1：12.90±0.88  vs 8.31±1.40,P<0.05）、（多肽P10：17.50±3.85 vs 11.96±0.61,P<0.05）］。表位多肽P1、P10均具有免疫原性,P10的免疫活性更高。结论 预培养法ELISPOT检测多肽的免疫反应性较直接法ELISPOT灵敏性更高,多种软件联合ELISPOT预培养法可有效筛选出免疫反应性更强的卵巢癌相关抗原TM4SF1 HLA-A2限制性CTL优势表位,其中P10的免疫活性最高。     

肿瘤相关抗原CT45的HLA-A2/A3限制性CTL表位的预测

目的:预测肿瘤相关抗原CT45的HLA-A2/A3限制性CTL表位,为CTL表位肽的合成和活性筛选提供依据。方法:基于新近发现的CT45抗原的氨基酸序列,用NetCTL 1.2 Server人工神经网络法远程预测系统进行HLA-A2/A3限制性CTL表位预测打分,挑选出分值较高的作为候选表位。结果:共预测出了5个潜在的HLA-A2限制性CTL表位九肽,15个潜在的HLA-A3限制性CTL表位九肽,所有这些表位均为首次报道。结论:NetCTL 1.2 Server人工神经网络法能高效的预测CTL表位肽,基于新近发现的CT45抗原,我们通过该系统成功预测出了5个潜在的HLA-A2限制性CTL表位,15个潜在的HLA-A3限制性CTL表位,可以对其进行进一步的研究。     

Identification of an HLA-A*0201-restricted cytotoxic T lymphocyte epitope from the lung carcinoma antigen, Lengsin

Lengsin is an eye lens protein with a glutamine synthetase domain. We previously identified this protein as a lung carcinoma antigen through cDNA microarray analysis. Lengsin protein is overexpressed irrespective of the histological type of lung carcinoma, but not in normal tissues other than the lens. Therefore, to significantly extend the use of Lengsin-based T-cell immunotherapies for the treatment of patients with lung carcinoma, we searched for HLA-A*0201-restricted epitopes from this protein by screening predicted Lengsin-derived candidate peptides for the induction of tumor-reactive CTLs. Four Lengsin-derived peptides were selected by computerized algorithm based on a permissive HLA-A*0201 binding motif, and were used to immunize HLA-A*0201 transgenic (HHD) mice. Two of the immunizing peptides, Lengsin(206-215)(FIYDFCIFGV) and Lengsin(270-279)(FLPEFGISSA), induced peptide-specific cytotoxic T lymphocytes (CTLs) in HHD mice, and thus were used to stimulate human peripheral blood lymphocytes in?vitro. Lengsin(206-215) and Lengsin (270-279) also induced human peptide-specific CTLs, and we were able to generate Lengsin(206-215)- and Lengsin(270-279)-specific CTL clones. The Lengsin(270-279)-specific CTL clone specifically recognized peptide-pulsed T2 cells, COS-7 cells expressing HLA-A*0201 and Lengsin, and HLA-A*0201+/Lengsin+ lung carcinoma cells in an HLA-A*0201-restricted manner. On the other hand, the Lengsin(206-215)-specific CTL clone failed to recognize HLA-A*0201+/Lengsin+ target cells in the absence of cognate peptide. These results suggest that Lengsin(270-279) is naturally processed and presented by HLA-A*0201 molecules on the surface of lung carcinoma cells and may be a new target for antigen-specific T-cell immunotherapy against lung cancer.     

Two new tumor-specific antigenic peptides encoded by gene MAGE-C2 and presented to cytolytic T lymphocytes by HLA-A2

We have identified 2 antigens recognized by several melanoma-specific cytolytic T lymphocyte clones isolated from a melanoma patient with a clinical history of tumor regression after immunotherapy. Both antigens are presented by HLA-A2 and encoded by gene MAGE-C2, a cancer-germline gene shown previously to be silent in normal somatic tissues and expressed in 40% of melanomas and in other tumor types. One antigen corresponds to peptide ALKDVEERV(336-344), whereas the other corresponds to peptide LLFGLALIEV(191-200). The CTL clones recognizing these 2 peptides also recognized allogeneic tumor cell lines expressing MAGE-C2 and HLA-A2. These 2 new peptides are the first known MAGE-C antigens and represent promising targets for cancer immunotherapy.     

Partial tyrosinase-specific self tolerance by HLA-A*0201-restricted cytotoxic T lymphocytes in mice and man

The human tyrosinase (hTyr) (369-377) cytotoxic T lymphocyte (CTL) epitope is presented by malignant melanoma and various nontransformed cells in association with human leukocyte antigen (HLA)-A*0201 (A2.1) and used for vaccination-based immunotherapy of melanoma patients. Its mouse homologue, mTyr (369-377), is naturally processed and bound by A2.1 with equivalent efficacy and thus enabled us to explore the effect of self tolerance on Tyr-specific T cells in different lines of A2.1 transgenic (Tg) mice and man. We found that self Tyr-reactive CTL in Tg mice and, importantly, in man were affected by partial tolerance resulting in only residual T lymphocytes of higher avidity for self Tyr along with low-avidity T cells to be present in the periphery. Immunizing mice with the xenogeneic nonself Tyr peptide facilitated the generation of self Tyr-reactive CTL. As compared to Tyr-reactive CTL induced by high amounts of the self Tyr epitope, however, the nonself antigen (Ag) had no effect on improving the avidity of self Tyr-specific mouse and human T cells. Depleting mice of CD25(+) T cells with and without CTL-associated Ag 4 (CTLA-4) blockade demonstrated that tolerance of Tyr-specific CTL was not regulated by CD4(+)CD25(+) T regulatory cells (Treg) or CTLA-4. Our studies have important implications for the design of anti-Tyr-based immunotherapeutics.