Antibody-epitope conjugates deliver immunogenic T-cell epitopes more efficiently when close to cell surfaces.

Antibody-mediated delivery of immunogenic viral CD8+ T-cell epitopes to redirect virus-specific T cells toward cancer cells is a promising new therapeutic avenue to increase the immunogenicity of tumors. Multiple strategies for viral epitope delivery have been shown to be effective. So far, most of these have relied on a free C-terminus of the immunogenic epitope for extracellular delivery. Here, we demonstrate that antibody-epitope conjugates (AECs) with genetically fused epitopes to the N-terminus of the antibody can also sensitize tumors for attack by virus-specific CD8+ T cells. AECs carrying epitopes genetically fused at the N-terminus of the light chains of cetuximab and trastuzumab demonstrate an even more efficient delivery of the T-cell epitopes compared to AECs with the epitope fused to the C-terminus of the heavy chain. We demonstrate that this increased efficiency is not caused by the shift in location of the cleavage site from the N- to the C-terminus, but by its increased proximity to the cell surface. We hypothesize that this facilitates more efficient epitope delivery. These findings not only provide additional insights into the mechanism of action of AECs but also broaden the possibilities for genetically fused AECs as an avenue for the redirection of multiple virus-specific T cells toward tumors.

Differences in incidence and survival of oral cavity and pharyngeal cancers between Germany and the United States depend on the HPV-association of the cancer site.

INTRODUCTION: The epidemiology of squamous cell oral cavity and pharyngeal cancers (OCPC) has changed rapidly during the last years, possibly due to an increase of human papilloma virus (HPV) positive tumors and successes in tobacco prevention. Here, we compare incidence and survival of OCPC by HPV-relation of the site in Germany and the United States (US). MATERIALS AND METHODS: Age-standardized and age-specific incidence and 5-year relative survival was estimated using data from population-based cancer registries in Germany and the US Surveillance Epidemiology and End Results (SEER) 13 database. Incidence was estimated for each year between 1999 and 2013. Relative survival for 2002-2005, 2006-2009, and 2010-2013 was estimated using period analysis. RESULTS: The datasets included 52,787 and 48,861 cases with OCPC diagnosis between 1997 and 2013 in Germany and the US. Incidence was much higher in Germany compared to the US for HPV-unrelated OCPC and more recently also for HPV-related OCPC in women. Five-year relative survival differences between Germany and the US were small for HPV-unrelated OCPC. For HPV-related OCPC, men had higher survival in the US (62.1%) than in Germany (45.4%) in 2010-2013. These differences increased over time and were largest in younger patients and stage IV disease without metastasis. In contrast, women had comparable survival for HPV-related OCPC in both countries. CONCLUSIONS: Strong survival differences between Germany and the US were observed for HPV-related OCPC in men, which might be explained by differences in HPV-attributable proportions. Close monitoring of the epidemiology of OCPC in each country is needed.

Occupation and cancer of the larynx: a systematic review and meta-analysis.

The aim of the study was to explore the relationship between occupational exposure, defined by occupational categories and job title, and laryngeal cancer. A systematic review and meta-analysis of 21 tobacco and alcohol-adjusted case-control studies including data from 6,906 exposed cases and 10,816 exposed controls was performed to investigate the frequency of laryngeal cancer in different occupations. Job classifications were harmonized using the International Standard Classification of Occupations. Pooled odds ratios (OR [95 % confidence intervals (CI)]) were calculated for the different occupational groups. A significantly increased risk of laryngeal cancer was observed for the occupational category of 'production-related workers, transport equipment operators, and laborers' (OR=1.3 [1.2-1.4]); particularly at risk were occupations as: miners (OR=1.6 [1.2-2.1]), tailors (OR=1.7 [1.2-2.3]), blacksmith and toolmakers (OR=1.5 [1.2-1.7]), painters (OR=1.4 [1.1-1.9]), bricklayers and carpenters (OR=1.3 [1.2-1.5]), and transport equipment operators (OR=1.3 [1.2-1.5]). Individuals working as 'professional, technical, and related workers' (OR=0.7 [0.6- 0.8]), 'administrative and managerial workers' (OR=0.6 [0.4-0.7]), or 'clerical and related workers' (OR=0.8 [0.7-0.9]) had laryngeal cancer less frequently. Occupational exposure, defined by occupational categories and job title, is likely to be an independent risk factor for laryngeal cancer. Further research on specific occupations with increased risk of laryngeal cancer is warranted to explore the underlying mechanisms.

[Changes in Incidence and Mortality Trends of Head and Neck Cancer in Rhineland-Palatinate, 2000-2009]. / Veränderung von Inzidenz und Mortalität von Kopf-Hals-Malignomen in Rheinland-Pfalz, 2000-2009.

BACKGROUND: Epidemiological data on HNC are often reported aggregated despite their anatomical and histological heterogeneity. In Germany, few studies have analyzed incidence and mortality trends separately for specific anatomic sites. Furthermore, little is known about whether the incidence of HPV-associated tumour entities of the head and neck region has increased. METHODS: Based on cancer registry data from Rhineland-Palatinate from 2000 to 2009, age-standardized incidence and mortality rates were calculated for all HNC sites and localisation groups that might be HPV-associated according to the literature. Trends were analyzed by Joinpoint regression and reported as the annual percentage change (APC). RESULTS: Throughout the study period, 8 055 incident cases and 3 177 deaths were identified. The incidence rates of overall HNC increased among women (APC:+2.2%) and declined slightly among men (- 0.9%). Significantly increasing incidence rates among women were seen for tumours of the oral cavity (+2.7%) and the oropharynx (+3.6%). Among men, a significant decrease in incidence rates for tumours of the hypopharynx (-3.4%) and the larynx (-2.7%) are noteworthy. Cancers at HPV-associated sites showed increased incidence rates in men (+3.3%) and women (+4.3%). A decrease in mortality was found for tumours of the larynx in both sexes (-5.8% men,-9.1% women). CONCLUSIONS: A detailed analysis by localisation of HNC showed significant and often opposing trends for men and women regarding incidence and mortality.

[Comparison of ICD coding between mortality statistics and study-intern retrospective re-coding]. / Vergleich von ICD-Kodierungen zwischen Mortalitätsstatistik und studieninterner retrospektiver Nachkodierung.

OBJECTIVES: The aim of this study was to assess agreement between study-intern nosologist death certificate coding and official mortality statistics ICD codes. METHODS: During the follow-up of a historical cohort study conducted in Germany, original death certificates were obtained for deceased cohort members and ICD coded by a study-intern nosologist. Additionally, ICD codes for these study subjects were obtained from a state statistical office responsible for mortality statistics. A weighted inter-observer agreement for these two sources was calculated. RESULTS: In total, 406 ICD codes were available from both sources. 219 (53.9%) of these ICD codes completely agreed on the highest level possible (three- or four-digits). Agreement was found on the three-digit level with a difference at the fourth digit in 42 (10.3%) causes of death. Agreement within diagnosis groups or within chapters of disease was found in 21 (5.2%) and 49 (12.1%) causes of death, respectively. The weighted kappa for the overall inter-observer agreement was 0.67 [95% confidence interval (CI): 0.63-0.71]. Within chapters of disease, agreement was higher for neoplasms (kappa=0.88; 95% CI: 0.83-0.93) than for cardiovascular diseases (kappa=0.69; CI: 0.62-0.76). CONCLUSIONS: Overall level of agreement between the two coding sources was not very good. In Germany, the quality of coding has not improved substantially in the past two decades. The introduction of automatic coding systems, multicausal coding and a mortality register could improve the quality of ICD coding in Germany.

Bovine herpesvirus 1 interferes with TAP-dependent peptide transport and intracellular trafficking of MHC class I molecules in human cells.

Bovine herpesvirus 1 (BoHV-1), the cause of infectious bovine rhinotracheitis and infectious pustular vulvovaginitis in cattle, establishes a lifelong infection, despite the presence of antiviral immunity in the host. BoHV-1 has been shown to elude the host immune system, but the viral gene products responsible for this interference have not yet been identified. Studies aiming at the identification of BoHV-1-encoded immune evasion genes have been hampered by the lack of bovine-specific immunological reagents. Some of the immune evasion molecules identified for other herpesviruses are host species specific; others can act across the species barrier. In this study, experiments were performed to investigate whether BoHV-1 can infect human cells and interfere with antigen processing and presentation in these cells. A human melanoma cell line, Mel JuSo, appeared to be permissive for BoHV-1 infection. BoHV-1 induced expression of major viral glycoproteins at the surface of these cells and produced progeny virus up to 10(5) plaque forming units per ml. BoHV-1 infection resulted in impaired intracellular transport of human MHC class I molecules and inhibition of human TAP. These data indicate that the BoHV-1-encoded molecule(s) that block antigen presentation in bovine cells are able to interact with homologous components of the human MHC class I presentation pathway. The fact that immune evasion by BoHV-1 can be studied in human cells will facilitate the identification of the BoHV-1 gene products involved in this process. Moreover, the data presented here suggest that the BoHV-1 encoded inhibitors of antigen presentation represent potential immune suppressive agents for use in humans.

Natural T-helper immunity against human papillomavirus type 16 (HPV16) E7-derived peptide epitopes in patients with HPV16-positive cervical lesions: identification of 3 human leukocyte antigen class II-restricted epitopes.

Tumor-specific T-helper (Th) immunity was found to play a pivotal role in the natural and vaccine-induced immune defense against tumors. Since the majority of cervical cancers express human papillomavirus type 16 (HPV16) E7 oncoprotein, it is important to investigate the Th response against this target antigen in detail. By means of PBMC cultures from HLA-typed healthy donors, we identified the central part of HPV16 E7 (E7(41-72)) as the major immunogenic region within this antigen. Furthermore, we mapped 3 distinct Th epitopes within this region (DR15/E7(50-62), DR3/E7(43-77), DQ2/E7(35-50)). In a parallel approach, employing IFN-gamma ELISPOT analysis, we detected Th immunity against HPV16 E7 in subjects with HPV16+ lesions. Several of these responses matched with the 3 Th epitopes defined in our study. A number of other HPV16+ subjects did not display any E7-specific type 1 cytokine-producing T-cell immunity, indicating failure of the immune response. Our combined data argue for more extensive as well as longitudinal analysis of HPV16-specific T-cell immunity using the ELISPOT assay described, as well as for HPV-specific vaccination of individuals with HPV+ lesions.

Detection of T helper responses, but not of human papillomavirus-specific cytotoxic T lymphocyte responses, after peptide vaccination of patients with cervical carcinoma.

Human papillomavirus type 16 (HPV16)-encoded E7 oncoprotein is constitutively expressed in cervical carcinoma cells and is required for cellular transformation to be maintained. The E7 protein, therefore, forms an attractive target for T-cell-mediated immune intervention to prevent or treat HPV16+ tumors. The authors performed a peptide-based phase I/II vaccination trial to induce anti-tumor immune responses in patients with recurrent or residual cervical carcinoma. Fifteen HLA-A*0201+ patients with HPV16+ cervical carcinoma received vaccinations with synthetic peptides representing 2 HPV16 E7-encoded, HLA-A*0201-restricted cytotoxic T lymphocyte epitopes and a pan-HLA-DR-binding T-helper epitope, PADRE, in adjuvant. No signs of toxicity were observed. Two patients had stable disease for more than 1 year after vaccination, 3 patients died of the disease during or shortly after the vaccination period, and 10 patients maintained progressive cervical carcinoma. Specific immune responses directed against the vaccine components were analyzed in peripheral blood samples. No cytotoxic T lymphocyte responses against the HPV16 E7 peptides were detectable. After vaccination, strong PADRE helper peptide-specific proliferation was detected in 4 of 12 patients. In conclusion, peptide vaccination with 2 HPV16 E7 cytotoxic T lymphocyte epitopes and a universal T helper epitope is well tolerated by patients with advanced cervical carcinoma. Despite a reduction of in vitro cytolytic or proliferative recall responses to some, but not all, conventional antigens in this patient group, peptide-specific proliferative responses were induced in 4 patients. Based on the current study, it is now feasible to perform peptide vaccination in earlier stages of HPV16-induced cervical disease.

Vaccination with HPV16 peptides of patients with advanced cervical carcinoma: clinical evaluation of a phase I-II trial.

A phase I-II clinical trial was performed involving vaccination with HPV16 E7 peptides of patients suffering from HPV16 positive cervical carcinoma which was refractory to conventional treatment. Patients receiving the vaccine were HLA-A*0201 positive with HPV16 positive cervical carcinoma. The clinical trial was designed as a dose-escalation study, in which successive groups of patients received 100 micrograms, 300 micrograms or 1000 micrograms of each peptide, respectively. The vaccine consisted of two HPV16 E7 peptides and one helper peptide emulsified in Montanide ISA 51 adjuvant. 19 patients were included in the study, no adverse side-effects were observed. 2 patients showed stable disease for 1 year after vaccination; 15 patients showed progressive disease of whom 1 died during the vaccination treatment due to progressive disease; and 2 patients showed tumour-regression after chemotherapy following vaccination. A relative low count of lymphocytes before and after vaccination was present in 11/19 patients indicating that these patients were immunocompromised. This study shows that HPV16 E7 peptide vaccination is feasible, even in a group of patients with terminal disease. This paves the way for vaccinating patients with less advanced disease, whose immune system is less compromised by progressive disease.

Differential binding of viral peptides to HLA-A2 alleles. Implications for human papillomavirus type 16 E7 peptide-based vaccination against cervical carcinoma.

Several cancer immune intervention protocols aim at inducing T cell immunity against antigens presented by HLA-A2, the most common human MHC class I molecule. In the context of HLA-A*0201, we previously identified two cytotoxic T lymphocyte epitopes (E7(11-20) and E7(86-93)) encoded by the human papillomavirus type 16 E7 (HPV16 E7) oncoprotein, which is a tumor-specific antigen for cervical carcinoma. This study reports that the two HPV16 epitopes and a control hepatitis B virus epitope bind equally well to five HLA-A2 alleles (A*0201, A*0202, A*0203, A*0204, and A*0209). These HLA-A2 variants display comparable binding characteristics in accordance with the A2 supertype (M. F. Del Guercio et al., J. Immunol. 1995. 154: 685-693). Cervical carcinoma patients expressing these alleles may benefit from vaccination with the two HPV16 E7 peptides. In contrast, none of the peptides tested bound to A*0207 or A*0208, whereas heterogeneous binding was observed for A*0205 and A*0206. Therefore, the amino acid substitutions that discriminate these HLA-A2 variants from A*0201 affect antigen presentation. Taken together, our findings have implications for application of the A2 supertype concept and for vaccination with A*0201-binding peptides, in particular HPV16 E7 peptides.

Immunization with human papillomavirus type 16 (HPV16) oncoprotein-loaded dendritic cells as well as protein in adjuvant induces MHC class I-restricted protection to HPV16-induced tumor cells.

Human papillomavirus (HPV) E6 and E7 oncoproteins are attractive targets for T-cell-based immunotherapy of cervical cancer. In this study, we demonstrate that dendritic cells (DCs) pulsed with HPV16 E7 protein are not only recognized in vitro by E7-specific CTLs but also elicit E7-specific CTL responses in vivo, associated with protection against a challenge with syngeneic HPV16-induced tumor cells. Vaccination with soluble E7 protein in incomplete Freund's adjuvant likewise induces E7-specific CTL responses associated with tumor protection. The presence of HPV16 E7-specific CTLs in vivo and the observation that depletion of CD8+ cells completely abolishes tumor protection demonstrate that CTLs are the major effector cells in mediating antitumor activity. The in vivo involvement of DCs in the activation of protective CTLs is suggested by the surface display of E7 peptide-loaded MHC class I molecules on these cells after E7 protein immunization. These data show that HPV16 E7 protein-pulsed DCs, as well as the administration of E7 protein antigen in adjuvant, can effectively stimulate tumor-specific MHC class I-restricted CD8+ T-cell-mediated protective immunity to HPV16-induced cancers.

Protective anti-tumor immunity induced by vaccination with recombinant adenoviruses encoding multiple tumor-associated cytotoxic T lymphocyte epitopes in a string-of-beads fashion.

Vaccines harboring genes that encode functional oncoproteins are intrinsically hazardous, as their application may lead to introduction of these genes into normal cells and thereby to tumorigenesis. On the other hand, oncoproteins are especially attractive targets for immunotherapy of cancer, as their expression is generally required for tumor growth, making the arisal of tumor variants lacking these antigens unlikely. Using murine tumor models, we investigated the efficacy of polyepitope recombinant adenovirus (rAd) vaccines, which encode only the immunogenic T cell epitopes derived from several oncogenes, for the induction of protective anti-tumor immunity. We chose to employ rAd, as these are safe vectors that do not induce the side effects associated with, for example, vaccinia virus vaccines. A single polyepitope rAd was shown to give rise to presentation of both H-2 and human leukocyte antigen-restricted cytotoxic T lymphocyte (CTL) epitopes. Moreover, vaccination with a rAd encoding H-2-restricted CTL epitopes, derived from human adenovirus type 5 early region 1 and human papilloma virus type 16-induced tumors, elicited strong tumor-reactive CTL and protected the vaccinated animals against an otherwise lethal challenge with either of these tumors. The protection induced was superior compared with that obtained by vaccination with irradiated tumor cells. Thus, vaccination with polyepitope rAd is a powerful approach for the induction of protective anti-tumor immunity that allows simultaneous immunization against multiple tumor-associated T cell epitopes, restricted by various major histocompatibility complex haplotypes.

The current status of therapeutic HPV vaccine.

Cervical carcinoma is the second most common cause of cancer-related deaths in women worldwide. Recurrences occur in 15% of the patients after optimal treatment of low-risk early-stage disease. Treatment results of recurrent disease are relatively poor and for this reason new therapeutic strategies are warranted. Viral infection with human papillomavirus seems to have an essential part in the aetiology of cervical carcinoma. Evidence for the assumption that cervical carcinoma, among other malignancies such as melanomas, renal malignancies and Kaposi sarcoma, are immunogenic is provided by the fact that these malignancies grow more rapidly in the presence of systemic immunosuppression. Spontaneous regression for these tumour types is also described and immunohistochemical studies show extensive infiltrates in the tumour, consisting of immunocompetent cells. It is thus postulated that cellular immunity, and mainly the T-cell system plays an important role in the antitumour defence in cervical carcinoma. This review describes the rationale for the use of immunotherapy as treatment for cervical carcinoma as well as the results of recent developments in tumour immunology and its implications for the clinical use of immunotherapeutical approaches.

Decreased expression of signal-transducing zeta chain in peripheral T cells and natural killer cells in patients with cervical cancer.

An impaired immune response is frequently observed in patients and experimental animals with advanced cancer. We and others have shown alterations in CD3-associated signal-transducing zeta molecules in tumor-infiltrating T cells and peripheral blood lymphocytes (PBLs) of patients with advanced cancer. By using flow cytometric analysis of permeabilized cells with a monoclonal antibody (TIA-2) that reacts with the cytoplasmic domain of the zeta chain, here we demonstrate a marked decrease (P < 0.01) in the expression of the signal-transducing CD3 zeta chain of PBLs in patients with cervical cancer (n = 22) as compared to PBLs from healthy donors (n = 21). In addition, PBLs isolated from patients (n = 23) with cervical intraepithelial neoplasia (CIN), to a lesser but significant (P < 0. 01) extent, expressed reduced CD3 zeta levels as compared to those from healthy donors. This decreased expression of zeta chains was also observed on CD16(+) natural killer cells in PBLs from patients with cervical cancer. Surface expression of CD3 epsilon on PBLs was also decreased in cervical cancer patients as compared to healthy donors, but not on PBLs from patients with CIN. CD3 zeta chain expression significantly (r = 0.53, P < 0.01) correlated with the ability of the PBLs to produce tumor necrosis factor in response to anti-CD3 stimulation. These findings suggest that alterations of signal-transducing zeta molecules commonly occur in patients with cervical cancer and to a lesser extent with CIN, and that they are associated with reduced cellular functions such as production of tumor necrosis factor.

Immunotherapy of cancer by peptide-based vaccines for the induction of tumor-specific T cell immunity.

Recent progress in defining the molecular nature of antigens and in finding ways to manipulate T cell-mediated immune responses may provide new modalities for cancer treatment. In this report, we review preclinical studies as well as the first clinical trials with vaccination strategies aiming at the induction of anti-tumor immunity. In particular, we focus on the development of a vaccine against human papillomavirus-induced cervical carcinoma.

Occasional memory cytotoxic T-cell responses of patients with human papillomavirus type 16-positive cervical lesions against a human leukocyte antigen-A *0201-restricted E7-encoded epitope.

Most cervical carcinoma (Cxca) cells constitutively express human papillomavirus type 16 (HPV16) E6 and E7 oncoproteins. These proteins are, therefore, attractive targets for T cell-based immunotherapy. Previously, we identified HVP16 E7-encoded CTL epitopes. In patients with cervical intraepithelial neoplasia or Cxca, little is known concerning T-cell activity against viruses in general and against HPV16 in particular. Here, we have screened the blood of 10 healthy donor controls and of 22 patients with HPV16+ cervical lesions for the presence of CTLs directed against HPV16 E7- and control influenza virus matrix-derived epitopes presented by HLA-A *0201. We detected influenza virus-specific CTLs in all donors and in the majority of patients, indicating that most patients have functioning T-cell responses despite their lesions or therapeutic interventions. Moreover, we show that patients with HPV16+ lesions occasionally have memory CTLs against a HPV16 E7-encoded epitope (sequence YMLD-LQPETT), providing evidence for natural CTL immunity against HPV16 in patients with cervical lesions. Combined, these findings raise possibilities for vaccination with HPV16 E7-encoded peptides to induce or augment CTL responses for treatment or prevention of Cxca.

Human CTL epitopes encoded by human papillomavirus type 16 E6 and E7 identified through in vivo and in vitro immunogenicity studies of HLA-A*0201-binding peptides.

Human papillomavirus type 16 (HPV16) is strongly associated with cervical carcinogenesis. The HPV16 E6 and E7 oncoproteins are constitutively expressed in the majority of cervical tumor cells and are, therefore, attractive targets for CTL-mediated immunotherapy. In mice, the outgrowth of a lethal dose of HPV16-induced tumor cells has been prevented by vaccination with a CTL epitope encoded by HPV16 E7, indicating the feasibility of peptide immunization to obtain antitumor CTL responses. In the present study, the immunogenicity of 9 HLA-A*0201-binding peptides encoded by HPV16 E6 and E7 was analyzed in vivo in HLA-A*0201Kb transgenic mice and in vitro in CTL cultures induced from PBMC of HLA-A*0201+ healthy donors. Four peptides with a good binding affinity were immunogenic in HLA-A*0201Kb transgenic mice, and three of them were also highly immunogenic in CTL induction experiments with PBMC of HLA-A*0201+ healthy donors. Human CTL clones specific for these three peptides were capable of lysing the HPV16 E7-containing HLA-A*0201+ cervical carcinoma cell line CaSki. These E7-derived peptides (11-20, YMLDLQPETT; 82-90, LLMGTLGIV; 86-93, TLGIVCPI), therefore, are likely to represent naturally processed human CTL epitopes of HPV16. Additionally, these three HPV16-encoded peptides have the highest affinity of binding to the HLA-A*0201 molecule. In this study, peptides with a lower binding affinity were less immunogenic. Therefore, our data illustrate that the HLA-binding affinity of a peptide has a major impact on its immunogenicity. In conclusion, we have identified immunogenic peptides encoded by HPV16 E6 and E7 that could be used in vaccines for the prevention and treatment of cervical carcinoma.

CTL specific for the tyrosinase autoantigen can be induced from healthy donor blood to lyse melanoma cells.

CTL that lyse melanoma cells were previously isolated from several melanoma patients. Such CTL recognize autologous proteins, indicating the occurrence of autoreactive T cells in melanoma patients. We have now raised CTL, using responding T lymphocytes from healthy donor blood, that lysed not only cells incubated with an HLA-A*0201-binding tyrosinase peptide but also melanoma cells endogenously processing and presenting the epitope. Our results suggest that autoreactive CTL precursors are present in healthy donor blood and can be activated in vitro with synthetic peptides presented on appropriate APCs. Therefore, tissue-specific, autoantigen-derived peptides might be useful in immunotherapy of both poorly and nonimmunogenic tumors.

Identification of an H-2 Kb-presented Moloney murine leukemia virus cytotoxic T-lymphocyte epitope that displays enhanced recognition in H-2 Db mutant bm13 mice.

Upon infection with the Moloney murine sarcoma virus-murine leukemia virus (MuLV) complex, H-2b C57BL/6 (B6) mice respond with a class I Db-restricted cytotoxic T-lymphocyte (CTL) response, which protects against virus-induced tumorigenesis. In the B6-derived Db mutant B6.CH-2bm13 (bm13) strain, part of the class I Db antigen-presenting groove is shaped by a class I Kb-encoded sequence. Like B6 mice, bm13 mice reject Moloney virus-induced tumors, but the protective CTL response is Kb restricted. In this study we show enhanced levels of Moloney MuLV-specific CTLp with a restriction for Kb in bm13 mice. Through the use of CTL clones from Moloney virus-immunized bm13 mice, the class I Kb-presented CTL epitope was identified. The epitope is located in the Moloney virus gp70 envelope protein region (Moloney envelope, amino acids 189 to 196 [Mol env (189-196)]), SSWDFITV and has the Kb allele-specific binding motif. The Dbm13 molecule does not present the env(189 to 196) epitope to Kb-restricted bm13 CTL. In B6 mice, Mol env(189-196)-specific CTL could be induced by peptide vaccination. B6 mice thus have CTL precursors specific for this epitope but at considerably lower levels than do bm13 mice. We hypothesize that additional positive selection of Kb-restricted CTL on the Dbm13 molecule in bm13 mice explains this difference in precursor frequencies. We examined related strains of MuLV for the presence of Mol env(189-196) sequence equivalents. Rauscher, Friend, and AKV MuLV-encoded Mol env(189-196) epitope equivalents were properly recognized in cytotoxicity assays, both as synthetic and as endogenously expressed (Rauscher MuLV) peptides. In contrast, the mink cell focus-forming virus MuLV-encoded epitope equivalent, lacking a Kb anchor residue, was not presented for CTL recognition and hence can be excluded as an important CTL epitope for mink cell focus-forming viruses.