A clinical trial of therapeutic vaccination in lymphoma with serial tumor sampling and single-cell analysis.

ABSTRACT: In situ vaccination (ISV) triggers an immune response to tumor-associated antigens at 1 tumor site, which can then tackle the disease throughout the body. Here, we report clinical and biological results of a phase 1/2 ISV trial in patients with low-grade lymphoma, combining an intratumoral toll-like receptor 9 (TLR9) agonist with local low-dose radiation and ibrutinib (an inhibitor of B- and T-cell kinases). Adverse events were predominately low grade. The overall response rate was 50%, including 1 complete response. All patients experienced tumor reduction at distant sites. Single-cell analyses of serial fine needle aspirates from injected and uninjected tumors revealed correlates of clinical response, such as lower CD47 and higher major histocompatibility complex class II expression on tumor cells, enhanced T-cell and natural killer cell effector function, and reduced immune suppression from transforming growth factor ß and inhibitory T regulatory 1 cells. Although changes at the local injected site were more pronounced, changes at distant uninjected sites were more often associated with clinical responses. Functional immune response assays and tracking of T-cell receptor sequences provided evidence of treatment-induced tumor-specific T-cell responses. Induction of immune effectors and reversal of negative regulators were both important in producing clinically meaningful tumor responses. The trial was registered at www.clinicaltrials.gov as #NCT02927964.

CD20-Targeted Therapy Ablates De Novo Antibody Response to Vaccination but Spares Preestablished Immunity.

To obtain a deeper understanding of poor responses to COVID-19 vaccination in patients with lymphoma, we assessed blocking antibodies, total anti-spike IgG, and spike-specific memory B cells in the peripheral blood of 126 patients with lymphoma and 20 age-matched healthy controls 1 and 4 months after COVID-19 vaccination. Fifty-five percent of patients developed blocking antibodies postvaccination, compared with 100% of controls. When evaluating patients last treated from days to nearly 18 years prior to vaccination, time since last anti-CD20 was a significant independent predictor of vaccine response. None of 31 patients who had received anti-CD20 treatment within 6 months prior to vaccination developed blocking antibodies. In contrast, patients who initiated anti-CD20 treatment shortly after achieving a vaccine-induced antibody response tended to retain that response during treatment, suggesting a policy of immunizing prior to treatment whenever possible. SIGNIFICANCE: In a large cohort of patients with B-cell lymphoma, time since anti-CD20 treatment was an independent predictor of neutralizing antibody response to COVID-19 vaccination. Comparing patients who received anti-CD20 treatment before or after vaccination, we demonstrate that vaccinating first can generate an antibody response that endures through anti-CD20-containing treatment. This article is highlighted in the In This Issue feature, p. 85.

Characteristics and Outcome of Extranodal NK/T-cell Lymphoma in North America: A Retrospective Multi-Institutional Experience.

BACKGROUND: Extranodal natural killer/T-cell lymphoma (ENKTL) is rare and clinical data from non-Asian countries are lacking. It is unclear whether outcomes and disease natural history is similar to reported Asian series. We assessed characteristics and outcomes of patients with ENKTL from major North American centers. PATIENTS AND METHODS: We retrospectively identified patients with newly-diagnosed CD56 + ENKTL and studied disease characteristics and clinical outcomes. RESULTS: One hundred and twenty-one patients with ENKTL diagnosed between June 1990 and November 2012 were identified. Eighty-three patients (69%) had stage I/II disease and were treated with combined modality therapy (CMT) (n = 53), chemotherapy alone (CT) (n = 14) or radiotherapy alone (RT) (n = 16).  Thirty-eight patients (31%) had stage III/IV disease and were treated with CMT (n = 12), CT (n = 23), or RT (n = 3).  The median follow-up for the entire cohort was 51 months. Patients with stage I/II disease, compared to those with stage III/IV disease, had superior 2-year progression free survival (PFS) 43% vs 19% (P = .03) and overall survival (OS) 59% vs. 29% (P= .004). Outcomes were similar for stage I/II patients who received CMT vs. RT alone with 2-year PFS (53% vs. 47%; P= .91) and OS (67% vs. 67%; P= .58). No significant differences in outcomes were noted based on race/ethnicity. CONCLUSIONS: This series represents a large experience of ENKTL treated at several major North American academic centers.  Our data are consistent with Asian studies: (1) majority of patients present with early-stage disease; (2) overall poor outcome regardless of race/ethnicity; (3) CMT likely yields favorable outcomes for suitable candidates with early-stage disease.

Characteristics and Outcome of Extranodal NK/T-Cell Lymphoma in North America: A Retrospective Multi-Institutional Experience.

BACKGROUND: Extranodal natural killer/T-cell lymphoma (ENKTL) is rare and clinicaldata from non-Asian countries are lacking. It is unclear whether outcomes and diseasenatural history is similar to reported Asian series. We assessed characteristics and outcomes of patients with ENKTL from major North American centers. PATIENTS AND METHODS: We retrospectively identified patients with newly-diagnosedCD56 + ENKTL and studied disease characteristics and clinical outcomes. RESULTS: 121 patients with ENKTL diagnosed between June 1990 and November 2012 were identified. Eighty-three patients (69%) had stage I/II disease and were treatedwith combined modality therapy (CMT) (n=53), chemotherapy alone (CT) (n=14) orradiotherapy alone (RT) (n=16). Thirty-eight patients (31%) had stage III/IV diseaseand were treated with CMT (n=12), CT (n=23), or RT (n=3). The median follow-up forthe entire cohort was 51 months. Patients with stage I/II disease, compared to thosewith stage III/IV disease, had superior 2-year progression free survival (PFS) 43% vs19% (p=0.03) and overall survival (OS) 59% vs 29% (p=0.004). Outcomes were similarfor stage I/II patients who received CMT vs RT alone with 2-year PFS (53% vs 47%;p=0.91) and OS (67% vs 67%; p=0.58). No significant differences in outcomes werenoted based on race/ethnicity. CONCLUSIONS: This series represents a large experience of ENKTL treated at several major North American academic centers. OUR DATA ARE CONSISTENT WITH ASIAN STUDIES: 1) majority of patients present with early-stage disease; 2) overall poor outcome regardless of race/ethnicity; 3) CMT likely yields favorable outcomes for suitable candidates with early-stage disease.

Stereotactic Radiotherapy for Recurrent Post-Transplant Primary Central Nervous System Lymphoma.

Post-transplant primary central nervous system lymphoma (PCNSL) is a rare complication of solid organ transplantation. The optimal therapy for post-transplant PCNSL is not well established and generally includes reduction of immunosuppression and chemotherapy. Progression after front-line chemotherapy is common, and whole-brain radiotherapy (WBRT) is a standard salvage treatment as there is a concern that localized treatment fields would not prevent out-of-field recurrences. However, WBRT is associated with neurotoxicity and morbidity in these patients with inherently poor prognoses. Here, we report a patient with local recurrence of post-transplant PCNSL who was treated with fractionated stereotactic radiotherapy (SRT). He had no clinical toxicity from treatment and maintained pre-treatment neurocognition and performance status. Local control was achieved for 20 months following SRT, at which point he developed an in-field recurrence. He restarted lymphoma therapy but died one month later from fungal pneumonia. For central nervous system (CNS) lymphoma, further data are needed to optimize tumor control and toxicity outcomes and identify patients in whom localized radiotherapy fields may be beneficial, avoiding the potential toxicity of WBRT.

Autologous tumor cell vaccine induces antitumor T cell immune responses in patients with mantle cell lymphoma: A phase I/II trial.

Here, we report on the results of a phase I/II trial (NCT00490529) for patients with mantle cell lymphoma who, having achieved remission after immunochemotherapy, were vaccinated with irradiated, CpG-activated tumor cells. Subsequently, vaccine-primed lymphocytes were collected and reinfused after a standard autologous stem cell transplantation (ASCT). The primary endpoint was detection of minimal residual disease (MRD) within 1 yr after ASCT at the previously validated threshold of ≥1 malignant cell per 10,000 leukocyte equivalents. Of 45 evaluable patients, 40 (89%) were found to be MRD negative, and the MRD-positive patients experienced early subsequent relapse. The vaccination induced antitumor CD8 T cell immune responses in 40% of patients, and these were associated with favorable clinical outcomes. Patients with high tumor PD-L1 expression after in vitro exposure to CpG had inferior outcomes. Vaccination with CpG-stimulated autologous tumor cells followed by the adoptive transfer of vaccine-primed lymphocytes after ASCT is feasible and safe.

Circulating Tumor DNA Measurements As Early Outcome Predictors in Diffuse Large B-Cell Lymphoma.

PURPOSE: Outcomes for patients with diffuse large B-cell lymphoma remain heterogeneous, with existing methods failing to consistently predict treatment failure. We examined the additional prognostic value of circulating tumor DNA (ctDNA) before and during therapy for predicting patient outcomes. PATIENTS AND METHODS: We studied the dynamics of ctDNA from 217 patients treated at six centers, using a training and validation framework. We densely characterized early ctDNA dynamics during therapy using cancer personalized profiling by deep sequencing to define response-associated thresholds within a discovery set. These thresholds were assessed in two independent validation sets. Finally, we assessed the prognostic value of ctDNA in the context of established risk factors, including the International Prognostic Index and interim positron emission tomography/computed tomography scans. RESULTS: Before therapy, ctDNA was detectable in 98% of patients; pretreatment levels were prognostic in both front-line and salvage settings. In the discovery set, ctDNA levels changed rapidly, with a 2-log decrease after one cycle (early molecular response [EMR]) and a 2.5-log decrease after two cycles (major molecular response [MMR]) stratifying outcomes. In the first validation set, patients receiving front-line therapy achieving EMR or MMR had superior outcomes at 24 months (EMR: EFS, 83% v 50%; P = .0015; MMR: EFS, 82% v 46%; P < .001). EMR also predicted superior 24-month outcomes in patients receiving salvage therapy in the first validation set (EFS, 100% v 13%; P = .011). The prognostic value of EMR and MMR was further confirmed in the second validation set. In multivariable analyses including International Prognostic Index and interim positron emission tomography/computed tomography scans across both cohorts, molecular response was independently prognostic of outcomes, including event-free and overall survival. CONCLUSION: Pretreatment ctDNA levels and molecular responses are independently prognostic of outcomes in aggressive lymphomas. These risk factors could potentially guide future personalized risk-directed approaches.

Distinct biological subtypes and patterns of genome evolution in lymphoma revealed by circulating tumor DNA.

Patients with diffuse large B cell lymphoma (DLBCL) exhibit marked diversity in tumor behavior and outcomes, yet the identification of poor-risk groups remains challenging. In addition, the biology underlying these differences is incompletely understood. We hypothesized that characterization of mutational heterogeneity and genomic evolution using circulating tumor DNA (ctDNA) profiling could reveal molecular determinants of adverse outcomes. To address this hypothesis, we applied cancer personalized profiling by deep sequencing (CAPP-Seq) analysis to tumor biopsies and cell-free DNA samples from 92 lymphoma patients and 24 healthy subjects. At diagnosis, the amount of ctDNA was found to strongly correlate with clinical indices and was independently predictive of patient outcomes. We demonstrate that ctDNA genotyping can classify transcriptionally defined tumor subtypes, including DLBCL cell of origin, directly from plasma. By simultaneously tracking multiple somatic mutations in ctDNA, our approach outperformed immunoglobulin sequencing and radiographic imaging for the detection of minimal residual disease and facilitated noninvasive identification of emergent resistance mutations to targeted therapies. In addition, we identified distinct patterns of clonal evolution distinguishing indolent follicular lymphomas from those that transformed into DLBCL, allowing for potential noninvasive prediction of histological transformation. Collectively, our results demonstrate that ctDNA analysis reveals biological factors that underlie lymphoma clinical outcomes and could facilitate individualized therapy.

Dose-adjusted EPOCH-rituximab therapy in primary mediastinal B-cell lymphoma.

BACKGROUND: Primary mediastinal B-cell lymphoma is a distinct subtype of diffuse large-B-cell lymphoma that is closely related to nodular sclerosing Hodgkin's lymphoma. Patients are usually young and present with large mediastinal masses. There is no standard treatment, but the inadequacy of immunochemotherapy alone has resulted in routine consolidation with mediastinal radiotherapy, which has potentially serious late effects. We aimed to develop a strategy that improves the rate of cure and obviates the need for radiotherapy. METHODS: We conducted a single-group, phase 2, prospective study of infusional dose-adjusted etoposide, doxorubicin, and cyclophosphamide with vincristine, prednisone, and rituximab (DA-EPOCH-R) and filgrastim without radiotherapy in 51 patients with untreated primary mediastinal B-cell lymphoma. We used results from a retrospective study of DA-EPOCH-R from another center to independently verify the outcomes. RESULTS: The patients had a median age of 30 years (range, 19 to 52) and a median tumor diameter of 11 cm; 59% were women. During a median of 5 years of follow-up, the event-free survival rate was 93%, and the overall survival rate was 97%. Among the 16 patients who were involved in the retrospective analysis at another center, over a median of 3 years of follow-up, the event-free survival rate was 100%, and no patients received radiotherapy. No late morbidity or cardiac toxic effects were found in any patients. After follow-up ranging from 10 months to 14 years, all but 2 of the 51 patients (4%) who received DA-EPOCH-R alone were in complete remission. The 2 remaining patients received radiotherapy and were disease-free at follow-up. CONCLUSIONS: Therapy with DA-EPOCH-R obviated the need for radiotherapy in patients with primary mediastinal B-cell lymphoma. (Funded by the National Cancer Institute; ClinicalTrials.gov number, NCT00001337.).

Clinical outcome prediction by microRNAs in human cancer: a systematic review.

BACKGROUND: MicroRNA (miR) expression may have prognostic value for many types of cancers. However, the miR literature comprises many small studies. We systematically reviewed and synthesized the evidence. METHODS: Using MEDLINE (last update December 2010), we identified English language studies that examined associations between miRs and cancer prognosis using tumor specimens for more than 10 patients during classifier development. We included studies that assessed a major clinical outcome (nodal disease, disease progression, response to therapy, metastasis, recurrence, or overall survival) in an agnostic fashion using either polymerase chain reaction or hybridized oligonucleotide microarrays. RESULTS: Forty-six articles presenting results on 43 studies pertaining to 20 different types of malignancy were eligible for inclusion in this review. The median study size was 65 patients (interquartile range [IQR] = 34-129), the median number of miRs assayed was 328 (IQR = 250-470), and overall survival or recurrence were the most commonly measured outcomes (30 and 19 studies, respectively). External validation was performed in 21 studies, 20 of which reported at least one nominally statistically significant result for a miR classifier. The median hazard ratio for poor outcome in externally validated studies was 2.52 (IQR = 2.26-5.40). For all classifier miRs in studies that evaluated overall survival across diverse malignancies, the miRs most frequently associated with poor outcome after accounting for differences in miR assessment due to platform type were let-7 (decreased expression in patients with cancer) and miR 21 (increased expression). CONCLUSIONS: MiR classifiers show promising prognostic associations with major cancer outcomes and specific miRs are consistently identified across diverse studies and platforms. These types of classifiers require careful external validation in large groups of cancer patients that have adequate protection from bias. -

A retrospective study evaluating the efficacy and safety of bendamustine in the treatment of mantle cell lymphoma.

Bendamustine is approved in the United States for relapsed indolent lymphoma. However, it has not been widely studied in mantle cell lymphoma (MCL). We retrospectively reviewed the records of all patients with MCL who were treated with bendamustine at three centers. The primary endpoint was overall response rate (ORR). Thirty patients with MCL received bendamustine, 25 for relapsed disease. After a median follow-up of 12 months, there were 15 complete responses (CRs) with an ORR of 83% (95% confidence interval [CI] 70-97%). Factors significantly associated with longer survival were achieving a CR and classical (versus blastic) variant of MCL. Grade 3 or 4 neutropenia, anemia and thrombocytopenia occurred in 23%, 3% and 20%, respectively. There was one case of progressive multifocal leukoencephalopathy 10 months after therapy completion. Bendamustine in combination with rituximab demonstrated a high response rate in this study of patients with predominantly relapsed MCL.

Current concepts and controversies in the management of early stage Hodgkin lymphoma.

Over the past three decades, due to the recognition of late effects related to high-dose extended field radiotherapy and heavy alkylator chemotherapy, combined modality therapy with abbreviated chemotherapy and limited field radiotherapy has emerged as the standard of care for early stage Hodgkin lymphoma, with cure rates in excess of 80%. Currently, however, controversy remains over identifying the most appropriate criteria to risk-stratify patients with early stage disease, so that those with a favorable prognosis receive limited treatment without compromising cure rates and those with unfavorable risk receive more intensified therapy. The optimal risk stratification system remains unclear, with variable definitions of favorable and unfavorable disease used by research groups in North America and Europe. Thus, comparison of clinical trial results has been challenging, and additional controversies persist regarding optimal chemotherapy regimens, duration of therapy, and the role of radiotherapy. Investigations are ongoing to assess the potential of functional imaging and biomarkers as tools for risk stratification. The collective goal is to further refine current stratification strategies to allow for an individualized, risk-adapted treatment approach that minimizes long-term late effects without compromising high cure rates.

Stage I-IIA non-bulky Hodgkin's lymphoma. Is further distinction based on prognostic factors useful? The Stanford experience.

PURPOSE: In the United States, early-stage Hodgkin's lymphoma (HL) is defined as asymptomatic stage I/II non-bulky disease. European groups stratify patients to more intense treatment by considering additional unfavorable factors, such as age, number of nodal sites, sedimentation rate, extranodal disease, and elements of the international prognostic score for advanced HL. We sought to determine the prognostic significance of these factors in patients with early-stage disease treated at Stanford University Medical Center. METHODS AND MATERIALS: This study was a retrospective analysis of 101 patients treated with abbreviated Stanford V chemotherapy (8 weeks) and 30-Gy (n=84 patients) or 20-Gy (n=17 patients) radiotherapy to involved sites. Outcomes were assessed after applying European risk factors. RESULTS: At a median follow-up of 8.5 years, freedom from progression (FFP) and overall survival (OS) rates were 94% and 97%, respectively. From 33% to 60% of our patients were unfavorable per European criteria (i.e., German Hodgkin Study Group [GHSG], n=55%; European Organization for Research and Treatment of Cancer, n=33%; and Groupe d'Etudes des Lymphomes de l'Adulte, n=61%). Differences in FFP rates between favorable and unfavorable patients were significant only for GHSG criteria (p=0.02) with there were no differences in OS rates for any criteria. Five of 6 patients who relapsed were successfully salvaged. CONCLUSIONS: The majority of our patients deemed unfavorable had an excellent outcome despite undergoing a significantly abbreviated regimen. Application of factors used by the GHSG defined a less favorable subset for FFP but with no impact on OS. As therapy for early-stage disease moves to further reductions in therapy, these factors take on added importance in the interpretation of current trial results and design of future studies.

The emerging role for rituximab in the treatment of nodular lymphocyte predominant Hodgkin lymphoma.

PURPOSE OF REVIEW: Nodular lymphocyte predominant Hodgkin lymphoma (NLPHL) is a rare subset of Hodgkin lymphoma that is distinct from classical Hodgkin lymphoma (cHL). The unique malignant 'popcorn' cells express the B-cell antigen CD20 and lack expression of the cHL markers CD15 and CD30. Traditionally, NLPHL has been included with cHL in clinical trials with excellent prognosis reported in several series. The reliable expression of CD20 has led to the evaluation of the chimeric monoclonal anti-CD20 antibody rituximab in several recent trials. RECENT FINDINGS: Three series have reported the efficacy of 4 weekly doses of rituximab in all stages of NLPHL, both in the treatment-naive and relapsed settings. Emerging data also suggest that longer courses of antibody therapy may improve the duration of response. SUMMARY: Rituximab appears to offer a nonchemotherapy-based effective treatment option, which is well tolerated. Ongoing studies are required to further define the optimal patient population who may benefit from rituximab and evaluate its role in maintenance as well as in combination with radiotherapy and chemotherapy.

Transcription arrest at an abasic site in the transcribed strand of template DNA.

A dedicated excision repair pathway, termed transcription-coupled repair (TCR), targets the removal of DNA lesions from transcribed strands of expressed genes. Transcription arrest at the site of the lesion has been proposed as the first step for initiation of TCR. In support of this model, a strong correlation between arrest of transcription by a lesion in vitro and TCR of that lesion in vivo has been found in most cases analyzed. TCR has been reported for oxidative DNA damage; however, very little is known about how frequently occurring and spontaneous DNA damage, such as depurination and base deamination, affects progression of the transcription complex. We have previously determined that the oxidative lesion, thymine glycol, is a significant block to transcription by T7 RNA polymerase (T7 RNAP) but has no detectable effect on transcription by RNA polymerase II (RNAP II) in a reconstituted system with all of the required factors. Another oxidative lesion, 8-oxoguanine, only slightly blocked T7 RNAP and caused RNAP II to briefly pause at the lesion before bypassing it. Because an abasic site is an intermediate in the repair of oxidative damage, it was of interest to learn whether it arrested transcription. Using in vitro transcription assays and substrates containing a specifically positioned lesion, we found that an abasic site in the transcribed strand is a 60% block to transcription by T7 RNAP but nearly a complete block to transcription by mammalian RNAP II. An abasic site in the nontranscribed strand did not block either polymerase. Our results clearly indicate that an abasic site is a much stronger block to transcription than either a thymine glycol or an 8-oxoguanine. Because the predominant model for TCR postulates that only lesions that block RNAP will be subject to TCR, our findings suggest that the abasic site may be sufficient to initiate TCR in vivo.

Transcriptional inhibition by an oxidized abasic site in DNA.

2-Deoxyribonolactone (dL) is an oxidized abasic site in DNA that can be induced by gamma-radiolysis, ultraviolet irradiation, and numerous antitumor drugs. Although this lesion is incised by AP endonucleases, suggesting a base-excision repair mechanism for dL removal, subsequent excision and repair synthesis by DNA polymerase beta is inhibited due to accumulation of a protein-DNA cross-link. This raises the possibility that additional repair pathways might be required to eliminate dL from the genome. Transcription-coupled repair (TCR) is a pathway of excision repair specific to DNA lesions present in transcribed strands of expressed genes. A current model proposes that transcription arrest at the site of DNA damage is required to initiate TCR. In support of this model, a strong correlation between transcription arrest by a lesion in vitro and TCR of the lesion in vivo has been found in most cases analyzed. To assess whether dL might be subject to TCR, we have studied the behavior of bacteriophage T3 and T7 RNA polymerases (T3RNAP, T7RNAP) and of mammalian RNA polymerase II (RNAPII) when they encounter a dL lesion or its "caged" precursor located either in the transcribed or in the nontranscribed strand of template DNA. DNA plasmids containing a specifically located dL downstream of the T3, T7 promoter or the Adenovirus major late promoter were constructed and used for in vitro transcription with purified proteins. We found that both dL and its caged precursor located in the transcribed strand represented a complete block to transcription by T3- and T7RNAP. Similarly, they caused more than 90% arrest when transcription was carried out with mammalian RNAPII. Furthermore, RNAPII complexes arrested at dL were subject to the transcript cleavage reaction mediated by elongation factor TFIIS, indicating that these complexes were stable. A dL in the nontranscribed strand did not block either polymerase.

Effect of 8-oxoguanine on transcription elongation by T7 RNA polymerase and mammalian RNA polymerase II.

8-Oxoguanine (8-oxoG) is a major oxidative lesion produced in DNA by normal cellular metabolism or after exposure to exogenous sources such as ionizing radiation. Persistence of this lesion in DNA causes G to T transversions, with deleterious consequences for the cell. As a result, several repair processes have evolved to remove this lesion from the genome. It has been reported that 8-oxoG is subject to transcription-coupled repair (TCR), a process dedicated to removal of lesions from transcribed strands of expressed genes. A current model assumes that RNA polymerase arrest at the site of the lesion is required for initiation of TCR. As a first step to understand how TCR of 8-oxoG occurs, we have studied the effect of 8-oxoG on transcription elongation by T7 RNA polymerase (T7 RNAP) and rat liver RNA polymerase II (RNAPII). We have utilized an in vitro transcription system with purified RNA polymerase and initiation factors, and substrates containing a single 8-oxoG in the transcribed or in the non-transcribed strand downstream of the T7 promoter or the Adenovirus major late promoter. We found that 8-oxoG only slightly inhibited T7 RNAP transcription, with a readthrough frequency of up to 95%. Similarly, this lesion only transiently blocked transcription by RNAPII. However, changes in nucleotide concentration affected the extent of RNAPII blockage at the 8-oxoG. When this lesion was positioned in the non-transcribed strand, complete lesion bypass was observed with either polymerase. Binding of the Saccharomyces cerevisiae MSH2-MSH6 complex to 8-oxoG containing substrates did not increase the frequency of RNAPII arrest at the site of the lesion, suggesting that this complex was displaced by the elongating polymerase. These results are discussed in the context of possible models for TCR.