AAV5 delivery of CRISPR/Cas9 mediates genome editing in the lungs of young rhesus monkeys

Genome editing has the potential to treat genetic diseases in a variety of tissues including the lung. We have previously developed and validated a dual adeno-associated virus (AAV) CRISPR platform that supports effective editing in the airways of mice. To validate this delivery vehicle in a large animal model, we have shown that intratracheal instillation of CRISPR/Cas9 in AAV5 can edit a housekeeping gene or a disease-related gene in the lungs of young rhesus monkeys. We observed up to 8% editing of ACE2 in lung lobes after single-dose administration. Single-nuclear RNA-sequencing revealed that AAV5 transduces multiple cell types in the caudal lung lobes, including alveolar cells, macrophages, fibroblasts, endothelial cells, and B cells. These results demonstrate that AAV5 is efficient in the delivery of CRISPR/Cas9 in the lung lobes of young rhesus monkeys.

B-cell-directed CAR-T cell therapy activates CD8+ cytotoxic CARneg bystander T-cells in non-human primates and patients

CAR-T cells hold promise as a therapy for B-cell-derived malignancies, yet despite their impressive initial response rates, a significant proportion of patients ultimately experience relapse. While recent studies have explored the mechanisms of in vivo CAR-T cell function, little is understood about the activation of surrounding CARneg bystander T-cells and their potential to enhance tumor responses. We performed single-cell RNA-Seq (scRNA-Seq) on non-human primate (NHP) and patient-derived T-cells to identify the phenotypic and transcriptomic hallmarks of bystander activation of CARneg T-cells following B-cell targeted CAR-T cell therapy. Utilizing a highly translatable CD20 CAR NHP model, we observed a distinct population of activated CD8+ CARneg T-cells emerging during CAR-T cell expansion. These bystander CD8+ CARneg T-cells exhibited a unique transcriptional signature with upregulation of NK-cell markers (KIR3DL2, CD160, KLRD1), chemokines and chemokine receptors (CCL5, XCL1, CCR9), and downregulation of naive T-cell-associated genes (SELL, CD28). A transcriptionally similar population was identified in patients following Tisagenlecleucel infusion. Mechanistic studies revealed that IL-2 and IL-15 exposure induced bystander-like CD8+ T-cells in a dose dependent manner. In vitro activated and patient-derived T-cells with the bystander phenotype efficiently killed leukemic cells through a TCR-independent mechanism. Collectively, this dataset provides the first comprehensive identification and profiling of CARneg bystander CD8+ T-cells following B-cell targeting CAR-T cell therapy and suggests a novel mechanism through which CAR-T cell infusion might trigger enhanced anti-leukemic responses.

IMMU-02. GENOMIC AND TRANSCRIPTOMIC CORRELATES OF IMMUNOTHERAPY RESPONSE WITHIN THE TUMOR MICROENVIRONMENT OF LEPTOMENINGEAL METASTASES

Leptomeningeal disease (LMD) is a devastating complication of solid tumor malignancies, with dire prognosis and no effective systemic treatment options. Over the past decade, the incidence of LMD has steadily increased due to therapeutics that have extended the survival of cancer patients, highlighting the need for new interventions. To examine the efficacy of immune checkpoint inhibitors (ICI) in patients with LMD, we completed two phase II clinical trials utilizing either Pembrolizumab alone or the combination of Ipilimumab and Nivolumab. We investigated the cellular and molecular features underpinning observed patient trajectories in these trials by applying single-cell RNA and cell-free DNA profiling to longitudinal cerebrospinal fluid (CSF) draws from enrolled patients. We isolated and sequenced 34,742 cells from both the malignant and immune compartment within CSF. Amongst the 19 patients included in the cohort, there were 13 pre-treatment and 24 post-treatment samples, and 9 patients were sampled across multiple timepoints. We detected dynamic changes in immune cell recruitment into the CSF and activation within 30 days of ICI, including increased effector T cell activation and IFN-gamma response pathways within T cells. Moreover, the overall level of IFN-gamma response and antigen processing within 30 days of ICI in malignant cells correlated with survival past clinical trial primary endpoint. Lastly, we observed evidence of longitudinal outgrowth of distinct immunogenic clones over the course of ICI. Overall, our study describes the liquid LMD tumor microenvironment prior to and following ICI treatment and provides unique insights into the compartmental and temporal variation during the course of ICI. Moreover, our findings demonstrate the clinical utility of cell- free and single-cell genomic measurements for LMD research.

Endothelial immune surveillance by intravascular antigen-experienced CD8+ T cells as a novel mechanism of antiviral adaptive immunity

Following acute infection, pathogen-specific CD8+ T cells proliferate and divide in a heterogeneous manner, giving rise to effector and memory T cells with distinct migratory patterns. This migratory division of labor ensures that the host is efficiently scanned for ongoing or recurring infections. Recent evidence suggests that the presumed migratory behavior of the classical effector memory T cell subset needs revision. Here, we report that terminally differentiated effector and effector memory T cells adhere to and patrol the endothelium, while less differentiated T cells migrate into peripheral tissues. Interestingly, patrolling T cells have been observed at vascular beds such as the dermal microvasculature, where they concentrate in arterioles and preferentially migrate against the blood flow. Patrolling T cells survey the endothelium in the search for cognate antigen and, once encountered, cease their migration and establish long-lived interactions with peptide-presenting endothelial cells without inducing cytotoxicity. Finally, single-cell RNA-Seq analysis of patrolling CD8+ T effector cells and blockade experiments to prevent patrolling have unveiled a key and unexpected role for patrolling in the generation and maintenance of the intravascular CD8+ CX3CR1high KLRG-1+ effector memory T cell subset, which in turn is relevant for long-term protective immunity.

Single-cell analysis of cellular state heterogeneity in human localized prostate cancer

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Background: Prostate cancer is the second most common malignancy in men worldwide. The development of cancer from prostate tissue involves complex interactions of tumor cells with surrounding epithelial and stromal cells and can occur multifocally, suggesting that prostate epithelial cells may undergo cellular state transitions towards carcinogenesis. Previous studies on localized prostate cancer molecular changes have focused on unsorted bulk tissue samples, leaving a gap in our understanding of the cellular heterogeneity in the tumor microenvironment. Single-cell analyses of tumor specimens have the potential to reveal, at unprecedented resolution, cellular composition, as well as instructive intercellular interactions. Methods: To characterize the localized prostate cancer tumor microenvironment, we performed single-cell RNA-sequencing (scRNA-seq) on prostate biopsies, radical prostatectomy specimens, and matched patient-derived organoids from localized prostate cancer patients. Results: Within prostate epithelial cells, we identified a population of club cells that may act as progenitor cells. Furthermore, we uncovered luminal-like epithelial cellular states augmented in androgen signaling across basal and club cell populations. By classifying tumor cells based on ERG expression status, we found that ERG- tumor cells, in contrast to ERG+ cells, share transcriptomic heterogeneity with surrounding luminal epithelial cells and are associated with common stromal and immune microenvironment responses. These results suggest that specific immune niches may arise based on TMPRSS2-ERG fusion status. Finally, we generated prostate epithelial organoids derived from matched localized prostate cancer patients and characterized their transcriptomic profiles by scRNA-seq. These patient-derived organoids recapitulated tumor-associated epithelial cell states but also harbored distinct cell types and states from their parent tissues. Conclusions: Our data from localized prostate cancer specimens and organoids provide diagnostically relevant insights and will help advance our understanding of the cancer cellular states associated with prostate carcinogenesis.

Abstract PR03: Subtype-specific microenvironmental crosstalk and tumor cell plasticity in metastatic pancreatic cancer

The majority of patients with pancreatic ductal adenocarcinoma (PDAC) present at diagnosis with metastatic disease and have median survival times of less than 12 months. Recent studies have demonstrated that PDAC tumors with distinct transcriptional phenotypes are associated with different clinical outcomes. However, the mechanisms underlying this survival difference, the degree of cellular heterogeneity within a given tumor, and the subtype-specific contributions from the local immune microenvironment are not understood. In addition, there are ongoing efforts to understand if patient-derived organoid models can be used as functional surrogates for an individual patient’s disease. It remains unclear if patient transcriptional phenotypes are preserved in their matched organoid models. Here, we describe a pipeline that permits both direct characterization of the PDAC liver metastatic niche via single-cell RNA-sequencing and functional assessment of PDAC tumor biology in patient-matched organoid models. Starting from core needle biopsies of metastatic PDAC lesions containing 50-100k viable cells, we simultaneously perform: (1) single-cell RNA-sequencing using Seq-Well and (2) three-dimensional organoid culture generation. We have applied this approach to profile 23 patients and their matched early passage organoid models. Our pipeline yields high-quality single-cell measurements across diverse cell types—both malignant and non-malignant—enabling a principled dissection of tumor intrinsic and extrinsic factors. Evaluation of clinically relevant transcriptional signatures (e.g., Basal-like vs Classical) revealed extensive heterogeneity at the single-cell level. Single malignant cells are capable of co-expressing markers of both Basal-like and Classical states suggesting these phenotypes lie on a continuum rather than as discrete types. Basal cells express more stem-like features and inhabit a distinct microenvironment compared to their Classical counterparts. Microenvironmental composition differed on several levels between the two types, most notably their T/NK cell and macrophage populations with specific implications for subtype-specific microenvironmental directed therapy. Finally, we found that the microenvironment in traditional organoid culture selects against the Basal-like subtype and that these tumors are capable of significant phenotypic plasticity in vitro. We are able to recover Basal-like features by altering the organoid growth conditions. These findings suggest the need for distinct environments to support specific transcriptional subtypes in PDAC. Overall, our work provides a framework for the analysis of human cancers and their matched models using single-cell methods, and reveals novel, actionable insights into the heterogeneity and plasticity underlying survival in transcriptionally distinct forms of PDAC.

Citation Format: Peter S. Winter, Srivatsan Raghavan, Andrew Navia, Hannah Williams, Alan DenAdel, Radha Kalekar, Jennyfer Galvez-Reyes, Kristen Lowder, Nolawit Mulugeta, Manisha Raghavan, Ashir Borah, Raymond Ng, Junning Wang, Emma Reilly, Dorisanne Ragon, Lauren Brais, Kimmie Ng, James Cleary, Lorin Crawford, Scott Manalis, Jonathan Nowak, Brian Wolpin, William Hahn, Andrew Aguirre, Alex Shalek. Subtype-specific microenvironmental crosstalk and tumor cell plasticity in metastatic pancreatic cancer [abstract]. In: Proceedings of the AACR Virtual Special Conference on Tumor Heterogeneity: From Single Cells to Clinical Impact; 2020 Sep 17-18. Philadelphia (PA): AACR; Cancer Res 2020;80(21 Suppl):Abstract nr PR03.

Defining the role of CD8 T cell subsets in tuberculosis in non-human primates

Tuberculosis (TB) is the number one cause of death by a single infectious agent with no protective vaccine available. Challenges in vaccine development are partially attributed to a deficiency in knowledge regarding specific immune cells’ contribution to protection. CD8 T cells comprise ~30–40% of T cells in the lung granuloma, an organized collection of immune cells that is a hallmark of Mycobacterium tuberculosis (Mtb) infection. However, few studies have explored the diversity, function and contribution to protection of CD8 subsets during TB. We investigated the roles of conventional CD8αβ T cells and unconventional CD8αα T cells during early Mtb infection. Cynomolgus macaques were CD8-depleted using anti-CD8α or anti-CD8β antibodies prior to and during Mtb infection and necropsied at 6 weeks post-infection. CD8α depleted NHPs (CD8αα and CD8αβ depletion) had higher bacterial burdens, worse pathology and overall poorer disease outcome compared to IgG controls, while CD8β depleted (CD8αβ only depletion) displayed an intermediate phenotype. This provides the first evidence for an important early role for non-conventional CD8 T cells in TB. Using spectral flow cytometry we identified multiple CD8 subsets and assessed their functions in granulomas for the first time in NHPs. After CD8α depletion, the repertoire of CD8 subsets dramatically changed in granulomas. This was associated with functional alterations in other immune cell populations which we validated and expanded upon using single-cell RNA sequencing. These studies provide a functional immune landscape of early events in Mtb granulomas as well as the effects of CD8 depletion, shedding light on the protective potential of CD8 T cell subsets during primary Mtb infection.

973. Single-cell RNA Sequencing Analysis of Zika Virus Infection in Human Stem Cell-Derived Cerebral Organoids

Background

The molecular mechanisms underpinning the neurologic and congenital pathologies caused by Zika virus (ZIKV) infection remain poorly understood. One barrier has been the lack of relevant model systems for the developing human brain; however, thanks to advances in the stem cell field, we can now evaluate ZIKV central nervous system infections in human stem cell-derived cerebral organoids which recapitulate complex 3-dimensional neural architecture.

Methods

We apply Seq-Well—a simple, portable platform for massively parallel single-cell RNA sequencing—to characterize cerebral organoids infected with ZIKV. Using this sequencing method, and published transcriptional profiles, we identify multiple cellular populations in our organoids, including neuroprogenitor cells, intermediate progenitor cells, and terminally differentiated neurons. We detect and quantify host mRNA transcripts and viral RNA with single-cell resolution, defining transcriptional features of uninfected cells and infected cells.

Results

In this model of the developing brain, we identify preferred tropisms of ZIKV infection and pronounced effects on cell division, differentiation, and death. Our data additionally reveal differences in cellular populations and gene expression within organoids infected by historic and contemporary ZIKV strains from a variety of geographic locations. This finding might help explain phenotypic differences attributed to the viruses, including variable propensity to cause microcephaly.

Conclusion

Overall, our work provides insight into normal and diseased human brain development, and suggests that both virus replication and host response mechanisms underlie the neuropathology of ZIKV infection.

Disclosures

All Authors: No reported Disclosures.

The Human Cell Atlas

The recent advent of methods for high-throughput single-cell molecular profiling has catalyzed a growing sense in the scientific community that the time is ripe to complete the 150-year-old effort to identify all cell types in the human body. The Human Cell Atlas Project is an international collaborative effort that aims to define all human cell types in terms of distinctive molecular profiles (such as gene expression profiles) and to connect this information with classical cellular descriptions (such as location and morphology). An open comprehensive reference map of the molecular state of cells in healthy human tissues would propel the systematic study of physiological states, developmental trajectories, regulatory circuitry and interactions of cells, and also provide a framework for understanding cellular dysregulation in human disease. Here we describe the idea, its potential utility, early proofs-of-concept, and some design considerations for the Human Cell Atlas, including a commitment to open data, code, and community.

The Human Cell Atlas

The recent advent of methods for high-throughput single-cell molecular profiling has catalyzed a growing sense in the scientific community that the time is ripe to complete the 150-year-old effort to identify all cell types in the human body. The Human Cell Atlas Project is an international collaborative effort that aims to define all human cell types in terms of distinctive molecular profiles (such as gene expression profiles) and to connect this information with classical cellular descriptions (such as location and morphology). An open comprehensive reference map of the molecular state of cells in healthy human tissues would propel the systematic study of physiological states, developmental trajectories, regulatory circuitry and interactions of cells, and also provide a framework for understanding cellular dysregulation in human disease. Here we describe the idea, its potential utility, early proofs-of-concept, and some design considerations for the Human Cell Atlas, including a commitment to open data, code, and community.

Abstract AP19: SINGLE–CELL RNA–SEQUENCING OF PATIENT–DERIVED OVARIAN CANCER CELLS AND PATIENT–DERIVED XENOGRAFT MODELS

BACKGROUND AND PURPOSE: Genetic heterogeneity is a hallmark of ovarian cancer (OvCa) biology and underlies treatment resistance. Macroscopic and or treatment resistant microscopic residual disease (MRD) after debulking surgery and chemotherapy are the source for disease recurrence and death in these patients. Current profiling methods are unable adequately reflect heterogeneity, and transcriptional programs associated with treatment resistance and MRD remain elusive. To elucidate transcriptional heterogeneity and potential mechanisms of drug-resistance, we isolated OvCa cells from patients with malignant ascites using flow-cytometry. We applied single-cell RNA-sequencing (sc-RNA-seq) to ascites-derived cells from patients with OvCa. We picked OvCa spheroids and profiled these separately. To investigate MRD, we used three PDX-models stably expressing mCherry, treated with carboplatin, and harvested tumor cells for sc-RNA-seq at three time points (pre-treatment, at time of MRD as determined by bio-luminescence imaging, and disease relapse).

SUMMARY OF RESULTS: We successfully sequenced 770 single-cell transcriptomes from 6 individuals with treatment-resistant OvCa, including 3 patients with sequential samples. We mapped the landscape of chromosomal aberrations by inferred large-scale copy number variations (CNVs) at a single-cell level. We observed significant inter-tumor heterogeneity and started to deconstruct the genomic architecture of individual patients. Using experimentally validated gene sets, we determined the cell cycle state of individual cells and identified transcriptional programs related to the cell cycle as significant bias in publically available bulk RNA-sequencing data. Principal component analysis revealed the expression of a stem-ness signature, including CD133, ALDH1A and AXL, in a sub-set of non-cycling cells. An important driver of transcriptional heterogeneity common to patients included in this study was the expression of gene sets related to inflammatory pathways, such as the NFkB and JAK/STAT pathways. Hierarchical clustering of 42 spheroid profiles identified four major clusters, including a highly “inflamed” phenotype. Therapeutic inhibition of the STAT pathway abrogated the capacity of spheroid formation on an ultra-low attachment surface, indicating its importance for metastasis. We have successfully isolated thousands of individual cells for single-cell profiling from PDX models treated with carboplatin. These cells were collected at three time points, including at the MRD stage. We have successfully sequenced 100 cells from this collection and were able to generate whole-transcriptome data comparable to that of freshly isolated patient cells and thousands of single cells are currently undergoing sequencing.

CONCLUSION: We have successfully applied single-cell RNA-sequencing to patient-derived ovarian cancer cells and PDX-models. Single-cell transcriptomes enabled inference of genomic information, genetic and transcriptional heterogeneity, cell cycle state, and programs related to stem-ness and inflammation, providing a unique and comprehensive perspective on ovarian cancer cell states. Ongoing profiling of carboplatin-resistant cells captured at the minimal residual disease stage in PDX-models will provide a unique opportunity to understand treatment resistance which ultimately leads to cancer recurrence.

Citation Format: Benjamin Izar, Elizabeth Stover, Itay Tirosh, Asaf Rotem, Parin Shah, Chris Rodman, Sanjay Prakadan, Marc Wadsworth, Mei-Ju Su, Rachel Leeson, Sangeetha Palakurthi, Joyce Liu, Ursula Matulonis, Alex Shalek, Orit Rozenblatt-Rosen, Aviv Regev, Levi Garraway. SINGLE–CELL RNA–SEQUENCING OF PATIENT–DERIVED OVARIAN CANCER CELLS AND PATIENT–DERIVED XENOGRAFT MODELS [abstract]. In: Proceedings of the 11th Biennial Ovarian Cancer Research Symposium; Sep 12-13, 2016; Seattle, WA. Philadelphia (PA): AACR; Clin Cancer Res 2017;23(11 Suppl):Abstract nr AP19.

Abstract 4380: Dissecting the multicellular ecosystem of metastatic melanoma by single-cell RNA-sequencing

Tumors are heterogeneous ecosystems composed of genetically and epigenetically distinct cancer cell populations embedded in an intricate tumor microenvironment. The complexity and cell-to-cell interactions within this system pose a tremendous therapeutic challenge and opportunity. Due to technical constraints, current profiling technologies only provide average signals that do not reflect this intrinsic genetic and phenotypic variability.

Here, we applied single-cell RNA-sequencing to examine 4,645 single cells isolated from 19 freshly procured melanomas, profiling malignant, immune and stromal cells. Malignant cells within the same tumor displayed transcriptional heterogeneity associated with the cell cycle, stem-like cells, spatial context, and a drug treatment resistance program. All tumors harbored malignant cells from two distinct transcriptional cell states, such that treatment-sensitive “MITF-high” tumors also contained drug-resistant “AXL-high” tumor cells; similar heterogeneity was present in 18 established melanoma cell lines. The frequency of AXL-high cells increased in post-relapse resistant tumors following treatment with BRAF/MEK inhibitors. Using multiplexed, quantitative single-cell immunofluorescence analysis and FACS, we validated these observations in melanoma cell lines treated with BRAF±MEK inhibitors. Signatures of cell types identified from single-cell analysis revealed distinct patterns of the tumor microenvironment. We inferred cell-to-cell interactions between stromal, immune and malignant cells, and identified factors, including known secreted gene products (e.g. CXCL12) and several complement factors. We validated the association between cancer-associated fibroblast (CAF)-expressed complement factor 3 (C3) and TIL infiltration in an independent set of 308 melanomas. Finally, analysis of TILs revealed T-cell activation dependent and independent exhaustion programs that varied among patients dependent on their exposure to treatment with immune checkpoint-inhibitors. In addition to co-expression of several known co-inhibitory receptors, including PD1, CTLA-4, and TIM-3, we identified common markers associated with cytotoxicity-independent T-cell exhaustion across patients. To identify potential T-cell clones, we classified single T-cells by their isoforms of the V and J segments of the alpha and beta TCR chains, allowing us to identify expanded T-cell clones. We found that clonally expanded T-cells expressed a strong exhaustion program, while non-expanded T-cells lacked this phenotype.

This study represents the most comprehensive single-cell genomics analysis in humans to date and begins to unravel the cellular ecosystem of tumors. Single-cell genomics offer new insights with implications for both targeted and immune therapies by simultaneously profiling numerous aspects of a tumor with a single assay.

Citation Format: Benjamin Izar, Itay Tirsh, Sanjay Prakadan, Marc Wadsworth, Daniel Treacy, John Trombetta, Asaf Rotem, Christine Lian, George Murphy, Mohammad Fallahi-Sichani, Ken Dutton-Regester, Jia-Ren Lin, Judit Jane-Valbuena, Orit Rozenblatt-Rosen, Charles Yoon, Alex Shalek, Aviv Regev, Levi Garraway. Dissecting the multicellular ecosystem of metastatic melanoma by single-cell RNA-sequencing. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 4380.

Oct1 and OCA-B are selectively required for CD4 memory T cell function

Shakya et al. identify the transcription factor Oct1 and its cofactor OCA-B as central mediators for generating memory T cell responses in mice.

Epigenetic changes are crucial for the generation of immunological memory. Failure to generate or maintain these changes will result in poor memory responses. Similarly, augmenting or stabilizing the correct epigenetic states offers a potential method of enhancing memory. Yet the transcription factors that regulate these processes are poorly defined. We find that the transcription factor Oct1 and its cofactor OCA-B are selectively required for the in vivo generation of CD4⁺ memory T cells. More importantly, the memory cells that are formed do not respond properly to antigen reencounter. In vitro, both proteins are required to maintain a poised state at the Il2 target locus in resting but previously stimulated CD4⁺ T cells. OCA-B is also required for the robust reexpression of multiple other genes including Ifng. ChIPseq identifies ∼50 differentially expressed direct Oct1 and OCA-B targets. We identify an underlying mechanism involving OCA-B recruitment of the histone lysine demethylase Jmjd1a to targets such as Il2, Ifng, and Zbtb32. The findings pinpoint Oct1 and OCA-B as central mediators of CD4⁺ T cell memory.

Characterizing the T cell receptor clonotype repertoire of an atypical HLA class II-restricted CD8 T cell response in HIV-1 infection (VIR6P.1170)

CD8 T cells targeting peptides presented by human leukocyte antigen (HLA) class II are atypical. Little is known about how the CD8 T cell receptor (TCR) recognizes peptides on HLA class II. We analyzed the TCR repertoire of a class II-restricted CD8 T cells targeting a HIV peptide. We identified an atypical HIV-specific CD8 response to DV16 peptide on HLA-DRB1*11. We sequenced the TCR of the class II-tetramer sorted cells. To measure the binding kinetics of the TCR with the peptide-MHC we used surface plasmon resonance (SPR). Analysis of 68 sequences showed that TCR-beta repertoire has a single TRBV2*01 clonotype. We looked at 64 sequences of TCRalpha and found that the repertoire had two clonotypes: TRAV26-1*02 and TRAV6*02. SPR revealed that only the TRAV6 was able to bind to the peptide-MHC. We studied 27 sequences of the CD4 T cells targeting the same MHC class II-peptide. We observed a polyclonal response of 16 clonotypes. 13 clonotypes used the same TRBV2*01 gene but had different rearrangements. Interestingly, two of these sequences are exactly the same as the dominant clonotype from the CD8 response. We found that the TCR is shared between CD8 and CD4 T cells targeting the same class II HLA-peptide. These data suggest that atypical CD8 cells express two different TCR alpha, possibly due to inefficient allelic exclusion during development. The use of the same TRBV2*01 by different CD4 clonotypes may suggest an atypical docking of TCR in the binding with peptide-HLA.