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Billiet et al. identify the post-thymic progeny of the agonist-selected CD10(+) PD-1(+) precursors in humans based on shared characteristics of the T cell receptor repertoire and the transcriptome. This lineage represents a well-defined but heterogeneous, unconventional TCR alpha beta(+) lineage mostly confined within the CD8(+) Helios(+) T cells. In the human thymus, a CD10(+) PD-1(+) TCR alpha beta(+) differentiation pathway diverges from the conventional single positive T cell lineages at the early double-positive stage. Here, we identify the progeny of this unconventional lineage in antigen-inexperienced blood. These unconventional T cells (UTCs) in thymus and blood share a transcriptomic profile, characterized by hallmark transcription factors (i.e., ZNF683 and IKZF2), and a polyclonal TCR repertoire with autoreactive features, exhibiting a bias toward early TCR alpha chain rearrangements. Single-cell RNA sequencing confirms a common developmental trajectory between the thymic and blood UTCs and clearly delineates this unconventional lineage in blood. Besides MME+ recent thymic emigrants, effector-like clusters are identified in this heterogeneous lineage. Expression of Helios and KIR and a decreased CD8 beta expression are characteristics of this lineage. This UTC lineage could be identified in adult blood and intestinal tissues. In summary, our data provide a comprehensive characterization of the polyclonal unconventional lineage in antigen-inexperienced blood and identify the adult progeny.

In the human thymus, a CD10+ PD-1+ TCRαβ+ differentiation pathway diverges from the conventional single positive T cell lineages at the early double positive stage. These cells are phenotypically and functionally similar to murine unconventional intraepithelial lymphocyte (uIEL) precursors. Here, the progeny of the human uIEL lineage was identified in antigen-inexperienced blood. The uIELs in thymus and peripheral blood share a transcriptomic profile, characterized by hallmark transcription factors (i.e. ZNF683 and IKZF2), and polyclonal TCR repertoire with autoreactive features, exhibiting a bias towards early TCR alpha chain rearrangements. Single-cell RNA sequencing confirmed a common developmental trajectory between the thymic and peripheral uIELs, and clearly delineated this unconventional lineage in peripheral blood. This population is phenotypically defined as CD3+ TCRαβ+ CD4- CCR7- CD26-. It contains CD10+ recent thymic emigrants, Helios+ KIR+ CD8+ Tregs and CD8αα+ T cells. Thus, the uIEL lineage represents a well-defined but heterogeneous, unconventional TCRαβ+ lineage mostly confined in human within the CD8 single positive T cells.SummaryBilliet et al. identify the postthymic progeny of the intraepithelial lymphocyte precursors in human based on shared characteristics of the T cell receptor repertoire and the transcriptome. This lineage represents a well-defined but heterogeneous, unconventional TCRαβ+ lineage mostly confined within the CD8 single positive T cells.

T cell receptor (TCR)-redirected T cells target intracellular antigens such as Wilms’ tumor 1 (WT1), a tumor-associated antigen overexpressed in several malignancies, including acute myeloid leukemia (AML). For both chimeric antigen receptor (CAR)- and TCR-redirected T cells, several clinical studies indicate that T cell subsets with a less-differentiated phenotype (e.g. stem cell memory T cells, TSCM) survive longer and mediate superior anti-tumor effects in vivo as opposed to more terminally differentiated T cells. Cytokines added during in vitro and ex vivo culture of T cells play an important role in driving the phenotype of T cells for adoptive transfer. Using the OP9-DL1 co-culture system, we have shown previously that we are able to generate in vitro, starting from clinically relevant stem cell sources, T cells with a single tumor antigen (TA)-specific TCR. This method circumvents possible TCR chain mispairing and unwanted toxicities that might occur when introducing a TA-specific TCR in peripheral blood lymphocytes. We now show that we are able to optimize our in vitro culture protocol, by adding IL-21 during maturation, resulting in generation of TA-specific T cells with a less-differentiated phenotype and enhanced in vitro anti-tumor effects. We believe the favorable TSCM-like phenotype of these in vitro generated T cells preludes superior in vivo persistence and anti-tumor efficacy. Therefore, these TA-specific T cells could be of use as a valuable new form of patient-tailored T cell immunotherapy for malignancies for which finding a suitable CAR-T target antigen is challenging, such as AML.

Messenger RNA (mRNA) has become a promising tool in therapeutic cancer vaccine strategies. Owing to its flexible design and rapid production, mRNA is an attractive antigen delivery format for cancer vaccines targeting mutated peptides expressed in a tumor-the so-called neoantigens. These neoantigens are rarely shared between patients, and inclusion of these antigens in a vaccine requires the production of individual batches of patient-tailored mRNA. The authors have developed MIDRIXNEO, a personalized mRNA-loaded dendritic cell vaccine targeting tumor neoantigens, which is currently being evaluated in a phase 1 clinical study in lung cancer patients. To facilitate this study, the authors set up a Good Manufacturing Practice (GMP)-compliant production process for the manufacture of small batches of personalized neoantigenencoding mRNA. In this article, the authors describe the complete mRNA production process and the extensive quality assessment to which the mRNA is subjected. Validation runs have shown that the process delivers mRNA of reproducible, high quality. This process is now successfully applied for the production of neoantigen-encoding mRNA for the clinical evaluation of MIDRIXNEO. To the authors' knowledge, this is the first time that a GMP-based production process of patient-tailored neoantigen mRNA has been described. (c) 2021 International Society for Cell & Gene Therapy. Published by Elsevier Inc. This is an open access article

Human thymic CD8αα+ CD10+ PD-1+ αβ T cells selected through early agonist selection have been proposed as the putative thymic precursors of the human CD8αα+ intestinal intraepithelial lymphocytes (IELs). However, the progeny of these thymic precursor cells in human blood or tissues has not yet been characterized. Here, we studied the phenotypical and transcriptional differentiation of the thymic IEL precursor (IELp) lineage upon in vitro exposure to cytokines prominent in the peripheral tissues such as interleukin-15 (IL-15) and the inflammatory cytokines interleukin-12 (IL-12) and interleukin-18 (IL-18). We showed that only the CD1a− fraction of the CD10+ PD-1+ IELp population was able to proliferate with IL-15, suggesting that this subset had acquired functionality. These cells downregulated PD-1 expression and completely lost CD10 expression, whereas other surface markers such as CD95 and CXCR3 remained highly expressed. RNA-seq analysis of the IL-15-cultured cells clearly showed induction of innate-like and effector genes. Induction of the cytotoxic machinery by the CD10+ PD-1+ population was acquired in the presence of IL-15 and was further augmented by inflammatory cytokines. Our data suggest that only the CD1a− CD10+ PD-1+ population exits the thymus and survives in the periphery. Furthermore, PD-1 and CD10 expression is not an intrinsic property of this lineage, but rather characterizes a transient stage in differentiation. CD95 and CXCR3 expression combined with the absence of CD28, CCR7, and CD6 expression might be more powerful markers to define this lineage in the periphery.

Recent approval of chimeric antigen receptor (CAR) T cell therapy by the European Medicines Agency (EMA)/Federal and Drug Administration (FDA) and the remarkable results of CAR T clinical trials illustrate the curative potential of this therapy. While CARs against a multitude of different antigens are being developed and tested (pre)clinically, there is still a need for optimization. The use of single-chain variable fragments (scFvs) as targeting moieties hampers the quick generation of functional CARs and could potentially limit the efficacy. Instead, nanobodies may largely circumvent these difficulties. We used an available nanobody library generated after immunization of llamas against Cluster of Differentiation (CD) 20 through DNA vaccination or against the ectodomain of CD33 using soluble protein. The nanobody specific sequences were amplified by PCR and cloned by Gibson Assembly into a retroviral vector containing two different second-generation CAR constructs. After transduction in T cells, we observed high cell membrane nanoCAR expression in all cases. Following stimulation of nanoCAR-expressing T cells with antigen-positive cell lines, robust T cell activation, cytokine production and tumor cell lysis both in vitro and in vivo was observed. The use of nanobody technology in combination with PCR and Gibson Assembly allows for the rapid and effective generation of compact CARs.

Human thymic CD8αα+ CD10+ PD-1+ αβ T cells selected through early agonist selection have been proposed as the putative thymic precursors of the human CD8αα+ intestinal intraepithelial lymphocytes (IELs). However, the progeny of these thymic precursor cells in human blood or tissues has not yet been characterized. Here, we studied the phenotypical and transcriptional differentiation of the thymic IEL precursor (IELp) lineage upon in vitro exposure to cytokines prominent in the peripheral tissues such as interleukin-15 (IL-15) and the inflammatory cytokines interleukin-12 (IL-12) and interleukin-18 (IL-18). We showed that only the CD1a− fraction of the CD10+ PD-1+ IELp population was able to proliferate with IL-15, suggesting that this subset had acquired functionality. These cells downregulated PD-1 expression and completely lost CD10 expression, whereas other surface markers such as CD95 and CXCR3 remained highly expressed. RNA-seq analysis of the IL-15-cultured cells clearly showed induction of innate-like and effector genes. Induction of the cytotoxic machinery by the CD10+ PD-1+ population was acquired in the presence of IL-15 and was further augmented by inflammatory cytokines. Our data suggest that only the CD1a− CD10+ PD-1+ population exits the thymus and survives in the periphery. Furthermore, PD-1 and CD10 expression is not an intrinsic property of this lineage, but rather characterizes a transient stage in differentiation. CD95 and CXCR3 expression combined with the absence of CD28, CCR7, and CD6 expression might be more powerful markers to define this lineage in the periphery.

Chimeric antigen receptor (CAR) T-cells have shown great promise in the treatment of B-cell malignancies. For acute myeloid leukemia (AML), however, the optimal target surface antigen has yet to be discovered. Alternatively, T-cell receptor (TCR)-redirected T-cells target intracellular antigens, marking a broader territory of available target antigens. Currently, adoptive TCR T-cell therapy uses peripheral blood lymphocytes for the introduction of a transgenic TCR. However, this can cause graft-versus-host disease, due to mispairing of introduced and endogenous TCR chains. Therefore, we started from hematopoietic stem and progenitor cells (HSPC), that do not express a TCR yet, isolated from healthy donors, patients in remission after chemotherapy and AML patients at diagnosis. Using the OP9-DL1 in vitro co-culture system and agonist selection, TCR-transduced HSPC develop into mature tumor antigen-specific T-cells with only one TCR. We show here that this approach is feasible with adult HSPC from clinically relevant sources, albeit with slower maturation and lower cell yield compared to cord blood HSPC. Moreover, cryopreservation of HSPC does not have an effect on cell numbers or functionality of the generated T-cells. In conclusion, we show here that it is feasible to generate TA-specific T-cells from HSPC from adult healthy donors and patients and we believe these T-cells could be of use as a very valuable form of patient-tailored T-cell immunotherapy.

We formerly demonstrated that vaccination with Wilms tumor 1 (WT1)-loaded autologous monocyte-derived dendritic cells (mo-DCs) can be a well-tolerated effective treatment in acute myeloid leukemia (AML) patients. Here, we investigated whether we could introduce the receptor for hyaluronic acid-mediated motility (RHAMM/HMMR/CD168), another clinically relevant tumor-associated antigen, into these mo-DCs through mRNA electroporation and elicit RHAMM-specific immune responses. While RHAMM mRNA electroporation significantly increased RHAMM protein expression by mo-DCs, our data indicate that classical mo-DCs already express and present RHAMM at sufficient levels to activate RHAMM-specific T cells, regardless of electroporation. Moreover, we found that RHAMM-specific T cells are present at vaccination sites in AML patients. Our findings implicate that we and others who are using classical mo-DCs for cancer immunotherapy are already vaccinating against RHAMM.

Recent clinical trials have shown that adoptive chimeric antigen receptor (CAR) T cell therapy is a very potent and possibly curative option in the treatment of B cell leukemias and lymphomas. However, targeting a single antigen may not be sufficient, and relapse due to the emergence of antigen negative leukemic cells may occur. A potential strategy to counter the outgrowth of antigen escape variants is to broaden the specificity of the CAR by incorporation of multiple antigen recognition domains in tandem. As a proof of concept, we here describe a bispecific CAR in which the single chain variable fragment (scFv) is replaced by a tandem of two single-antibody domains or nanobodies (nanoCAR). High membrane nanoCAR expression levels are observed in retrovirally transduced T cells. NanoCARs specific for CD20 and HER2 induce T cell activation, cytokine production and tumor lysis upon incubation with transgenic Jurkat cells expressing either antigen or both antigens simultaneously. The use of nanobody technology allows for the production of compact CARs with dual specificity and predefined affinity.

The thymus plays a central role in self-tolerance, partly by eliminating precursors with a T cell receptor (TCR) that binds strongly to self-antigens. However, the generation of self-agonist-selected lineages also relies on strong TCR signaling. How thymocytes discriminate between these opposite outcomes remains elusive. Here, we identified a human agonist-selected PD-1+ CD8αα+ subset of mature CD8αβ+ T cells that displays an effector phenotype associated with agonist selection. TCR stimulation of immature post-β-selection thymocyte blasts specifically gives rise to this innate subset and fixes early T cell receptor alpha variable (TRAV) and T cell receptor alpha joining (TRAJ) rearrangements in the TCR repertoire. These findings suggest that the checkpoint for agonist selection precedes conventional selection in the human thymus.

Recent clinical studies indicate that adoptive T-cell therapy and especially chimeric antigen receptor (CAR) T-cell therapy is a very potent and potentially curative treatment for B-lineage hematologic malignancies. Currently, autologous peripheral blood T cells are used for adoptive T-cell therapy. Adoptive T cells derived from healthy allogeneic donors may have several advantages; however, the expected occurrence of graft versus host disease (GvHD) as a consequence of the diverse allogeneic T-cell receptor (TCR) repertoire expressed by these cells compromises this approach. Here, we generated T cells from cord blood hematopoietic progenitor cells (HPCs) that were transduced to express an antigen receptor (AR): either a CAR or a TCR with or without built-in CD28 co-stimulatory domains. These AR-transgenic HPCs were culture-expanded on an OP9-DL1 feeder layer and subsequently differentiated to CD5(+)CD7(+) T-lineage precursors, to CD4(+) CD8(+) double positive cells and finally to mature AR(+) T cells. The AR(+) T cells were largely naive CD45RA(+)CD62L(+) T cells. These T cells had mostly germline TCR alpha and TCR beta loci and therefore lacked surface-expressed CD3/TCR alpha beta complexes. The CD3(-) AR-transgenic cells were mono-specific, functional T cells as they displayed specific cytotoxic activity. Cytokine production, including IL-2, was prominent in those cells bearing ARs with built-in CD28 domains. Data sustain the concept that cord blood HPC derived, in vitro generated allogeneic CD3(-) AR(+) T cells can be used to more effectively eliminate malignant cells, while at the same time limiting the occurrence of GvHD.

While in vitro models exist to study human T cell development, they still lack the precise environmental stimuli, such as the exact combination and levels of cytokines and chemokines, that are present in vivo. Moreover, studying the homing of hematopoietic stem (HSC) and progenitor (HPC) cells to the thymus can only be done using in vivo models. Although species-specific differences exist, "humanized" models are generated to circumvent these issues. In this chapter, we focus on the humanized mouse models that can be used to study early T cell development. Models that study solely mature T cells, such as the SCID-PBL (Tary-Lehmann et al., Immunol Today 16:529-533) are therefore not discussed here, but have recently been reviewed (Shultz et al., Nat Rev Immunol 12:786-798).

While in vitro models exist to study human T cell development, they still lack the precise environmental stimuli, such as the exact combination and levels of cytokines and chemokines, that are present in vivo. Moreover, studying the homing of hematopoietic stem (HSC) and progenitor (HPC) cells to the thymus can only be done using in vivo models. Although species-specific differences exist, "humanized" models are generated to circumvent these issues. In this chapter, we focus on the humanized mouse models that can be used to study early T cell development. Models that study solely mature T cells, such as the SCID-PBL (Tary-Lehmann et al., Immunol Today 16:529-533) are therefore not discussed here, but have recently been reviewed (Shultz et al., Nat Rev Immunol 12:786-798).

Peripheral blood T cells transduced with a tumor-specific T-cell receptor (TCR) face problems of auto-reactivity and lack of efficacy caused by cross-pairing of exogenous and endogenous TCR chains, as well as short term in vivo survival due to activation and growth factor-induced differentiation. We here studied an alternative strategy for the efficient generation of naive CD8(+) T cells with a single TCR. TCR-transduced human postnatal thymus-derived and adult mobilized blood-derived hematopoietic progenitor cells (HPCs) were differentiated to CD4(+)CD8(+) double-positive T cells using OP9-Delta-like 1 (OP9-DL1) cultures. Addition of the agonist peptide induced double positive cells to cross-present the peptide, leading, in the absence of co-stimulation, to cell cycle arrest and differentiation into mature CD8(+) T cells. Comprehensive phenotypic, molecular and functional analysis revealed the generation of naive and resting CD8(+) T cells through a process similar to thymic positive selection. These mature T cells show a near complete inhibition of endogenous TCRA and TCRB rearrangements and express high levels of the introduced multimer-reactive TCR. Upon activation, specific cytokine production and efficient killing of tumor cells were induced. Using this strategy, large numbers of high-avidity tumor-specific naive T cells can be generated from readily available HPCs without TCR chain cross-pairing.

Autologous T cells transduced to express chimeric antigen receptors (CAR) directed against CD19, a B cell antigen, are reported to induce complete remission in patients with leukemia or lymphoma of the B cell lineage. Although potentially very effective, this treatment strategy has major drawbacks. First, CAR therapy is based on autologous T cells and therefore dependent on the nature and quality of T cells present in the peripheral blood of these patients at the time of treatment. Poor quality of the T cells may cause treatment failure in some patients. In addition, therapy based on autologous cells is tailor-made i.e. CAR+ T cells have to be generated de novo for every patient. Finally, autologous cell therapy requires different, more complicated logistics than conventional therapy. We therefore investigate whether a general purpose, allogeneic CAR therapy based on HLA-matched cord blood obtained from cord blood banks can be devised. Here, we investigated whether functional CAR+ T cells can be generated in vitro that do not express an endogenous T cell receptor to avoid alloreactivity causing graft versus host reactions. We compared carcino-embryonic antigen (CEA)- specific CARs of the first generation (intracellular CD3ζ signaling chain), of the 2nd generation (intracellular CD3ζ and CD28 signaling chain) and of the 3rd generation (intracellular CD3ζ, CD28 and OX40 signaling chain). CD34+ progenitor cells were isolated from human cord blood or postnatal thymus and subsequently transduced with one of the three green fluorescent protein (GFP)-encoding CAR constructs. Transduced cells were subsequently co-cultured on OP9DL1 in the presence of stem cell factor, Flt3-ligand and interleukin-7. Unlike T cell receptor transduced precursors (1), expansion was not enhanced by transduction of the chimeric receptor. Expansion was highest with first generation CARs whereas second and third generation CARs displayed only restricted expansion. Similar to T cell receptor transduced progenitors, CAR transduced cells show an accelerated differentiation during co-culture compared to the non-transduced cells: first committed CD5+ CD7+ T precursors appear, then CD4+ CD8+ double positive cells (DP) and finally CD1- CD27- single positive or double negative (DN) mature T cells. In cultures transduced with 2nd and 3rd generation CARs, few transduced cells passed through the proliferative DP pathway but rather differentiated to mature CD1- CD27- non-proliferative DN cells without passing through the DP stage. This phenomenon is responsible for the limited expansion seen with precursors transgenic for 2nd or 3rd generation CARs. However, in all cultures CAR+ DP cells were generated and, as shown for TCR transgenic cells (1), we were able to induce CEA specific maturation after co-culturing these DP cells with a cell line expressing CEA or by antibody-induced cross-linking of the CAR, giving rise to CD1- CD27+ matured cells. The observations described above are compatible with data obtained in mice showing that strong T cell receptor (TCR) activation during thymocyte differentiation inhibits the generation of DP cells and induces maturation to DN cells. Both the spontaneously and induced mature CAR+ cells were TCR and CD3 negative, suggesting that the expression of a CAR in early T cell precursors shuts down rearrangements of the endogenous TCR chains. Moreover, these cells lack NK marker expression (CD56, NKG2D) and show expression of T cell markers (CD5, CD7, CD2), confirming their T cell nature. In conclusion, the CAR+ CD3/TCR negative cells are T cells as these are derived from T cell precursors (CD5+, DP cells) and express various membrane and nuclear T cell markers. Mature CD1- CD27- CAR+ cells can be expanded to large cell numbers using T cell expansion protocols. They displayed cytokine production specific for CEA+ tumor lines as well as specific cytotoxicity. Moreover, the 2nd and 3rd generation CAR expressing cells showed increased specific cytokine production when compared to the first generation CAR expressing cells. These results show that the cord blood-derived CAR+ cells have potent functional activity similar to peripheral blood derived CAR+ T cells. We believe that these in vitro generated CAR+ cells developed from HLA-matched cord blood progenitors may be ideal as an adjunct to cord blood transplantation. (1) Snauwaert et al, Leukemia, 2014 Disclosures No relevant conflicts of interest to declare.

Introduction: Chemotherapy leads to cure of acute myeloid leukemia (AML) in less than half of the patients. Stem cell transplantation can be used as an immunotherapeutic treatment to cure the patient, but carries a high risk of toxicity and mortality. Moreover, not all patients have a suitable donor. We have developed a novel immunotherapeutic treatment, in which we generate in vitro, starting from hematopoietic precursor cells, T-cells that recognize WT1, a tumor antigen that is overexpressed on 70% of the AMLs. Aims: In this study we have evaluated the functionality and specificity of the generated WT1-directed T-cells both in vitro and in vivo. The ultimate goal is to use these cells in patients, as this form of immunotherapy is promising and could be an option for cure in patients who are not eligible for stem cell transplantation. In contrast to the more widely used immunotherapy using TCR-transduced peripheral T-cells, our therapy is expected to be more effective and carry less risk of autoreactivity. Methods: CD34+ cells isolated from cord blood and mobilized peripheral blood mononuclear cells were cultured on OP9-DL1 in the presence of the cytokines IL-7, Flt3-L and SCF, for 2 weeks, until T-cell commitment. Subsequently, they were transduced with a WT1-TCR (H. Stauss) or a CMV-TCR (M. Heemskerk), and again co-cultured until CD4+CD8+ double positive cells were abundantly present (generally after another 2-3 weeks). At that point, the agonist peptide WT1 or CMV resp was added to the culture together with IL-7, and 5 days later cells were harvested and expanded (polyclonally or using agonist peptide), in the presence of IL-2, or IL-7+IL-15. T-cells were evaluated using a 51Chromium release assay, for cytotoxicity against WT1 and HLA-A2 positive and negative targets. Also, upon activation, production of IFN-g was evaluated using ELISA. Immunodeficient 6-8 weeks old NSG mice were irradiated (200 cGy), and 24 hours later injected intravenously with either a luciferase-positive, WT1, HLA-A2 transduced K562 cell line (R. Stripecke), or luciferase-transduced, HLA-A2+, WT1+ primary AML cells (expanded on MS-5 in the presence of cytokines) in the current experiments, and 24 hours later, with 5x106 or 107WT1-TCR T-cells or CMV-TCR T-cells (negative control). Mice were evaluated using the IVIS bioluminescence assay. Results: We observed that a nice mix of WT1-TCR CD8+ and CD4+ T-cells (50%/50%) was generated with cells expanded after harvest from the coculture using the combination of the agonist peptide, IL-7 and IL-15. Using 51Cr release assay and ELISA, we could show that upon activation, the T-cells showed specific cytokine production and efficient killing of tumor cells. We observed that the luciferase+, WT1, HLA-A2 transduced K562 cell line homed to ovaria and brain (female mice) or liver, testes and brain (male mice) when injected intravenously, and these are largely sanctuary sites, not reached by the T-cells, therefore resulting in low efficiency. When this cell line was injected subcutaneously in the hind flank, mice showed significant swelling of the resp limb and needed to be euthanized for ethical reasons before full evaluation was possible. Currently, experiments are ongoing evaluating the efficacy of the WT1 T-cells against luciferase transduced primary AML cells (after long term expansion culture on MS-5), as these cells are expected to home to the bone marrow and blood of the mice, and therefore reflect more the physiological situation, and can be more easily reached by the T-cells. Results of these experiments will be presented at ASH. Summary/Conclusion: We have shown that, using the OP9-DL1 model, we were able to generate large numbers of high-avidity tumor-specific naïve and resting T-cells, after a process similar to thymic positive selection. After expansion (polyclonal or antigen specific -in the presence of the agonist peptide-) and activation, these cells show specificity and functionality in vitro and are currently evaluated in an in vivo immunodeficient mouse model. Disclosures No relevant conflicts of interest to declare.