Modular pooled discovery of synthetic knockin sequences to program durable cell therapies.

Chronic stimulation can cause T cell dysfunction and limit the efficacy of cellular immunotherapies. Improved methods are required to compare large numbers of synthetic knockin (KI) sequences to reprogram cell functions. Here, we developed modular pooled KI screening (ModPoKI), an adaptable platform for modular construction of DNA KI libraries using barcoded multicistronic adaptors. We built two ModPoKI libraries of 100 transcription factors (TFs) and 129 natural and synthetic surface receptors (SRs). Over 30 ModPoKI screens across human TCR- and CAR-T cells in diverse conditions identified a transcription factor AP4 (TFAP4) construct that enhanced fitness of chronically stimulated CAR-T cells and anti-cancer function in vitro and in vivo. ModPoKI's modularity allowed us to generate an ∼10,000-member library of TF combinations. Non-viral KI of a combined BATF-TFAP4 polycistronic construct enhanced fitness. Overexpressed BATF and TFAP4 co-occupy and regulate key gene targets to reprogram T cell function. ModPoKI facilitates the discovery of complex gene constructs to program cellular functions.

Epigenetic regulation of T cell exhaustion.

Chronic antigen stimulation during viral infections and cancer can lead to T cell exhaustion, which is characterized by reduced effector function and proliferation, and the expression of inhibitory immune checkpoint receptors. Recent studies have demonstrated that T cell exhaustion results in wholescale epigenetic remodeling that confers phenotypic stability to these cells and prevents T cell reinvigoration by checkpoint blockade. Here, we review foundational technologies to profile the epigenome at multiple scales, including mapping the locations of transcription factors and histone modifications, DNA methylation and three-dimensional genome conformation. We discuss how these technologies have elucidated the development and epigenetic regulation of exhausted T cells and functional implications across viral infection, cancer, autoimmunity and engineered T cell therapies. Finally, we cover emerging multi-omic and genome engineering technologies, current and upcoming opportunities to apply these to T cell exhaustion, and therapeutic opportunities for T cell engineering in the clinic.

Divergent clonal differentiation trajectories of T cell exhaustion.

Chronic antigen exposure during viral infection or cancer promotes an exhausted T cell (Tex) state with reduced effector function. However, whether all antigen-specific T cell clones follow the same Tex differentiation trajectory remains unclear. Here, we generate a single-cell multiomic atlas of T cell exhaustion in murine chronic viral infection that redefines Tex phenotypic diversity, including two late-stage Tex subsets with either a terminal exhaustion (Texterm) or a killer cell lectin-like receptor-expressing cytotoxic (TexKLR) phenotype. We use paired single-cell RNA and T cell receptor sequencing to uncover clonal differentiation trajectories of Texterm-biased, TexKLR-biased or divergent clones that acquire both phenotypes. We show that high T cell receptor signaling avidity correlates with Texterm, whereas low avidity correlates with effector-like TexKLR fate. Finally, we identify similar clonal differentiation trajectories in human tumor-infiltrating lymphocytes. These findings reveal clonal heterogeneity in the T cell response to chronic antigen that influences Tex fates and persistence.

Distinct epigenomic landscapes underlie tissue-specific memory T cell differentiation.

The memory CD8+ T cell pool contains phenotypically and transcriptionally heterogeneous subsets with specialized functions and recirculation patterns. Here, we examined the epigenetic landscape of CD8+ T cells isolated from seven non-lymphoid organs across four distinct infection models, alongside their circulating T cell counterparts. Using single-cell transposase-accessible chromatin sequencing (scATAC-seq), we found that tissue-resident memory T (TRM) cells and circulating memory T (TCIRC) cells develop along distinct epigenetic trajectories. We identified organ-specific transcriptional regulators of TRM cell development, including FOSB, FOS, FOSL1, and BACH2, and defined an epigenetic signature common to TRM cells across organs. Finally, we found that although terminal TEX cells share accessible regulatory elements with TRM cells, they are defined by TEX-specific epigenetic features absent from TRM cells. Together, this comprehensive data resource shows that TRM cell development is accompanied by dynamic transcriptome alterations and chromatin accessibility changes that direct tissue-adapted and functionally distinct T cell states.

BCL6-dependent TCF-1+ progenitor cells maintain effector and helper CD4+ T cell responses to persistent antigen.

Soon after activation, CD4+ T cells are segregated into BCL6+ follicular helper (Tfh) and BCL6- effector (Teff) T cells. Here, we explored how these subsets are maintained during chronic antigen stimulation using the mouse chronic LCMV infection model. Using single cell-transcriptomic and epigenomic analyses, we identified a population of PD-1+ TCF-1+ CD4+ T cells with memory-like features. TCR clonal tracing and adoptive transfer experiments demonstrated that these cells have self-renewal capacity and continue to give rise to both Teff and Tfh cells, thus functioning as progenitor cells. Conditional deletion experiments showed Bcl6-dependent development of these progenitors, which were essential for sustaining antigen-specific CD4+ T cell responses to chronic infection. An analogous CD4+ T cell population developed in draining lymph nodes in response to tumors. Our study reveals the heterogeneity and plasticity of CD4+ T cells during persistent antigen exposure and highlights their population dynamics through a stable, bipotent intermediate state.

The epigenetic state of IL-4-polarized macrophages enables inflammatory cistromic expansion and extended synergistic response to TLR ligands.

Prior exposure to microenvironmental signals could fundamentally change the response of macrophages to subsequent stimuli. It is believed that T helper-2 (Th2)-cell-type cytokine interleukin-4 (IL-4) and Toll-like receptor (TLR) ligand-activated transcriptional programs mutually antagonize each other, and no remarkable convergence has been identified between them. In contrast, here, we show that IL-4-polarized macrophages established a hyperinflammatory gene expression program upon lipopolysaccharide (LPS) exposure. This phenomenon, which we termed extended synergy, was supported by IL-4-directed epigenomic remodeling, LPS-activated NF-κB-p65 cistrome expansion, and increased enhancer activity. The EGR2 transcription factor contributed to the extended synergy in a macrophage-subtype-specific manner. Consequently, the previously alternatively polarized macrophages produced increased amounts of immune-modulatory factors both in vitro and in vivo in a murine Th2 cell-type airway inflammation model upon LPS exposure. Our findings establish that IL-4-induced epigenetic reprogramming is responsible for the development of inflammatory hyperresponsiveness to TLR activation and contributes to lung pathologies.

Macrophage inflammatory and regenerative response periodicity is programmed by cell cycle and chromatin state.

Cell cycle (CC) facilitates cell division via robust, cyclical gene expression. Protective immunity requires the expansion of pathogen-responsive cell types, but whether CC confers unique gene expression programs that direct the subsequent immunological response remains unclear. Here, we demonstrate that single macrophages (MFs) adopt different plasticity states in CC, which leads to heterogeneous cytokine-induced polarization, priming, and repolarization programs. Specifically, MF plasticity to interferon gamma (IFNG) is substantially reduced during S-G2/M, whereas interleukin 4 (IL-4) induces S-G2/M-biased gene expression, mediated by CC-biased enhancers. Additionally, IL-4 polarization shifts the CC-phase distribution of MFs toward the G2/M phase, providing a subpopulation-specific mechanism for IL-4-induced, dampened IFNG responsiveness. Finally, we demonstrate CC-dependent MF responses in murine and human disease settings in vivo, including Th2-driven airway inflammation and pulmonary fibrosis, where MFs express an S-G2/M-biased tissue remodeling gene program. Therefore, MF inflammatory and regenerative responses are gated by CC in a cyclical, phase-dependent manner.

FOXO1 is a master regulator of memory programming in CAR T cells.

A major limitation of chimeric antigen receptor (CAR) T cell therapies is the poor persistence of these cells in vivo1. The expression of memory-associated genes in CAR T cells is linked to their long-term persistence in patients and clinical efficacy2-6, suggesting that memory programs may underpin durable CAR T cell function. Here we show that the transcription factor FOXO1 is responsible for promoting memory and restraining exhaustion in human CAR T cells. Pharmacological inhibition or gene editing of endogenous FOXO1 diminished the expression of memory-associated genes, promoted an exhaustion-like phenotype and impaired the antitumour activity of CAR T cells. Overexpression of FOXO1 induced a gene-expression program consistent with T cell memory and increased chromatin accessibility at FOXO1-binding motifs. CAR T cells that overexpressed FOXO1 retained their function, memory potential and metabolic fitness in settings of chronic stimulation, and exhibited enhanced persistence and tumour control in vivo. By contrast, overexpression of TCF1 (encoded by TCF7) did not enforce canonical memory programs or enhance the potency of CAR T cells. Notably, FOXO1 activity correlated with positive clinical outcomes of patients treated with CAR T cells or tumour-infiltrating lymphocytes, underscoring the clinical relevance of FOXO1 in cancer immunotherapy. Our results show that overexpressing FOXO1 can increase the antitumour activity of human CAR T cells, and highlight memory reprogramming as a broadly applicable approach for optimizing therapeutic T cell states.

Latent human herpesvirus 6 is reactivated in CAR T cells.

Cell therapies have yielded durable clinical benefits for patients with cancer, but the risks associated with the development of therapies from manipulated human cells are understudied. For example, we lack a comprehensive understanding of the mechanisms of toxicities observed in patients receiving T cell therapies, including recent reports of encephalitis caused by reactivation of human herpesvirus 6 (HHV-6)1. Here, through petabase-scale viral genomics mining, we examine the landscape of human latent viral reactivation and demonstrate that HHV-6B can become reactivated in cultures of human CD4+ T cells. Using single-cell sequencing, we identify a rare population of HHV-6 'super-expressors' (about 1 in 300-10,000 cells) that possess high viral transcriptional activity, among research-grade allogeneic chimeric antigen receptor (CAR) T cells. By analysing single-cell sequencing data from patients receiving cell therapy products that are approved by the US Food and Drug Administration2 or are in clinical studies3-5, we identify the presence of HHV-6-super-expressor CAR T cells in patients in vivo. Together, the findings of our study demonstrate the utility of comprehensive genomics analyses in implicating cell therapy products as a potential source contributing to the lytic HHV-6 infection that has been reported in clinical trials1,6-8 and may influence the design and production of autologous and allogeneic cell therapies.

Aberrant activation of TCL1A promotes stem cell expansion in clonal haematopoiesis.

Mutations in a diverse set of driver genes increase the fitness of haematopoietic stem cells (HSCs), leading to clonal haematopoiesis1. These lesions are precursors for blood cancers2-6, but the basis of their fitness advantage remains largely unknown, partly owing to a paucity of large cohorts in which the clonal expansion rate has been assessed by longitudinal sampling. Here, to circumvent this limitation, we developed a method to infer the expansion rate from data from a single time point. We applied this method to 5,071 people with clonal haematopoiesis. A genome-wide association study revealed that a common inherited polymorphism in the TCL1A promoter was associated with a slower expansion rate in clonal haematopoiesis overall, but the effect varied by driver gene. Those carrying this protective allele exhibited markedly reduced growth rates or prevalence of clones with driver mutations in TET2, ASXL1, SF3B1 and SRSF2, but this effect was not seen in clones with driver mutations in DNMT3A. TCL1A was not expressed in normal or DNMT3A-mutated HSCs, but the introduction of mutations in TET2 or ASXL1 led to the expression of TCL1A protein and the expansion of HSCs in vitro. The protective allele restricted TCL1A expression and expansion of mutant HSCs, as did experimental knockdown of TCL1A expression. Forced expression of TCL1A promoted the expansion of human HSCs in vitro and mouse HSCs in vivo. Our results indicate that the fitness advantage of several commonly mutated driver genes in clonal haematopoiesis may be mediated by TCL1A activation.

Charting a shared epigenetic pathway to CD8+ T cell dysfunction in infection and cancer.

Pritykin et al. (2021) establish a comprehensive chromatin atlas of CD8+ T cell dysfunction in chronic viral infection and cancer via analysis of bulk and single-cell ATAC-seq datasets across immune challenges. These results unify the classification scheme and molecular programs driving CD8+ T cell dysfunction across disease settings and will facilitate basic discovery and translational efforts in T cell immunity.

RASA2 ablation in T cells boosts antigen sensitivity and long-term function.

The efficacy of adoptive T cell therapies for cancer treatment can be limited by suppressive signals from both extrinsic factors and intrinsic inhibitory checkpoints1,2. Targeted gene editing has the potential to overcome these limitations and enhance T cell therapeutic function3-10. Here we performed multiple genome-wide CRISPR knock-out screens under different immunosuppressive conditions to identify genes that can be targeted to prevent T cell dysfunction. These screens converged on RASA2, a RAS GTPase-activating protein (RasGAP) that we identify as a signalling checkpoint in human T cells, which is downregulated upon acute T cell receptor stimulation and can increase gradually with chronic antigen exposure. RASA2 ablation enhanced MAPK signalling and chimeric antigen receptor (CAR) T cell cytolytic activity in response to target antigen. Repeated tumour antigen stimulations in vitro revealed that RASA2-deficient T cells show increased activation, cytokine production and metabolic activity compared with control cells, and show a marked advantage in persistent cancer cell killing. RASA2-knockout CAR T cells had a competitive fitness advantage over control cells in the bone marrow in a mouse model of leukaemia. Ablation of RASA2 in multiple preclinical models of T cell receptor and CAR T cell therapies prolonged survival in mice xenografted with either liquid or solid tumours. Together, our findings highlight RASA2 as a promising target to enhance both persistence and effector function in T cell therapies for cancer treatment.

The transcription factor EGR2 is the molecular linchpin connecting STAT6 activation to the late, stable epigenomic program of alternative macrophage polarization.

Macrophages polarize into functionally distinct subtypes while responding to microenvironmental cues. The identity of proximal transcription factors (TFs) downstream from the polarization signals are known, but their activity is typically transient, failing to explain the long-term, stable epigenomic programs developed. Here, we mapped the early and late epigenomic changes of interleukin-4 (IL-4)-induced alternative macrophage polarization. We identified the TF, early growth response 2 (EGR2), bridging the early transient and late stable gene expression program of polarization. EGR2 is a direct target of IL-4-activated STAT6, having broad action indispensable for 77% of the induced gene signature of alternative polarization, including its autoregulation and a robust, downstream TF cascade involving PPARG. Mechanistically, EGR2 binding results in chromatin opening and the recruitment of chromatin remodelers and RNA polymerase II. Egr2 induction is evolutionarily conserved during alternative polarization of mouse and human macrophages. In the context of tissue resident macrophages, Egr2 expression is most prominent in the lung of a variety of species. Thus, EGR2 is an example of an essential and evolutionarily conserved broad acting factor, linking transient polarization signals to stable epigenomic and transcriptional changes in macrophages.

The Nuclear Receptor PPARγ Controls Progressive Macrophage Polarization as a Ligand-Insensitive Epigenomic Ratchet of Transcriptional Memory.

Macrophages polarize into distinct phenotypes in response to complex environmental cues. We found that the nuclear receptor PPARγ drove robust phenotypic changes in macrophages upon repeated stimulation with interleukin (IL)-4. The functions of PPARγ on macrophage polarization in this setting were independent of ligand binding. Ligand-insensitive PPARγ bound DNA and recruited the coactivator P300 and the architectural protein RAD21. This established a permissive chromatin environment that conferred transcriptional memory by facilitating the binding of the transcriptional regulator STAT6 and RNA polymerase II, leading to robust production of enhancer and mRNAs upon IL-4 re-stimulation. Ligand-insensitive PPARγ binding controlled the expression of an extracellular matrix remodeling-related gene network in macrophages. Expression of these genes increased during muscle regeneration in a mouse model of injury, and this increase coincided with the detection of IL-4 and PPARγ in the affected tissue. Thus, a predominantly ligand-insensitive PPARγ:RXR cistrome regulates progressive and/or reinforcing macrophage polarization.

The Transcription Factor STAT6 Mediates Direct Repression of Inflammatory Enhancers and Limits Activation of Alternatively Polarized Macrophages.

The molecular basis of signal-dependent transcriptional activation has been extensively studied in macrophage polarization, but our understanding remains limited regarding the molecular determinants of repression. Here we show that IL-4-activated STAT6 transcription factor is required for the direct transcriptional repression of a large number of genes during in vitro and in vivo alternative macrophage polarization. Repression results in decreased lineage-determining transcription factor, p300, and RNA polymerase II binding followed by reduced enhancer RNA expression, H3K27 acetylation, and chromatin accessibility. The repressor function of STAT6 is HDAC3 dependent on a subset of IL-4-repressed genes. In addition, STAT6-repressed enhancers show extensive overlap with the NF-κB p65 cistrome and exhibit decreased responsiveness to lipopolysaccharide after IL-4 stimulus on a subset of genes. As a consequence, macrophages exhibit diminished inflammasome activation, decreased IL-1ß production, and pyroptosis. Thus, the IL-4-STAT6 signaling pathway establishes an alternative polarization-specific epigenenomic signature resulting in dampened macrophage responsiveness to inflammatory stimuli.

Macrophage PPARγ, a Lipid Activated Transcription Factor Controls the Growth Factor GDF3 and Skeletal Muscle Regeneration.

Tissue regeneration requires inflammatory and reparatory activity of macrophages. Macrophages detect and eliminate the damaged tissue and subsequently promote regeneration. This dichotomy requires the switch of effector functions of macrophages coordinated with other cell types inside the injured tissue. The gene regulatory events supporting the sensory and effector functions of macrophages involved in tissue repair are not well understood. Here we show that the lipid activated transcription factor, PPARγ, is required for proper skeletal muscle regeneration, acting in repair macrophages. PPARγ controls the expression of the transforming growth factor-ß (TGF-ß) family member, GDF3, which in turn regulates the restoration of skeletal muscle integrity by promoting muscle progenitor cell fusion. This work establishes PPARγ as a required metabolic sensor and transcriptional regulator of repair macrophages. Moreover, this work also establishes GDF3 as a secreted extrinsic effector protein acting on myoblasts and serving as an exclusively macrophage-derived regeneration factor in tissue repair.

A Multi-Omics Approach Reveals Features That Permit Robust and Widespread Regulation of IFN-Inducible Antiviral Effectors.

The antiviral state, an initial line of defense against viral infection, is established by a set of IFN-stimulated genes (ISGs) encoding antiviral effector proteins. The effector ISGs are transcriptionally regulated by type I IFNs mainly via activation of IFN-stimulated gene factor 3 (ISGF3). In this study, the regulatory elements of effector ISGs were characterized to determine the (epi)genetic features that enable their robust induction by type I IFNs in multiple cell types. We determined the location of regulatory elements, the DNA motifs, the occupancy of ISGF3 subunits (IRF9, STAT1, and STAT2) and other transcription factors, and the chromatin accessibility of 37 effector ISGs in murine dendritic cells. The IFN-stimulated response element (ISRE) and its tripartite version occurred most frequently in the regulatory elements of effector ISGs than in any other tested ISG subsets. Chromatin accessibility at their promoter regions was similar to most other ISGs but higher than at the promoters of inflammation-related cytokines, which were used as a reference gene set. Most effector ISGs (81.1%) had at least one ISGF3 binding region proximal to the transcription start site (TSS), and only a subset of effector ISGs (24.3%) was associated with three or more ISGF3 binding regions. The IRF9 signals were typically higher, and ISRE motifs were "stronger" (more similar to the canonical sequence) in TSS-proximal versus TSS-distal regulatory regions. Moreover, most TSS-proximal regulatory regions were accessible before stimulation in multiple cell types. Our results indicate that "strong" ISRE motifs and universally accessible promoter regions that permit robust, widespread induction are characteristic features of effector ISGs.

Identification of a T-bethi Quiescent Exhausted CD8 T Cell Subpopulation That Can Differentiate into TIM3+CX3CR1+ Effectors and Memory-like Cells.

Persistent Ag induces a dysfunctional CD8 T cell state known as "exhaustion" characterized by PD-1 expression. Nevertheless, exhausted CD8 T cells retain functionality through continued differentiation of progenitor into effector cells. However, it remains ill-defined how CD8 T cell effector responses are sustained in situ. In this study, we show using the mouse chronic lymphocytic choriomeningitis virus infection model that CX3CR1+ CD8 T cells contain a T-bet-dependent TIM3-PD-1lo subpopulation that is distinct from the TIM3+CX3CR1+PD-1+ proliferative effector subset. The TIM3-CX3CR1+ cells are quiescent and express a low but significant level of the transcription factor TCF-1, demonstrating similarity to TCF-1hi progenitor CD8 T cells. Furthermore, following the resolution of lymphocytic choriomeningitis virus viremia, a substantial proportion of TCF-1+ memory-like CD8 T cells show evidence of CX3CR1 expression during the chronic phase of the infection. Our results suggest a subset of the CX3CR1+ exhausted population demonstrates progenitor-like features that support the generation of the CX3CR1+ effector pool from the TCF-1hi progenitors and contribute to the memory-like pool following the resolution of viremia.

An Atlas of Promoter Chromatin Modifications and HiChIP Regulatory Interactions in Human Subcutaneous Adipose-Derived Stem Cells.

The genome of human adipose-derived stem cells (ADSCs) from abdominal and gluteofemoral adipose tissue depots are maintained in depot-specific stable epigenetic conformations that influence cell-autonomous gene expression patterns and drive unique depot-specific functions. The traditional approach to explore tissue-specific transcriptional regulation has been to correlate differential gene expression to the nearest-neighbor linear-distance regulatory region defined by associated chromatin features including open chromatin status, histone modifications, and DNA methylation. This has provided important information; nonetheless, the approach is limited because of the known organization of eukaryotic chromatin into a topologically constrained three-dimensional network. This network positions distal regulatory elements in spatial proximity with gene promoters which are not predictable based on linear genomic distance. In this work, we capture long-range chromatin interactions using HiChIP to identify remote genomic regions that influence the differential regulation of depot-specific genes in ADSCs isolated from different adipose depots. By integrating these data with RNA-seq results and histone modifications identified by ChIP-seq, we uncovered distal regulatory elements that influence depot-specific gene expression in ADSCs. Interestingly, a subset of the HiChIP-defined chromatin loops also provide previously unknown connections between waist-to-hip ratio GWAS variants with genes that are known to significantly influence ADSC differentiation and adipocyte function.