CRISPR screens are a powerful source of biological discovery, enabling the unbiased interrogation of gene function in a wide range of applications and species. In pooled CRISPR screens, various genetically encoded perturbations are introduced into pools of cells. The targeted cells proliferate under a biological challenge such as cell competition, drug treatment or viral infection. Subsequently, the perturbation-induced effects are evaluated by sequencing-based counting of the guide RNAs that specify each perturbation. The typical results of such screens are ranked lists of genes that confer sensitivity or resistance to the biological challenge of interest. Contributing to the broad utility of CRISPR screens, adaptations of the core CRISPR technology make it possible to activate, silence or otherwise manipulate the target genes. Moreover, high-content read-outs such as single-cell RNA sequencing and spatial imaging help characterize screened cells with unprecedented detail. Dedicated software tools facilitate bioinformatic analysis and enhance reproducibility. CRISPR screening has unravelled various molecular mechanisms in basic biology, medical genetics, cancer research, immunology, infectious diseases, microbiology and other fields. This Primer describes the basic and advanced concepts of CRISPR screening and its application as a flexible and reliable method for biological discovery, biomedical research and drug development — with a special emphasis on high-content methods that make it possible to obtain detailed biological insights directly as part of the screen. CRISPR screening is a high-throughput approach for identifying genes, pathways and mechanisms involved in a given phenotype or biological process. High-content read-outs of these screens, such as imaging and single-cell sequencing techniques, have further broadened its applicability. This Primer by Bock et al. describes the main concepts of CRISPR screening and gives examples of its application as a method for biological discovery, with a focus on the use of high-content read-outs. 

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High-content CRISPR screening

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Genome-wide CRISPR screens of T cell exhaustion identify chromatin remodeling factors that limit T cell persistence

T follicular helper (TFH) cells are crucial for B cell-mediated humoral immunity1. Although transcription factors such as BCL6 drive the differentiation of TFH cells2,3, it is unclear whether and how post-transcriptional and metabolic programs enforce TFH cell programming. Here we show that the cytidine diphosphate (CDP)–ethanolamine pathway co-ordinates the expression and localization of CXCR5 with the responses of TFH cells and humoral immunity. Using in vivo CRISPR–Cas9 screening and functional validation in mice, we identify ETNK1, PCYT2, and SELENOI—enzymes in the CDP–ethanolamine pathway for de novo synthesis of phosphatidylethanolamine (PE)—as selective post-transcriptional regulators of TFH cell differentiation that act by promoting the surface expression and functional effects of CXCR5. TFH cells exhibit unique lipid metabolic programs and PE is distributed to the outer layer of the plasma membrane, where it colocalizes with CXCR5. De novo synthesis of PE through the CDP–ethanolamine pathway co-ordinates these events to prevent the internalization and degradation of CXCR5. Genetic deletion of Pcyt2, but not of Pcyt1a (which mediates the CDP–choline pathway), in activated T cells impairs the differentiation of TFH cells, and this is associated with reduced humoral immune responses. Surface levels of PE and CXCR5 expression on B cells also depend on Pcyt2. Our results reveal that phospholipid metabolism orchestrates post-transcriptional mechanisms for TFH cell differentiation and humoral immunity, highlighting the metabolic control of context-dependent immune signalling and effector programs. Enzymes in the cytidine diphosphate–ethanolamine metabolic pathway, which promotes de novo synthesis of phosphatidylethanolamine, are shown to act as post-transcriptional mediators of the differentiation of T follicular helper (TFH) cells, by regulating the chemokine receptor CXCR5.

Metabolic control of TFH cells and humoral immunity by phosphatidylethanolamine.

Epigenetic regulation of T cell exhaustion

In vivo CRISPR screening reveals nutrient signaling processes underpinning CD8+ T cell fate decisions.

Nutrients are emerging regulators of adaptive immunity1. Selective nutrients interplay with immunological signals to activate mechanistic target of rapamycin complex 1 (mTORC1), a key driver of cell metabolism2–4, but how these environmental signals are integrated for immune regulation remains unclear. Here we use genome-wide CRISPR screening combined with protein–protein interaction networks to identify regulatory modules that mediate immune receptor- and nutrient-dependent signalling to mTORC1 in mouse regulatory T (Treg) cells. SEC31A is identified to promote mTORC1 activation by interacting with the GATOR2 component SEC13 to protect it from SKP1-dependent proteasomal degradation. Accordingly, loss of SEC31A impairs T cell priming and Treg suppressive function in mice. In addition, the SWI/SNF complex restricts expression of the amino acid sensor CASTOR1, thereby enhancing mTORC1 activation. Moreover, we reveal that the CCDC101-associated SAGA complex is a potent inhibitor of mTORC1, which limits the expression of glucose and amino acid transporters and maintains T cell quiescence in vivo. Specific deletion of Ccdc101 in mouse Treg cells results in uncontrolled inflammation but improved antitumour immunity. Collectively, our results establish epigenetic and post-translational mechanisms that underpin how nutrient transporters, sensors and transducers interplay with immune signals for three-tiered regulation of mTORC1 activity and identify their pivotal roles in licensing T cell immunity and immune tolerance. CRISPR screening and protein–protein interaction networks identify components and mechanisms of nutrient-dependent mTORC1 signalling in regulatory T cells and reveal how mTORC1 integrates immunological cues and nutrient signals for adaptive immunity.

Guotong Fu

Papers

In vivo CRISPR screening reveals nutrient signaling processes underpinning CD8+ T cell fate decisions.

Metabolic control of TFH cells and humoral immunity by phosphatidylethanolamine.

CRISPR screens unveil signal hubs for nutrient licensing of T cell immunity