Summary: Rare, monogenetic diseases present unique models to dissect gene functions and biological pathways, concomitantly enhancing our understanding of the etiology of complex (and often more common) traits. Although inflammatory bowel disease (IBD) is a generally prototypic complex disease, it can also manifest in an early‐onset, monogenic fashion, often following Mendelian modes of inheritance. Recent advances in genomic technologies have spurred the identification of genetic defects underlying rare, very early‐onset IBD (VEO‐IBD) as a disease subgroup driven by strong genetic influence, pinpointing key players in the delicate homeostasis of the immune system in the gut and illustrating the intimate relationships between bowel inflammation, systemic immune dysregulation, and primary immunodeficiency with increased susceptibility to infections. As for other human diseases, it is likely that adult‐onset diseases may represent complex diseases integrating the effects of host genetic susceptibility and environmental triggers. Comparison of adult‐onset IBD and VEO‐IBD thus provides beautiful models to investigate the relationship between monogenic and multifactorial/polygenic diseases. This review discusses the present and novel findings regarding monogenic IBD as well as key questions and future directions of IBD research.

Somekh and colleagues identify CD137, a member of the tumor necrosis factor superfamily, as a novel cause of immunodeficiency associated with a risk of autoimmunity and lymphoid malignancy.

Exocytosis of cytotoxic granules (CG) by lymphocytes is required for the elimination of infected and malignant cells. Impairments in this process underly a group of diseases with dramatic hyperferritinemic inflammation termed hemophagocytic lymphohistiocytosis (HLH). Although genetic and functional studies of HLH have identified proteins controlling distinct steps of CG exocytosis, the molecular mechanisms that spatiotemporally coordinate CG release remain partially elusive. We studied a patient exhibiting characteristic clinical features of HLH associated with markedly impaired cytotoxic T lymphocyte (CTL) and natural killer (NK) cell exocytosis functions, who beared biallelic deleterious mutations in the gene encoding the small GTPase RhoG. Experimental ablation of RHOG in a model cell line and primary CTLs from healthy individuals uncovered a hitherto unappreciated role of RhoG in retaining CGs in the vicinity of the plasma membrane (PM), a fundamental prerequisite for CG exocytotic release. We discovered that RhoG engages in a protein–protein interaction with Munc13-4, an exocytosis protein essential for CG fusion with the PM. We show that this interaction is critical for docking of Munc13-4+ CGs to the PM and subsequent membrane fusion and release of CG content. Thus, our study illuminates RhoG as a novel essential regulator of human lymphocyte cytotoxicity and provides the molecular pathomechanism behind the identified here and previously unreported genetically determined form of HLH.

The immunological synapse is a complex structure that decodes stimulatory signals into adapted lymphocyte responses. It is a unique window to monitor lymphocyte activity because of development of systematic quantitative approaches. Here we demonstrate the applicability of high-content imaging to human T and natural killer (NK) cells and develop a pipeline for unbiased analysis of high-definition morphological profiles. Our approach reveals how distinct facets of actin cytoskeleton remodeling shape immunological synapse architecture and affect lytic granule positioning. Morphological profiling of CD8⁺ T cells from immunodeficient individuals allows discrimination of the roles of the ARP2/3 subunit ARPC1B and the ARP2/3 activator Wiskott-Aldrich syndrome protein (WASP) in immunological synapse assembly. Single-cell analysis further identifies uncoupling of lytic granules and F-actin radial distribution in ARPC1B-deficient lymphocytes. Our study provides a foundation for development of morphological profiling as a scalable approach to monitor primary lymphocyte responsiveness and to identify complex aspects of lymphocyte micro-architecture.

Regulated exocytosis controls key cellular functions ranging from neurotransmitter release to the secretion of immune mediators, and its disruption is associated with numerous pathologies. The cytotoxic activity of lymphocytes is particularly dependent on regulated and polarized lytic granule delivery toward infected or malignant cells. Although genetic and mechanistic studies have identified factors regulating exocytosis in cytotoxic lymphocytes, a systematic mapping of the relevant factors and their relationships is lacking. Through a genome-scale CRISPR knockout screen in a human natural killer cell line, we characterized a complex genetic network regulating cytotoxic granule exocytosis, with lipid metabolism and protein lipidation among the most prominent pathways. By combining global protein palmitoylation and lipidomic studies, we found that ZDHHC17 drives palmitoylation of the core SNARE complex protein SNAP23 to target cytotoxic granules to GM1-rich lipid rafts whose assembly is controlled by serine palmitoyltransferase. In summary, our study identifies previously unrecognized factors essential for cytotoxic function in human lymphocytes and uncovers how lipid metabolism and protein palmitoylation are involved in the process of regulated exocytosis.