Ann Y. Park

Inflammatory bowel disease (IBD) causes chronic suffering from gastrointestinal inflammation and dysfunction that can progress to colon cancer ^1,2 . The prevalence of the disease is increasing, and there is an urgent need to better understand its pathogenic mechanisms to improve treatment. We show that GPR15—a G-protein-coupled receptor expressed in immune cells and described previously as an entry co-factor for human and simian immunodeficiency viruses ³ —is a marker and homing receptor for a subset of intramucosal GPR15-guided regulatory CD8 ⁺ T lymphocytes (CD8 ⁺ T _IGR cells). Deleterious GPR15 gene variants in humans cause defective homing of CD8 ⁺ T _IGR cells and are associated with severe early-onset IBD. Moreover, CD8 ⁺ T _IGR cells are reduced in the intestinal mucosa of individuals with sporadic IBD. In mice, GPR15 deficiency impairs colonic homing of CD8 ⁺ T _IGR cells, leading to accumulation of inflammatory macrophages and increased susceptibility to colitis. CD8 ⁺ T _IGR cells potently kill macrophages activated by intestinal damage or disease using Fas ligand and TNF-related weak inducer of apoptosis (TWEAK). The identification of CD8 ⁺ T _IGR cells yields new insights into organ-specific immune regulation and potential therapeutics for IBD.

Inborn errors of immunity (IEIs) are rare genetic anomalies that cause defective immune function. Over 500 IEIs have been identified to date, affecting millions of patients globally. These IEIs reveal the complex interplay between genetics, the environment and microorganisms that determine immune disease phenotypes. Progress in understanding the molecular and cellular mechanisms of IEIs provides a genetic framework for a functional understanding of the human immune system, disease pathogenesis and successful therapeutic interventions. This Review describes how IEIs impact infectious diseases, particularly coronavirus disease 2019, inflammatory bowel disease and cancer.

Preserving cells in a functional, non-senescent state is a major goal for extending human healthspans. Model organisms reveal that longevity and senescence are genetically controlled, but how genes control longevity in different mammalian tissues is unknown. Here, we report a new human genetic disease that causes cell senescence, liver and immune dysfunction, and early mortality that results from deficiency of GIMAP5, an evolutionarily conserved GTPase selectively expressed in lymphocytes and endothelial cells. We show that GIMAP5 restricts the pathological accumulation of long-chain ceramides (CERs), thereby regulating longevity. GIMAP5 controls CER abundance by interacting with protein kinase CK2 (CK2), attenuating its ability to activate CER synthases. Inhibition of CK2 and CER synthase rescues GIMAP5-deficient T cells by preventing CER overaccumulation and cell deterioration. Thus, GIMAP5 controls longevity assurance pathways crucial for immune function and healthspan in mammals.

Inborn errors of immunity (IEIs) unveil regulatory pathways of human immunity. We describe a new IEI caused by mutations in the GTPase of the immune-associated protein 6 (GIMAP6) gene in patients with infections, lymphoproliferation, autoimmunity, and multiorgan vasculitis. Patients and Gimap6−/− mice show defects in autophagy, redox regulation, and polyunsaturated fatty acid (PUFA)–containing lipids. We find that GIMAP6 complexes with GABARAPL2 and GIMAP7 to regulate GTPase activity. Also, GIMAP6 is induced by IFN-γ and plays a critical role in antibacterial immunity. Finally, we observed that Gimap6−/− mice died prematurely from microangiopathic glomerulosclerosis most likely due to GIMAP6 deficiency in kidney endothelial cells.

We report a pleiotropic disease due to loss-of-function mutations in RHBDF2, the gene encoding iRHOM2, in two kindreds with recurrent infections in different organs. One patient had recurrent pneumonia but no colon involvement, another had recurrent infectious hemorrhagic colitis but no lung involvement and the other two experienced recurrent respiratory infections. Loss of iRHOM2, a rhomboid superfamily member that regulates the ADAM17 metalloproteinase, caused defective ADAM17-dependent cleavage and release of cytokines, including tumor-necrosis factor and amphiregulin. To understand the diverse clinical phenotypes, we challenged Rhbdf2^−/− mice with Pseudomonas aeruginosa by nasal gavage and observed more severe pneumonia, whereas infection with Citrobacter rodentium caused worse inflammatory colitis than in wild-type mice. The fecal microbiota in the colitis patient had characteristic oral species that can predispose to colitis. Thus, a human immunodeficiency arising from iRHOM2 deficiency causes divergent disease phenotypes that can involve the local microbial environment.