Stefan Feske

Pathogenic T helper 17 (pT _H 17) cells are a subset of CD4 ⁺ T cells driving autoimmune diseases including multiple sclerosis (MS). Compared to homeostatic T _H 17 cells and other T _H subsets, pT _H 17 have enhanced mitochondrial function and oxidative phosphorylation (OXPHOS) that supports their differentiation and pathogenic function. Here we identify Copper Transporter 1 (CTR1), encoded by Slc31a1, as essential for copper uptake in CD4 ⁺ T cells, OXPHOS and pT _H 17 cell differentiation and function. While copper levels are known to be higher in the cerebrospinal fluid of patients with MS compared to healthy individuals, and excess copper contributes to oligodendrocyte loss in murine models of MS, the effect of copper on T cell function and pathogenicity in MS are unclear. We demonstrate that deletion of Slc31a1 in CD4 ⁺ T cells decreased intracellular copper levels, disrupting mitochondrial respiration and rewiring metabolism. These changes altered the epigenetic landscape of pT _H 17 cells by impairing DNA demethylation capacity, leading to hypermethylated DNA and altered chromatin accessibility at key binding sites for AP-1 transcription factors essential for pT _H 17 differentiation. As a result, CTR1-deficient T cells showed defective differentiation into pT _H 17 cells, with decreased production of IL-17A and expression of T _H 17 signature genes, while the differentiation of other CD4 ⁺ T cell subsets remained largely unaffected. Moreover, T cell-specific deletion of Slc31a1 protected mice from central nervous system (CNS) inflammation in the experimental autoimmune encephalomyelitis (EAE) model of MS by suppressing clonal expansion of autoreactive CD4 ⁺ T cells. These findings establish copper as a critical regulator of pT _H 17 differentiation and function, revealing a previously unknown molecular link between copper homeostasis, metabolism and epigenetic regulation governing pT _H 17-mediated autoimmunity.

The Ca²⁺ release-activated Ca²⁺ (CRAC) channel mediates store-operated calcium entry (SOCE), a ubiquitous pathway essential for many cell types, including immune cells. Three Orai (Orai1/2/3) proteins constitute the plasma membrane pore-forming units of CRAC channels that are activated by the endoplasmic reticulum (ER) Ca2+-sensing STIM1/2 proteins when ER Ca2+ stores are depleted. Orai1/2/3 are differentially expressed across primary cells with discernible differences in their structures and biophysical properties. Further, Orai1 has two alternatively translated isoforms: long mammalian-specific Orai1α and the 63-residue shorter Orai1β, which is evolutionarily older and conserved across vertebrates. Whether Orai1α/1β/2/3 produce unique cytosolic Ca²⁺ signatures that bias transcriptional responses through effectors like NFAT is unclear. Here, we used HEK293 cells engineered to express one native Orai isoform and show that all Orai isoforms couple to NFAT1/4 induction. The magnitude of NFAT1/4 induction for each Orai isoform matches that of SOCE, with the following profile: Orai1β>Orai1α>>Orai2>Orai3. Near-native re-expression of either Orai1α or Orai1β in primary murine Orai1−/− CD4⁺ T cells restored SOCE, NFAT activation, cytokine production and promoted near identical transcriptional responses enriched for immune activation pathways. An analysis of genetic and clinical data of human individuals showed that homozygous null mutations selectively abolishing Orai1α are not associated with disease resembling CRAC channelopathy. Primary T cells from individuals homozygous or heterozygous for an Orai1α null mutation showed enhanced, rather than impaired, SOCE and NFAT induction. Our data indicate that NFAT activation and transcriptional outputs are primarily driven by the graded strength of SOCE mediated by each isoform of the Orai quartet.

Antibody-producing plasmablasts (PB) and plasma cells (PC) are critical for humoral immunity, autoimmunity and vaccine responses. Despite the importance of environmental stressors in regulating humoral immune responses, the influence of pH on PB and PC differentiation remains elusive. Here, we identify SLC4A7/NBCn1, a Na⁺/HCO₃^– cotransporter, as a selective regulator of PB differentiation in vitro. SLC4A7 deletion also impairs the formation of antibody secreting cells (ASCs) and antibody responses in mice in vivo following immunization and influenza A virus infection. Mechanistically, SLC4A7 deletion results in intracellular acidification and lysosomal alkalinization, and is associated with impaired function of the mechanistic target of rapamycin complex 1 (mTORC1). Enforcing mTORC1 activation in SLC4A7-deficient B cells or B cells in which intracellular pH is acidified by blocking Na⁺/H⁺ exchanger (NHE) function restores PB differentiation in vitro. Moreover, ASC differentiation and antibody responses are impaired under conditions of extracellular acidosis in vitro and in a mouse model of metabolic acidosis. Altogether, we identify a critical relationship between intracellular pH regulation through SLC4A7 and mTORC1-dependent ASC differentiation and humoral immunity.

The Ca²⁺ release-activated Ca²⁺ (CRAC) channel mediates store-operated calcium entry (SOCE), a ubiquitous pathway essential for many cell types, including immune cells. Three Orai (Orai1, Orai2, Orai3) proteins constitute the plasma membrane pore-forming units of CRAC channels that are activated by the endoplasmic reticulum (ER) Ca ²⁺ -sensing STIM1 and STIM2 proteins when ER Ca ²⁺ stores are depleted. Orai1/2/3 are differentially expressed across primary cells with discernible differences in their structures and biophysical properties. Further, Orai1 has two alternatively translated isoforms: the long mammalian-specific Orai1α and the 63-residue shorter Orai1β, which is evolutionarily older and conserved across vertebrates. Whether Orai1α/1β/2/3 at native expression levels produce unique cytosolic Ca²⁺ signatures that bias transcriptional responses through effectors like NFAT is unclear. Here we used HEK293 cells engineered to express one native Orai isoform and show that all Orai isoforms couple to NFAT1 and NFAT4 induction. The magnitude of NFAT1/4 induction tracked the magnitude of SOCE, with the following profile: Orai1β>Orai1α>>Orai2>Orai3. Near-native ectopic expression of either Orai1α or Orai1β in primary murine Orai1 ^−/− CD4⁺ T cells restored SOCE, NFAT activation, cytokine production and promoted near identical transcriptional responses enriched for immune activation pathways. An analysis of genetic and clinical data of human individuals showed that homozygous null mutations selectively abolishing Orai1α are not associated with disease resembling CRAC channelopathy. Primary T cells from individuals homozygous or heterozygous for an Orai1α null mutation showed enhanced, rather than impaired, SOCE and NFAT induction. Collectively, our data indicate that Orai isoforms regulate NFAT activation and downstream transcription primarily through the magnitude of SOCE they support, with Orai1α and Orai1β being the most potent isoforms coupling SOCE to NFAT responses.

B cells play critical roles in humoral immunity to infection, vaccination, and autoimmunity. The differentiation of B cells into antibody-producing plasma cells (PCs) has been extensively studied, but the role of metabolic transporters that mediate nutrient uptake during PC differentiation is not well-understood. Here, we characterized the dependence of B cells and PC differentiation on the neutral amino acid transporter SLC7A5. We demonstrate that SLC7A5 promotes B cell functions including proliferation and PC differentiation in vitro and in vivo after immunization with T dependent and independent antigens. Deletion of SLC7A5 in B cells suppressed the function of mTORC1 and enforced mTORC1 activity rescued PC differentiation. The role of SLC7A5 in B cells appears to be unrelated to leucine uptake because B cells were insensitive to extracellular leucine depletion. Defects in SLC7A5-deficient B cells could, however, be rescued by extracellular methionine supplementation, suggesting a role for methionine in SLC7A5-dependent B cell function and PC differentiation. Our study provides evidence for a leucine-independent role of SLC7A5 in B cell function and PC differentiation.

Chaperone-mediated autophagy (CMA) contributes to proteostasis maintenance by selectively degrading a subset of proteins in lysosomes. CMA declines with age in most tissues, including skeletal muscle. However, the role of CMA in skeletal muscle and the consequences of its decline remain poorly understood. Here we demonstrate that CMA regulates skeletal muscle function. We show that CMA is upregulated in skeletal muscle in response to starvation, exercise and tissue repair, but declines in ageing and obesity. Using a muscle-specific CMA-deficient mouse model, we show that CMA loss leads to progressive myopathy, including reduced muscle force and degenerative myofibre features. Comparative proteomic analyses reveal CMA-dependent changes in the mitochondrial proteome and identify the sarcoplasmic–endoplasmic reticulum Ca²⁺-ATPase (SERCA) as a CMA substrate. Impaired SERCA turnover in CMA-deficient skeletal muscle is associated with defective calcium (Ca²⁺) storage and dysregulated Ca²⁺ dynamics. We confirm that CMA is also downregulated with age in human skeletal muscle. Remarkably, genetic upregulation of CMA activity in old mice partially ameliorates skeletal muscle ageing phenotypes. Together, our work highlights the contribution of CMA to skeletal muscle homoeostasis and myofibre integrity.

Loss-of-function mutations of ORAI1 suppress store-operated Ca2+ entry (SOCE) and cause an immunodeficiency disorder called Ca2+ release–activated Ca2+ channelopathy. Here, we report an infant patient who is compound-heterozygous for p.His134Pro and p.Leu194Pro mutations in ORAI1 and whose T cells have strongly reduced SOCE. Whereas the p.Leu194Pro mutant ORAI1 protein is not expressed at the plasma membrane, the p.His134Pro mutation results in a constitutively open channel that is unresponsive to activation by stromal interaction molecule 1. The patient suffered from a severe form of combined immunodeficiency, hemophagocytic lymphohistiocytosis, and fatal chronic cytomegalovirus infection. His immunodeficiency was characterized by an altered composition of T and NK cell compartments, impaired stimulation-induced cytokine production and signs of CD4+ T cell and NK cell activation, but attenuated CD8+ T effector memory cell function. Our findings demonstrate that small constitutive SOCE through a mutant ORAI1 channel is not sufficient to provide immunity to viral infection.

Pathogenic CD4 T cells drive autoimmunity in diseases such as multiple sclerosis (MS) and inflammatory bowel disease (IBD). Through a forward genetic screen, we identified chloride nucleotide-sensitive channel 1A (CLNS1A) as a key regulator of inflammation in the experimental autoimmune encephalomyelitis (EAE) model of MS. CLNS1A is expressed in several subsets of CD4 T cells, including pathogenic T helper 17 (pT _H 17) cells. Deletion of Clns1a in T cells resulted in DNA damage, cell cycle arrest, impaired T cell proliferation, and effector function, thereby protecting mice from both EAE and IBD. We found that CLNS1A interacts with protein arginine methyl transferase 5 (PRMT5). Moreover, CLNS1A regulates symmetric histone dimethylation and the expression of genes involved in DNA repair, replication, and cell cycle progression. Thus, CLNS1A plays an important role in CD4 T cells by promoting genome stability and cell cycle progression.