Rare SH2B3 coding variants in lupus patients impair B cell tolerance and predispose to autoimmunity.

Systemic lupus erythematosus (SLE) is a heterogeneous autoimmune disease with a clear genetic component. While most SLE patients carry rare gene variants in lupus risk genes, little is known about their contribution to disease pathogenesis. Amongst them, SH2B3-a negative regulator of cytokine and growth factor receptor signaling-harbors rare coding variants in over 5% of SLE patients. Here, we show that unlike the variant found exclusively in healthy controls, SH2B3 rare variants found in lupus patients are predominantly hypomorphic alleles, failing to suppress IFNGR signaling via JAK2-STAT1. The generation of two mouse lines carrying patients' variants revealed that SH2B3 is important in limiting the number of immature and transitional B cells. Furthermore, hypomorphic SH2B3 was shown to impair the negative selection of immature/transitional self-reactive B cells and accelerate autoimmunity in sensitized mice, at least in part due to increased IL-4R signaling and BAFF-R expression. This work identifies a previously unappreciated role for SH2B3 in human B cell tolerance and lupus risk.

DECTIN-1: A modifier protein in CTLA-4 haploinsufficiency.

Autosomal dominant loss-of-function (LoF) variants in cytotoxic T-lymphocyte associated protein 4 (CTLA4) cause immune dysregulation with autoimmunity, immunodeficiency and lymphoproliferation (IDAIL). Incomplete penetrance and variable expressivity are characteristic of IDAIL caused by CTLA-4 haploinsufficiency (CTLA-4h), pointing to a role for genetic modifiers. Here, we describe an IDAIL proband carrying a maternally inherited pathogenic CTLA4 variant and a paternally inherited rare LoF missense variant in CLEC7A, which encodes for the ß-glucan pattern recognition receptor DECTIN-1. The CLEC7A variant led to a loss of DECTIN-1 dimerization and surface expression. Notably, DECTIN-1 stimulation promoted human and mouse regulatory T cell (Treg) differentiation from naïve αß and γδ T cells, even in the absence of transforming growth factor-ß. Consistent with DECTIN-1's Treg-boosting ability, partial DECTIN-1 deficiency exacerbated the Treg defect conferred by CTL4-4h. DECTIN-1/CLEC7A emerges as a modifier gene in CTLA-4h, increasing expressivity of CTLA4 variants and acting in functional epistasis with CTLA-4 to maintain immune homeostasis and tolerance.

A common human MLKL polymorphism confers resistance to negative regulation by phosphorylation.

Across the globe, 2-3% of humans carry the p.Ser132Pro single nucleotide polymorphism in MLKL, the terminal effector protein of the inflammatory form of programmed cell death, necroptosis. Here we show that this substitution confers a gain in necroptotic function in human cells, with more rapid accumulation of activated MLKLS132P in biological membranes and MLKLS132P overriding pharmacological and endogenous inhibition of MLKL. In mouse cells, the equivalent Mlkl S131P mutation confers a gene dosage dependent reduction in sensitivity to TNF-induced necroptosis in both hematopoietic and non-hematopoietic cells, but enhanced sensitivity to IFN-ß induced death in non-hematopoietic cells. In vivo, MlklS131P homozygosity reduces the capacity to clear Salmonella from major organs and retards recovery of hematopoietic stem cells. Thus, by dysregulating necroptosis, the S131P substitution impairs the return to homeostasis after systemic challenge. Present day carriers of the MLKL S132P polymorphism may be the key to understanding how MLKL and necroptosis modulate the progression of complex polygenic human disease.

De Novo PACSIN1 Gene Variant Found in Childhood Lupus and a Role for PACSIN1/TRAF4 Complex in Toll-like Receptor 7 Activation.

OBJECTIVE: Increased Toll-like receptor 7 (TLR-7) signaling leading to the production of type I interferon (IFN) is an important contributor to human systemic lupus erythematosus (SLE). Protein kinase C and casein kinase substrate in neurons 1 (PACSIN1), a molecule that regulates synaptic vesicle recycling, has been linked to TLR-7/TLR-9-mediated type I IFN production in humans and mice, but the underlying mechanism is unknown. We undertook this study to explore the pathogenicity and underlying mechanism of a de novo PACSIN1 missense variant identified in a child with SLE. METHODS: PACSIN1 Q59K de novo and null variants were introduced into a human plasmacytoid dendritic cell line and into mice using CRISPR/Cas9 editing. The effects of the variants on TLR-7/TLR-9 signaling in human and mouse cells, as well as PACSIN1 messenger RNA and IFN signature in SLE patients, were assessed using real-time polymerase chain reaction and flow cytometry. Mechanisms were investigated using luciferase reporter assays, RNA interference, coimmunoprecipitation, and immunofluorescence. RESULTS: We established that PACSIN1 forms a trimolecular complex with tumor necrosis factor receptor-associated factor 4 (TRAF4) and TRAF6 that is important for the regulation of type I IFN. The Q59K mutation in PACSIN1 augments binding to neural Wiskott-Aldrich syndrome protein while it decreases binding to TRAF4, leading to unrestrained TRAF6-mediated activation of type I IFN. Intriguingly, PACSIN1 Q59K increased TLR-7 but not TLR-9 signaling in human cells, leading to elevated expression of IFNß and IFN-inducible genes. Untreated SLE patients had high PACSIN1 expression in peripheral blood cells that correlated positively with IFN-related genes. Introduction of the Pacsin1 Q59K mutation into mice caused increased surface TLR-7 and TRAIL expression in B cells. CONCLUSION: PACSIN1 Q59K increases IFNß activity through the impairment of TRAF4-mediated inhibition of TLR-7 signaling, possibly contributing to SLE risk.

A protocol to evaluate immunoglobulin deposits in mouse glomeruli.

Accumulation of immunoglobulin and complement components within the kidneys is a hallmark of glomerulonephritis. Staining and detection of IgG, IgA, IgM, and C3 deposits can assist in diagnosing the underlying causes of nephritis and has implications for the pathological processes underpinning glomerulonephritis. Here, we describe a protocol to detect immune deposits within biological specimens such as mouse kidneys. We detail tissue isolation and processing, immunostaining, and fluorescence microscopy to characterize and quantify the extent of immunological deposits contributing to kidney injury. For complete details on the use and execution of this protocol, please refer to Jiang et al. (2021).

TLR7 gain-of-function genetic variation causes human lupus.

Although circumstantial evidence supports enhanced Toll-like receptor 7 (TLR7) signalling as a mechanism of human systemic autoimmune disease1-7, evidence of lupus-causing TLR7 gene variants is lacking. Here we describe human systemic lupus erythematosus caused by a TLR7 gain-of-function variant. TLR7 is a sensor of viral RNA8,9 and binds to guanosine10-12. We identified a de novo, previously undescribed missense TLR7Y264H variant in a child with severe lupus and additional variants in other patients with lupus. The TLR7Y264H variant selectively increased sensing of guanosine and 2',3'-cGMP10-12, and was sufficient to cause lupus when introduced into mice. We show that enhanced TLR7 signalling drives aberrant survival of B cell receptor (BCR)-activated B cells, and in a cell-intrinsic manner, accumulation of CD11c+ age-associated B cells and germinal centre B cells. Follicular and extrafollicular helper T cells were also increased but these phenotypes were cell-extrinsic. Deficiency of MyD88 (an adaptor protein downstream of TLR7) rescued autoimmunity, aberrant B cell survival, and all cellular and serological phenotypes. Despite prominent spontaneous germinal-centre formation in Tlr7Y264H mice, autoimmunity was not ameliorated by germinal-centre deficiency, suggesting an extrafollicular origin of pathogenic B cells. We establish the importance of TLR7 and guanosine-containing self-ligands for human lupus pathogenesis, which paves the way for therapeutic TLR7 or MyD88 inhibition.

Increased burden of rare variants in genes of the endosomal Toll-like receptor pathway in patients with systemic lupus erythematosus.

OBJECTIVE: To compare the frequency of rare variants in genes of the pathophysiologically relevant endosomal Toll-like receptor (eTLR) pathway and any quantifiable differences in variant rarity, predicted deleteriousness, or molecular proximity in patients with systemic lupus erythematosus (SLE) and healthy controls. PATIENTS AND METHODS: 65 genes associated with the eTLR pathway were identified by literature search and pathway analysis. Using next generation sequencing techniques, these were compared in two randomised cohorts of patients with SLE (n = 114 and n = 113) with 197 healthy controls. Genetically determined ethnicity was used to normalise minor allele frequencies (MAF) for the identified genetic variants and these were then compared by their frequency: rare (MAF < 0.005), uncommon (MAF 0.005-0.02), and common (MAF >0.02). This was compared to the results for 65 randomly selected genes. RESULTS: Patients with SLE are more likely to carry a rare nonsynonymous variant affecting proteins within the eTLR pathway than healthy controls. Furthermore, individuals with SLE are more likely to have multiple rare variants in this pathway. There were no differences in rarity, Combined Annotation Dependent Depletion (CADD) score, or molecular proximity for rare eTLR pathway variants. CONCLUSIONS: Rare non-synonymous variants are enriched in patients with SLE in the eTLR pathway. This supports the hypothesis that SLE arises from several rare variants of relatively large effect rather than many common variants of small effect.

A Point Mutation in IKAROS ZF1 Causes a B Cell Deficiency in Mice.

IKZF1 (IKAROS) is essential for normal lymphopoiesis in both humans and mice. Previous Ikzf1 mouse models have demonstrated the dual role for IKZF1 in both B and T cell development and have indicated differential requirements of each zinc finger. Furthermore, mutations in IKZF1 are known to cause common variable immunodeficiency in patients characterized by a loss of B cells and reduced Ab production. Through N-ethyl-N-nitrosourea mutagenesis, we have discovered a novel Ikzf1 mutant mouse with a missense mutation (L132P) in zinc finger 1 (ZF1) located in the DNA binding domain. Unlike other previously reported murine Ikzf1 mutations, this L132P point mutation (Ikzf1L132P ) conserves overall protein expression and has a B cell-specific phenotype with no effect on T cell development, indicating that ZF1 is not required for T cells. Mice have reduced Ab responses to immunization and show a progressive loss of serum Igs compared with wild-type littermates. IKZF1L132P overexpressed in NIH3T3 or HEK293T cells failed to localize to pericentromeric heterochromatin and bind target DNA sequences. Coexpression of wild-type and mutant IKZF1, however, allows for localization to pericentromeric heterochromatin and binding to DNA indicating a haploinsufficient mechanism of action for IKZF1L132P Furthermore, Ikzf1+/L132P mice have late onset defective Ig production, similar to what is observed in common variable immunodeficiency patients. RNA sequencing revealed a total loss of Hsf1 expression in follicular B cells, suggesting a possible functional link for the humoral immune response defects observed in Ikzf1L132P/L132P mice.

Infanticide vs. inherited cardiac arrhythmias.

AIMS: In 2003, an Australian woman was convicted by a jury of smothering and killing her four children over a 10-year period. Each child died suddenly and unexpectedly during a sleep period, at ages ranging from 19 days to 18 months. In 2019 we were asked to investigate if a genetic cause could explain the children's deaths as part of an inquiry into the mother's convictions. METHODS AND RESULTS: Whole genomes or exomes of the mother and her four children were sequenced. Functional analysis of a novel CALM2 variant was performed by measuring Ca2+-binding affinity, interaction with calcium channels and channel function. We found two children had a novel calmodulin variant (CALM2 G114R) that was inherited maternally. Three genes (CALM1-3) encode identical calmodulin proteins. A variant in the corresponding residue of CALM3 (G114W) was recently reported in a child who died suddenly at age 4 and a sibling who suffered a cardiac arrest at age 5. We show that CALM2 G114R impairs calmodulin's ability to bind calcium and regulate two pivotal calcium channels (CaV1.2 and RyR2) involved in cardiac excitation contraction coupling. The deleterious effects of G114R are similar to those produced by G114W and N98S, which are considered arrhythmogenic and cause sudden cardiac death in children. CONCLUSION: A novel functional calmodulin variant (G114R) predicted to cause idiopathic ventricular fibrillation, catecholaminergic polymorphic ventricular tachycardia, or mild long QT syndrome was present in two children. A fatal arrhythmic event may have been triggered by their intercurrent infections. Thus, calmodulinopathy emerges as a reasonable explanation for a natural cause of their deaths.

Deletions in VANGL1 are a risk factor for antibody-mediated kidney disease.

We identify an intronic deletion in VANGL1 that predisposes to renal injury in high risk populations through a kidney-intrinsic process. Half of all SLE patients develop nephritis, yet the predisposing mechanisms to kidney damage remain poorly understood. There is limited evidence of genetic contribution to specific organ involvement in SLE.1,2 We identify a large deletion in intron 7 of Van Gogh Like 1 (VANGL1), which associates with nephritis in SLE patients. The same deletion occurs at increased frequency in an indigenous population (Tiwi Islanders) with 10-fold higher rates of kidney disease compared with non-indigenous populations. Vangl1 hemizygosity in mice results in spontaneous IgA and IgG deposition within the glomerular mesangium in the absence of autoimmune nephritis. Serum transfer into B cell-deficient Vangl1+/- mice results in mesangial IgG deposition indicating that Ig deposits occur in a kidney-intrinsic fashion in the absence of Vangl1. These results suggest that Vangl1 acts in the kidney to prevent Ig deposits and its deficiency may trigger nephritis in individuals with SLE.

A missense mutation in the MLKL brace region promotes lethal neonatal inflammation and hematopoietic dysfunction.

MLKL is the essential effector of necroptosis, a form of programmed lytic cell death. We have isolated a mouse strain with a single missense mutation, MlklD139V, that alters the two-helix 'brace' that connects the killer four-helix bundle and regulatory pseudokinase domains. This confers constitutive, RIPK3 independent killing activity to MLKL. Homozygous mutant mice develop lethal postnatal inflammation of the salivary glands and mediastinum. The normal embryonic development of MlklD139V homozygotes until birth, and the absence of any overt phenotype in heterozygotes provides important in vivo precedent for the capacity of cells to clear activated MLKL. These observations offer an important insight into the potential disease-modulating roles of three common human MLKL polymorphisms that encode amino acid substitutions within or adjacent to the brace region. Compound heterozygosity of these variants is found at up to 12-fold the expected frequency in patients that suffer from a pediatric autoinflammatory disease, chronic recurrent multifocal osteomyelitis (CRMO).

Functional rare and low frequency variants in BLK and BANK1 contribute to human lupus.

Systemic lupus erythematosus (SLE) is the prototypic systemic autoimmune disease. It is thought that many common variant gene loci of weak effect act additively to predispose to common autoimmune diseases, while the contribution of rare variants remains unclear. Here we describe that rare coding variants in lupus-risk genes are present in most SLE patients and healthy controls. We demonstrate the functional consequences of rare and low frequency missense variants in the interacting proteins BLK and BANK1, which are present alone, or in combination, in a substantial proportion of lupus patients. The rare variants found in patients, but not those found exclusively in controls, impair suppression of IRF5 and type-I IFN in human B cell lines and increase pathogenic lymphocytes in lupus-prone mice. Thus, rare gene variants are common in SLE and likely contribute to genetic risk.

ROQUIN signalling pathways in innate and adaptive immunity.

ROQUIN is an RNA-binding protein that plays important roles in both the innate and adaptive immune systems. ROQUIN binds to several key immune-relevant messenger RNA (mRNA) targets through its ROQ domain modulating their stability and influencing macrophage function and the peripheral homeostasis of T cells and B cells. More recently, the E3 ubiquitin ligase activity of the ROQUIN RING domain has been shown to be crucial for T-cell-dependent B-cell responses against infection. Defective ROQUIN activity can culminate in a range of diseases, such as systemic autoimmunity, immunodeficiency, and inflammatory bowel disorder. Here, we provide a current overview of the immunomodulatory role of ROQUIN defined by its ribonucleoprotein-like structure, its repertoire of mRNA targets shared by related RNA-binding enzymes, and its involvement in a range of intracellular signalling pathways central to shaping immune responses.

Attenuation of AMPK signaling by ROQUIN promotes T follicular helper cell formation.

T follicular helper cells (Tfh) are critical for the longevity and quality of antibody-mediated protection against infection. Yet few signaling pathways have been identified to be unique solely to Tfh development. ROQUIN is a post-transcriptional repressor of T cells, acting through its ROQ domain to destabilize mRNA targets important for Th1, Th17, and Tfh biology. Here, we report that ROQUIN has a paradoxical function on Tfh differentiation mediated by its RING domain: mice with a T cell-specific deletion of the ROQUIN RING domain have unchanged Th1, Th2, Th17, and Tregs during a T-dependent response but show a profoundly defective antigen-specific Tfh compartment. ROQUIN RING signaling directly antagonized the catalytic α1 subunit of adenosine monophosphate-activated protein kinase (AMPK), a central stress-responsive regulator of cellular metabolism and mTOR signaling, which is known to facilitate T-dependent humoral immunity. We therefore unexpectedly uncover a ROQUIN-AMPK metabolic signaling nexus essential for selectively promoting Tfh responses.

MicroRNA-146a regulates ICOS-ICOSL signalling to limit accumulation of T follicular helper cells and germinal centres.

Tight control of T follicular helper (Tfh) cells is required for optimal maturation of the germinal centre (GC) response. The molecular mechanisms controlling Tfh-cell differentiation remain incompletely understood. Here we show that microRNA-146a (miR-146a) is highly expressed in Tfh cells and peak miR-146a expression marks the decline of the Tfh response after immunization. Loss of miR-146a causes cell-intrinsic accumulation of Tfh and GC B cells. MiR-146a represses several Tfh-cell-expressed messenger RNAs, and of these, ICOS is the most strongly cell autonomously upregulated target in miR-146a-deficient T cells. In addition, miR-146a deficiency leads to increased ICOSL expression on GC B cells and antigen-presenting cells. Partial blockade of ICOS signalling, either by injections of low dose of ICOSL blocking antibody or by halving the gene dose of Icos in miR-146a-deficient T cells, prevents the Tfh and GC B-cell accumulation. Collectively, miR-146a emerges as a post-transcriptional brake to limit Tfh cells and GC responses.

Roquin binds microRNA-146a and Argonaute2 to regulate microRNA homeostasis.

Roquin is an RNA-binding protein that prevents autoimmunity and inflammation via repression of bound target mRNAs such as inducible costimulator (Icos). When Roquin is absent or mutated (Roquin(san)), Icos is overexpressed in T cells. Here we show that Roquin enhances Dicer-mediated processing of pre-miR-146a. Roquin also directly binds Argonaute2, a central component of the RNA-induced silencing complex, and miR-146a, a microRNA that targets Icos mRNA. In the absence of functional Roquin, miR-146a accumulates in T cells. Its accumulation is not due to increased transcription or processing, rather due to enhanced stability of mature miR-146a. This is associated with decreased 3' end uridylation of the miRNA. Crystallographic studies reveal that Roquin contains a unique HEPN domain and identify the structural basis of the 'san' mutation and Roquin's ability to bind multiple RNAs. Roquin emerges as a protein that can bind Ago2, miRNAs and target mRNAs, to control homeostasis of both RNA species.

Roquin-2 shares functions with its paralog Roquin-1 in the repression of mRNAs controlling T follicular helper cells and systemic inflammation.

Accumulation of T follicular helper (Tfh) cells and proinflammatory cytokines drive autoantibody-mediated diseases. The RNA-binding protein Roquin-1 (Rc3h1) represses the inducible costimulator ICOS and interferon-γ (IFN-γ) in T cells to prevent Tfh cell accumulation. Unlike Rc3h1(san) mice with a mutation in the ROQ domain of Roquin-1, mice lacking the protein, paradoxically do not display increased Tfh cells. Here we have analyzed mice with mutations that eliminate the RING domain from Roquin-1 or its paralog, Roquin-2 (Rc3h2). RING or ROQ mutations both disrupted Icos mRNA regulation by Roquin-1, but, unlike the ROQ mutant that still occupied mRNA-regulating stress granules, RING-deficient Roquin-1 failed to localize to stress granules and allowed Roquin-2 to compensate in the repression of ICOS and Tfh cells. These paralogs also targeted tumor necrosis factor (TNF) in nonlymphoid cells, ameliorating autoantibody-induced arthritis. The Roquin family emerges as a posttranscriptional brake in the adaptive and innate phases of antibody responses.

The ROQUIN family of proteins localizes to stress granules via the ROQ domain and binds target mRNAs.

Roquin is an E3 ubiquitin ligase with a poorly understood but essential role in preventing T-cell-mediated autoimmune disease and in microRNA-mediated repression of inducible costimulator (Icos) mRNA. Roquin and its mammalian paralogue membrane-associated nucleic acid binding protein (MNAB) define a protein family distinguished by an approximately 200 amino acid domain of unknown function, ROQ, that is highly conserved from mammals to invertebrates and is flanked by a RING-1 zinc finger and a CCCH zinc finger. Here we show that human, Drosophila and Caenorhabditis elegans Roquin and human MNAB localize to the cytoplasm and upon stress are concentrated in stress granules, where stalled mRNA translation complexes are stored. The ROQ domain is necessary and sufficient for localization to arsenite-induced stress granules and to induce these structures upon overexpression, and is required to trigger Icos mRNA decay. Gel-shift, SPR and footprinting studies show that an N-terminal fragment centred on the ROQ domain binds RNA from the Icos 3'-untranslated region comprising the minimal sequence for Roquin-mediated repression, adjacent to the miR-101 sequence complementarity. These findings identify Roquin as an RNA-binding protein and establish a specific function for the ROQ protein domain in mRNA homeostasis. Structured digital abstract * MINT-7711163: TIA-1 (uniprotkb:P31483) and Roquin (uniprotkb:Q4VGL6) colocalize (MI:0403) by fluorescence microscopy (MI:0416) * MINT-7711475: RLE-1 (uniprotkb:O45962) and TIA-1 (uniprotkb:P31483) colocalize (MI:0403) by fluorescence microscopy (MI:0416) * MINT-7711487: DmRoquin (uniprotkb:Q9VV48) and TIA-1 (uniprotkb:P31483) colocalize (MI:0403) by fluorescence microscopy (MI:0416) * MINT-7711447, MINT-7711460: MNAB (uniprotkb:Q9HBD1) and TIA-1 (uniprotkb:P31483) colocalize (MI:0403) by fluorescence microscopy (MI:0416) * MINT-7711176: eIF3 (uniprotkb:P55884) and Roquin (uniprotkb:Q4VGL6) colocalize (MI:0403) by fluorescence microscopy (MI:0416) * MINT-7711192: DCP1A (uniprotkb:Q9NPI6) and TIA-1 (uniprotkb:P31483) colocalize (MI:0403) by fluorescence microscopy (MI:0416).

Identification and characterization of a novel component of the human minichromosome maintenance complex.

Minichromosome maintenance (MCM) complex replicative helicase complexes play essential roles in DNA replication in all eukaryotes. Using a tandem affinity purification-tagging approach in human cells, we discovered a form of the MCM complex that contains a previously unstudied protein, MCM binding protein (MCM-BP). MCM-BP is conserved in multicellular eukaryotes and shares limited homology with MCM proteins. MCM-BP formed a complex with MCM3 to MCM7, which excluded MCM2; and, conversely, hexameric complexes of MCM2 to MCM7 lacked MCM-BP, indicating that MCM-BP can replace MCM2 in the MCM complex. MCM-BP-containing complexes exhibited increased stability under experimental conditions relative to those containing MCM2. MCM-BP also formed a complex with the MCM4/6/7 core helicase in vitro, but, unlike MCM2, did not inhibit this helicase activity. A proportion of MCM-BP bound to cellular chromatin in a cell cycle-dependent manner typical of MCM proteins, and, like other MCM subunits, preferentially associated with a cellular origin in G(1) but not in S phase. In addition, down-regulation of MCM-BP decreased the association of MCM4 with chromatin, and the chromatin association of MCM-BP was at least partially dependent on MCM4 and cdc6. The results indicate that multicellular eukaryotes contain two types of hexameric MCM complexes with unique properties and functions.

Roquin represses autoimmunity by limiting inducible T-cell co-stimulator messenger RNA.

Immune responses are normally targeted against microbial pathogens and not self-antigens by mechanisms that are only partly understood. Here we define a newly discovered pathway that prevents autoimmunity by limiting the levels on T lymphocytes of aco-stimulatory receptor, the inducible T-cell co-stimulator(ICOS). In sanroque mice homozygous for an M199R mutation in the ROQ domain of Roquin (also known as Rc3h1), increased Icos expression on T cells causes the accumulation of lymphocytes that is associated with a lupus-like autoimmune syndrome. Roquin normally limits Icos expression by promoting the degradation of Icos messenger RNA.A conserved segment in the unusually long ICOS 3' untranslated mRNA is essential for regulation by Roquin. This segment comprises a 47-base-pair minimal region complementary to T-cell-expressed microRNAs including miR-101, the repressive activity of which is disrupted by base-pair inversions predicted to abrogate miR-101 binding. These findings illuminate a critical post-transcriptional pathway within T cells that regulates lymphocyte accumulation and autoimmunity, and highlights the therapeutic potential of partially antagonising the ICOS pathway.