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ObjectiveNeutrophils are typically the most abundant leucocyte in arthritic synovial fluid. We sought to understand changes that occur in neutrophils as they migrate from blood to joint.MethodsWe performed RNA sequencing of neutrophils from healthy human blood, arthritic blood and arthritic synovial fluid, comparing transcriptional signatures with those from murine K/BxN serum transfer arthritis. We employed mass cytometry to quantify protein expression and sought to reproduce the synovial fluid phenotype ex vivo in cultured healthy blood neutrophils.ResultsBlood neutrophils from healthy donors and patients with active arthritis showed largely similar transcriptional signatures. By contrast, synovial fluid neutrophils exhibited more than 1600 differentially expressed genes. Gene signatures identified a prominent response to interferon gamma (IFN-γ), as well as to tumour necrosis factor, interleukin-6 and hypoxia, in both humans and mice. Mass cytometry confirmed that healthy and arthritic donor blood neutrophils are largely indistinguishable but revealed a range of neutrophil phenotypes in synovial fluid defined by downregulation of CXCR1 and upregulation of FcγRI, HLA-DR, PD-L1, ICAM-1 and CXCR4. Reproduction of key elements of this signature in cultured blood neutrophils required both IFN-γ and prolonged culture.ConclusionsCirculating neutrophils from patients with arthritis resemble those from healthy controls, but joint fluid cells exhibit a network of changes, conserved across species, that implicate IFN-γ response and ageing as complementary drivers of the synovial fluid neutrophil phenotype.

We study the classification problem of simple transitive 2-representations of the 2-category of finite-dimensional bimodules over a radical square zero Nakayama algebra. This results in a complete classification of simple transitive 2-representations whose apex is a finitary two-sided cell. We define a notion of localization of 2-representations. We construct previously unknown simple transitive 2-representations as localizations of cell 2-representations. Using the universal property of our construction we prove that any simple transitive 2-representation with finitary apex is equivalent to a localization of a cell 2-representation., Comment: 24 pages

BACKGROUND: Pro-inflammatory fibroblasts are critical for pathogenesis in rheumatoid arthritis, inflammatory bowel disease, interstitial lung disease, and Sjögren’s syndrome and represent a novel therapeutic target for chronic inflammatory disease. However, the heterogeneity of fibroblast phenotypes, exacerbated by the lack of a common cross-tissue taxonomy, has limited our understanding of which pathways are shared by multiple diseases. METHODS: We profiled fibroblasts derived from inflamed and non-inflamed synovium, intestine, lungs, and salivary glands from affected individuals with single-cell RNA sequencing. We integrated all fibroblasts into a multi-tissue atlas to characterize shared and tissue-specific phenotypes. FINDINGS: Two shared clusters, CXCL10(+)CCL19(+) immune-interacting and SPARC(+)COL3A1(+) vascular-interacting fibroblasts, were expanded in all inflamed tissues and mapped to dermal analogs in a public atopic dermatitis atlas. We confirmed these human pro-inflammatory fibroblasts in animal models of lung, joint, and intestinal inflammation. CONCLUSIONS: This work represents a thorough investigation into fibroblasts across organ systems, individual donors, and disease states that reveals shared pathogenic activation states across four chronic inflammatory diseases. FUNDING: Grant from F. Hoffmann-La Roche (Roche) AG.

Genetic studies have revealed that autoimmune susceptibility variants are over-represented in memory CD4+ T cell regulatory elements1–3. Understanding how genetic variation affects gene expression in different T cell physiological states is essential for deciphering genetic mechanisms of autoimmunity4,5. Here, we characterized the dynamics of genetic regulatory effects at eight time points during memory CD4+ T cell activation with high-depth RNA-seq in healthy individuals. We discovered widespread, dynamic allele-specific expression across the genome, where the balance of alleles changes over time. These genes were enriched fourfold within autoimmune loci. We found pervasive dynamic regulatory effects within six HLA genes. HLA-DQB1 alleles had one of three distinct transcriptional regulatory programs. Using CRISPR–Cas9 genomic editing we demonstrated that a promoter variant is causal for T cell–specific control of HLA-DQB1 expression. Our study shows that genetic variation in cis-regulatory elements affects gene expression in a manner dependent on lymphocyte activation status, contributing to the interindividual complexity of immune responses.

SummaryRheumatoid arthritis (RA) is a prototypical autoimmune disease that causes destructive tissue inflammation in joints and elsewhere. Clinical challenges in RA include the empirical selection of drugs to treat patients, inadequate responders with incomplete disease remission, and lack of a cure. We profiled the full spectrum of cells in inflamed synovium from patients with RA with the goal of deconstructing the cell states and pathways characterizing pathogenic heterogeneity in RA. Our multicenter consortium effort used multi-modal CITE-seq, RNA-seq, and histology of synovial tissue from 79 donors to build a >314,000 single-cell RA synovial cell atlas with 77 cell states from T, B/plasma, natural killer, myeloid, stromal, and endothelial cells. We stratified tissue samples into six distinct cell type abundance phenotypes (CTAPs) individually enriched for specific cell states. These CTAPs demonstrate the striking diversity of RA synovial inflammation, ranging from marked enrichment of T and B cells (CTAP-TB) to a congregation of specific myeloid, fibroblast, and endothelial cells largely lacking lymphocytes (CTAP-EFM). Disease-relevant cytokines, histology, and serology metrics are associated with certain CTAPs. This comprehensive RA synovial atlas and molecular, tissue-based CTAP stratification reveal new insights into RA pathology and heterogeneity, which could lead to novel targeted-treatment approaches in RA.

Immune checkpoint inhibitor (ICI) therapies that promote T cell activation have improved outcomes for advanced malignancies yet also elicit harmful autoimmune reactions. The T cell mechanisms mediating these iatrogenic autoimmune events remain unclear. Here we assayed T cells from joints of patients affected by ICI-induced inflammatory arthritis (ICI-arthritis), which can present clinically indistinguishable from rheumatoid arthritis (RA). Compared to the autoimmune arthritides RA and psoriatic arthritis (PsA), ICI-arthritis joints contained an expanded CD38hi CD127− CD8+ T cell subset that displays cytotoxic, effector, and interferon (IFN) response signatures. The abundance of CD38hi CD8 T cells in ICI-arthritis resulted from a limited number of clones that could be found proliferating in the joint. Exposure of synovial T cells to Type I IFN, more so than IFN-γ, induces the CD38hi cytotoxic phenotype. Relative to other CD8+ T cell subsets in the joints, the CD38hi population is distinct from a dysfunctional population and clonally most related to TCF7+ memory populations. Examination of synovial tissue from bilateral knee arthroplasty demonstrated considerable sharing of TCR clonotypes in the CD38hi CD8 T cell fraction from both knees. These results define a distinct CD8 T cell subset that may be directly activated by ICI therapy and mediate a tissue-specific autoimmune cellular reaction in patient joints.

ObjectivesIn lupus nephritis the pathological diagnosis from tissue retrieved during kidney biopsy drives treatment and management. Despite recent approval of new drugs, complete remission rates remain well under aspirational levels, necessitating identification of new therapeutic targets by greater dissection of the pathways to tissue inflammation and injury. This study assessed the safety of kidney biopsies in patients with SLE enrolled in the Accelerating Medicines Partnership, a consortium formed to molecularly deconstruct nephritis.Methods475 patients with SLE across 15 clinical sites in the USA consented to obtain tissue for research purposes during a clinically indicated kidney biopsy. Adverse events (AEs) were documented for 30 days following the procedure and were determined to be related or unrelated by all site investigators. Serious AEs were defined according to the National Institutes of Health reporting guidelines.Results34 patients (7.2%) experienced a procedure-related AE: 30 with haematoma, 2 with jets, 1 with pain and 1 with an arteriovenous fistula. Eighteen (3.8%) experienced a serious AE requiring hospitalisation; four patients (0.8%) required a blood transfusion related to the kidney biopsy. At one site where the number of cores retrieved during the biopsy was recorded, the mean was 3.4 for those who experienced a related AE (n=9) and 3.07 for those who did not experience any AE (n=140). All related AEs resolved.ConclusionsProcurement of research tissue should be considered feasible, accompanied by a complication risk likely no greater than that incurred for standard clinical purposes. In the quest for targeted treatments personalised based on molecular findings, enhanced diagnostics beyond histology will likely be required.

More than 1 million apheresis platelet collections are performed annually in the United States. After 2 healthy plateletpheresis donors were incidentally found to have low CD4+ T-lymphocyte counts, we investigated whether plateletpheresis causes lymphopenia. We conducted a cross-sectional single-center study of platelet donors undergoing plateletpheresis with the Trima Accel, which removes leukocytes continuously with its leukoreduction system chamber. We recruited 3 groups of platelet donors based on the total number of plateletpheresis sessions in the prior 365 days: 1 or 2, 3 to 19, or 20 to 24. CD4+ T-lymphocyte counts were

T cells acquire a regulatory phenotype when their T cell receptors (TCRs) experience an intermediate-high affinity interaction with a self-peptide presented on MHC. Using TCR sequences from FACS-sorted human cells, we identified TCR features that shape affinity to these self-peptide-MHC complexes, finding that 1) CDR3β hydrophobicity and 2) certain TRBV genes promote Treg fate. We developed a scoring system for TCR-intrinsic regulatory potential (TiRP) and found that within the tumor microenvironment clones exhibiting Treg-Tconv plasticity had higher TiRP than expanded clones maintaining the Tconv phenotype. To elucidate drivers of these predictive TCR features, we examined the two elements of the Treg TCR ligand separately: the self-peptide via murine Tregs, and the human MHC II molecule via human memory Tconvs. These analyses revealed that CDR3β hydrophobicity promotes reactivity to self-peptides, while TRBV gene usage shapes the TCR’s general propensity for MHC II-restricted activation.

T cells acquire a regulatory phenotype when their T cell antigen receptors (TCRs) experience an intermediate- to high-affinity interaction with a self-peptide presented via the major histocompatibility complex (MHC). Using TCRβ sequences from flow-sorted human cells, we identified TCR features that promote regulatory T cell (T

SARS-CoV-2 in an Immunocompromised Host This letter describes an immunocompromised patient who had persistent infection with SARS-CoV-2 over a period of months, despite several courses of remdesivi...

Systemic juvenile idiopathic arthritis (sJIA) begins with fever, rash, and high-grade systemic inflammation but commonly progresses to a persistent afebrile arthritis. The basis for this transition is unknown. To evaluate a role for lymphocyte polarization, we characterized T cells from patients with acute and chronic sJIA using flow cytometry, mass cytometry, and RNA sequencing. Acute and chronic sJIA each featured an expanded population of activated Tregs uncommon in healthy controls or in children with nonsystemic JIA. In acute sJIA, Tregs expressed IL-17A and a gene expression signature reflecting Th17 polarization. In chronic sJIA, the Th17 transcriptional signature was identified in T effector cells (Teffs), although expression of IL-17A at the protein level remained rare. Th17 polarization was abrogated in patients responding to IL-1 blockade. These findings identify evolving Th17 polarization in sJIA that begins in Tregs and progresses to Teffs, likely reflecting the impact of the cytokine milieu and consistent with a biphasic model of disease pathogenesis. The results support T cells as a potential treatment target in sJIA.

Significance The study addresses a basic immunology topic with considerable clinical relevance, namely the nature of the pathogenic T helper cells that drive autoimmune disorders like multiple sclerosis (MS) and inflammatory arthritis. Using mouse MS, we identify precursors of the encephalitogenic/pathogenic T cells based on their dependence on the protease inhibitor SerpinB1. Newly identified signature genes reveal the unusual nature of these T cells that combine ( i ) inflammatory cytokine secretion, ( ii ) a cytolytic system, and ( iii ) extreme rapid proliferation. We demonstrate that their survival/expansion depends on SerpinB1 and involves regulation of a proliferation-associated protease-mediated cell suicide mechanism. Importantly, we discovered that cell surface CXCR6 is an exquisite marker of pathogenic T helper cells and demonstrated that anti-CXCR6 treatment has potential to prevent or mitigate MS.

Lupus nephritis is a potentially fatal autoimmune disease for which the current treatment is ineffective and often toxic. To develop mechanistic hypotheses of disease, we analyzed kidney samples from patients with lupus nephritis and from healthy control subjects using single-cell RNA sequencing. Our analysis revealed 21 subsets of leukocytes active in disease, including multiple populations of myeloid cells, T cells, natural killer cells and B cells that demonstrated both pro-inflammatory responses and inflammation-resolving responses. We found evidence of local activation of B cells correlated with an age-associated B-cell signature and evidence of progressive stages of monocyte differentiation within the kidney. A clear interferon response was observed in most cells. Two chemokine receptors, CXCR4 and CX3CR1, were broadly expressed, implying a potentially central role in cell trafficking. Gene expression of immune cells in urine and kidney was highly correlated, which would suggest that urine might serve as a surrogate for kidney biopsies.

Understanding how genetic regulatory variation affects gene expression in different T cell states is essential to deciphering autoimmunity. We conducted a high-resolution RNA-seq time course analysis of stimulated memory CD4+T cells from 24 healthy individuals. We identified 186 genes with dynamic allele-specific expression, where the balance of alleles changes over time. These genes were four fold enriched in autoimmune loci. We found pervasive dynamic regulatory effects within six HLA genes, particularly for a major autoimmune risk gene,HLA-DQB1. EachHLA-DQB1allele had one of three distinct transcriptional regulatory programs. Using CRISPR/Cas9 genomic editing we demonstrated that a single nucleotide variant at the promoter is causal for T cell-specific control ofHLA-DQB1expression. Our study in CD4+T cells shows that genetic variation incisregulatory elements may affect gene expression in a lymphocyte activation status-dependent manner contributing to the inter-individual complexity of immune responses.

Objective:To develop an evidence-based guideline for the pharmacologic and nonpharmacologic treatment of psoriatic arthritis (PsA), as a collaboration between the American College of Rheumatology (ACR) and the National Psoriasis Foundation (NPF).Methods:We identified critical outcomes in PsA and clinically relevant PICO (population/intervention/comparator/outcomes) questions. A Literature Review Team performed a systematic literature review to summarize evidence supporting the benefits and harms of available pharmacologic and nonpharmacologic therapies for PsA. GRADE (Grading of Recommendations Assessment, Development and Evaluation) methodology was used to rate the quality of the evidence. A voting panel, including rheumatologists, dermatologists, other health professionals, and patients, achieved consensus on the direction and the strength of the recommendations.Results:The guideline covers the management of active PsA in patients who are treatment-naive and those who continue to have active PsA despite treatment, and addresses the use of oral small molecules, tumor necrosis factor inhibitors, interleukin-12/23 inhibitors (IL-12/23i), IL-17 inhibitors, CTLA4-Ig (abatacept), and a JAK inhibitor (tofacitinib). We also developed recommendations for psoriatic spondylitis, predominant enthesitis, and treatment in the presence of concomitant inflammatory bowel disease, diabetes, or serious infections. We formulated recommendations for a treat-to-target strategy, vaccinations, and nonpharmacologic therapies. Six percent of the recommendations were strong and 94% conditional, indicating the importance of active discussion between the health care provider and the patient to choose the optimal treatment.Conclusion:The 2018 ACR/NPF PsA guideline serves as a tool for health care providers and patients in the selection of appropriate therapy in common clinical scenarios. Best treatment decisions consider each individual patient situation. The guideline is not meant to be proscriptive and should not be used to limit treatment options for patients with PsA.

Lupus nephritis is a potentially fatal autoimmune disease, whose current treatment is ineffective and often toxic. To gain insights into disease mechanisms, we analyzed kidney samples from lupus nephritis patients and healthy controls using single-cell RNA-seq. Our analysis revealed 21 subsets of leukocytes active in disease, including multiple populations of myeloid, T, NK and B cells, demonstrating both pro-inflammatory and resolving responses. We found evidence of local activation of B cells correlated with an age-associated B cell signature, and of progressive stages of monocyte differentiation within the kidney. A clear interferon response was observed in most cells. Two chemokine receptors, CXCR4 and CX3CR1, were broadly expressed, pointing to potential therapeutic targets. Gene expression of immune cells in urine and kidney was highly correlated, suggesting urine may be a surrogate for kidney biopsies. Our results provide a first comprehensive view of the complex network of leukocytes active in lupus nephritis kidneys.

In this paper, we describe the combinatorics of the cell structure of the tensor category of bimodules over a radical square zero Nakayama algebra. This accounts to an explicit description of left, right and two-sided cells.

Natural killer (NK) cells have important functions in cancer immunosurveillance, BM allograft rejection, fighting infections, tissue homeostasis, and reproduction. NK cell–based therapies are promising treatments for blood cancers. Overcoming their currently limited efficacy requires a better understanding of the molecular mechanisms controlling NK cell development and dampening their effector functions. NK cells recognize the loss of self-antigens or up-regulation of stress-induced ligands on pathogen-infected or tumor cells through invariant NK cell receptors (NKRs), and then kill such stressed cells. Two second-messenger pathways downstream of NKRs are required for NK cell maturation and effector responses: PIP3 generation by PI3K and generation of diacylglycerol and IP3 by phospholipase-Cγ (PLCγ). In the present study, we identify a novel role for the phosphorylated IP3 metabolite inositol (1,3,4,5)tetrakisphosphate (IP4) in NK cells. IP4 promotes NK cell terminal differentiation and acquisition of a mature NKR repertoire. However, in mature NK cells, IP4 limits NKR-induced IFNγ secretion, granule exocytosis, and target-cell killing, in part by inhibiting the PIP3 effector-kinase Akt. This identifies IP4 as an important novel regulator of NK cell development and function and expands our understanding of the therapeutically important mechanisms dampening NK cell responses. Our results further suggest that PI3K regulation by soluble IP4 is a broadly important signaling paradigm.

NK cells are innate immune lymphocytes that can react to cells lacking self-MHC class I. However, NK cells that cannot engage self-MHC through an inhibitory receptor are resistant to stimulation through their activation receptors. To become licensed (i.e., functionally competent to be triggered through its activation receptors), an NK cell must engage host MHC class I via a MHC class I-specific inhibitory receptor, such as a member of the murine Ly49 family. To explore potential determinants of NK cell licensing on a single Ly49 receptor, we have investigated the relative licensing impacts of the b, d, k, q, r, and s H2 haplotypes on Ly49A+ NK cells. The results indicate that licensing is essentially analog but is saturated by moderate-binding MHC class I ligands. Interestingly, licensing exhibited a strong inverse correlation with a measure of cis engagement of Ly49A. Finally, licensing of Ly49A+ NK cells was found to be less sensitive to MHC class I engagement than Ly49A-mediated effector inhibition, suggesting that licensing establishes a margin of safety against NK cell autoreactivity.

The adaptive immune response is tightly regulated to limit responding cells in an Ag-specific manner. On B cells, coreceptors CD21/CD19 modulate the strength of BCR signals, potentially influencing cell fate. The importance of the CD95 pathway was examined in response of B cells to moderate affinity Ag using an adoptive transfer model of lysozyme-specific Ig transgenic (HEL immunoglobulin transgene (MD4) strain) B cells. Although adoptively transferred Cr2+/+ MD4 B cells are activated and persist within splenic follicles of duck egg lysozyme-immunized mice, Cr2−/− MD4 B cells do not. In contrast, Cr2−/− MD4 lpr B cells persist after transfer, suggesting that lack of CD21/CD35 signaling results in CD95-mediated elimination. Cr2 deficiency did not affect CD95 levels, but cellular FLIP (c-FLIP) protein and mRNA levels were reduced 2-fold compared with levels in Cr2+/+ MD4 B cells. In vitro culture with Cr2+/+ MD4 B cells demonstrated that equimolar amounts of rHEL-C3d3 were more effective than hen egg lysozyme alone in up-regulating c-FLIP levels and for protection against CD95-mediated apoptosis. Collectively, this study implies a mechanism for regulating B cell survival in vivo whereby the strength of BCR signaling (including coreceptor) determines c-FLIP levels and protection from CD95-induced death.

In inflammatory arthridities such as rheumatoid arthritis, cognate lymphocytes have long been considered instigators of autoimmunity, but accumulating evidence indicates that innate immune cells such as neutrophils and mast cells are responsible for a vast majority of acute and ongoing inflammation; however, the molecular mechanisms that govern them remain largely unknown. Here we show that such inflammation requires the forkhead transcription factor Foxo3a: Foxo3a-deficient mice are resistant to two models of neutrophilic inflammation, immune complex-mediated inflammatory arthritis and thioglycollate-induced peritonitis. This reflects a need for Foxo3a to maintain neutrophil vitality during inflammation by suppressing Fas ligand; because Foxo3a can bind and suppress the Fasl promoter, Foxo3a-deficient neutrophils upregulate Fas ligand and undergo apoptosis in response to TNF-alpha and IL-1, and Fas ligand blockade renders Foxo3a-deficient mice susceptible to both arthritis and peritonitis. Thus, Foxo3a ensures neutrophil survival during inflammation, identifying Foxo3a as therapeutic target in inflammation.

Mouse natural killer (NK) cells acquire effector function by an education process termed "licensing" mediated by inhibitory Ly49 receptors which recognize self-MHC class I. Ly49 receptors can bind to MHC class I on targets (in trans) and also to MHC class I on the NK-cell surface (in cis). Which of these interactions regulates NK-cell licensing is not yet clear. Moreover, there are no clear phenotypic differences between licensed and unlicensed NK cells, perhaps because of the previously limited ability to study NK cells with synchronized licensing. Here, we produced MHC class I-deficient mice with inducible MHC class I consisting of a single-chain trimer (SCT), ovalbumin peptide-β2 microgloblin-H2K(b) (SCT-K(b)). Only NK cells with a Ly49 receptor with specificity for SCT-K(b) were licensed after MHC class I induction. NK cells were localized consistently in red pulp of the spleen during induced NK-cell licensing, and there were no differences in maturation or activation markers on recently licensed NK cells. Although MHC class I-deficient NK cells were licensed in hosts following SCT-K(b) induction, NK cells were not licensed after induced SCT-K(b) expression on NK cells themselves in MHC class I-deficient hosts. Furthermore, hematopoietic cells with induced SCT-K(b) licensed NK cells more efficiently than stromal cells. These data indicate that trans interaction with MHC class I on hematopoietic cells regulates NK-cell licensing, which is not associated with other obvious phenotypic changes.

Natural killer (NK) cells express receptors to detect and kill target cells based on expression of target cell surface molecules. Through a process termed NK cell licensing, only NK cells that express inhibitory receptors (e.g., Ly49 receptors in the mouse) for self-major histocompatibility complex (MHC) class I molecules become functionally competent to be triggered through their activation receptors. To determine the licensing status of particular Ly49(+) murine NK cell subsets, splenocytes are stimulated with plate-bound anti-NK1.1 monoclonal antibody in the presence of brefeldin A and then assessed for NK cell activation on a single-cell basis using intracellular cytokine interferon-gamma staining and flow cytometry.

Natural killer (NK) cells express receptors to detect and kill target cells based on expression of target cell surface molecules. Through a process termed NK cell licensing, only NK cells that express inhibitory receptors (e.g., Ly49 receptors in the mouse) for self-major histocompatibility complex (MHC) class I molecules become functionally competent to be triggered through their activation receptors. To determine the licensing status of particular Ly49(+) murine NK cell subsets, splenocytes are stimulated with plate-bound anti-NK1.1 monoclonal antibody in the presence of brefeldin A and then assessed for NK cell activation on a single-cell basis using intracellular cytokine interferon-gamma staining and flow cytometry.

SummaryNKG2D is a potent activating receptor on natural killer (NK) cells and acts as a molecular sensor for stressed cells expressing NKG2D ligands such as infected or tumor-transformed cells. Although NKG2D is expressed on NK cell precursors, its role in NK cell development is not known. We have generated NKG2D-deficient mice by targeting the Klrk1 locus. Here we provide evidence for an important regulatory role of NKG2D in the development of NK cells. The absence of NKG2D caused faster division of NK cells, perturbation in size of some NK cell subpopulations, and their augmented sensitivity to apoptosis. As expected, Klrk1−/− NK cells are less responsive to tumor targets expressing NKG2D ligands. Klrk1−/− mice, however, showed an enhanced NK cell-mediated resistance to mouse cytomegalovirus infection as a consequence of NK cell dysregulation. Altogether, these findings provide evidence for regulatory function of NKG2D in NK cell physiology.

Armed with potent cytotoxic and immunostimulatory effector functions, natural killer (NK) cells have the potential to cause significant damage to normal self cells unless controlled by self-tolerance mechanisms. NK cells identify and attack target cells based on integration of signals from activation and inhibitory receptors, whose ligands exhibit complex expression and/or binding patterns. Preservation of NK cell self-tolerance must therefore go beyond mere engagement of inhibitory receptors during effector functions. Herein, we review recent work that has uncovered a number of mechanisms to ensure self-tolerance of NK cells. For example, licensing of NK cells allows only NK cells that can engage self-MHC to become functionally competent, or licensed. The molecular mechanism of this phenomenon appears to require signaling by receptors that were originally identified in effector inhibition. However, the nature of the signaling event has not yet been defined, but new interpretations of several published experiments provide valuable clues. In addition, several other cell-intrinsic and -extrinsic mechanisms of NK cell tolerance are discussed, including activation receptor cooperation and synergy, cytokine stimulation, and the opposing roles of accessory and regulatory cells. Finally, NK cell tolerance is discussed as it relates to the clinic, such as KIR-HLA disease associations, tumor immunotherapy, and fetal tolerance.

Natural killer (NK) cells are innate immune cells that mediate resistance against viruses and tumors. They express multiple activating receptors that couple to immunoreceptor tyrosine-based activation motif (ITAM)–containing signaling chains for downstream cell activation. Ligation of activating NK-cell receptors induces NK-cell cytotoxicity and cytokine release. How these distinct events are selectively controlled is not well defined. Here we report the identification of a specific signaling pathway that operates downstream of the ITAM-coupled NK-cell receptors NK1.1, Ly49D, Ly49H, and NKG2D. Using primary NK cells from Bcl10−/−, Malt1−/−, Carma1−/−, and Card9−/− mice, we demonstrate a key role for Bcl10 signalosomes in the activation of canonical NF-κB signaling as well as JNK and p38 MAPK upon NK-cell triggering. Bcl10 directly cooperates with Malt1 and depends on Carma1 (Card11) but not on Card9 for NK-cell activation. These Bcl10-dependent cascades selectively control cytokine and chemokine production but do not affect NK-cell differentiation or killing. Thus, we identify a molecular basis for the segregation of NK-cell receptor–induced signals for cytokine release and target cell killing and extend the previously recognized roles for CARD-protein/Bcl10/Malt1 complexes in ITAM receptor signaling in innate and adaptive immune cells.

NK cells have important functions in cancer immunosurveillance, BM allograft rejection, fighting infections and reproduction. NK cell-based therapies are promising blood cancer treatments. Overcoming their limited efficacy requires a better understanding of the molecular mechanisms dampening their effector functions. NK cells recognize and kill pathogen-infected or tumor cells through invariant NK cell receptors (NKR). Two second-messenger pathways downstream of NKRs are required for NK cell maturation and effector responses: PIP3-generation by PI3K, and generation of DAG and IP3 by PLCγ. IP3 plays a key signaling role by mobilizing Calcium, but can also be converted into soluble IP4. We and others previously showed that IP4 can act as a soluble analog of the PI3K lipid-product PIP3 and control PIP3-mediated protein membrane recruitment and activation in thymocytes and granulocytes. Here, we show data which suggest a novel IP4 function in promoting NK cell terminal differentiation and acquisition of a mature NKR repertoire. However, in mature NK cells, IP4 limits NKR induced IFNγ secretion, granule exocytosis and target-cell clearance, in part by inhibiting the PIP3 effector Akt. This identifies IP4 as an important novel regulator of NK cell development and function, and expands our understanding of the therapeutically important mechanisms dampening NK cell responses. Our results further suggest that PI3K regulation by soluble IP4 is a broadly important signaling paradigm.

NK cells react to cells that lack self-MHC class I. Yet, since many NK cells cannot recognize self-MHC, mechanisms such as NK cell licensing protect against autoreactivity. To become licensed, i.e. functionally competent to be triggered through its activation receptors, an NK cell must engage host MHC class I via at least one of its MHC class I-specific inhibitory receptors, such as the Ly49 family of receptors in the mouse. However, the general determinants of this process remain largely unknown. Herein, we investigated the licensing impact of the b, d, f, k, q, r, and s H2 haplotypes on Ly49A+ NK cells in MHC-congenic mice. Ex vivo PK136 (anti-NK1.1) stimulation assays indicated that licensing may not be a binary phenomenon as some Ly49A-MHC class I haplotype combinations produced an intermediate licensing phenotype. Ly49A surface accessibility, a measure of cis binding with MHC class I, and Ly49A tetramer binding displayed a similar variability among the MHC haplotypes. Taken together, the data indicate a correlation between strength of licensing and strength of binding of Ly49A receptors with MHC class I, both of which are inversely correlated with surface accessibility of Ly49A receptors. These findings suggest that NK cell licensing is modulated by the strength of the interaction of inhibitory receptors with host MHC class I. Supported by HHMI and grants from the NIH.

We study the problem of classification of simple transitive 2-representations for the (non-finitary) 2-category of bimodules over the dual numbers. We show that simple transitive 2-representations with finitary apex are necessarily of rank 1 or 2, and those of rank 2 are exactly the cell 2-representations. For 2-representations of rank 1, we show that they cannot be constructed using the approach of (co)algebra 1-morphisms. We also propose an alternative definition of (co-)Duflo 1-morphisms for finitary 2-categories and describe them in the case of bimodules over the dual numbers.