Genetic associations of LYN with systemic lupus erythematosus

The abnormal signaling of the B cell receptor and co-receptors of lupus B cells

It is easily understood that studying the physiology and pathophysiology of the BCRtriggered cascade is of importance, particularly in such diseases as systemic lupus erythematosus (SLE) that are considered by many as a "B cell disease". Even though B cells are not considered as the only players in lupus pathogenesis, and other immune and non-immune cells are certainly involved, it is the success of recent B cell-targeting treatment strategies that ascribe a critical role to the lupus B cell.

B-1a Cells, but Not Marginal Zone B Cells, Are Implicated in the Accumulation of Autoreactive Plasma Cells in Lyn-/- Mice

Mice deficient in Lyn, a tyrosine kinase that limits B cell activation, develop a lupus-like autoimmune disease characterized by the accumulation of splenic plasma cells and the production of autoantibodies. Lyn-/- mice have reduced numbers of marginal zone (MZ) B cells, a B cell subset that is enriched in autoreactivity and prone to plasma cell differentiation. We hypothesized that this is due to unchecked terminal differentiation of this potentially pathogenic B cell subpopulation. However, impairing MZ B cell development in Lyn-/- mice did not reduce plasma cell accumulation or autoantibodies, and preventing plasma cell differentiation did not restore MZ B cell numbers. Instead, Lyn-/- mice accumulated B-1a cells when plasma cell differentiation was impaired. Similar to MZ B cells, B-1a cells tend to be polyreactive or weakly autoreactive and are primed for terminal differentiation. Our results implicate B-1a cells, but not MZ B cells, as contributors to the autoreactive plasma cell pool in Lyn-/- mice.

Identification of new candidate drugs for primary Sjögren's syndrome using a drug repurposing transcriptomic approach

OBJECTIVES

To date, no immunomodulatory drug has demonstrated its efficacy in primary SS (pSS). We sought to analyse potential commonalities between pSS transcriptomic signatures and signatures of various drugs or specific knock-in or knock-down genes.

METHODS

Gene expression from peripheral blood samples of patients with pSS was compared with that of healthy controls in two cohorts and three public databases. In each of the five datasets, we analysed the 150 most up- and downregulated genes between pSS patients and controls with regard to the differentially expressed genes resulting from the biological action on nine cell lines of 2837 drugs, 2160 knock-in and 3799 knock-down genes in the Connectivity Map database.

RESULTS

We analysed 1008 peripheral blood transcriptomes from five independent studies (868 patients with pSS and 140 healthy controls). Eleven drugs could represent potential candidate drugs, with histone deacetylases and PI3K inhibitors among the most significantly associated. Twelve knock-in genes were associated with a pSS-like profile and 23 knock-down genes were associated with a pSS-revert profile. Most of those genes (28/35, 80%) were interferon-regulated.

CONCLUSION

This first drug repositioning transcriptomic approach in SS confirms the interest of targeting interferons and identifies histone deacetylases and PI3K inhibitors as potential therapeutic targets.

T-bet-expressing B cells contribute to the autoreactive plasma cell pool in Lyn-/- mice

Systemic Lupus Erythematosus (SLE) is characterized by pathogenic autoantibodies against nucleic acid-containing antigens. Understanding which B-cell subsets give rise to these autoantibodies may reveal therapeutic approaches for SLE that spare protective responses. Mice lacking the tyrosine kinase Lyn, which limits B and myeloid cell activation, develop lupus-like autoimmune diseases characterized by increased autoreactive plasma cells (PCs). We used a fate-mapping strategy to determine the contribution of T-bet+ B cells, a subset thought to be pathogenic in lupus, to the accumulation of PCs and autoantibodies in Lyn-/- mice. Approximately, 50% of splenic PCs in Lyn-/- mice originated from T-bet+ cells, a significant increase compared to WT mice. In vitro, splenic PCs derived from T-bet+ B cells secreted both IgM and IgG anti-dsDNA antibodies. To determine the role of these cells in autoantibody production in vivo, we prevented T-bet+ B cells from differentiating into PCs or class switching in Lyn-/- mice. This resulted in a partial reduction in splenic PCs and anti-dsDNA IgM and complete abrogation of anti-dsDNA IgG. Thus, T-bet+ B cells make an important contribution to the autoreactive PC pool in Lyn-/- mice.

An Optimized Flow Cytometric Method to Demonstrate the Differentiation Stage-Dependent Ca2+ Flux Responses of Peripheral Human B Cells

Calcium (Ca²⁺) flux acts as a central signaling pathway in B cells, and its alterations are associated with autoimmune dysregulation and B-cell malignancies. We standardized a flow-cytometry-based method using various stimuli to investigate the Ca²⁺ flux characteristics of circulating human B lymphocytes from healthy individuals. We found that different activating agents trigger distinct Ca²⁺ flux responses and that B-cell subsets show specific developmental-stage dependent Ca²⁺ flux response patterns. Naive B cells responded with a more substantial Ca²⁺ flux to B cell receptor (BCR) stimulation than memory B cells. Non-switched memory cells responded to anti-IgD stimulation with a naive-like Ca²⁺ flux pattern, whereas their anti-IgM response was memory-like. Peripheral antibody-secreting cells retained their IgG responsivity but showed reduced Ca²⁺ responses upon activation, indicating their loss of dependence on Ca²⁺ signaling. Ca²⁺ flux is a relevant functional test for B cells, and its alterations could provide insight into pathological B-cell activation development.

B cells from anti-thyroid antibody positive, infertile women show hyper-reactivity to BCR stimulation

Anti-thyroid antibody (ATA) positivity affects 1 out of 9 women in childbearing age and presents a significant risk for infertility. Emerging evidence indicates that alterations in the B cell receptor induced calcium (Ca2+) signaling could be key in the development of autoimmunity. We aimed to investigate the Ca2+ flux response of B lymphocyte subsets to BCR stimulation in Hashimoto’s thyroiditis and related infertility. We collected peripheral blood samples from ATA+, infertile, euthyroid patients (HIE), hypothyroid, ATA+ patients before (H1) and after levothyroxine treatment (H2), and age-matched healthy controls (HC). All B cell subsets of ATA+, infertile, euthyroid patients showed elevated basal Ca2+ level and hyper-responsivity to BCR ligation compared to the other groups, which could reflect altered systemic immune function. The Ca2+ flux of hypothyroid patients was similar to healthy controls. The levothyroxine-treated patients had decreased prevalence of CD25+ B cells and lower basal Ca2+ level compared to pre-treatment. Our results support the role of altered Ca2+ flux of B cells in the early phase of thyroid autoimmunity and infertility.

The interactions of CAP and LYN with the insulin signaling transducer CBL play an important role in polycystic ovary syndrome

BACKGROUND

Polycystic ovary syndrome (PCOS) is a hormonal disorder characterized by hyperandrogenism, ovulatory dysfunction, and insulin resistance. Evidence suggests that aberrations in insulin signaling-associated pathways may underlie PCOS pathogenesis. Our aim was to investigate the molecular mechanisms underlying PCOS and associated insulin resistance using in silico analyses, in vitro cell models, and in vivo murine models.

METHODS

R-based bioinformatics analysis was performed on granulosa cell microarray data from three human cohorts: healthy control, PCOS patients without insulin resistance, and PCOS patients with insulin resistance. Transgenic human granulosa cell models were utilized for in vitro studies. Transgenic murine models of dehydroepiandrosterone (DHEA)-induced PCOS were utilized for in vivo studies.

RESULTS

Sorbin and SH3 Domain Containing 1 (SORBS1), the parent gene of the insulin receptor-associated Casitas B-lineage lymphoma protein (CBL)-associated protein (CAP), is a key downregulated gene in PCOS patients with insulin resistance. CAP binding to CBL reduced CBL^Y731 phosphorylation, CBL-phosphoinositide 3-kinase (PI3K) p85α interactivity, protein kinase B (Akt)^S473 phosphorylation, and NFκB-induced inflammatory marker expression but enhanced CRKII-mediated membrane GLUT4 translocation in granulosa cells. In contrast, the tyrosine kinase Lck/Yes-Related Novel Protein (LYN) is upregulated in PCOS patients with insulin resistance. LYN binding to CBL enhanced CBL^Y731 phosphorylation, CBL-PI3K p85α interactivity, Akt^S473 phosphorylation, and NFκB-induced inflammatory marker expression but did not impact membrane GLUT4 translocation. In PCOS mice, Cap overexpression, Cap transactivation by metformin, or enhancing Cbl-CrkII binding improved insulin sensitivity and ovarian dysfunction (i.e., estrous cycle disruption, cyst-like follicle formation, and sex hormone dysregulation). In contrast, Lyn knockdown, Lyn inhibition by PP2, or CBL-PI3K p85α blockade improved only ovarian dysfunction. Cbl^3YF phosphomutant overexpression (which enhances Cbl-CrkII binding but blocks Cbl-PI3K p85α binding) ameliorated both ovarian dysfunction and insulin resistance.

CONCLUSIONS

The interactions of CAP and LYN with CBL, and the resulting effects on CBL phosphorylation and activity, may play an important role in PCOS pathogenesis. Targeting these players may be a viable therapeutic strategy for PCOS.

Decreased BAFF Receptor Expression and Unaltered B Cell Receptor Signaling in Circulating B Cells from Primary Sjögren's Syndrome Patients at Diagnosis

Animal models of autoimmunity and human genetic association studies indicate that the dysregulation of B-cell receptor (BCR) signaling is an important driver of autoimmunity. We previously showed that in circulating B cells from primary Sjögren’s syndrome (pSS) patients with high systemic disease activity, protein expression of the BCR signaling molecule Bruton’s tyrosine kinase (BTK) was increased and correlated with T-cell infiltration in the target organ. We hypothesized that these alterations could be driven by increased B-cell activating factor (BAFF) levels in pSS. Here, we investigated whether altered BCR signaling was already present at diagnosis and distinguished pSS from non-SS sicca patients. Using (phospho-)flow cytometry, we quantified the phosphorylation of BCR signaling molecules, and investigated BTK and BAFF receptor (BAFFR) expression in circulating B cell subsets in an inception cohort of non-SS sicca and pSS patients, as well as healthy controls (HCs). We found that both BTK protein levels and BCR signaling activity were comparable among groups. Interestingly, BAFFR expression was significantly downregulated in pSS, but not in non-SS sicca patients, compared with HCs, and correlated with pSS-associated alterations in B cell subsets. These data indicate reduced BAFFR expression as a possible sign of early B cell involvement and a diagnostic marker for pSS.

A dominant function of LynB kinase in preventing autoimmunity

Here, we report that the LynB splice variant of the Src-family kinase Lyn exerts a dominant immunosuppressive function in vivo, whereas the LynA isoform is uniquely required to restrain autoimmunity in female mice. We used CRISPR-Cas9 gene editing to constrain lyn splicing and expression, generating single-isoform LynA knockout (LynAKO) or LynBKO mice. Autoimmune disease in total LynKO mice is characterized by production of antinuclear antibodies, glomerulonephritis, impaired B cell development, and overabundance of activated B cells and proinflammatory myeloid cells. Expression of LynA or LynB alone uncoupled the developmental phenotype from the autoimmune disease: B cell transitional populations were restored, but myeloid cells and differentiated B cells were dysregulated. These changes were isoform-specific, sexually dimorphic, and distinct from the complete LynKO. Despite the apparent differences in disease etiology and penetrance, loss of either LynA or LynB had the potential to induce severe autoimmune disease with parallels to human systemic lupus erythematosus (SLE).

Copy Number Variation and Frequency of rs179008 in TLR7 Gene Associated with Systemic Lupus Erythematosus in Two Mexican Populations

Systemic Lupus Erythematosus (SLE) is an autoimmune disease in which genetic factors play a role in the susceptibility to develop it. Genes related to the synthesis of interferons such as TLR7 and genetics factors such as single nucleotide polymorphisms (SNPs) or copies number variation (CNV) in the gene have been involved with the development of the disease. The genetic differences between the populations contribute to the complexity of LES. Mexico has a mestizo population with a genetic load of at least three origins: Amerindian, Caucasian, and African. The mestizo of Yucatán is the only group whose contribution Amerindian is mainly Mayan, geographically distant from other Mexican Amerindians. We analyzed the CNV and the frequency of SNP rs179008 of the TLR7 as genetic risk factors in developing the disease in patients from Yucatán and Central Mexico. Results show that 14% of the cases of the Yucatecan population showed significantly >2 CNV and a higher risk of developing the disease (OR: 34.364), concerning 4% of those coming from Central Mexico (OR: 10.855). T allele and the A/T and T/T risk genotypes of rs179008 were more frequent in patients of Central Mexico than in those of Yucatán (50% vs. 30%, 93% vs. 30%, 4% vs. 1%), and association with susceptibility to develop SLE was observed (OR: 1.5 vs. 0.58, 9.54 vs. 0.66, 12 vs. 0.14). Data support the genetic differences between and within Mexican mestizo populations and the role of the TLR7 in the pathogenesis of SLE.

The Src-family Kinase Lyn in Immunoreceptor Signaling

Effective regulation of immune-cell activation is critical for ensuring that the immune response, and inflammation generated for the purpose of pathogen elimination, are limited in space and time to minimize tissue damage. Autoimmune disease can occur when immunoreceptor signaling is dysregulated, leading to unrestrained inflammation and organ damage. Conversely, tumors can coopt the tissue healing and immunosuppressive functions of hematopoietic cells to promote metastasis and evade therapy. The Src-family kinase Lyn is an essential regulator of immunoreceptor signaling, initiating both proinflammatory and suppressive signaling pathways in myeloid immune cells (eg, neutrophils, dendritic cells, monocytes, macrophages) and in B lymphocytes. Defects in Lyn signaling are implicated in autoimmune disease, but mechanisms by which Lyn, expressed along with a battery of other Src-family kinases, may uniquely direct both positive and negative signaling remain incompletely defined. This review describes our current understanding of the activating and inhibitory contributions of Lyn to immunoreceptor signaling and how these processes contribute to myeloid and B-cell function. We also highlight recent work suggesting that the 2 proteins generated by alternative splicing of lyn, LynA and LynB, differentially regulate both immune and cancer-cell signaling. These principles may also extend to other Lyn-expressing cells, such as neuronal and endocrine cells. Unraveling the common and cell-specific aspects of Lyn function could lead to new approaches to therapeutically target dysregulated pathways in pathologies ranging from autoimmune and neurogenerative disease to cancer.

Bruton's Tyrosine Kinase Inhibition as an Emerging Therapy in Systemic Autoimmune Disease

Systemic autoimmune disorders are complex heterogeneous chronic diseases involving many different immune cells. A significant proportion of patients respond poorly to therapy. In addition, the high burden of adverse effects caused by "classical" anti-rheumatic or immune modulatory drugs provides a need to develop more specific therapies that are better tolerated. Bruton's tyrosine kinase (BTK) is a crucial signaling protein that directly links B-cell receptor (BCR) signals to B-cell activation, proliferation, and survival. BTK is not only expressed in B cells but also in myeloid cells, and is involved in many different signaling pathways that drive autoimmunity. This makes BTK an interesting therapeutic target in the treatment of autoimmune diseases. The past decade has seen the emergence of first-line BTK small-molecule inhibitors with great efficacy in the treatment of B-cell malignancies, but with unfavorable safety profiles for use in autoimmunity due to off-target effects. The development of second-generation BTK inhibitors with superior BTK specificity has facilitated the investigation of their efficacy in clinical trials with autoimmune patients. In this review, we discuss the role of BTK in key signaling pathways involved in autoimmunity and provide an overview of the different inhibitors that are currently being investigated in clinical trials of systemic autoimmune diseases, including rheumatoid arthritis and systemic lupus erythematosus, as well as available results from completed trials.

Therapeutic implications of the anergic/postactivated status of B cells in systemic lupus erythematosus

Systemic lupus erythematosus (SLE) is characterised by numerous abnormalities in B lineage cells, including increased CD27++ plasmablasts/plasma cells, atypical CD27-IgD- B cells with increased CD95, spleen tyrosine kinase (Syk)++, CXCR5- and CXCR5+ subsets and anergic CD11c+Tbet+ age-associated B cells. Most findings, together with preclinical lupus models, support the concept of B cell hyperactivity in SLE. However, it remains largely unknown whether these specific B cell subsets have pathogenic consequences and whether they provide relevant therapeutic targets. Recent findings indicate a global distortion of B cell functional capability, in which the entire repertoire of naïve and memory B cells in SLE exhibits an anergic or postactivated (APA) functional phenotype. The APA status of SLE B cells has some similarities to the functional derangement of lupus T cells. APA B cells are characterised by reduced global cytokine production, diminished B cell receptor (BCR) signalling with decreased Syk and Bruton's tyrosine kinase phosphorylation related to repeated in vivo BCR stimulation as well as hyporesponsiveness to toll-like receptor 9 engagement, but intact CD40 signalling. This APA status was related to constitutive co-localisation of CD22 linked to phosphatase SHP-1 and increased overall protein phosphatase activities. Notably, CD40 co-stimulation could revert this APA status and restore BCR signalling, downregulate protein tyrosine phosphatase transcription and promote B cell proliferation and differentiation. The APA status and their potential rescue by bystander help conveyed through CD40 stimulation not only provides insights into possible mechanisms of escape of autoreactive clones from negative selection but also into novel ways to target B cells therapeutically.

Structural and immunogenomic insights into B-cell receptor activation

B cells express B-cell receptors (BCRs) which recognize antigen to trigger signaling cascades for B-cell activation and subsequent antibody production. BCR activation has a crucial influence on B-cell fate. How BCR is activated upon encountering antigen remains to be solved, although tremendous progresses have been achieved in the past few years. Here, we summarize the models that have been proposed to explain BCR activation, including the cross-linking model, the conformation-induced oligomerization model, the dissociation activation model, and the conformational change model. Especially, we elucidate the partially resolved structures of antibodies and/or BCRs by far and discusse how these current structural and further immunogenomic messages and more importantly the future studies may shed light on the explanation of BCR activation and the relevant diseases in the case of dysregulation.

Juvenile-onset systemic lupus erythematosus: Update on clinical presentation, pathophysiology and treatment options

Juvenile-onset systemic lupus erythematosus (jSLE) accounts for up to 20% of all SLE patients. Key differences between juvenile- and adult-onset (aSLE) disease include higher disease activity, earlier development of damage, and increased use of immunosuppressive treatment in jSLE suggesting (at least partial) infectivity secondary to variable pathomechanisms. While the exact pathophysiology of jSLE remains unclear, genetic factors, immune complex deposition, complement activation, hormonal factors and immune cell dysregulation are involved to variable extents, promising future patient stratification based on immune phenotypes. Though less effective and potentially toxic, jSLE patients are treated based upon evidence from studies in aSLE cohorts. Here, age-specific clinical features of jSLE, underlying pathomechanisms, treatment options and disease outcomes will be addressed. Future directions to improve the care of jSLE patients, including implementation of the Single Hub and Access point for pediatric Rheumatology in Europe (SHARE) recommendations, biomarkers, treat to target and personalized medicine approaches are discussed.

Identification and Characterization of Post-activated B Cells in Systemic Autoimmune Diseases

Autoimmune diseases (AID) such as systemic lupus erythematosus (SLE), primary Sjögren's syndrome (pSS), and rheumatoid arthritis (RA) are chronic inflammatory diseases in which abnormalities of B cell function play a central role. Although it is widely accepted that autoimmune B cells are hyperactive in vivo, a full understanding of their functional status in AID has not been delineated. Here, we present a detailed analysis of the functional capabilities of AID B cells and dissect the mechanisms underlying altered B cell function. Upon BCR activation, decreased spleen tyrosine kinase (Syk) and Bruton's tyrosine kinase (Btk) phosphorylation was noted in AID memory B cells combined with constitutive co-localization of CD22 and protein tyrosine phosphatase (PTP) non-receptor type 6 (SHP-1) along with hyporesponsiveness to TLR9 signaling, a Syk-dependent response. Similar BCR hyporesponsiveness was also noted specifically in SLE CD27⁻ B cells together with increased PTP activities and increased transcripts for PTPN2, PTPN11, PTPN22, PTPRC, and PTPRO in SLE B cells. Additional studies revealed that repetitive BCR stimulation of normal B cells can induce BCR hyporesponsiveness and that tissue-resident memory B cells from AID patients also exhibited decreased responsiveness immediately ex vivo, suggesting that the hyporesponsive status can be acquired by repeated exposure to autoantigen(s) in vivo. Functional studies to overcome B cell hyporesponsiveness revealed that CD40 co-stimulation increased BCR signaling, induced proliferation, and downregulated PTP expression (PTPN2, PTPN22, and receptor-type PTPs). The data support the conclusion that hyporesponsiveness of AID and especially SLE B cells results from chronic in vivo stimulation through the BCR without T cell help mediated by CD40–CD154 interaction and is manifested by decreased phosphorylation of BCR-related proximal signaling molecules and increased PTPs. The hyporesponsiveness of AID B cells is similar to a form of functional anergy.

Non-receptor tyrosine kinase signaling in autoimmunity and therapeutic implications

Autoimmune diseases are characterized by impaired immune tolerance towards self-antigens, leading to enhanced immunity to self by dysfunctional B cells and/or T cells. The activation of these cells is controlled by non-receptor tyrosine kinases (NRTKs), which are critical mediators of antigen receptor and cytokine receptor signaling pathways. NRTKs transduce, amplify and sustain activating signals that contribute to autoimmunity, and are counter-regulated by protein tyrosine phosphatases (PTPs). The function of and interaction between NRTKs and PTPs during the development of autoimmunity could be key points of therapeutic interference against autoimmune diseases. In this review, we summarize the current state of knowledge of the functions of NRTKs and PTPs involved in B cell receptor (BCR), T cell receptor (TCR), and cytokine receptor signaling pathways that contribute to autoimmunity, and discuss their targeting for therapeutic approaches against autoimmune diseases.

A Precision B Cell-Targeted Therapeutic Approach to Autoimmunity Caused by Phosphatidylinositol 3-Kinase Pathway Dysregulation

The inositol lipid phosphatases PTEN and SHIP-1 play a crucial role in maintaining B cell anergy and are reduced in expression in B cells from systemic lupus erythematosus and type 1 diabetes patients, consequent to aberrant regulation by miRNA-7 and 155. With an eye toward eventual use in precision medicine therapeutic approaches in autoimmunity, we explored the ability of p110δ inhibition to compensate for PI3K pathway dysregulation in mouse models of autoimmunity. Low dosages of the p110δ inhibitor idelalisib, which spare the ability to mount an immune response to exogenous immunogens, are able to block the development of autoimmunity driven by compromised PI3K pathway regulation resultant from acutely induced B cell-targeted haploinsufficiency of PTEN and SHIP-1. These conditions do not block autoimmunity driven by B cell loss of the regulatory tyrosine phosphatase SHP-1. Finally, we show that B cells in NOD mice express reduced PTEN, and low-dosage p110δ inhibitor therapy blocks disease progression in this model of type 1 diabetes. These studies may aid in the development of precision treatments that act by enforcing PI3K pathway regulation in patients carrying specific risk alleles.

Cutting Edge: Deletion of Ezrin in B Cells of Lyn-Deficient Mice Downregulates Lupus Pathology

Genetic deletion of the Src family tyrosine kinase Lyn in mice recapitulates human systemic lupus erythematosus, characterized by hyperactive BCR signaling, splenomegaly, autoantibody generation, and glomerulonephritis. However, the molecular regulators of autoimmunity in Lyn-deficient mice and in human lupus remain poorly characterized. In this study, we report that conditional deletion of the membrane-cytoskeleton linker protein ezrin in B cells of Lyn-deficient mice (double knockout [DKO] mice) ameliorates B cell activation and lupus pathogenesis. B cells from DKO mice respond poorly to BCR stimulation, with severe downregulation of major signaling pathways. DKO mice exhibit reduced splenomegaly as well as significantly lower levels of autoantibodies against a variety of autoantigens, including dsDNA, histone, and chromatin. Leukocyte infiltration and deposition of IgG and complement component C3 in the kidney glomeruli of DKO mice are markedly reduced. Our data demonstrate that ezrin is a novel molecular regulator of B cell-associated lupus pathology.

Putting on the Brakes: Regulatory Kinases and Phosphatases Maintaining B Cell Anergy

B cell antigen receptor (BCR) signaling is a tightly regulated process governed by both positive and negative mediators/regulators to ensure appropriate responses to exogenous and autologous antigens. Upon naïve B cell recognition of antigen CD79 [the immunoreceptor tyrosine-based activation motif (ITAM)-containing signaling subunit of the BCR] is phosphorylated and recruits Src and Syk family kinases that then phosphorylate proximal intermediaries linked to downstream activating signaling circuitry. This plasma membrane localized signalosome activates PI3K leading to generation of PIP3 critical for membrane localization and activation of plecktrin homology domain-containing effectors. Conversely, in anergic B cells, chronic antigen stimulation drives biased monophosphorylation of CD79 ITAMs leading to recruitment of Lyn, but not Syk, which docks only to bi-phosphorylated ITAMS. In this context, Lyn appears to function primarily as a driver of inhibitory signaling pathways promoting the inhibition of the PI3K pathway by inositol phosphatases, SHIP-1 and PTEN, which hydrolyze PIP3 to PIP2. Lyn may also exert negative regulation of signaling through recruitment of SHP-1, a tyrosine phosphatase that dephosphorylates activating signaling molecules. Alleles of genes that encode or regulate expression of components of this axis, including SHIP-1, SHP-1, Csk/PTPn22, and Lyn, have been shown to confer risk of autoimmunity. This review will discuss functional interplay of components of this pathway and the impact of risk alleles on its function.

Lyn, Lupus, and (B) Lymphocytes, a Lesson on the Critical Balance of Kinase Signaling in Immunity

Systemic lupus erythematosus (SLE) is a progressive autoimmune disease characterized by increased sensitivity to self-antigens, auto-antibody production, and systemic inflammation. B cells have been implicated in disease progression and as such represent an attractive therapeutic target. Lyn is a Src family tyrosine kinase that plays a major role in regulating signaling pathways within B cells as well as other hematopoietic cells. Its role in initiating negative signaling cascades is especially critical as exemplified by Lyn-/- mice developing an SLE-like disease with plasma cell hyperplasia, underscoring the importance of tightly regulating signaling within B cells. This review highlights recent advances in our understanding of the function of the Src family tyrosine kinase Lyn in B lymphocytes and its contribution to positive and negative signaling pathways that are dysregulated in autoimmunity.

Bruton's Tyrosine Kinase, a Component of B Cell Signaling Pathways, Has Multiple Roles in the Pathogenesis of Lupus

Systemic lupus erythematosus (SLE) is an autoimmune disease characterized by the loss of adaptive immune tolerance to nucleic acid-containing antigens. The resulting autoantibodies form immune complexes that promote inflammation and tissue damage. Defining the signals that drive pathogenic autoantibody production is an important step in the development of more targeted therapeutic approaches for lupus, which is currently treated primarily with non-specific immunosuppression. Here, we review the contribution of Bruton’s tyrosine kinase (Btk), a component of B and myeloid cell signaling pathways, to disease in murine lupus models. Both gain- and loss-of-function genetic studies have revealed that Btk plays multiple roles in the production of autoantibodies. These include promoting the activation, plasma cell differentiation, and class switching of autoreactive B cells. Small molecule inhibitors of Btk are effective at reducing autoantibody levels, B cell activation, and kidney damage in several lupus models. These studies suggest that Btk may promote end-organ damage both by facilitating the production of autoantibodies and by mediating the inflammatory response of myeloid cells to these immune complexes. While Btk has not been associated with SLE in GWAS studies, SLE B cells display signaling defects in components both upstream and downstream of Btk consistent with enhanced activation of Btk signaling pathways. Taken together, these observations indicate that limiting Btk activity is critical for maintaining B cell tolerance and preventing the development of autoimmune disease. Btk inhibitors, generally well-tolerated and approved to treat B cell malignancy, may thus be a useful therapeutic approach for SLE.

Intracellular B Lymphocyte Signalling and the Regulation of Humoral Immunity and Autoimmunity

B lymphocytes are critical for effective immunity; they produce antibodies and cytokines, present antigens to T lymphocytes and regulate immune responses. However, because of the inherent randomness in the process of generating their vast repertoire of antigen-specific receptors, B cells can also cause diseases through recognizing and reacting to self. Therefore, B lymphocyte selection and responses require tight regulation at multiple levels and at all stages of their development and activation to avoid diseases. Indeed, newly generated B lymphocytes undergo rigorous tolerance mechanisms in the bone marrow and, subsequently, in the periphery after their migration. Furthermore, activation of mature B cells is regulated through controlled expression of co-stimulatory receptors and intracellular signalling thresholds. All these regulatory events determine whether and how B lymphocytes respond to antigens, by undergoing apoptosis or proliferation. However, defects that alter regulated co-stimulatory receptor expression or intracellular signalling thresholds can lead to diseases. For example, autoimmune diseases can result from altered regulation of B cell responses leading to the emergence of high-affinity autoreactive B cells, autoantibody production and tissue damage. The exact cause(s) of defective B cell responses in autoimmune diseases remains unknown. However, there is evidence that defects or mutations in genes that encode individual intracellular signalling proteins lead to autoimmune diseases, thus confirming that defects in intracellular pathways mediate autoimmune diseases. This review provides a synopsis of current knowledge of signalling proteins and pathways that regulate B lymphocyte responses and how defects in these could promote autoimmune diseases. Most of the evidence comes from studies of mouse models of disease and from genetically engineered mice. Some, however, also come from studying B lymphocytes from patients and from genome-wide association studies. Defining proteins and signalling pathways that underpin atypical B cell response in diseases will help in understanding disease mechanisms and provide new therapeutic avenues for precision therapy.

Altered B cell signalling in autoimmunity

Recent work has provided new insights into how altered B cell-intrinsic signals - through the B cell receptor (BCR) and key co-receptors - function together to promote the pathogenesis of autoimmunity. These combined signals affect B cells at two distinct stages: first, in the selection of the naive repertoire; and second, during extrafollicular or germinal centre activation responses. Thus, dysregulated signalling can lead to both an altered naive BCR repertoire and the generation of autoantibody-producing B cells. Strikingly, high-affinity autoantibodies predate and predict disease in several autoimmune disorders, including type 1 diabetes and systemic lupus erythematosus. This Review summarizes how, rather than being a downstream consequence of autoreactive T cell activation, dysregulated B cell signalling can function as a primary driver of many human autoimmune diseases.

Expanding the B Cell-Centric View of Systemic Lupus Erythematosus

Systemic lupus erythematosus (SLE) is an autoimmune disorder characterized by a breakdown of self-tolerance in B cells and the production of antibodies against nuclear self-antigens. Increasing evidence supports the notion that additional cellular contributors beyond B cells are important for lupus pathogenesis. In this review we consider recent advances regarding both the pathogenic and the regulatory role of lymphocytes in SLE beyond the production of IgG autoantibodies. We also discuss various inflammatory effector cell types involved in cytokine production, removal of self-antigens, and responses to autoreactive IgE antibodies. We aim to integrate these ideas to expand the current understanding of the cellular components that contribute to disease progression and ultimately help in the design of novel, targeted therapeutics.

Kruppel-like factor4 regulates PRDM1 expression through binding to an autoimmune risk allele

A SNP identified as rs548234, which is found in PRDM1, the gene that encodes BLIMP1, is a risk allele associated with systemic lupus erythematosus (SLE). BLIMP1 expression was reported to be decreased in women with the PRDM1 rs548234 risk allele compared with women with the nonrisk allele in monocyte-derived DCs (MO-DCs). In this study, we demonstrate that BLIMP1 expression is regulated by the binding of Kruppel-like factor 4 (KLF4) to the risk SNP. KLF4 is highly expressed in MO-DCs but undetectable in B cells, consistent with the lack of altered expression of BLIMP1 in B cells from risk SNP carriers. Female rs548234 risk allele carriers, but not nonrisk allele carriers, exhibited decreased levels of BLIMP1 in MO-DCs, showing that the regulatory function of KLF4 is influenced by the risk allele. In addition, KLF4 directly recruits histone deacetylases (HDAC4, HDAC6, and HDAC7), established negative regulators of gene expression. Finally, the knock down of KLF4 expression reversed the inhibitory effects of the risk SNP on promoter activity and BLIMP1 expression. Therefore, the binding of KLF4 and the subsequent recruitment of HDACs represent a mechanism for reduced BLIMP1 expression in MO-DCs bearing the SLE risk allele rs548234.

BLK pathway-associated rs13277113 GA genotype is more frequent in SLE patients and associated with low gene expression and increased flares

We aimed to evaluate the relationship between some important genetic variations and expressions of these genes in our SLE population. We also determined their association with clinical parameters. Eighty-four SLE patients (79 F, 5 M) and 105 healthy controls (98 F, 7 M) were included in the study. rs13277113, rs2736340, rs7829816, rs6983130, rs2613310, and rs704853 polymorphisms, gene expressions of Src family kinases (Blk, Hck, Lck, and Lyn), and Syk kinases (Syk, ZAP70) were studied by real-time PCR. The heterozygous genotypic pattern (GA) for rs13277113 polymorphism was more frequent in patients with SLE when compared to that in controls (48.8 vs. 31.4%, p = 0.035). Other genotype variants were similar in SLE patients and controls. In the SLE group, the heterozygous genotype for rs13277113 was significantly less frequent in active SLE patients (58.8 vs. 26.7%, p = 0.01). SLE flares according to the SELENA-SLEDAI flare index were significantly more frequent in GA (rs13277113) (70 vs. 37%) and CT (rs2736340) genotypes (66.7 vs. 35.2%) than those in other genotypes (p values <0.01). The relative expression of Blk gene was significantly decreased in the SLE group as compared to that in controls (0.52 times, 95%CI 0.19-0.85). The gene expressions of Blk and ZAP70 were significantly lower in SLE patients who had flares according to the SELENA-SLEDAI flare index when compared to those in others (p values 0.01 and 0.017). We observed more frequent heterozygous GA genotypic pattern (rs13277113) in our SLE patients compared to that in controls; and it was associated with disease flares. Blk gene expression in SLE was lower, especially in relapsing patients.

LYN- and AIRE-mediated tolerance checkpoint defects synergize to trigger organ-specific autoimmunity

Studies of the genetic factors associated with human autoimmune disease suggest a multigenic origin of susceptibility; however, how these factors interact and through which tolerance pathways they operate generally remain to be defined. One key checkpoint occurs through the activity of the autoimmune regulator AIRE, which promotes central T cell tolerance. Recent reports have described a variety of dominant-negative AIRE mutations that likely contribute to human autoimmunity to a greater extent than previously thought. In families with these mutations, the penetrance of autoimmunity is incomplete, suggesting that other checkpoints play a role in preventing autoimmunity. Here, we tested whether a defect in LYN, an inhibitory protein tyrosine kinase that is implicated in systemic autoimmunity, could combine with an Aire mutation to provoke organ-specific autoimmunity. Indeed, mice with a dominant-negative allele of Aire and deficiency in LYN spontaneously developed organ-specific autoimmunity in the eye. We further determined that a small pool of retinal protein-specific T cells escaped thymic deletion as a result of the hypomorphic Aire function and that these cells also escaped peripheral tolerance in the presence of LYN-deficient dendritic cells, leading to highly destructive autoimmune attack. These findings demonstrate how 2 distinct tolerance pathways can synergize to unleash autoimmunity and have implications for the genetic susceptibility of autoimmune disease.

Immunological function of Blimp-1 in dendritic cells and relevance to autoimmune diseases

Previous studies have identified the immunological functions of transcription factor B lymphocyte-induced maturation protein-1 (Blimp-1) in various adaptive immune cell types such as T and B lymphocytes. More recently, it has been shown that Blimp-1 extends its functional roles to dendritic cells (DCs) and macrophages, two cell types belonging to the innate immune system. The protein acts as a direct and indirect regulator of target genes by recruiting chromatin modification factors and by regulating microRNA expression, respectively. In DCs, Blimp-1 has been identified as one of the components involved in antigen presentation. Genome-wide association studies identified polymorphisms associated with multiple autoimmune diseases such as system lupus erythematosus, rheumatoid arthritis, and inflammatory bowel disease in PRDM1, the gene encoding Blimp-1 protein. In this review, we will discuss the immune regulatory functions of Blimp-1 in DCs with a main focus on the tolerogenic mechanisms of Blimp-1 required to protect against the development of autoimmune diseases.

Genetics of Lupus Nephritis: Clinical Implications

Systemic lupus erythematosus is a heterogeneous autoimmune disease marked by the presence of pathogenic autoantibodies, immune dysregulation, and chronic inflammation that may lead to increased morbidity and early mortality from end-organ damage. More than half of all systemic lupus erythematosus patients will develop lupus nephritis. Genetic-association studies have identified more than 50 polymorphisms that contribute to lupus nephritis pathogenesis, including genetic variants associated with altered programmed cell death and defective immune clearance of programmed cell death debris. These variants may support the generation of autoantibody-containing immune complexes that contribute to lupus nephritis. Genetic variants associated with lupus nephritis also affect the initial phase of innate immunity and the amplifying, adaptive phase of the immune response. Finally, genetic variants associated with the kidney-specific effector response may influence end-organ damage and the progression to end-stage renal disease and death. This review discusses genetic insights of key pathogenic processes and pathways that may lead to lupus nephritis, as well as the clinical implications of these findings as they apply to recent advances in biologic therapies.

The genetic basis of systemic lupus erythematosus: What are the risk factors and what have we learned

The genome-wide association study is a free-hypothesis approach based on screening of thousands or even millions of genetic variants distributed throughout the whole human genome in relation to a phenotype. The relevant role of the genome-wide association studies in the last decade is undisputed because it has permitted to elucidate multiple risk genetic factors associated with the susceptibility to several human complex diseases. Regarding systemic lupus erythematosus (SLE) this approach has allowed to identify more than 60 risk loci for SLE susceptibility across populations to date, increasing our understanding on the pathogenesis of this disease. We present the latest findings in the genetic of SLE across populations using genome-wide approaches. These studies revealed that most of the genetic risk is shared across borders and ethnicities. Finally, we focus on describing the most important risk loci for SLE attempting to cover the genetic findings in relation to functional polymorphisms, such as missense single nucleotide polymorphisms (SNPs) or regulatory variants involved in the development of the disease. The functional studies try to identify the causality of some GWAS-associated variants, many of which fall in non-coding regions of the genome, suggesting a regulatory role. Many loci show an environmental interaction, another aspect revealed by the studies of epigenetic modifications and those associated with genetic variants. Finally, new-generation sequencing technologies can open other paths in the research on SLE genetics, the role of rare variants and the detailed identification of causal regulatory variation. The clinical relevance of the genetic factors will be shown when we are able to use them or in combination with other molecular measurements to re-classify a heterogeneous disease such as SLE.

Breakdown of Immune Tolerance in Systemic Lupus Erythematosus by Dendritic Cells

Dendritic cells (DC) play an important role in the pathogenesis of systemic lupus erythematosus (SLE), an autoimmune disease with multiple tissue manifestations. In this review, we summarize recent studies on the roles of conventional DC and plasmacytoid DC in the development of both murine lupus and human SLE. In the past decade, studies using selective DC depletions have demonstrated critical roles of DC in lupus progression. Comprehensive in vitro and in vivo studies suggest activation of DC by self-antigens in lupus pathogenesis, followed by breakdown of immune tolerance to self. Potential treatment strategies targeting DC have been developed. However, many questions remain regarding the mechanisms by which DC modulate lupus pathogenesis that require further investigations.

Autoantibodies in SLE: Specificities, Isotypes and Receptors

Systemic Lupus Erythematosus (SLE) is characterized by a wide spectrum of auto-antibodies which recognize several cellular components. The production of these self-reactive antibodies fluctuates during the course of the disease and the involvement of different antibody-secreting cell populations are considered highly relevant for the disease pathogenesis. These cells are developed and stimulated through different ways leading to the secretion of a variety of isotypes, affinities and idiotypes. Each of them has a particular mechanism of action binding to a specific antigen and recognized by distinct receptors. The effector responses triggered lead to a chronic tissue inflammation. DsDNA autoantibodies are the most studied as well as the first in being characterized for its pathogenic role in Lupus nephritis. However, others are of growing interest since they have been associated with other organ-specific damage, such as anti-NMDAR antibodies in neuropsychiatric clinical manifestations or anti-β2GP1 antibodies in vascular symptomatology. In this review, we describe the different auto-antibodies reported to be involved in SLE. How autoantibody isotypes and affinity-binding to their antigen might result in different pathogenic responses is also discussed.

The Role of the Transcription Factor Ets1 in Lupus and Other Autoimmune Diseases

Systemic lupus erythematosus (SLE) is a chronic autoimmune disease characterized by excess B- and T-cell activation, the development of autoantibodies against self-antigens including nuclear antigens, and immune complex deposition in target organs, which triggers an inflammatory response and tissue damage. The genetic and environmental factors that contribute to the development of SLE have been studied extensively in both humans and mouse models of the disease. One of the important genetic contributions to SLE development is an alteration in the expression of the transcription factor Ets1, which regulates the functional differentiation of lymphocytes. Here, we review the genetic, biochemical, and immunological studies that have linked low levels of Ets1 to aberrant lymphocyte differentiation and to the pathogenesis of SLE.

Genetic Interaction between Lyn, Ets1, and Btk in the Control of Antibody Levels

Tight control of B cell differentiation into plasma cells (PCs) is critical for proper immune responses and the prevention of autoimmunity. The Ets1 transcription factor acts in B cells to prevent PC differentiation. Ets1(-/-) mice accumulate PCs and produce autoantibodies. Ets1 expression is downregulated upon B cell activation through the BCR and TLRs and is maintained by the inhibitory signaling pathway mediated by Lyn, CD22 and SiglecG, and SHP-1. In the absence of these inhibitory components, Ets1 levels are reduced in B cells in a Btk-dependent manner. This leads to increased PCs, autoantibodies, and an autoimmune phenotype similar to that of Ets1(-/-) mice. Defects in inhibitory signaling molecules, including Lyn and Ets1, are associated with human lupus, although the effects are more subtle than the complete deficiency that occurs in knockout mice. In this study, we explore the effect of partial disruption of the Lyn/Ets1 pathway on B cell tolerance and find that Lyn(+/-)Ets1(+/-) mice demonstrate greater and earlier production of IgM, but not IgG, autoantibodies compared with Lyn(+/-) or Ets1(+/-) mice. We also show that Btk-dependent downregulation of Ets1 is important for normal PC homeostasis when inhibitory signaling is intact. Ets1 deficiency restores the decrease in steady state PCs and Ab levels observed in Btk(-/-) mice. Thus, depending on the balance of activating and inhibitory signals to Ets1, there is a continuum of effects on autoantibody production and PC maintenance. This ranges from full-blown autoimmunity with complete loss of Ets1-maintaining signals to reduced PC and Ab levels with impaired Ets1 downregulation.

Genetics and novel aspects of therapies in systemic lupus erythematosus

Autoimmune diseases, such as rheumatoid arthritis, multiple sclerosis, autoimmune hepatitis and inflammatory bowel disease, have complex pathogeneses and the factors which cause these disorders are not well understood. But all have in common that they arise from a dysfunction of the immune system, interpreting self components as foreign antigens. Systemic lupus erythematosus (SLE) is one of these complex inflammatory disorders that mainly affects women and can lead to inflammation and severe damage of virtually any tissue and organ. Recently, the application of advanced techniques of genome-wide scanning revealed more genetic information about SLE than previously possible. These case-control or family-based studies have provided evidence that SLE susceptibility is based (with a few exceptions) on an individual accumulation of various risk alleles triggered by environmental factors and also help to explain the discrepancies in SLE susceptibility between different populations or ethnicities. Moreover, during the past years new therapies (autologous stem cell transplantation, B cell depletion) and improved conventional treatment options (corticosteroids, traditional and new immune-suppressants like mycophenolate mofetile) changed the perspective in SLE therapeutic approaches. Thus, this article reviews genetic aspects of this autoimmune disease, summarizes clinical aspects of SLE and provides a general overview of conventional and new therapeutic approaches in SLE.

Linking susceptibility genes and pathogenesis mechanisms using mouse models of systemic lupus erythematosus

Systemic lupus erythematosus (SLE) represents a challenging autoimmune disease from a clinical perspective because of its varied forms of presentation. Although broad-spectrum steroids remain the standard treatment for SLE, they have many side effects and only provide temporary relief from the symptoms of the disease. Thus, gaining a deeper understanding of the genetic traits and biological pathways that confer susceptibility to SLE will help in the design of more targeted and effective therapeutics. Both human genome-wide association studies (GWAS) and investigations using a variety of mouse models of SLE have been valuable for the identification of the genes and pathways involved in pathogenesis. In this Review, we link human susceptibility genes for SLE with biological pathways characterized in mouse models of lupus, and discuss how the mechanistic insights gained could advance drug discovery for the disease.

Genetics and pathogenesis of systemic lupus erythematosus and lupus nephritis

Systemic lupus erythematosus (SLE) is a multisystem autoimmune disorder that has a broad spectrum of effects on the majority of organs, including the kidneys. Approximately 40-70% of patients with SLE will develop lupus nephritis. Renal assault during SLE is initiated by genes that breach immune tolerance and promote autoantibody production. These genes might act in concert with other genetic factors that augment innate immune signalling and IFN-I production, which in turn can generate an influx of effector leucocytes, inflammatory mediators and autoantibodies into end organs, such as the kidneys. The presence of cognate antigens in the glomerular matrix, together with intrinsic molecular abnormalities in resident renal cells, might further accentuate disease progression. This Review discusses the genetic insights and molecular mechanisms for key pathogenic contributors in SLE and lupus nephritis. We have categorized the genes identified in human studies of SLE into one of four pathogenic events that lead to lupus nephritis. We selected these categories on the basis of the cell types in which these genes are expressed, and the emerging paradigms of SLE pathogenesis arising from murine models. Deciphering the molecular basis of SLE and/or lupus nephritis in each patient will help physicians to tailor specific therapies.

B cells take the front seat: dysregulated B cell signals orchestrate loss of tolerance and autoantibody production

A significant proportion of autoimmune-associated genetic variants are expressed in B cells, suggesting that B cells may play multiple roles in autoimmune pathogenesis. In this review, we highlight recent studies demonstrating that even modest alterations in B cell signaling are sufficient to promote autoimmunity. First, we describe several examples of genetic variations promoting B cell-intrinsic initiation of autoimmune germinal centers and autoantibody production. We highlight how dual antigen receptor/toll-like receptor signals greatly facilitate this process and how activated, self-reactive B cells may function as antigen presenting cells, leading to loss of T cell tolerance. Further, we propose that B cell-derived cytokines may initiate and/or sustain autoimmune germinal centers, likely also contributing, in parallel, to programing of self-reactive T cells.

A balance between B cell receptor and inhibitory receptor signaling controls plasma cell differentiation by maintaining optimal Ets1 levels

Signaling through the BCR can drive B cell activation and contribute to B cell differentiation into Ab-secreting plasma cells. The positive BCR signal is counterbalanced by a number of membrane-localized inhibitory receptors that limit B cell activation and plasma cell differentiation. Deficiencies in these negative signaling pathways may cause autoantibody generation and autoimmune disease in both animal models and human patients. We have previously shown that the transcription factor Ets1 can restrain B cell differentiation into plasma cells. In this study, we tested the roles of the BCR and inhibitory receptors in controlling the expression of Ets1 in mouse B cells. We found that Ets1 is downregulated in B cells by BCR or TLR signaling through a pathway dependent on PI3K, Btk, IKK2, and JNK. Deficiencies in inhibitory pathways, such as a loss of the tyrosine kinase Lyn, the phosphatase Src homology region 2 domain-containing phosphatase 1 (SHP1) or membrane receptors CD22 and/or Siglec-G, result in enhanced BCR signaling and decreased Ets1 expression. Restoring Ets1 expression in Lyn- or SHP1-deficient B cells inhibits their enhanced plasma cell differentiation. Our findings indicate that downregulation of Ets1 occurs in response to B cell activation via either BCR or TLR signaling, thereby allowing B cell differentiation and that the maintenance of Ets1 expression is an important function of the inhibitory Lyn → CD22/SiglecG → SHP1 pathway in B cells.

Clinical perspectives on lupus genetics: advances and opportunities

In recent years, genome-wide association studies have led to an expansion in the identification of regions containing confirmed genetic risk variants within complex human diseases, such as systemic lupus erythematosus (SLE). Many of the strongest SLE genetic associations can be divided into groups based on their potential roles in different processes implicated in lupus pathogenesis, including ubiquitination, DNA degradation, innate immunity, cellular immunity, lymphocyte development, and antigen presentation. Recent advances have also shown several genetic associations with SLE subphenotypes and subcriteria. Many areas for further exploration remain to move lupus genetic studies toward clinically informative end points.