Suppressing Synthesis of the Long Isoform of the Prolactin Receptor Is a Targeted Strategy to Prevent and Treat B Cell Malignancies

Rationale B cell malignancies, including leukemia and lymphoma, are high-risk lymphoid neoplasms. B cell malignancies predispose to autoimmune diseases including systemic lupus erythematosus (SLE) which increase the risk of developing these malignancies by >5-fold. Increased prolactin (PRL) expression is known to exacerbate SLE and promote the survival of autoreactive B cells. Furthermore, PRL induces expression of the protooncogenes, MYC and BCL2, in lymphoid tissues. However, whether PRL drives the initiation and maintenance of B cell malignancies was not known. Results We first tested our hypothesis that PRL, specifically signaling through the pro-proliferative and anti-apoptotic long isoform (LF) of the PRL receptor (PRLR), drives the progression of SLE to B cell malignancies. To this end, we knocked down the LF PRLR in MRL-lpr mice predisposed to developing SLE using a splice-modulating oligomer (SMO) that blocks splicing to produce the LF PRLR without affecting the short isoforms. LF PRLR knockdown reduced splenic and circulating B cell numbers in MRL-lpr SLE mice (Fig.1a). Consistent with reduced B cell numbers, BCL2 expression in B cells of SLE mice was suppressed after LF PRLR knockdown, although MYC was unaltered (Fig.1b). By sequencing the immunoglobulin heavy chains (IGH), we compared the composition of the splenic B cell repertoire between control- and LF PRLR SMO-treated SLE mice. Control oligomer treated SLE mice accumulated splenic B cells with long complementary determining region 3 (CDR3) and B cells with non-functional IGH, characteristics of autoreactive B cells. Treatment with the LF PRLR SMO reduced both. We then measured the expression of enzymes known to induce malignant transformation of B cells, namely recombination activating genes 1/2 (RAG1/2) and activation-induced cytidine deaminase (AID), in B cells of SLE mice in controls versus LF PRLR knockdown. Importantly, LF PRLR knockdown significantly reduced RAG1 (Fig.1c) and AID expression in splenic B cells of SLE mice (Fig.1d,e). Our findings thus underscore a causal role for LF PRLR signaling in promoting of malignant transformation of B cells in SLE. Because PRL induces the expression of BCL2 and MYC in lymphocytes, we next determined whether LF PRLR promotes the survival of overt B cell malignancies that overexpress MYC and BCL2, including diffuse large B cell lymphoma (DLBCL) and B-cell acute lymphoblastic leukemia (B-ALL). We observed that B-lymphoblasts expressed significantly higher levels of PRL and the LF PRLR as compared to normal B cells (Fig.1f). We also found that higher expression of PRL at diagnosis predicts poor clinical outcome in DLBCL patients (P=0.0244), and that patients with MYC/BCL2-overexpressing ALLs with a poor prognosis had significantly higher expression of the LF PRLR compared to their MYC lowBCL2 low counterparts (P<0.0001). These observations suggested that LF PRLR may modulate MYC and BCL2 expression. Knockdown of the LF PRLR using the LF PRLR SMO in MYC/BCL2-driven human B cell malignancies killed lymphoblasts and reduced MYC and BCL2 protein levels (Fig.1g). Because we previously showed that MYC-driven lymphoid malignancies are sensitive to natural killer (NK) cell-mediated immune clearance, we also examined whether LF PRLR knockdown synergized with NK cells in killing DLBCL. We found that LF PRLR knockdown enhanced NK cell-mediated killing of B-lymphoblasts (Fig.1h). Of note, no reductions were observed in NK cell viability or MYC levels within NK cells upon LF PRLR knockdown, suggesting that LF PRLR selectively kills B-lymphoblasts without negatively impacting NK homeostasis. Conclusion Our studies identify the specific knockdown of LF PRLR as a potentially safe and targeted strategy to prevent the onset of B cell malignancies in SLE patients and to treat flagrant DLBCL and B-ALL. Figure 1 Figure 1. Disclosures No relevant conflicts of interest to declare.

Pharmacological Targeting of PI3K-Dependent Central Tolerance Mechanisms in Refractory Pre-Germinal Center B-Cell Malignancies

Rationale: About 75% of newly formed B-cells are autoreactive and express potentially harmful autoantibodies (Wardemann 2003). Hence, a powerful mechanism, termed central tolerance, is in place to eliminate millions of newly formed autoreactive B-cells every day. Results: B-ALL, mantle cell lymphoma (MCL) and unmutated chronic lymphocytic leukemia (U-CLL) originate from early, pre-germinal center (pre-GC) stages of B-cell development that are subject to negative B-cell selection and central tolerance mechanisms. While designed to eliminate autoreactive clones during early B-cell development, we recently discovered that B-ALL, MCL and U-CLL fully retained sensitivity to central tolerance mechanisms, which are triggered by persistent PI3K-hyperactivation. PI3K-signaling code to distinguish between normal and pathological signaling. Studying short transient pulses and chronic activation of PI3K-signaling, we discovered that pre-GC B-cells have evolved a "PI3K-signaling code" to distinguish between normal B-cell activation by antigen and pathological signaling: thereby, antigen encounter induces a short transient pulse of PI3K-activation which promotes survival and proliferation. Conversely, persistent activation of PI3K-activation reflects pathological signaling, either from an autoreactive B-cell receptor (BCR) or a transforming oncogene. Pre-GC B-cell malignancies are exempt from oncogenic PI3K-lesions. PI3K-lesions in cancer result in permanent hyperactivation as in autoreactive B-cells. The PI3K pathway is targeted by oncogenic lesions in ~25% of human cancer. The phosphatases PTEN, SHIP1 and PP2A function as negative regulators of PI3K signaling and are frequently mutated in a broad range of cancers and also occur in some GC- and post-GC lymphomas (e.g. Burkitt's, DLBCL). However, our analysis in six clinical cohorts revealed that pre-GC B-cell malignancies, including B-ALL, MCL and U-CLL critically depend on PTEN, SHIP1 and PP2A function and do not tolerate persistent hyperactivation of PI3K-signaling for more than three hours. Loss-of-function mutations of these phosphatases and activating PI3K lesions were not detected in large clinical cohorts of patients with B-ALL, MCL and CLL. Likewise, phosphorylation of AKT-S473, reflecting PI3K signaling strength, is elevated throughout multiple cancer types including post-GC DLBCL, but not in B-ALL and MCL. This is in line with previous work demonstrating that inherited mutations that cause PI3K-activation predispose to various cancers but cause profound defects in human B-lymphopoiesis (Fruman 2014). Pharmacological targeting of PI3K-dependent central tolerance mechanisms. We tested the hypothesis that PI3K-hyperactivation represents a unique vulnerability in pre-GC B-cell tumors including B-ALL, MCL and U-CLL. Sensitivity to PI3K-hyperactivation of reflects their pre-GC origin and central tolerance mechanisms during early B-cell development that are designed to eliminate autoreactive B-cells based on hyperactive PI3K-signaling. For this reason, we tested pharmacological PI3K-hyperactivation as a novel strategy to selectively target pre-GC B-cell malignancies. To this end, we tested 144 compounds for their ability to engage PI3K-dependent central tolerance mechanism in B-ALL, MCL and CLL. Small molecule inhibitors of SHIP1 (3AC, K118), PTEN (SF-1670), PP2A (LB-100) and a direct PI3K-agonist (SC79) achieved strong phosphorylation of known PI3K-substrates (AKT, S6K) in vitro and prolonged overall survival in NSG mice transplanted with refractory B-ALL and MCL PDX in vivo. Conclusions and future directions: Current treatment regimens (kinase-inhibitor paradigm) use agents that apply selective pressure in one direction (e.g., PI3K-inhibitors; BCR-ABL1, SYK- or BTK-inhibitors). Here, we are pursuing a new concept (central tolerance paradigm) based on sequential treatment regimens that alternate between kinase-inhibitors (e.g., dasatinib, ibrutinib, idelalisib) and PI3K-hyperactivation (3AC, K118, LB100). By sequentially applying selective pressures in opposite directions, our approach will subvert clonal evolution and selection for drug-resistant mutants. Figure 1 Figure 1. Disclosures No relevant conflicts of interest to declare.

Regulation of B Cell Responses in SLE by Three Classes of Interferons

There are three classes of interferons (type 1, 2, and 3) that can contribute to the development and maintenance of various autoimmune diseases, including systemic lupus erythematosus (SLE). Each class of interferons promotes the generation of autoreactive B cells and SLE-associated autoantibodies by distinct signaling mechanisms. SLE patients treated with various type 1 interferon-blocking biologics have diverse outcomes, suggesting that additional environmental and genetic factors may dictate how these cytokines contribute to the development of autoreactive B cells and SLE. Understanding how each class of interferons controls B cell responses in SLE is necessary for developing optimized B cell- and interferon-targeted therapeutics. In this review, we will discuss how each class of interferons differentially promotes the loss of peripheral B cell tolerance and leads to the development of autoreactive B cells, autoantibodies, and SLE.

The expansion of activated naive DNA autoreactive B cells and its association with disease activity in systemic lupus erythematosus patients

Background Autoreactive B cells are well recognized as key participants in the pathogenesis of systemic lupus erythematosus (SLE). However, elucidating the particular subset of B cells in producing anti-dsDNA antibodies is limited due to their B cell heterogeneity. This study aimed to identify peripheral B cell subpopulations that display autoreactivity to DNA and contribute to lupus pathogenesis. Methods Flow cytometry was used to detect total B cell subsets (n = 20) and DNA autoreactive B cells (n = 15) in SLE patients’ peripheral blood. Clinical disease activities were assessed in SLE patients using modified SLEDAI-2 K and used for correlation analyses with expanded B cell subsets and DNA autoreactive B cells. Results The increases of circulating double negative 2 (DN2) and activated naïve (aNAV) B cells were significantly observed in SLE patients. Expanded B cell subsets and DNA autoreactive B cells represented a high proportion of aNAV B cells with overexpression of CD69 and CD86. The frequencies of aNAV B cells in total B cell populations were significantly correlated with modified SLEDAI-2 K scores. Further analysis showed that expansion of aNAV DNA autoreactive B cells was more related to disease activity and serum anti-dsDNA antibody levels than to total aNAV B cells. Conclusion Our study demonstrated an expansion of aNAV B cells in SLE patients. The association between the frequency of aNAV B cells and disease activity patients suggested that these expanded B cells may play a role in SLE pathogenesis.

Dermatan Sulfate Is a Potential Regulator of IgH via Interactions With Pre-BCR, GTF2I, and BiP ER Complex in Pre-B Lymphoblasts

Dermatan sulfate (DS) and autoantigen (autoAg) complexes are capable of stimulating autoreactive CD5+ B1 cells. We examined the activity of DS on CD5+ pre-B lymphoblast NFS-25 cells. CD19, CD5, CD72, PI3K, and Fas possess varying degrees of DS affinity. The three pre-BCR components, Ig heavy chain mu (IgH), VpreB, and lambda 5, display differential DS affinities, with IgH having the strongest affinity. DS attaches to NFS-25 cells, gradually accumulates in the ER, and eventually localizes to the nucleus. DS and IgH co-localize on the cell surface and in the ER. DS associates strongly with 17 ER proteins (e.g., BiP/Grp78, Grp94, Hsp90ab1, Ganab, Vcp, Canx, Kpnb1, Prkcsh, Pdia3), which points to an IgH-associated multiprotein complex in the ER. In addition, DS interacts with nuclear proteins (Ncl, Xrcc6, Prmt5, Eftud2, Supt16h) and Lck. We also discovered that DS binds GTF2I, a required gene transcription factor at the IgH locus. These findings support DS as a potential regulator of IgH in pre-B cells at protein and gene levels. We propose a (DS•autoAg)-autoBCR dual signal model in which an autoBCR is engaged by both autoAg and DS, and, once internalized, DS recruits a cascade of molecules that may help avert apoptosis and steer autoreactive B cell fate. Through its affinity with autoAgs and its control of IgH, DS emerges as a potential key player in the development of autoreactive B cells and autoimmunity.

Modifications of the BAFF/BAFF-Receptor Axis in Patients With Pemphigus Treated With Rituximab Versus Standard Corticosteroid Regimen

The efficacy of the B-cell-depleting agent rituximab has been reported in immune diseases but relapses are frequent, suggesting the need for repeated infusions. The B-cell activating factor (BAFF) is an important factor for B cell survival, class switch recombination and selection of autoreactive B cells, as well as maintaining long-lived plasma cells. It has been hypothesized that relapses after rituximab might be due to the increase of serum BAFF levels. From the Ritux3 trial, we showed that baseline serum BAFF levels were higher in pemphigus patients than in healthy donors (308 ± 13 pg/mL versus 252 ± 28 pg/mL, p=0.037) and in patients with early relapse compared who didn’t (368 ± 92 vs 297 ± 118 pg/mL, p=0.036). Rituximab and high doses of CS alone have different effects on the BAFF/BAFF-R axis. Rituximab led to an increase of BAFF levels associated to a decreased mRNA (Day 0: 12.3 ± 7.6 AU vs Month 36: 3.3 ± 4.3 AU, p=0.01) and mean fluorescence intensity of BAFF-R in non-autoreactive (Day 0: 3232 vs Month 36: 1527, mean difference: 1705, 95%CI: 624 to 2786; p=0.002) as well as on reappearing autoreactive DSG-specific B cells (Day 0: 3873 vs Month 36: 2688, mean difference: 1185, 95%CI: -380 to 2750; p=0.20). Starting high doses of corticosteroids allowed a transitory decrease of serum BAFF levels that re-increased after doses tapering whereas it did not modify BAFF-R expression in autoreactive and non-autoreactive B cells. Our results suggest that the activation of autoreactive B cells at the onset of pemphigus is likely to be related to the presence of high BAFF serum levels and that the decreased BAFF-R expression after rituximab might be responsible for the delayed generation of memory B cells, resulting in a rather long period of mild pemphigus activity after rituximab therapy. Conversely, the incomplete B cell depletion and persistent BAFF-R expression associated with high BAFF serum levels might explain the high number of relapses in patients treated with CS alone.

B Cell Activation and Escape of Tolerance Checkpoints: Recent Insights from Studying Autoreactive B Cells

Autoreactive B cells are key drivers of pathogenic processes in autoimmune diseases by the production of autoantibodies, secretion of cytokines, and presentation of autoantigens to T cells. However, the mechanisms that underlie the development of autoreactive B cells are not well understood. Here, we review recent studies leveraging novel techniques to identify and characterize (auto)antigen-specific B cells. The insights gained from such studies pertaining to the mechanisms involved in the escape of tolerance checkpoints and the activation of autoreactive B cells are discussed. In addition, we briefly highlight potential therapeutic strategies to target and eliminate autoreactive B cells in autoimmune diseases.

Central human B cell tolerance manifests with a distinctive cell phenotype and is enforced via CXCR4 signaling in hu-mice

Central B cell tolerance, the process restricting the development of many newly generated autoreactive B cells, has been intensely investigated in mouse cells while studies in humans have been hampered by the inability to phenotypically distinguish autoreactive and nonautoreactive immature B cell clones and the difficulty in accessing fresh human bone marrow samples. Using a human immune system mouse model in which all human Igκ+ B cells undergo central tolerance, we discovered that human autoreactive immature B cells exhibit a distinctive phenotype that includes lower activation of ERK and differential expression of CD69, CD81, CXCR4, and other glycoproteins. Human B cells exhibiting these characteristics were observed in fresh human bone marrow tissue biopsy specimens, although differences in marker expression were smaller than in the humanized mouse model. Furthermore, the expression of these markers was slightly altered in autoreactive B cells of humanized mice engrafted with some human immune systems genetically predisposed to autoimmunity. Finally, by treating mice and human immune system mice with a pharmacologic antagonist, we show that signaling by CXCR4 is necessary to prevent both human and mouse autoreactive B cell clones from egressing the bone marrow, indicating that CXCR4 functionally contributes to central B cell tolerance.

Evidence For The Involvement of B Cells and Antibody in The Pathogenesis of Multiple Sclerosis in Immunized Mouse With Recombinant Myelin Basic Protein Peptide

Over the years, regarding great progresses in knowledge of immunology and neuroscience, the treatment of multiple sclerosis (MS) has been changed. The earlier strategies were focused mainly on T lymphocytes as pioneer cells responsible to inflammatory damage in the central nervous system lesions, whereas B cells, plasma cells and antibodies are also found in the active nerve lesions in MS patients. Despite the accumulating evidence, the role of Myelin basic protein (MBP) antibodies in progression of lesions in nervous system is not completely clear yet. In this regard, here, we present data on B cells and antibody level after MBP immunization of MS mice model. Recombinant fusion protein harboring Myelin basic protein peptide (amino acids 83–99) and CFP was produced in E. coli and purified with chromatography. Then, the C57BL / 6 mice were immunized by rMBP-CFP. Antibody-based assay was used to quantify the level of reactivity to the MBP in mice serum. Subsequently, humoral immunity was analyzed by immunohistochemistry (IHC), ELISA, and Flow cytometry. Our data indicated an increase in autoreactive B cells and MBP specific antibodies after immunization. IHC analysis revealed an increasing penetration rate of immune cells and the nerve lesions development in the nervous system following increasing in MBP antibody titers.This study represented data to support this idea that reactive B cells and antibodies to MBP may contribute to MS pathogenesis. Hence, targeting of these autoreactive B cells and antibodies can be used as potential tools in treatment of MS patients.