Intravital Imaging of B Cell Responses in Lymph Nodes.

Humoral immune responses depend on B cells encountering antigen (Ag) in lymph nodes (LNs) draining infection sites, getting activated, interacting with different cells, proliferating and differentiating into antibody (Ab)-secreting cells. Each of these events occurs in distinct LN sub-compartments, requiring the migration of B cells from niche to niche in a fast and tightly coordinated fashion. While some of the rules that characterize B cell behavior in secondary lymphoid organs have been elucidated at the population level, we have only limited knowledge of the precise dynamics of B cell interactions with different kinds of LN cells at the single-cell level. Here, we describe in detail an intravital microscopy technique that allows the analysis of B cell dynamic behavior in the popliteal lymph node of anesthetized mice at high spatial and temporal resolution. A detailed understanding of the spatiotemporal dynamics of B cells within secondary lymphoid organs may lead to novel, rational vaccine strategies aimed at inducing rapid and long-lived humoral immune responses.

External signals regulate germinal center fate-determining transcription factors in the A20 lymphoma cell line.

Due to the apoptosis-prone nature of primary germinal center B (GCB) cells, it remains a huge challenge to dissect signals that guide their differentiation towards memory B cells and plasma cells in vitro. Here we show that the murine lymphoma cell line A20 resembles primary GCB cells in expression of GC-specific surface markers and the master transcription factor BCL6 and may serve as a useful system to model certain GCB cell behaviors in vitro. Using these cells, we found that both CD40 and B cell receptor (BCR) signaling are able to drive BCL6 downregulation, which is a prerequisite of post-GC B-cell differentiation. Under the steady state, BCL6 is constantly and rapidly degraded in A20 cells by the proteasome in a strictly FBXO11-dependent manner. This process can be further enhanced by signals downstream of the BCR. Both CD40 and BCR stimulation can upregulate IRF4, a transcription factor that suppresses BCL6 expression. However, only BCR signaling downregulate PAX5 and BACH2, two transcription factors that help maintain the GCB identity. Together, these results validate the A20 cell line as an experimental system suitable for studying regulation of BCL6 and potentially other transcription factors relevant to post-GC fate determination, and they support that combined signaling from BCR and CD40 receptors would drive termination of the GC program.

Cortactin, a Lyn substrate, is a checkpoint molecule at the intersection of BCR and CXCR4 signalling pathway in chronic lymphocytic leukaemia cells.

Cortactin (CTTN) is a substrate of the Src kinase Lyn that is known to play an actin cytoskeletal regulatory role involved in cell migration and cancer progression following its phosphorylation at Y421. We recently demonstrated that Cortactin is overexpressed in patients with chronic lymphocytic leukaemia (CLL). This work was aimed at defining the functional role of Cortactin in these patients. We found that Cortactin is variably expressed in CLL patients both in the peripheral blood and lymph nodes and that its expression correlates with the release of matrix metalloproteinase 9 (MMP-9) and the motility of neoplastic cells. Cortactin knockdown, by siRNA, induced a reduction in MMP-9 release as well as a decrease of migration capability of leukaemic B cells in vitro, also after chemotactic stimulus. Furthermore, Cortactin phosphorylation was lowered by the Src kinase-inhibitor PP2 with a consequent decrease of MMP-9 release in culture medium. An impaired migration, as compared to control experiments without Cortactin knockdown, was observed following CXCL12 triggering. Reduced Cortactin expression and phosphorylation were also detected both in vivo and in vitro after treatment with Ibrutinib, a Btk inhibitor. Our results highlight the role of Cortactin in CLL as a check-point molecule between the BCR and CXCR4 signalling pathways.

One-way membrane trafficking of SOS in receptor-triggered Ras activation.

SOS is a key activator of the small GTPase Ras. In cells, SOS-Ras signaling is thought to be initiated predominantly by membrane recruitment of SOS via the adaptor Grb2 and balanced by rapidly reversible Grb2-SOS binding kinetics. However, SOS has multiple protein and lipid interactions that provide linkage to the membrane. In reconstituted-membrane experiments, these Grb2-independent interactions were sufficient to retain human SOS on the membrane for many minutes, during which a single SOS molecule could processively activate thousands of Ras molecules. These observations raised questions concerning how receptors maintain control of SOS in cells and how membrane-recruited SOS is ultimately released. We addressed these questions in quantitative assays of reconstituted SOS-deficient chicken B-cell signaling systems combined with single-molecule measurements in supported membranes. These studies revealed an essentially one-way trafficking process in which membrane-recruited SOS remains trapped on the membrane and continuously activates Ras until being actively removed via endocytosis.

Loss of B-cell receptor expression defines a subset of diffuse large B-cell lymphoma characterized by silent BCR/PI3K/AKT signaling and a germinal center phenotype displaying low-risk clinicopathologic features.

B-cell receptor (BCR) signaling is crucial for the survival of normal and neoplastic B cells, and inhibitors targeting BCR signaling pathways have shown promising therapeutic outcomes for patients with B-cell lymphomas. In the current study, we analyzed de novo diffuse large B-cell lymphoma without BCR expression (DLBCL, BCR) in 25 cases to determine the BCR/phosphatidylinositol-3-kinase/AKT (BCR/PI3K/AKT) signaling status, clinicopathologic features, and underlying causes leading to the loss of BCR. On the basis of clinical features, 15 (60%) DLBCL, BCR patients were classified into the low-risk group, and 18 (86%) experienced complete remission. Morphologically and immunophenotypically, DLBCL, BCR demonstrated centroblastic cytology (21/25, 84%) and germinal center B-cell-like cell origin (18/25, 72%). Other components in BCR complexity remained intact, on the basis of immunohistochemical findings. Epstein-Barr virus infection, deficiency in B-lineage transcription factors (PAX5, Oct-2, and Bob.1), and oncogene rearrangement did not seem to be associated with BCR loss. The activated form of signaling proteins (pSYK and pAKT) involved in the BCR/PI3K/AKT pathway were expressed at low levels in DLBCL, BCR tissue. In vitro validation revealed that in DLBCL, BCR cell lines, the BCR/PI3K/AKT pathway did not respond to BCR stimulation or inhibition. Our findings suggest that DLBCL, BCR was characterized by a silent BCR/PI3K/AKT pathway, germinal center phenotype, and low risk and may not be a candidate for BCR-targeted therapies.

Dominant negative form of alpha4 inhibits the BCR crosslinking-induced phosphorylation of Bcl-xL and apoptosis in an immature B cell line WEHI-231.

We previously demonstrated that c-Jun N-terminal kinase (JNK) phosphorylates serine 62 of Bcl-xL to induce the degradation of Bcl-xL and apoptosis in WEHI-231 cells upon BCR crosslinking. In order to elucidate the regulatory mechanisms underlying the phosphorylation of Bcl-xL, we prepared an assay system in which JNK phosphorylated Bcl-xL in HEK293T cells. Consequently, we found that a signal transduction molecule, alpha4, enhanced the phosphorylation of Bcl-xL by JNK, while the co-expression of C-terminal alpha4 (220-340) diminished the phosphorylation of Bcl-xL induced by JNK. Furthermore, full-length alpha4 associated with both JNK and Bcl-xL, whereas C-terminal alpha4 (220-340) associated only with Bcl-xL, not JNK. In addition, WEHI-231 cells transfected with the cDNA of C-terminal alpha4 (220-340) exhibited decreased phosphorylation of Bcl-xL and stronger resistance to apoptosis induced by BCR crosslinking. These results indicate that alpha4 is an important regulatory molecule of apoptosis induced by BCR crosslinking in WEHI-231 cells and that C-terminal alpha4 (220-340) functions as a dominant negative form.

Amplification of IL-21 signalling pathway through Bruton's tyrosine kinase in human B cell activation.

OBJECTIVE: B cells play an important role in the pathogenesis of autoimmune diseases. The role of Bruton's tyrosine kinase (Btk) in cytokine-induced human B cell differentiation and class-switch recombination remains incompletely defined. This study analysed the effect of Btk on human activated B cells. METHODS: Purified B cells from healthy subjects were stimulated with B cell receptor (BCR) and other stimuli with or without a Btk inhibitor and gene expression was measured. The B cell line BJAB was used to assess Btk-associated signalling cascades. Phosphorylated Btk (p-Btk) in peripheral blood B cells obtained from 10 healthy subjects and 41 patients with RA was measured by flow cytometry and compared with patient backgrounds. RESULTS: IL-21 signalling, in concert with BCR, CD40 and BAFF signals, led to robust expression of differentiation- and class-switch DNA recombination-related genes and IgG production in human B cells, all of which were significantly suppressed by the Btk inhibitor. Although phosphorylation of STAT1 and STAT3 was induced by co-stimulation with IL-21, BCR and CD40, STAT1 phosphorylation in the nucleus, but not in the cytoplasm, was exclusively impaired by Btk blockade. High levels of p-Btk were noted in B cells of RA patients compared with controls and they correlated significantly with titres of RF among RF-positive patients. CONCLUSION: The findings elucidate a model in which Btk not only plays a fundamental role in the regulation of BCR signalling, but may also mediate crosstalk with cytokine signalling pathways through regulation of IL-21-induced phosphorylation of STAT1 in the nuclei of human B cells. Btk appears to have pathological relevance in RA.

BCR-JAK2 drives a myeloproliferative neoplasm in transplanted mice.

BCR-JAK2 is an infrequent gene fusion found in chronic/acute, myeloid/lymphoid Philadelphia chromosome-negative leukaemia. In this study, we demonstrated that in vivo expression of BCR-JAK2 in mice induces neoplasia, with fatal consequences. Transplantation of BCR-JAK2 bone marrow progenitors promoted splenomegaly, with megakaryocyte infiltration and elevated leukocytosis of myeloid origin. Analysis of peripheral blood revealed the presence of immature myeloid cells, platelet aggregates and ineffective erythropoiesis. A possible molecular mechanism for these observations involved inhibition of apoptosis by deregulated expression of the anti-apoptotic mediator Bcl-xL and the serine/threonine kinase Pim1. Together, these data provide a suitable in vivo molecular mechanism for leukaemia induction by BCR-JAK2 that validates the use of this model as a relevant preclinical tool for the design of new targeted therapies in Philadelphia chromosome-negative leukaemia involving BCR-JAK2-driven activation of the JAK2 pathway.

The SH2 domain of Abl kinases regulates kinase autophosphorylation by controlling activation loop accessibility.

The activity of protein kinases is regulated by multiple molecular mechanisms, and their disruption is a common driver of oncogenesis. A central and almost universal control element of protein kinase activity is the activation loop that utilizes both conformation and phosphorylation status to determine substrate access. In this study, we use recombinant Abl tyrosine kinases and conformation-specific kinase inhibitors to quantitatively analyse structural changes that occur after Abl activation. Allosteric SH2-kinase domain interactions were previously shown to be essential for the leukemogenesis caused by the Bcr-Abl oncoprotein. We find that these allosteric interactions switch the Abl activation loop from a closed to a fully open conformation. This enables the trans-autophosphorylation of the activation loop and requires prior phosphorylation of the SH2-kinase linker. Disruption of the SH2-kinase interaction abolishes activation loop phosphorylation. Our analysis provides a molecular mechanism for the SH2 domain-dependent activation of Abl that may also regulate other tyrosine kinases.

Dynamic control of excitatory synapse development by a Rac1 GEF/GAP regulatory complex.

The small GTPase Rac1 orchestrates actin-dependent remodeling essential for numerous cellular processes including synapse development. While precise spatiotemporal regulation of Rac1 is necessary for its function, little is known about the mechanisms that enable Rac1 activators (GEFs) and inhibitors (GAPs) to act in concert to regulate Rac1 signaling. Here, we identify a regulatory complex composed of a Rac-GEF (Tiam1) and a Rac-GAP (Bcr) that cooperate to control excitatory synapse development. Disruption of Bcr function within this complex increases Rac1 activity and dendritic spine remodeling, resulting in excessive synaptic growth that is rescued by Tiam1 inhibition. Notably, EphB receptors utilize the Tiam1-Bcr complex to control synaptogenesis. Following EphB activation, Tiam1 induces Rac1-dependent spine formation, whereas Bcr prevents Rac1-mediated receptor internalization, promoting spine growth over retraction. The finding that a Rac-specific GEF/GAP complex is required to maintain optimal levels of Rac1 signaling provides an important insight into the regulation of small GTPases.

Metadherin contribute to BCR signaling in chronic lymphocytic leukemia.

We have reported Metadherin (MTDH) was proven to be overexpression and involved in malignance of chronic lymphocytic leukemia (CLL) via Wnt signaling pathway. In this study, we further investigate the role of MTDH in regulation of BCR signaling pathway in CLL. Six CLL samples whose cells were proliferation after BCR activation were chosen from patients with unmutated IgVH. CCK-8 method used to evaluate the proliferation rate. MTDH expression was measured by quantitative PCR and Western blot. After BCR activation, there exist upregulation of MTDH expression in mRNA and protein level in all six CLL patients (P<0.05). In cell line MEC-1, we observed the same pro-proliferation effect accompanying with elevated MTDH expression. The proliferation effects of BCR activation to MEC-1 can be inhibited by MTDH interference. The results of this study indicate that MTDH involved in the pro-proliferation effect of BCR activation in CLL. And the results imply that MTDH can be a potential therapy target of CLL.

Altered BCR signalling quality predisposes to autoimmune disease and a pre-diabetic state.

The spleen tyrosine kinase family members Syk and Zap-70 are pivotal signal transducers downstream of antigen receptors and exhibit overlapping expression patterns at early lymphocytic developmental stages. To assess their differential kinase fitness in vivo, we generated mice, which carry a Zap-70 cDNA knock-in controlled by intrinsic Syk promoter elements that disrupts wild-type Syk expression. Kinase replacement severely compromised Erk1/2-mediated survival and proper selection of developing B cells at central and peripheral checkpoints, demonstrating critical dependence on BCR signalling quality. Furthermore, ITAM- and hemITAM-mediated activation of platelets and neutrophils was completely blunted, while surprisingly FcγR-mediated phagocytosis in macrophages was retained. The alteration in BCR signalling quality resulted in preferential development and survival of marginal zone B cells and prominent autoreactivity, causing the generation of anti-insulin antibodies and age-related glomerulonephritis. Development of concomitant fasting glucose intolerance in knock-in mice highlights aberrant B cell selection as a potential risk factor for type 1 diabetes, and suggests altered BCR signalling as a mechanism to cause biased cellular and Ig repertoire selection, ultimately contributing to B cell-mediated autoimmune predisposition.

Overexpression of TCL1 activates the endoplasmic reticulum stress response: a novel mechanism of leukemic progression in mice.

Chronic lymphocytic leukemia (CLL) represents 30% of adult leukemia. TCL1 is expressed in ~ 90% of human CLL. Transgenic expression of TCL1 in murine B cells (Eµ-TCL1) results in mouse CLL. Here we show for the first time that the previously unexplored endoplasmic reticulum (ER) stress response is aberrantly activated in Eµ-TCL1 mouse and human CLL. This includes activation of the IRE-1/XBP-1 pathway and the transcriptionally up-regulated expression of Derlin-1, Derlin-2, BiP, GRP94, and PDI. TCL1 associates with the XBP-1 transcription factor, and causes the dysregulated expression of the transcription factors, Pax5, IRF4, and Blimp-1, and of the activation-induced cytidine deaminase. In addition, TCL1-overexpressing CLL cells manufacture a distinctly different BCR, as we detected increased expression of membrane-bound IgM and altered N-linked glycosylation of Igα and Igß, which account for the hyperactive BCR in malignant CLL. To demonstrate that the ER stress-response pathway is a novel molecular target for the treatment of CLL, we blocked the IRE-1/XBP-1 pathway using a novel inhibitor, and observed apoptosis and significantly stalled growth of CLL cells in vitro and in mice. These studies reveal an important role of TCL1 in activating the ER stress response in support for malignant progression of CLL.

Absence of mature peripheral B cell populations in mice with concomitant defects in B cell receptor and BAFF-R signaling.

Generation of mature B lymphocytes from early (T1) and late transitional (T2) precursors requires cooperative signaling through BCR and B cell-activating factor receptor 3 (BR3). Recent studies have shown that BCR signaling positively regulates NF-kappaB2, suggesting BCR regulation of BR3 signaling. To investigate the significance of signal integration from BCR and BR3 in B cell development and function, we crossed Btk-deficient mice (btk(-/-)), which are developmentally blocked between the T2 and the mature follicular B cell stage as a result of a partial defect in BCR signaling, and A/WySnJ mice, which possess a mutant BR3 defective in propagating intracellular signals that results in a severely reduced peripheral B cell compartment, although all B cell subsets are present in relatively normal ratios. A/WySnJ x btk(-/-) mice display a B cell-autonomous defect, resulting in a developmental block at an earlier stage (T1) than either mutation alone, leading to the loss of mature splenic follicular and marginal zone B cells, as well as the loss of peritoneal B1 and B2 cell populations. The competence of the double mutant T1 B cells to respond to TLR4 and CD40 survival and activation signals is further attenuated compared with single mutations as evidenced by severely reduced humoral immune responses in vivo and proliferation in response to anti-IgM, LPS, and anti-CD40 stimulation in vitro. Thus, BCR and BR3 independently and in concert regulate the survival, differentiation, and function of all B cell populations at and beyond T1, earliest transitional stage.

Stereotyped B-cell receptor is an independent risk factor of chronic lymphocytic leukemia transformation to Richter syndrome.

PURPOSE: Few biological prognosticators are useful for prediction of Richter syndrome (RS), representing the transformation of chronic lymphocytic leukemia (CLL) to aggressive lymphoma. Stereotyped B-cell receptors (BCR) may have prognostic effect in CLL progression. We tested the prognostic effect of stereotyped BCR for predicting RS transformation. EXPERIMENTAL DESIGN: The prevalence of stereotyped BCR was compared in RS (n = 69) versus nontransformed CLL (n = 714) by a case-control analysis. Subsequently, the effect of stereotyped BCR at CLL diagnosis on risk of RS transformation was actuarially assessed in a consecutive CLL series (n = 753). RESULTS: RS (n = 69) displayed a higher prevalence of stereotyped BCR (P < 0.001) compared with nontransformed CLL. The actuarial risk of RS transformation was significantly higher in CLL carrying stereotyped BCR (P < 0.001). Among BCR subsets most represented in CLL, subset 8 using IGHV4-39/IGHD6-13/IGHJ5 carried the highest risk of RS transformation [hazard ratio (HR), 24.50; P < 0.001]. Multivariate analysis selected stereotyped BCR (HR, 3.33; P = 0.001) and IGHV4-39 usage (HR, 4.03; P = 0.004) as independent predictors of RS transformation. The combination of IGHV4-39 usage and stereotyped BCR in the same patient identified CLL with a very high risk of RS transformation (5-year risk, 68.7%). The risk carried by stereotyped BCR and IGHV4-39 usage was specific for RS transformation and had no effect on CLL progression without transformation. CONCLUSIONS: Analysis of BCR features may help identify CLL patients at risk of RS. A close monitoring and a careful biopsy policy may help early recognition of RS in CLL patients using stereotyped BCR, particularly if combined with IGHV4-39.

The BCL6 transcriptional program features repression of multiple oncogenes in primary B cells and is deregulated in DLBCL.

The BCL6 transcriptional repressor is required for development of germinal center (GC) B cells and when expressed constitutively causes diffuse large B-cell lymphomas (DLBCLs). We examined genome-wide BCL6 promoter binding in GC B cells versus DLBCLs to better understand its function in these settings. BCL6 bound to both distinct and common sets of functionally related gene in normal GC cells versus DLBCL cells. Certain BCL6 target genes were preferentially repressed in GC B cells, but not DLBCL cells. Several such genes have prominent oncogenic functions, such as BCL2, MYC, BMI1, EIF4E, JUNB, and CCND1. BCL6 and BCL2 expression was negatively correlated in primary DLBCLs except in the presence of BCL2 translocations. The specific BCL6 inhibitor retro-inverso BCL6 peptidomimetic inhibitor-induced expression of BCL2 and other oncogenes, consistent with direct repression effects by BCL6. These data are consistent with a model whereby BCL6 can directly silence oncogenes in GC B cells and counterbalance its own tumorigenic potential. Finally, a BCL6 consensus sequence and binding sites for other physiologically relevant transcription factors were highly enriched among target genes and distributed in a pathway-dependent manner, suggesting that BCL6 forms specific regulatory circuits with other B-cell transcriptional factors.

CD21 signaling via C3 regulates Purkinje cell protein 4 expression.

Complement receptor proteins CR2 (CD21) and CR1 (CD35) have been identified as components of the murine B cell co-receptor complex. Gene expression profiles between naïve WT, C3-/-, and CD21/35-/- B cells demonstrate enhanced expression of a Ca(2+)-modulating gene, Pcp4, in WT mice compared to the complement-deficient animals. Increased expression of Pcp4 is also coincident with B cell maturation into end stage phenotypes. Prolonged activation of B cells via cross-linking of the BCR (but not CR1/CR2 alone) leads to increased expression of Pcp4 and suppressed Ca(2+) release. In total these data demonstrate that the expression of Pcp4 in naïve resting mature B cells is dependent upon tonic stimulation from the CR1/CR2 proteins via a C3 ligand, and that antigen specific B cell activation can also elevate Pcp4 expression that is coincident with suppression of calcium-dependent responses.

Bcr kinase activation by angiotensin II inhibits peroxisome-proliferator-activated receptor gamma transcriptional activity in vascular smooth muscle cells.

Bcr is a serine/threonine kinase activated by platelet-derived growth factor that is highly expressed in the neointima after vascular injury. Here, we demonstrate that Bcr is an important mediator of angiotensin (Ang) II and platelet-derived growth factor-mediated inflammatory responses in vascular smooth muscle cells (VSMCs). Among transcription factors that might regulate Ang II-mediated inflammatory responses we found that ligand-mediated peroxisome proliferator-activated receptor (PPAR)gamma transcriptional activity was significantly decreased by Ang II. Ang II increased Bcr expression and kinase activity. Overexpression of Bcr significantly inhibited PPARgamma activity. In contrast, knockdown of Bcr using Bcr small interfering RNA and a dominant-negative form of Bcr (DN-Bcr) reversed Ang II-mediated inhibition of PPARgamma activity significantly, suggesting the critical role of Bcr in Ang II-mediated inhibition of PPARgamma activity. Point-mutation and in vitro kinase analyses showed that PPARgamma was phosphorylated by Bcr at serine 82. Overexpression of wild-type Bcr kinase did not inhibit ligand-mediated PPARgamma1 S82A mutant transcriptional activity, indicating that Bcr regulates PPARgamma activity via S82 phosphorylation. DN-Bcr and Bcr small interfering RNA inhibited Ang II-mediated nuclear factor kappaB activation in VSMCs. DN-PPARgamma reversed DN-Bcr-mediated inhibition of nuclear factor kappaB activation, suggesting that PPARgamma is downstream from Bcr. Intimal proliferation in low-flow carotid arteries was decreased in Bcr knockout mice compared with wild-type mice, suggesting the critical role of Bcr kinase in VSMC proliferation in vivo, at least in part, via regulating PPARgamma/nuclear factor kappaB transcriptional activity.