Carbohydrate-dependent B cell activation by fucose-binding bacterial lectins

Bacterial lectins are typically multivalent and bind noncovalently to specific carbohydrates on host tissues to facilitate bacterial adhesion. Here, we analyzed the effects of two fucose-binding lectins, BambL from Burkholderia ambifaria and LecB from Pseudomonas aeruginosa, on specific signaling pathways in B cells. We found that these bacterial lectins induced B cell activation, which, in vitro, was dependent on the cell surface expression of the B cell antigen receptor (BCR) and its co-receptor CD19, as well as on spleen tyrosine kinase (Syk) activity. The resulting release of intracellular Ca²⁺ was followed by an increase in the cell surface abundance of the activation marker CD86, augmented cytokine secretion, and subsequent cell death, replicating all of the events that are observed in vitro upon canonical and antigen-mediated B cell activation. Moreover, injection of BambL in mice resulted in a substantial, BCR-independent loss of B cells in the bone marrow with simultaneous, transient enlargement of the spleen (splenomegaly), as well as an increase in the numbers of splenic B cells and myeloid cells. Together, these data suggest that bacterial lectins can initiate polyclonal activation of B cells through their sole capacity to bind to fucose.

Idelalisib for optimized CD19-specific chimeric antigen receptor T cells in chronic lymphocytic leukemia patients

Despite encouraging results with chimeric antigen receptor T (CART) cells, outcome can still be improved by optimization of the CART cell generation process. The proportion of less-differentiated T cells within the transfused product is linked to enhanced in vivo CART cell expansion and long-term persistence. The clinically approved PI3Kδ inhibitor idelalisib is well established in the treatment of B cell malignancies. Besides B cell receptor pathway inhibition, idelalisib can modulate T cell differentiation and function. Here, detailed longitudinal analysis of idelalisib-induced effects on T cell phenotype and function was performed during CART cell production. A third generation CD19.CAR.CD28.CD137zeta CAR vector system was used. CART cells were generated from peripheral blood mononuclear cells of healthy donors (HDs) and chronic lymphocytic leukemia (CLL) patients. Idelalisib-based CART cell generation resulted in an enrichment of less-differentiated naïve-like T cells (CD45RA+CCR7+), decreased expression of the exhaustion markers PD-1 and Tim-3, as well as upregulation of the lymph node homing marker CD62L. Idelalisib increased transduction efficiency, but did not impair viability and cell expansion. Strikingly, CD4:CD8 ratios that were altered in CART cells from CLL patients were approximated to ratios in HDs by idelalisib. Furthermore, in vivo efficacy of idelalisib-treated CART cells was validated in a xenograft mouse model. Intracellular TNF-α and IFN-γ production decreased in presence of idelalisib. This effect was reversible after resting CART cells without idelalisib. In summary, PI3Kδ inhibition with idelalisib can improve CART cell products, particularly when derived from CLL patients. Further studies with idelalisib-based CART cell generation protocols are warranted.

Differences in Self-Recognition between Secreted Antibody and Membrane-Bound B Cell Antigen Receptor

The random gene segment rearrangement during B cell development ensures Ab repertoire diversity. Because this process might generate autoreactive specificities, it has been proposed that stringent selection mechanisms prevent the development of autoreactive B cells. However, conventional assays to identify autoreactive B cells usually employ in vitro-generated Abs, which differ from membrane-bound BCRs. In this study, we used a cell-based assay to investigate the autoreactivity of membrane-bound BCRs derived from different B cell developmental stages of human peripheral blood. Contrasted to soluble Ab counterparts, only a few of the tested BCRs were autoreactive, although the cell-based assay sensitively detects feeble Ag recognition of a germline-reverted murine BCR that was selected after OVA immunization of mice, whereas conventional assays failed to do so. Together, these data suggest that proper identification of autoreactive B cells requires the membrane-bound BCR, as the soluble Ab may largely differ from its BCR counterpart in Ag binding.

Pathogen-specific B-cell receptors drive chronic lymphocytic leukemia by light-chain-dependent cross-reaction with autoantigens

Several lines of evidence indirectly suggest that antigenic stimulation through the B‐cell receptor (BCR) supports chronic lymphocytic leukemia (CLL) development. In addition to self‐antigens, a number of microbial antigens have been proposed to contribute to the selection of the immunoglobulins expressed in CLL. How pathogen‐specific BCRs drive CLL development remains, however, largely unexplored. Here, we utilized mouse models of CLL pathogenesis to equip B cells with virus‐specific BCRs and study the effect of antigen recognition on leukemia growth. Our results show that BCR engagement is absolutely required for CLL development. Unexpectedly, however, neither acute nor chronic exposure to virus‐derived antigens influenced leukemia progression. Rather, CLL clones preferentially selected light chains that, when paired with virus‐specific heavy chains, conferred B cells the ability to recognize a broad range of autoantigens. Taken together, our results suggest that pathogens may drive CLL pathogenesis by selecting and expanding pathogen‐specific B cells that cross‐react with one or more self‐antigens.

Distinct homotypic B-cell receptor interactions shape the outcome of chronic lymphocytic leukaemia

Cell-autonomous B-cell receptor (BcR)-mediated signalling is a hallmark feature of the neoplastic B lymphocytes in chronic lymphocytic leukaemia (CLL). Here we elucidate the structural basis of autonomous activation of CLL B cells, showing that BcR immunoglobulins initiate intracellular signalling through homotypic interactions between epitopes that are specific for each subgroup of patients with homogeneous clinicobiological profiles. The molecular details of the BcR–BcR interactions apparently dictate the clinical course of disease, with stronger affinities and longer half-lives in indolent cases, and weaker, short-lived contacts mediating the aggressive ones. The diversity of homotypic BcR contacts leading to cell-autonomous signalling reconciles the existence of a shared pathogenic mechanism with the biological and clinical heterogeneity of CLL and offers opportunities for innovative treatment strategies.

Chronic lymphocytic leukaemia (CLL) is characterized by cell-autonomous B-cell receptor (BcR)-mediated signalling of neoplastic B lymphocytes. Here the authors unveil the structural basis and diversity of activatory homotypic BcR contacts and link them with CLL heterogeneity and the clinical outcome.

Abstract LB-012: Autonomous, antigen-independent B-cell receptor signalling as a novel pathogenetic mechanism in non-GCB DLBCL

The activated B-cell (ABC) type of diffuse large B-cell lymphoma (DLBCL) is clinically more aggressive than the germinal center (GCB) type. The gene expression profile of ABC DLBCL resembles that of mature B cells upon stimulation via their B-cell receptor (BCR). In up to 30% of ABC DLBCL cases, this signature can be explained by gain-of-function mutations in CD79A, CD79B, or CARD11. Antigen-independent, constitutively active (BCR) signaling is a general oncogenic mechanism of CLL (Dühren-von Minden, Nature 2012). We hypothesized that autonomous BCR signaling akin to CLL may operate in ABC DLBCL and could explain the characteristic ABC gene expression signature.

BCR transcripts were identified from fresh-frozen biopsies from 12 histologically confirmed, IgM-expressing DLBCL by ARTISAN PCR, a novel anchored RT-PCR for unbiased amplification of BCR transcripts facilitated by template switching. Clonal full-length BCR sequences were identified by PacBio next-generation sequencing. Triple KO (TKO) cells were transduced with functional DLBCL BCR. TKO cells are arrested at the pre-B-cell stage due to lack of the rag2 and lambda5 genes. In addition, TKO cells have their wild-type SLP65 adaptor replaced with a tamoxifen-dependent SLP65. When transduced with expression constructs encoding a functional BCR, autonomous or antigen-induced BCR signalling can be measured as calcium flux upon induction with tamoxifen.

TKO cells transduced with seven of the twelve DLBCL BCR (58%) showed robust calcium flux and proliferation upon activation of SLP65 by tamoxifen without additional BCR crosslinking. By histomorphology and immunohistochemistry for CD10, PAX5, MUM1, and Bcl-6, these cases were classified as non-GCB-type DLBCL, including one primary CNS DLBCL and two primary mediastinal (PM-)DLBCL. In accordance with our hypothesis, BCR from three DLBCL classified as GCB lacked an autonomous BCR signal. The remaining two negative cases were non-GCB-type and are currently being analyzed for activating CD79/CARD11 mutations.

To further explore the detection of autonomous BCR signaling in DLBCL, PM-DLBCL and ABC DLBCL cell lines were tested in vitro by comparing calcium flux with and without blockade of Syk signaling by the tyrosine kinase inhibitor R406. All four PM-DLBCL and two of five ABC DLBCL cell lines had higher calcium flux in the absence of R406, indicating autonomous signaling activity without antigenic stimulation or artificial BCR crosslinking.

In summary, we demonstrate autonomous BCR activity in a substantial fraction (72%) of non-GCB DLBCL (including PM-DLBCL). These results point to an oncogenic role of structurally normal BCR in non-GCB DLBCL akin to CLL. Autonomous BCR signaling is a candidate alternative mechanism to the previously described activating mutations of components of the BCR signaling cascade to induce the characteristic gene expression signature of ABC DLBCL. Formal delineation of autonomous BCR signalling to activating mutations in the BCR signalling cascade is ongoing.

Citation Format: Marvyn T. Koning, Rudolf Übelhart, Arjen H.G. Cleven, Willem H. Zoutman, Sander A.J. van der Zeeuw, Philip Kluin, Marieke Griffioen, Hans G. Drexler, Cornelis P. Tensen, Maarten H. Vermeer, Rein Willemze, Szymon M. Kielbasa, Marcelo A. Navarrete, Hassan Jumaa, Hendrik Veelken. Autonomous, antigen-independent B-cell receptor signalling as a novel pathogenetic mechanism in non-GCB DLBCL. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr LB-012.

Chronic lymphocytic leukaemia is driven by antigen-independent cell-autonomous signalling

B-cell antigen receptor (BCR) expression is an important feature of chronic lymphocytic leukaemia (CLL), one of the most prevalent B-cell neoplasias in Western countries. The presence of stereotyped and quasi-identical BCRs in different CLL patients suggests that recognition of specific antigens might drive CLL pathogenesis. Here we show that, in contrast to other B-cell neoplasias, CLL-derived BCRs induce antigen-independent cell-autonomous signalling, which is dependent on the heavy-chain complementarity-determining region (HCDR3) and an internal epitope of the BCR. Indeed, transferring the HCDR3 of a CLL-derived BCR provides autonomous signalling capacity to a non-autonomously active BCR, whereas mutations in the internal epitope abolish this capacity. Because BCR expression was required for the binding of secreted CLL-derived BCRs to target cells, and mutations in the internal epitope reduced this binding, our results indicate a new model for CLL pathogenesis, with cell-autonomous antigen-independent signalling as a crucial pathogenic mechanism.