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.

Key Points• BCR variable-region mannoses in follicular lymphoma are recognized by lectins of common opportunistic bacteria.• Introduction of N-linked sugars into the BCR variable region interferes with antigen recognition.B-cell antigen receptor (BCR) expression is a key feature of most B-cell lymphomas, but the mechanisms of BCR signal induction and the involvement of autoantigen recognition remain unclear. In follicular lymphoma (FL) B cells, BCR expression is retained despite a chromosomal translocation that links the antiapoptotic gene BCL2 to the regulatory elements of immunoglobulin genes, thereby disrupting 1 heavy-chain allele. A remarkable feature of FL-BCRs is the acquisition of potential N-glycosylation sites during somatic hypermutation. The introduced glycans carry mannose termini, which create potential novel binding sites for mannose-specific lectins. Here, we investigated the effect of N-linked variable-region glycosylation for BCR interaction with cognate antigen and with lectins of different origins. N-glycans were found to severely impair BCR specificity and affinity to the initial cognate antigen. In addition, we found that lectins from Pseudomonas aeruginosa and Burkholderia cenocepacia bind and stimulate FL cells. Human exposure to these bacteria can occur by contact with soil and water. In addition, they represent opportunistic pathogens in susceptible hosts. Understanding the role of bacterial lectins might elucidate the pathogenesis of FL and establish novel therapeutic approaches. (Blood. 2015;125(21):3287-3296)

No abstract

2704 Follicular lymphoma (FL) is an indolent B-cell lymphoma characterized by apoptosis resistance due to overexpression of Bcl-2 as a consequence of the t(14;18) translocation, ongoing somatic hypermutation (SHM), and expression of B-cell receptors (BCR) with glycosylation of the antigen binding sites. Translocation and concomitant Bcl-2 overexpression can be found in healthy human blood B cells and is insufficient to drive lymphoma outgrowth in mouse models. Since most FL cells still express a surface B cell receptor (BCR) despite the disruption of one immunoglobulin heavy chain allele by the t(14;18) translocation, expression of an antigen receptor seems to be indispensable for FL development. Around 80% of FLs possess asparagine (N)-linked glycosylation sites (amino acid sequence: N-X-S/T) in their BCR variable regions that are not encoded in germ-line but are acquired through SHM. In contrast to germ-line-encoded glycosylation sites in the constant BCR region, where normal processing of the glycans results in termination on branched sugars like sialic acid, the variable region glycosylation sites carry mannose-terminating sugars. Recently, it has been shown that C-type lectins bind to and stimulate FL cells. Such lectins are normally expressed on cells of the innate immune system, e.g. dendritic cells (DCs), which also reside in close interaction with the transformed B cells in germinal centers. Importantly, previous studies point to an outstanding role of the tumor microenvironment in survival and proliferation of the FL cells. In this study, we demonstrate that the variable region glycosylation in FL BCRs contribute to stimulation of the cells as well as adhesion to cells of the innate immune system. The BCR from six FL and the appropriate glycosylation-defective controls in which the N-linked glycosylation sequons are removed by replacing the asparagine (N) residues with glutamine (Q) residues were expressed in the tko cellular reconstitution system. In tko cells, the BCR signaling cascade can be rendered functional at will through a tamoxifen-dependent mutant of the signal transducer SLP-65 (Meixlsperger et al., Immunity 2007; Dühren von Minden et al., Nature 2012). Tko cells expressing FL BCRs and their glycosylation-defective controls were tested for binding of a recombinant DC-SIGN/Fc fusion protein by flow cytometry. The mannosylated FL-derived BCR but not glycosylation-mutated receptors bound DC-SIGN. Stepwise mutation of individual glycosylation sites demonstrated variable contribution to the strength of lectin binding. Despite this specific binding to mannosylated FL BCRs, DC-SIGN/Fc failed to induce significant calcium mobilization of transduced tko cells. Crosslinking with anti-IgM, in contrast, led to a readily detectable BCR-mediated signal, thereby demonstrating functionality of the transduced BCR. To study the role of mannosylated FL receptors in interaction with their environment, we co-cultured cells expressing FL receptors containing or lacking N-linked glycans in the variable regions together with macrophages. Western blot analyses with a pan-phosphotyrosine antibody demonstrated higher global tyrosine phosphorylation in the lysates of cells expressing glycosylated receptors, thereby indicating a specific role for mannosylated V-regions in FL stimulation. Glycan-mediated interactions fulfill multiple important functions in the mammalian immune system including pathogen recognition and cell adhesion or trafficking. DC-SIGN serves as receptor for the uptake of mannosylated pathogens and contributes to cell-cell interaction by binding to the heavily glycosylated ICAM-2/3 (intracellular adhesion molecules-2/3). In the case of FL, it is therefore conceivable that DC-SIGN expressed on follicular DCs binds to the heavily mannosylated FL BCRs and serves thereby as adhesion molecule to keep the FL B cells within the follicular structure. We tested this hypothesis using live cell imaging on a DC sublayer and detected slightly slower movement and shorter tracks of cells expressing glycosylated FL BCRs as compared to control cells. Together, our results ascribe a role of the acquired glycosylation sites in FL BCRs for B-cell/DC interaction, thereby keeping the cells in the appropriate environment in a process that involves active signal transduction rather than triggering a classical antigen-induced BCR stimulation. Disclosures: No relevant conflicts of interest to declare.