DC-SIGN binding to mannosylated B-cell receptors in follicular lymphoma down-modulates receptor signaling capacity

The essential microenvironmental role of oligomannoses specifically inserted into the antigen-binding sites of lymphoma cells

ABSTRACT

There are 2 mandatory features added sequentially en route to classical follicular lymphoma (FL): first, the t(14;18) translocation, which upregulates BCL2, and second, the introduction of sequence motifs into the antigen-binding sites of the B-cell receptor (BCR), to which oligomannose-type glycan is added. Further processing of the glycan is blocked by complementarity-determining region-specific steric hindrance, leading to exposure of mannosylated immunoglobulin (Ig) to the microenvironment. This allows for interaction with the local lectin, dendritic cell-specific ICAM-3-grabbing nonintegrin (DC-SIGN), expressed by tissue macrophages and follicular dendritic cells. The major function of DC-SIGN is to engage pathogens, but this is subverted by FL cells. DC-SIGN induces tumor-specific low-level BCR signaling in FL cells and promotes membrane changes with increased adhesion to VCAM-1 via proximal kinases and actin regulators but, in contrast to engagement by anti-Ig, avoids endocytosis and apoptosis. These interactions appear mandatory for the early development of FL, before the acquisition of other accelerating mutations. BCR-associated mannosylation can be found in a subset of germinal center B-cell-like diffuse large B-cell lymphoma with t(14;18), tracking these cases back to FL. This category was associated with more aggressive behavior: both FL and transformed cases and, potentially, a significant number of cases of Burkitt lymphoma, which also has sites for N-glycan addition, could benefit from antibody-mediated blockade of the interaction with DC-SIGN.

Tinker, tailor, soldier, cell: the role of C-type lectins in the defense and promotion of disease

C-type lectins (CTLs) represent a large family of soluble and membrane-bound proteins which bind calcium dependently via carbohydrate recognition domains (CRDs) to glycan residues presented on the surface of a variety of pathogens. The deconvolution of a cell's glycan code by CTLs underpins several important physiological processes in mammals such as pathogen neutralization and opsonization, leukocyte trafficking, and the inflammatory response. However, as our knowledge of CTLs has developed it has become apparent that the role of this innate immune family of proteins can be double-edged, where some pathogens have developed approaches to subvert and exploit CTL interactions to promote infection and sustain the pathological state. Equally, CTL interactions with host glycoproteins can contribute to inflammatory diseases such as arthritis and cancer whereby, in certain contexts, they exacerbate inflammation and drive malignant progression. This review discusses the 'dual agent' roles of some of the major mammalian CTLs in both resolving and promoting infection, inflammation and inflammatory disease and highlights opportunities and emerging approaches for their therapeutic modulation.

B-cell receptor signaling induces proteasomal degradation of PDCD4 via MEK1/2 and mTORC1 in malignant B cells

B-cell receptor (BCR) signaling plays a major role in the pathogenesis of B-cell malignancies and is an established target for therapy, including in chronic lymphocytic leukemia cells (CLL), the most common B-cell malignancy. We previously demonstrated that activation of BCR signaling in primary CLL cells downregulated expression of PDCD4, an inhibitor of the translational initiation factor eIF4A and a potential tumor suppressor in lymphoma. Regulation of the PDCD4/eIF4A axis appeared to be important for expression of the MYC oncoprotein as MYC mRNA translation was increased following BCR stimulation and MYC protein induction was repressed by pharmacological inhibition of eIF4A. Here we show that MYC expression is also associated with PDCD4 down-regulation in CLL cells in vivo and characterize the signaling pathways that mediate BCR-induced PDCD4 down-regulation in CLL and lymphoma cells. PDCD4 downregulation was mediated by proteasomal degradation as it was inhibited by proteasome inhibitors in both primary CLL cells and B-lymphoma cell lines. In lymphoma cells, PDCD4 degradation was predominantly dependent on signaling via the AKT pathway. By contrast, in CLL cells, both ERK and AKT pathways contributed to PDCD4 down-regulation and dual inhibition using ibrutinib with either MEK1/2 or mTORC1 inhibition was required to fully reverse PDCD4 down-regulation. Consistent with this, dual inhibition of BTK with MEK1/2 or mTORC1 resulted in the strongest inhibition of BCR-induced MYC expression. This study provides important new insight into the regulation of mRNA translation in B-cell malignancies and a rationale for combinations of kinase inhibitors to target translation control and MYC expression.

The succinoglycan riclin restores beta cell function through the regulation of macrophages on Th1 and Th2 differentiation in type 1 diabetic mice

Bacterial succinoglycan is found suitable as a viscosifying and emulsifying agent in the food industry. Riclin is a de-succinyl succinoglycan from an Agrobacterium isolate. Our previous study has revealed that riclin exerts special anti-inflammatory effects in vitro and in vivo. This study aims to determine the effects of riclin on preventing against immunological injury of beta cells in a type 1 diabetic model. We found that orally riclin effectively restores beta-cell function and improves the complications of streptozotocin (STZ)-induced diabetes. Riclin also reduces STZ-induced liver and kidney damage, and balances the inappropriate ratio of T helper type 1 cell (Th1)/type 2 cell (Th2) in the spleen and pancreatic draining lymph nodes of the STZ-induced diabetic mice. In a co-culture system with the islet β cell MIN6 and macrophage RAW 264.7, riclin reduces the levels of IFN-γ and IL-1β, protecting against STZ-caused MIN6 cell injury. We identified that riclin specifically binds to the membrane of macrophages and regulates the ratio of IL-10 and IL-12, thereby inhibiting the macrophage-mediated polarization of Th1 cells and promoting the differentiation of Th2 cells, which depends on the dendritic cell-specific intercellular adhesion molecule-3-grabbing non-integrin (DC-SIGN) receptor. Moreover, orally riclin significantly decreases the incidence of STZ-induced hyperglycemia (7.1% in riclin vs. 92.9% in STZ), and prevents autoimmune diabetes in non-obese diabetic (NOD) mice, with 87.5% of mice free of diabetes compared to 46.6% of the control mice. These results suggest that riclin has potential to be a functional food to prevent and improve autoimmune diabetes and related diseases.

Insertion of atypical glycans into the tumor antigen-binding site identifies DLBCLs with distinct origin and behavior

Glycosylation of the surface immunoglobulin (Ig) variable region is a remarkable follicular lymphoma-associated feature rarely seen in normal B cells. Here, we define a subset of diffuse large B-cell lymphomas (DLBCLs) that acquire N-glycosylation sites selectively in the Ig complementarity-determining regions (CDRs) of the antigen-binding sites. Mass spectrometry and X-ray crystallography demonstrate how the inserted glycans are stalled at oligomannose-type structures because they are buried in the CDR loops. Acquisition of sites occurs in ∼50% of germinal-center B-cell-like DLBCL (GCB-DLBCL), mainly of the genetic EZB subtype, irrespective of IGHV-D-J use. This markedly contrasts with the activated B-cell-like DLBCL Ig, which rarely has sites in the CDR and does not seem to acquire oligomannose-type structures. Acquisition of CDR-located acceptor sites associates with mutations of epigenetic regulators and BCL2 translocations, indicating an origin shared with follicular lymphoma. Within the EZB subtype, these sites are associated with more rapid disease progression and with significant gene set enrichment of the B-cell receptor, PI3K/AKT/MTORC1 pathway, glucose metabolism, and MYC signaling pathways, particularly in the fraction devoid of MYC translocations. The oligomannose-type glycans on the lymphoma cells interact with the candidate lectin dendritic cell-specific intercellular adhesion molecule 3 grabbing non-integrin (DC-SIGN), mediating low-level signals, and lectin-expressing cells form clusters with lymphoma cells. Both clustering and signaling are inhibited by antibodies specifically targeting the DC-SIGN carbohydrate recognition domain. Oligomannosylation of the tumor Ig is a posttranslational modification that readily identifies a distinct GCB-DLBCL category with more aggressive clinical behavior, and it could be a potential precise therapeutic target via antibody-mediated inhibition of the tumor Ig interaction with DC-SIGN-expressing M2-polarized macrophages.