Lectins from opportunistic bacteria interact with acquired variable-region glycans of surface immunoglobulin in follicular lymphoma

Abstract

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)

DC-SIGN–expressing macrophages trigger activation of mannosylated IgM B-cell receptor in follicular lymphoma

“…This could be at least in part explained by the lack of BCR endocytosis after engagement by DC-SIGN. 40 Even if several lectins could mediate FL BCR activation in vitro, 17 DC-SIGN is a very good candidate as 33 and triggers direct signal transducer and activator of transcription (STAT) 6-dependent antiapoptotic activity on malignant FL B cells. 28,45 Our current results further suggest that IL-4 could also display indirect protumoral activity by promoting TAM polarization and B-cell supportive activity.…”

“…16 Recently, mannosylated V regions of FL BCR were reported to interact with opportunistic pathogen-derived lectins but were not sufficient to trigger functional interaction with human DC-SIGN. 17 Only a subset of FL samples was able to bind to a mannose-specific bacterial lectin, suggesting an additional level of heterogeneity in FL BCR signaling. Collectively, these observations raise 2 questions: what are the mechanisms underlying the functional heterogeneity of FL BCR, and what are the endogenous ligands for FL BCR in vivo?…”

The Emerging Importance of IgG Fab Glycosylation in Immunity

“…Signaling through interactions between N-linked glycans on follicular lymphoma BCRs and lectins, such as DC-SIGN or mannose-binding lectin, at the surface of macrophages and dendritic cells may free these cells from dependence on Ag and may contribute to tumor cell persistence or growth (48,49). Some lectins from opportunistic pathogens, such as Pseudomonas aeruginosa and Burkholderia cenocepacia, can also interact with high-mannose BCR Fab glycans (50) (Fig. 2A).…”

“…In addition, the removal of sialic acid residues from Fab glycans can decrease Ag-binding affinity (57). In contrast, insertion of follicular lymphoma-specific N-linked glycosylation sites into the model BCRs B1-8 and HyHEL10 strongly impairs the binding to their respective Ags (50), and the presence of N-linked glycans in the Fab of an anti-CD33 Ab decreases its binding to CD33 by 3-8-fold (58). Fab glycans may also leave Ag-binding affinity essentially unaffected (23).…”

“…Several studies showed that the presence of Nlinked glycosylation sites in the variable domains can increase (18,23,(54)(55)(56) or decrease (18,23,50) Ag-binding affinity. For instance, the anti-a(1→6)-dextran Ab 14.6b.1 has a 10-fold higher affinity compared with a single amino acid mutant that lacks the N-linked glycosylation site (54), presumably due to additional hydrophilic interactions of the Fab glycans with the carbohydrate Ag (Fig.…”