Bacterial lectin BambL acts as a B cell superantigen

Bacterial lectins: multifunctional tools in pathogenesis and possible drug targets

Glycans are vital macromolecules with diverse biological roles, decoded by lectins - specialized carbohydrate-binding proteins crucial in pathogenesis. The WHO identifies bacterial antimicrobial resistance (AMR) as a critical global health challenge, necessitating innovative strategies that also target non-antibiotic pathways. Recent studies highlight bacterial lectins as key players in pathogenesis and promising therapeutic targets, with early clinical success using glycomimetics and vaccines to treat and prevent AMR-related infections. This review covers the current knowledge on bacterial lectins, their classifications, and roles in host recognition and adhesion, biofilm formation, cytotoxicity, and host immune evasion, with examples of well-characterized lectins. It also explores their therapeutic potential and highlights novel lectins with unknown functions, encouraging further research.

Lectins and polysaccharide EPS I have flow-responsive roles in the attachment and biofilm mechanics of plant pathogenic Ralstonia

Bacterial biofilm formation and attachment to hosts are mediated by carbohydrate-binding lectins, exopolysaccharides, and their interactions in the extracellular matrix (ECM). During tomato infection Ralstonia pseudosolanacearum (Rps) GMI1000 highly expresses three lectins: LecM, LecF, and LecX. The latter two are uncharacterized. We evaluated the roles in bacterial wilt disease of LecF, a fucose-binding lectin, LecX, a xylose-binding lectin, and the Rps exopolysaccharide EPS I. Interestingly, single and double lectin mutants attached to tomato roots better and formed more biofilm under static conditions in vitro. Consistent with this finding, static bacterial aggregation was suppressed by heterologous expression of lecFGMI1000 and lecXGMI1000 in other Ralstonia strains that naturally lack these lectins. Crude ECM from a ΔlecF/X double mutant was more adhesive than the wild-type ECM, and LecF and LecX increased Rps attachment to ECM. The enhanced adhesiveness of the ΔlecF/X ECM could explain the double mutant's hyper-attachment in static conditions. Unexpectedly, mutating lectins decreased Rps attachment and biofilm viscosity under shear stress, which this pathogen experiences in plant xylem. LecF, LecX, and EPS I were all essential for biofilm development in xylem fluid flowing through cellulose-coated microfluidic channels. These results suggest that under shear stress, LecF and LecX increase Rps attachment by interacting with the ECM and plant cell wall components like cellulose. In static conditions such as on root surfaces and in clogged xylem vessels, the same lectins suppress attachment to facilitate pathogen dispersal. Thus, Rps lectins have a dual biological function that depends on the physical environment.

Novel insights into the immune response to bacterial T cell superantigens

Bacterial T cell superantigens (SAgs) are a family of microbial exotoxins that function to activate large numbers of T cells simultaneously. SAgs activate T cells by direct binding and crosslinking of the lateral regions of MHC class II molecules on antigen-presenting cells with T cell receptors (TCRs) on T cells; these interactions alter the normal TCR-peptide-MHC class II architecture to activate T cells in a manner that is independent of the antigen specificity of the TCR. SAgs have well-recognized, central roles in human diseases such as toxic shock syndrome and scarlet fever through their quantitative effects on the T cell response; in addition, numerous other consequences of SAg-driven T cell activation are now being recognized, including direct roles in the pathogenesis of endocarditis, bloodstream infections, skin disease and pharyngitis. In this Review, we summarize the expanding family of bacterial SAgs and how these toxins can engage highly diverse adaptive immune receptors. We highlight recent findings regarding how SAg-driven manipulation of the adaptive immune response may operate in multiple human diseases, as well as contributing to the biology and life cycle of SAg-producing bacterial pathogens.

Fucose-binding lectins: purification, characterization and potential biomedical applications

The property of lectins to specifically recognize and bind carbohydrates makes them an excellent candidate in biomedical research. Among them are fucose-binding lectins possessing the capacity to bind fucose are taxonomically, evolutionarily and ecologically significant class of lectins that are identified in a wide range of taxa. Purification of fucose-binding lectins dates back to 1967 when L-fucose binding protein from Lotus tetragonolobus was isolated using a dye that contained three α-L-fucopyranosyl residues. Beginning with that, several FBLs were purified from various animals as well as plant sources that were structurally and functionally characterised. This review focuses on fucose-binding lectins, their occurrence and purification with special emphasis on various strategies adopted to purify them followed by molecular and functional characterization. The exclusive ability to recognize and bind to fucose-containing glycans endows these lectins with the potential to act as anti-cancer agents, diagnostic markers and mitogens for immune cells. Though they have been in research focus for more than half a century with their occurrence reported in various taxa, they still need to be explored for their prospective functions to develop them as a biological tool in biomedical research.

Pseudomonas aeruginosa LecB suppresses immune responses by inhibiting transendothelial migration

Pseudomonas aeruginosa is a Gram-negative bacterium causing morbidity and mortality in immuno-compromised humans. It produces a lectin, LecB, that is considered a major virulence factor, however, its impact on the immune system remains incompletely understood. Here we show that LecB binds to endothelial cells in human skin and mice and disrupts the transendothelial passage of leukocytes in vitro. It impairs the migration of dendritic cells into the paracortex of lymph nodes leading to a reduced antigen-specific T cell response. Under the effect of the lectin, endothelial cells undergo profound cellular changes resulting in endocytosis and degradation of the junctional protein VE-cadherin, formation of an actin rim, and arrested cell motility. This likely negatively impacts the capacity of endothelial cells to respond to extracellular stimuli and to generate the intercellular gaps for allowing leukocyte diapedesis. A LecB inhibitor can restore dendritic cell migration and T cell activation, underlining the importance of LecB antagonism to reactivate the immune response against P. aeruginosa infection.