In silico mining and characterization of bifidobacterial lipoprotein with CHAP domain secreted in an aggregated form

Bifidobacterium breve C50 secretes a lipoprotein associated with glucose, acting in an aggregating form (>600 kDa) as an agonist of TLR2/6. Similar lipoproteins were sought for in bifidobacteria. In silico, the closest homology was shown with a Bifidobacterium longum protein containing CHAP and lipobox domains. Two strains secreted aggregates whose peptides sequences aligned with the mined protein. C16:0 and C18:0 fatty acids detected in the aggregates further supported a lipoprotein structure. Glucose and mannose detected by gas chromatography were likely ligands of the lipoprotein. The binding of aggregates to galectin-1 indicated that hexosamines and galactose surrounded them. However, unlike B. breve C50, aggregate secreted by B. longum CBi0703 was unable to bind TLR2/6 likely because of a more hydrophobic structure. In gnotobiotic mice, the intake of B. longum aggregate induced, in splenic dendritic cells, the expression of genes involved in antigen presentation. A positive correlation between the number of dendritic cells and CD4+CD25+ cells was observed in mice receiving these aggregates. In conclusion, B. longum secretes a lipoprotein forming aggregates which may influence dendritic and CD4+CD25+ cell interactions independently of the TLR2/6 pathway.