Bacterial polysaccharides with zwitterionic charge motifs: Toll-like receptor 2 agonists, T cell antigens, or both?

Bacterial capsular polysaccharides (PS) which naturally contain zwitterionic charge motifs (ZPS) possess specific immunostimulatory activity, leading to direct activation of antigen-presenting cells (APCs) through Toll-like receptor 2 (TLR2) and of T cells in co-culture systems. When administered intraperitoneally, ZPS and bacteria expressing them are involved in the induction or regulation of T-cell dependent inflammatory processes such as intra-abdominal abscess formation. To generate vaccine candidates with antigen and adjuvant properties in one molecule we have chemically introduced zwitterionic motifs into naturally anionic PS and find that the resulting ZPS are TLR2 agonists, able to activate human and mouse APCs. Since T-regulatory cells and other T-cell subsets express TLR2, and TLR2 engagement modifies functionality and activation state of these cells, we speculate that most effects induced by natural and chemically derived ZPS may be explained by their TLR2 agonist properties, presumably through the combined action on TLR2-expressing APCs and T cells.