Protection against Polymicrobial Sepsis by Chitin Oligomers is Fine-Tuned by N‑Acetyl‑d‑Glucosamine Residues

Chitin, poly-N-acetyl-d-glucosamine, is an abundant polysaccharide produced by fungal cell walls, insect epicuticles, and nematode cuticles. Its immunomodulatory function has curious attributes: paradoxically opposing host responses using different host receptors, resulting in immunostimulatory or immunosuppressive properties depending on the size/length of the oligomers as well as acetylation levels, have been reported in the literature. Here, we demonstrate that hepta-N-acetyl chitoheptaose (7-mer) is a TLR2 ligand, while the 8-mer activates immune cells through TLR4, and the 6-mer is a relatively poor ligand for both TLR2 and TLR4 in generating inflammatory host cytokines. A significantly enhanced inflammatory response, characterized by increased TNF-α, IL-1β, IL-6, and IL-10, was a feature of only the 8-mer, while the 6- and 7-mers induced modest activation in both HEK cells (transfected with specific TLRs) and human THP2 cells in vitro. The translational significance of these features was addressed in an experimental model of sepsis using the Cecal Ligation Puncture (CLP) protocol, a murine model considered a gold standard for human sepsis. More significantly, therapeutic rather than prophylactic administration, which simulates the real-life scenario of human and animal sepsis, of the 7 mer, rather than the 6- or 8-mer residues of chitin oligomer, significantly protected mice against sepsis, as shown by decreased mortality and decreased induction of inflammatory cytokines. These findings suggest that modulation of immune response by chito-polysaccharides in vivo is precisely calibrated.