The lipopolysaccharide-activated innate immune response network of the horseshoe crab

Abstract

Primary stimulation of the horseshoe crab innate immune system by bacterial lipopolysaccharide
(LPS) activates a network of responses to ensure host defense against invading pathogens. Granular
hemocytes selectively respond to LPS via a G protein-dependent exocytic pathway that critically
depends on the proteolytic activity of the LPS-responsive coagulation factor C. In response to
stimulation by LPS, the hemocyte secretes transglutaminase (TGase) and several kinds of defense
molecules, such as coagulation factors, lectins, antimicrobial peptides, and protein substrates for
TGase. LPS-induced hemocyte exocytosis is enhanced by a feedback mechanism in which the
antimicrobial peptide tachyplesin serves as an endogenous mediator. The coagulation cascade
triggered by LPS or β-1,3-D-glucans results in the formation of coagulin fibrils that are subsequently
stabilized by TGase-dependent cross-linking. A cuticle-derived chitin-binding protein additionally forms a TGase-stabilized mesh at sites of injury. Invading pathogens are agglutinated by both hemocyte- and plasma-derived lectins. In addition, the proclotting enzyme and tachyplesin functionally convert hemocyanin to phenoloxidase. In the plasma, coagulation factor C acts an LPS-sensitive complement C3 convertase on the surface of Gram-negative bacteria. In this manner, LPS-induced hemocyte exocytosis leads not only to coagulation but also activates a sophisticated innate immune response network that coordinately effects pathogen recognition, prophenoloxidase activation, pathogen clearance, and TGase-dependent wound healing.