| Throughout an organism's life it must successfully combat myriad potential pathogens to survive to sexual maturity. The American horseshoe crab, Limulus polyphemus, is a long-lived marine invertebrate that attains sexual maturity at approximately ten years of age and an ultimate lifespan in excess of twenty years. Limulus, as do all invertebrates, lacks the adaptive arm of the immune system. The sole mediator of immune defense is the innate immune system, the different elements of which typically recognize broad classes of pathogens, in contrast to the specificity of targeting enjoyed by the adaptive immune system. An important component of the innate immune system is direct cytolytic attack on the potential pathogen. Microbes that have gained entry into the internal milieu of host metazoans are, ideally, hindered in their dissemination throughout the organism and killed by elements of the plasma based innate immune system. The pentraxins are a family of highly conserved plasma proteins of metazoans known to function in immune defense. The canonical members, C-reactive protein and serum amyloid P component, have been identified in arthropods and humans. I show here that the pentraxin proteins from Limulus plasma participate in immunity by agglutinating and under the proper circumstances forming hydrophilic transmembrane pores in the lipid bilayers of target organisms. I have identified a Ca2+ -independent mode of membrane binding by Limulus C-reactive protein and suggest that this is the necessary mode of binding for the subsequent Ca2+-dependent transmembrane pore-formation. Electron microscopy reveals that Limulus C-reactive protein also agglutinates liposomes through the formation of large, hyper-oligomerized fibrils and sheets of protein that bind and encapsulated the liposomes. Immune defense also acts at epithelial boundaries, surfaces of the organism that contact the external environment. I identify here, a cytolytic activity that is present in a viscous surface secretion of Limulus, that may operate in protecting the chitinized-epithelium of the animal from deleterious fouling organisms, or epibionts. |
