THE EFFECTS OF β1,3-GLUCANS ON BLOOD COAGULATION AND AMEBOCYTE RELEASE IN THE HORSESHOE CRAB,LIMULUS POLYPHEMUS

The best-studied defense reaction in the horseshoe crab, Limulus polyphemus, involves the exocytotic release of a clotting system from the blood cells (amebocytes) and the formation of an extracellular clot in response to bacterial lipopolysaccharide. Although β1,3-glucans, apparently serving as signals for the presence of potentially pathogenic fungi, trigger release, blood coagulation, and activation of the prophenol-oxidase system in many arthropods, Limulus appears to be unresponsive to endotoxin-free preparations of naturally occurring β1,3-glucans. However, the carboxymethylated β1,3-glucan derivative CMPS induced gelation of amebocyte lysates. The induction of clotting enzyme activity and gelation of amebocyte lysate by CMPS followed a complex pattern. Enzymatic activity generated by CMPS was only 15-42% of that produced by bacterial lipopolysaccharide. Neither native β1,3-glucans nor CMPS induced exocytosis of living Limulus amebocytes. We conclude that although CMPS produced coagulation of amebocyte lysates, the response is of questionable biological significance since native β1,3-glucans were inactive. The blood of Limulus lacked detectable phenoloxidase activity.