Interactions of lipid a and liposome-associated lipid A with Limulus polyphemus amoebocytes.

Lipid A or lipid A fractions and liposomes containing lipid A were tested for the ability to gel Limulus amoebocyte lysates and for effects on intact Limulus amoebocytes. Liposomes having a relatively low concentration of lipid A did not produce coagulation of lysate and were designated as Limulus-negative, but liposomes having a high concentration of lipid A were Limulus-positive. Limulus-negative liposomes had no effect on intact amoebocytes. Limulus-positive liposomes caused a striking transformation in the appearance of amoebocytes in that the cells sent out long filamentous extensions that formed a tangled network of processes between cells. The filamentous projections were similar to those that have been previously observed in the presence of gram-negative bacteria. We conclude that amoebocytes have the ability to recognize Limulus-positive liposomes, but the lack of activation of Limulus lysate or the absence of amoebocyte recognition does not prove the absence of liposomal lipid A. We also found that individual lipid A fractions were heterogeneous in their ability to gel lysate. Of eight fractions tested, one (fraction 1) had no detectable activity above the background, and the seven others had activity that ranged from 10-fold to 10,000-fold above the background. The heterogeneity of lipid A fractions detected in assays with amoebocyte lysate was consistent with the finding of heterogeneity in other functional assays of lipid A fractions.