ELISA for antibodies to lipid A, lipopolysaccharides and other hydrophobic antigens.

A simple, sensitive and rapid ELISA method for the quantification and characterization of antibodies to lipid A has been developed, which can also be applied to other hydrophobic antigens. For coating, antigens were applied to the wells of ELISA plates as solutions in a mixture of chloroform and ethanol 1:9 (v/v), and the solvent evaporated in a stream of warm air. Under these conditions a high coating efficiency was achieved, which made the assay highly sensitive. The use of the above organic solvent considerably reduced the nonspecific adsorption of immunoglobulins to the solid phase, making the usual blocking of unspecific binding sites with BSA or gelatine unnecessary. For screening of lipid A antibodies in the sera of immunized animals, coating with 0.2 microgram of the corresponding antigen per well was found to be suitable. For optimal measurement of antibodies in pre-immune sera, sera of healthy human donors, and of monoclonal antibodies, higher amounts of antigen (1-2 micrograms/well) had to be used. The coating method described here proved excellent also for other antigens directly soluble in organic solvents, such as Re-lipopolysaccharide (LPS) or gangliosides. In addition, the method was successfully applied to less hydrophobic antigens, such as LPS of the classes Ra to Rd and S forms, and lipoteichoic acid. These could be brought into solution in chloroform/ethanol by diluting their aqueous solutions with a large volume of the organic mixture.