Radioimmunoassay for Gram-negative bacterial lipopolysaccharide O antigens: influence of antigen solubility.

We have developed a solid-phase radioimmunoassay technique for specific gram-negative bacterial lipopolysaccharide (LPS) O antigens. The method exploits the high-titer, specific immunoglobulin M response of the rabbit to LPS immunization to measure as little as 5 ng of homologous LPS per ml with less than 0.5% cross-reactivity toward heterologous LPS or culture supernatants. We found that O antigen in complete LPS was less available for antibody binding than O antigen in the soluble polysaccharide derived by mild acid hydrolysis of LPS and that triethylamine-induced disaggregation of complete LPS increased its activity in the assay. Quantitation of O antigen with the assay was thus influenced by the physical state of LPS or "free" O antigen.