Evidence for the presence of lipopolysaccharide in a ribonuclease-sensitive ribosomal vaccine of Pseudomonas aeruginosa.

To obtain information about the nature of the immunogens in the ribosomal vaccine (fraction II) of Pseudomonas aeruginosa, we studied the specificity of rabbit antibodies to fraction II. Crossed immunoelectrophoresis demonstrated the presence of antibodies which precipitated with ribosomal antigens, but not with lipopolysaccharide (LPS). By means of an enzyme-linked immunosorbent assay, antibodies to LPS were detected in antibodies to fraction II. Antibodies to fraction II could protect mice against a lethal challenge with P. aeruginosa. Absorption experiments demonstrated that the protective ability of antibodies to fraction II was due to antibodies to cell envelope antigens, whereas antibodies to ribosomal antigens did not contribute to the protection. Antibodies to LPS could be detected in mice 1 week after a single vaccination with fraction II. It was concluded that the protective activity of fraction II was due, at least in part, to the presence of LPS in the ribosomal vaccine. Treatment of fraction II with ribonuclease decreased the protective activity of the ribosomal vaccine. Addition of synthetic polyadenylic acid-polyuridylic acid restored the protective activity of ribonuclease-treated fraction II, indicating that RNA in the ribosomal vaccine might act as an adjuvant or a carrier in the presentation of the of the contaminating cell envelope antigens. The protective activity and the toxicity of fraction II were compared with the protective activity and the toxicity of fraction I, which contained cell envelope components, including LPS, and of purified LPS. The results indicated that protection by the ribosomal vaccine was associated with a slightly higher toxicity than was protection by fraction I, whereas purified LPS was the most toxic vaccine.