Protective immunity induced in mice by immunization with high-molecular-weight polysaccharide from Pseudomonas...

A high-molecular-weight alkali-labile polysaccharide (PS) isolated from the slime of immunotype 1 Pseudomonas aeruginosa was tested for its ability to protect mice from lethal challenge with the live, homologous organism. Intraperitoneal (i.p.) injection of 10 to 25 microgram of the PS protected 60 to 70% of the mice against challenge with up to 50 50% lethal dose units. Although single immunization of mice with up to 250 microgram of PS effected protective levels of only 70%, two successive immunizations provided 100% protection. Subcutaneous and intravenous immunization with PS also provided protection to i.p. challenges with immunotype 1 P. aeruginosa, but not to i.p. challenge with immunotype 4 P. aeruginosa. Although lipopolysaccharide (LPS) was found to be more immunogenic than PS in out studies, contamination of the alkali-labile PS with LPS did not account for the protection seen. Alkali treatment (0.1 N NaOH, 37 degrees C, 2 h) of the PS destroyed its protective effectiveness, while similarly treated LPS retained its capacity for inducing immunity in mice. Adsorption and passive protection studies with sera raised to either PS or a mixture of PS and LPS indicated that antibody directed to the alkali-labile PS antigen was capable of contributing to the protection of mice against challenge with P. aeruginosa.