Evaluation of the Immunogenic effect of lipopolysaccharide (LPS) and protein fractions of Legionella pneumophila in challenging with lethal dose of this bacterium in mice

Legionella pneumophila is the cause of legionellosis disease that can be fatal. Yet, no vaccine has been available for this infection. Also antigens of these bacteria can stimulate the immune system. The purpose of this study was to compare the immunogenic effect of lipopolysaccharide fraction with that of protein fraction of Legionella pneumophila in challenging with lethal dose of these bacteria in mice.

After preparation of bacterial biomass, LPS and protein fractions were separated by hot phenol method and precipitated by enzyme digestion. LPS and protein fraction electrophoresis on poly acrylamide gel was performed. For preparation of vaccines from LPS and protein fractions, 10µg of each antigen was solved in 0.5 ml of normal saline and used for injection. Six groups of female BALB/c mice (each group consisted of 15 mice) were selected. Four groups of mice were vaccinated by intraperitoneal (i.p) injections at fortnightly intervals for three times. The two control groups of mice received normal saline injections. Two weeks after the last immunization, two groups of vaccinated mice and one control group were challenged with LD100 of the virulent strain of L. pneumophila. Also six weeks after the last immunization the other three groups (2 vaccinated and one control groups) were challenged.

The results of the first challenge showed the immunogenic efficiency gains of 86.66% and 73.33% for protein fraction and LPS respectively, and after six weeks of the last immunization the immunogenic efficiency gains were 60% for LPS and 86.66% for protein fractions.

This study showed that the protein fraction and LPS of L.pneumophila have high immunogenic activity and can be proper candidates for vaccine studies.