Investigating the role of L. pnuemophila LPS derivatives in formation of specific cell-mediated immune responses against the pathogen

Legionella pneumophila is a Gram-negative intracellular bacterium and causes legionnaire's disease an –atypical pneumonia in humans. Lipopolysaccharide (LPS) is the main antigen of Gram-negative bacteria but is less studied because of its carbohydrate nature. Here, we immunized mice with detoxified LPS and O-antigen polysaccharide in combination with bovine serum albumin (BSA) and explored the immunological responses of mice to the bacterial infection. LPS of L. pneumophila was extracted by hot phenol-water method. Purified LPS was detoxified by sodium hydroxide alkaline procedure. O-polysaccharide antigen (OPS) obtained by acetic acid treatment of LPS. BALB/c mice were immunized mainly with non-covalent combination of detoxified LPS (dLPS) or OPS with BSA separately. Pure polysaccharide antigens did not elicit significant serum IgG against LPS. Combination of the dLPS and OPS with BSA resulted in risen IgG and its subclasses (IgG1 and IgG2a) against lipopolysaccharide. Mice were challenged intravenously with sublethal dose of L. pneumpphila. Then, splenocytes were cultured and cytokine responses of splenocytes to pathogenic Legionella was studied by ELISA. Mice immunized with combination of the dLPS or OPS and BSA showed significant elevation of cytokine responses to pathogenic L. pneumophila. Our results suggest that combination of the polysaccharide antigen derived from Legionella LPS may confer raised cell-mediated responses against the pathogen when combined with a protein antigen which is capable of eliciting cell-mediated responses. Although not covalently bond, Legionella polysaccharides combined with BSA effectively elicited Th-1 type cytokines and humoral responses against L. pneumophila in BALB/c mice.