Non-specific modulation of the immune response with liposomal meningococcal lipopolysaccharide: role of different cells and cytokines.

The immunomodulating action of Neisseria meningitidis lipopolysaccharide (LPS) incorporated into liposomes and the activation of different populations of immunocompetent cells or the secretion of cytokines were studied. LPS stimulated an anti-sheep red blood cell (SRBC) plaque-forming cell response in the spleen of mice after simultaneous injection of LPS and SRBC but if LPS was administered 3 days before the immunization with SRBC the response to SRBC was strongly suppressed. After the incorporation of LPS into liposomes the stimulation index was increased from 6 to 19 and the liposomal LPS did not suppress the immune response to SRBC. The incorporation of LPS into liposomes leads to enhancement of B-mitogenic properties of LPS, as liposomal LPS stimulated the proliferation of splenocytes in mice better than free LPS and has no influence on the thymocytes. The liposomal LPS induced more prolonged and significant accumulation of IgM-secreting cells in the spleen of mice in comparison with the free LPS. Liposomal LPS also induced more active accumulation of IFN-gamma in human peripheral blood mononuclear cells and less active accumulation of monokines, contributing to the realization of the toxic properties of endotoxin (IL-1 alpha, TNF-alpha, IL-6 and GM-CSF). These results demonstrated that the incorporation of N. meningitidis LPS into liposomes dramatically changed its immunomodulating activity. The data obtained are important for the construction of an adjuvant formulation for synthetic immunogens capable of inducing genetically unrestricted immune responses.