Immunization against Echis ocellatus (carpet viper) venom using liposomes incorporating immunostimulants: role of lipopolysaccharide in conferring protection in a mouse model.

Varying doses of whole West African Echis ocellatus venom were incorporated, with or without immunostimulants, into membrane-stabilized reverse phase evaporation (REV) liposomes. Preparations were given either subcutaneously (s.c.) or intravenously (i.v.) to mice and the immune responses compared. Addition of lipopolysaccharide (LPS) significantly increased the venom antibody response. Lipid A produced a less pronounced and less sustained effect and the addition of muramyl dipeptide analogues made no significant contribution to the antibody response. The protective ability of circulating venom antibodies was assessed by challenging the immunized mice with a minimum lethal dose of whole venom after 175 days. A dose of 250 micrograms E. ocellatus venom + 300 micrograms LPS in REVs injected s.c. conferred the highest protection against lethal venom effects. Orally administered venom/liposomes incorporated with the mucosal adjuvant avridine primed the antibody response and produced a classic secondary response following a sublethal boost of whole venom. Single injections of venom or venom fraction/liposome preparations which produce a high and sustained immune response have potential in commercial antivenom production and in active immunization of man in areas of high snakebite incidence and mortality.