Adjuvant effects of nonionic block polymer surfactants on liposome-induced humoral immune response

The ability of several surface-active agents to stimulate the humoral immune response in mice against haptenated liposomes was tested. The surfactants were block copolymers of hydrophilic polyoxyethylene (POE) and hydrophobic polyoxypropylene (POP) that differed in m.w., percentage of POE, and mode of linkage of POP to POE. The liposomes were haptenated with tripeptide-enlarged dinitrophenyl coupled to phosphatidylethanolamine, which was incorporated into the liposomal membrane. Additional injection of mice with surfactant stimulated serum hemagglutination titers and splenic plaque-forming cell (PFC) numbers to varying extents. Block polymers with POP chains flanking a POE center, as well as polymers with POE chains flanking a POP center, displayed high adjuvant activity. These block polymers stimulated the antibody response in a dose-dependent manner. They stimulated the antibody response with both high and low antigen doses. Furthermore, the addition of one of these adjuvants (25R1) reduced the amount of carrier lipid required in the liposome in order to obtain an optimal antibody response. The surfactants, which displayed high adjuvant activity, did not interfere with liposome stability as measured with a liposome lysis assay. Moreover, in vitro preincubation of liposomes with a block polymer did not affect their immunogenicity. Optimal adjuvant activity was observed when both adjuvant and liposomes were administered by the same route. Simultaneous injection of both components, however, is not a prerequisite. Conclusively, it can be stated that nonionic block polymer surfactants are potent adjuvants for stimulation of the antibody response against haptenated liposomes.