Liposomes as antigen vehicles to increase immunogenicity: effects of variation of structural characteristics

Liposomes may be formulated as antigen vehicles, whose properties may be varied at will to produce structures which elicit the desired effects on the immune response against the associated antigen. Amongst the factors influencing the efficacy and nature of the adjuvant effect of liposomes are bilayer lipid composition, change and size of the vesicles, site of association of the antigen with the lipid lamellae and lipid to antigen ratio. Owing to the complex structure of these vesicles, strict control of the many differing parameters, as regards the liposome, the antigen, their positional relationship and the time of antibody assay between separate studies is difficult to achieve. The optimal conditions for adjuvant effect against a particular antigen thus need to be individualised. Further enhancement of liposomal adjuvanticity may be obtained by co-entrapment of immunomodulators such as lipid A, muramyl dipeptide and interleukin-2 and targeting to antigen-presenting cells via surface-linked sugar ligands.