Monophosphoryl lipid A enhances specific CTL induction by a soluble protein antigen entrapped in liposomes.

Exogenous soluble antigen loaded in membranous vehicles including splenocytes and liposomes can induce a CD8+ cytotoxic T-lymphocyte (CTL) response in mice. Plain liposomes of simple composition, however, are not as effective as cellular vehicles such as splenocytes. In the present study it is shown that incorporation of monophosphoryl lipid A (MPL), a semisynthetic bacterial adjuvant, into liposomes enhanced the ability of liposomal ovalbumin (OVA) to prime for a specific CTL response. With the MPL formulation, the minimal antigen dose required for a detectable CTL induction was reduced about fivefold, and this approximated the required minimal dose of OVA loaded in the splenocytes. Moreover, liposomes containing MPL could induce a considerable level of CTL activity by either an intravenous, intramuscular or subcutaneous immunization protocol, whereas liposomes without MPL could only elicit such a response by an intravenous injection route. Subcutaneous injection of a mixture of liposomes containing MPL and liposomes containing antigen also elicited specific CTL activity. However, simultaneous subcutaneous administration of liposomal MPL and liposomal OVA at two distant sites did not prime the mice for a CTL response. These results indicate that MPL, although not necessarily incorporated in the same liposomes, must be in close proximity to the antigen to exert its adjuvant activity. Based on the results of this model antigen study, it is suggested that an optimal CTL inductive vaccine should include immunomodulatory adjuvant in addition to a class I pathway delivery vehicle such as liposomes.