Universal vaccine carrier. Liposomes that provide T-dependent help to weak antigens

The design of a thymus-dependent synthetic vaccine that will provide a universal T cell epitope for B cell epitopes is described in this study. Simultaneous incorporation into liposomes of both a peptide recognized by Th lymphocytes and a lipophilic hapten and the IgG antibody responses to this hapten were assessed in outbred mice. DNP-aminocaproyl phosphatidylethanolamine (DNP-CapPE) is a well characterized T-independent hapten Ag. HA2 peptide derived from the hemagglutinin protein of influenza virus contains amino acid sequences recognized by Th and T cytotoxic lymphocytes. In addition, HA2 contains a sequence of hydrophobic amino acids near the carboxyl terminus, allowing its incorporation into liposomes. Results of immunization show that (i), when DNP-CapPE is carried by liposomes without the HA2 peptide, an IgM antibody response is induced, (ii) liposomes carrying both HA2 and DNP-CapPE elicit an IgG antibody response to DNP in a dose-dependent fashion for both HA2 and DNP, (iii) the liposomes must be processed intracellularly in order to elicit a response, (iv) the system leads to a memory response for DNP, and (v) all of the IgG subclasses are elicited. These data suggest that liposomes containing the HA2 peptide exhibit a T-dependent carrier effect for a T-independent Ag. The significance of these findings is discussed in conjunction with the characteristics of the liposome model used.