Application of polyethyleneglycol (PEG)-modified liposomes for oral vaccine: effect of lipid dose on systemic and mucosal immunity

To examine the systemic and mucosal immunity towards a liposomal antigen in an oral vaccine, we prepared ovalbumin (OVA)-encapsulating polyethyleneglycol (PEG)-modified liposomes and unmodified ones, and orally administered two different concentrations of them to mice. Unmodified liposomes tended to induce a stronger systemic immune response than the PEG-modified ones especially at the higher concentration of liposomes. Whereas at the lower liposome concentration the mucosal immune response was stronger for the PEG-modified liposomes than for the unmodified ones but nearly the same at the higher concentration. The relative amount of immunoglobulin G (IgG) against OVA in the plasma was 1.7-fold higher for a 12.5 μmol phospholipid dose of PEG-liposomes encapsulating OVA than for a 5.0 μmol one encapsulating the same amount of OVA. On the contrary, the relative amount of IgA in the intestinal wash was 2.6-fold higher for the 5.0 μmol phospholipid dose than for the 12.5 μmol one. These results indicate that OVA encapsulated in a small number of liposomes, especially the PEG-modified ones, is favorable for inducing a mucosal immune response and that the same amount of OVA in a large number of liposomes tends to improve the systemic immune response. A possible explanation for this tendency is the differential release rate of OVA from the liposomes at the intestinal mucosa. Our present study suggests that the dose of liposomes containing antigen is an important factor for controlling the response of systemic and mucosal immune systems.