Qualitative and quantitative changes in the antibody producing cell response to lipopolysaccharide induced after incorporation of the antigen into bacterial membrane phospholipid vesicles.

Lipopolysaccharide (LPS) isolated from Proteus mirabilis and administered to mice i.p. induced a primary immune response that consisted of the proliferation of only IgM antibody-producing cells. The response to a second stimulus 14 days later was also predominantly IgM, although a smaller number of IgG-producing cells was detected. The strength of the responses depended upon the dosage of LPS administered. When mice received a primary injection of the same amount of LPS incorporated into P. mirabilis phospholipid vesicles, the number of IgM-producing cells was significantly increased over that induced by LPS alone. In addition, IgG-producing cells appeared on day 5 and increased during the time course measured up to day 14. After a booster injection of the same amount of LPS-phospholipid vesicles on day 14, the numbers of IgM-producing cells increased approximately 3-fold and the numbers of IgG-producing cells approximately 16-fold over those of mice given LPS alone. These results demonstrate a pronounced adjuvant effect of bacterial membrane phospholipids that are able to alter not only the strength but also the type of response to LPS.