Abstract
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.
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