Kinetics of memory B cell and plasma cell responses in the mice immunized with plague vaccines

In our previous studies, we found that plague vaccines can induce long-term antibody response, but no significant antibody boost was observed when the immunized mice were challenged with virulent Yersinia pestis. However, a booster vaccination of subunit vaccine on week 3 after primary immunization elicited a significantly higher antibody titre than a single dose, whereas no significant antibody titre difference was observed between a single dose and two doses of EV76 vaccination. To address these issues, in this study, we first investigated the kinetics of memory B cells and plasma cells in the mice immunized with EV76 or F1 protein by flow cytometry and then determined antibody titre in five groups of mice immunized with various vaccination strategy. The results showed that memory B cells dropped to a low level at day 56 after primary immunization. In contrast, plasma cells were maintained for more than 98 days. The group with primary immunization of EV76 and booster of F1 antigen developed a higher antibody titre than the group with immunization of F1 antigen and booster of EV76. This result supports a hypothesis that an excess of antigens can neutralize pre-existing antibodies, and then the redundant antigen induces antibody boost. Taken together, a boost of antibody titre after revaccination may be dependent on the existence of memory B cells and an excess of antigen vaccination. In addition, this study showed an ideal immunization strategy that involves first immunization with a live attenuated vaccine, such as EV76, and then with a subunit vaccine.