Cognate interaction with antigen-specific T cells is required for the response of IgG-type memory B cells. Cognate interaction with antigen-specific T cells is required for the response of IgG-type memory B cells. (A) B6 mice were immunized with alum-precipitated NP-CGG. After 60 days, mice were injected i.p. with NP-EαGFP (Prime + NP-EαGFP) or left untreated (Prime alone). After 24 h, splenocytes were analyzed by flow cytometry with Y-Ae mAb for estimating antigen presentation in the context of MHC class II. Percentages of CD38+Y-Ae+ cells among NP-binding IgG1+ cells are indicated in FACS profiles. (B and C) Memory B cells and CGG-primed T cells were purified from B6 mice immunized with alum-precipitated NP-CGG. (B) Cells were transferred to Rag1−/− mice. One day later, the mice were left untreated (None) or rechallenged with NP-CGG or NP-OVA without adjuvant. (C) Cells were transferred to Rag−/− or Rag−/− MHC class II−/− mice. After 1 day, mice were immunized with NP-CGG without adjuvant. On day 0 and day 3 after cell transfer, some mice were injected with 200 μg of Fab fragments of anti-MHC class II mAb. In both B and C, splenocytes were prepared and subjected to enzyme-linked immunospot (ELISPOT) assays for measuring the number of antibody-forming cells (AFCs) 7 days after cell transfer. Yuichi Aiba et al. PNAS 2010;107:27:12192-12197 ©2010 by National Academy of Sciences