Generation of rapidly cycling and apoptotic iPCs in the TD response. (A) A representative FACS analysis of CD138 and B220 expression in the low-density splenic B cells isolated from p18^+/+ mice. (B) Percentage of p18^+/+ and p18^−/− iPCs and PCs in the low-density fraction on indicated days after NP-CGG immunization. (C) FACS analysis of iPCs and PCs at indicated hours after sorting and incubation in B media; R1 (CD138^hi/B220^lo), R2 (CD138^hi/B220^hi), and R3 (CD138^m/B220^hi) are as marked. (D) The percentage of R1, R2, and R3 generated from iPCs at 2 and 20 hours as shown in panel C. (E) Percentage of iPCs and PCs in p18^+/+ and p18^−/− mice on day 4 of the secondary NP-CGG response, before or 6 hours after coculture with MC3T3 cells. (F) Percentage of TO-PRO-3⁺ and BrdU⁺ cells after a 2-hour-pulse in vivo in p18^+/+ and p18^−/− mice. (G) qPCR analysis of relative mRNA levels in sorted p18^+/+ and p18^−/− populations. rBC indicates resting B cell; aBC, activated B cell; iPC, intermediate plasma cell; PC, plasma cell.