p18 is required within B cells for differentiation of iPCs to PCs. (A) Differentiation of polyclonal PCs from resting B cells in vitro. p18^+/+ and p18^−/− iPCs and PCs were sorted on day 8 and cultured separately for 48 hours with MC3T3 stromal cells. (B) The recovery of live p18^−/− iPCs relative to p18^+/+ iPCs (left) and the percentage of live cells (right) (C) FACS analysis of sorted p18^+/+ and p18^−/− PCs and iPCs after 48 hours in coculture. Percentage of cells in each quadrant is indicated.