Decreased expression of Kruppel-like factors in memory B cells induces the rapid response typical of secondary antibody responses

Secondary antibody responses are characterized by the rapid kinetics of the responding cells, including the production of larger amounts of serum Ig compared with the primary response. Memory B cells, which are responsible for this phenomenon, undergo greater proliferation and differentiation into Ig-secreting plasma cells than naive B cells. We have found that memory cells rapidly enter cell division, irrespective of extrinsic stimuli. Microarray analysis of human splenic B cells revealed that naive cells express higher levels than memory B cells of Kruppel-like factor (KLF) 4, KLF9, and promyelocytic leukemia zinc finger (PLZF), transcription factors important in maintaining cellular quiescence. These genes were down-regulated after activation through CD40 and the B cell receptor. Enforced expression of KLF4, KLF9, and PLZF in memory B cells delayed their entry into division and reduced the number of proliferating cells, such that the behavior of transfected memory cells resembled that of naive B cells. Thus, the accelerated response of memory B cells correlates with reduced expression of KLF4, KLF9, and PLZF and the subsequent regulatory effects they exert on the cell cycle.