Krüppel-like factor 2 (KLF2) controls T lymphocyte egress from lymphoid organs by regulating sphingosin-1 phosphate receptor 1 (S1Pr1). Here we show that this is not the case for B cells. Instead, KLF2 controls homeostasis of B cells in peripheral lymphatic organs and homing of plasma cells to the bone marrow, presumably by controlling the expression of β 7 -integrin. In mice with a B cell-specific deletion of KLF2, S1Pr1 expression on B cells was only slightly affected. Accordingly, all splenic B cell subsets including B1 cells were present, but their numbers were increased with a clear bias for marginal zone (MZ) B cells. In contrast, fewer peyers patches harboring fewer B cells were found, and fewer B1 cells in the peritoneal cavity as well as recirculating B cells in the bone marrow were detected. Upon thymus-dependent immunization, IgG titers were diminished, and antigen-specific plasma cells were absent in the bone marrow, although numbers of antigen-specific splenic plasmablasts were normal. KLF2 plays also a role in determining the identity of follicular B cells, as KLF2-deficient follicular B cells showed calcium responses similar to those of MZ B cells and failed to down-regulate MZ B cell signature genes, such as CD21 and CXCR7.cell trafficking | S1P1 | LKLF | B cell development | knockout mouse M aturation of lymphocytes in primary lymphoid organs, controlled egress into the periphery, and proper positioning in lymphoid tissues are critical for efficient adaptive immune responses. Differential expression of Krüppel-like factor 2 (KLF2) promotes egress of T cells from lymphoid organs into the blood and T-cell migration to lymph nodes (1, 2). Accordingly, KLF2 is expressed in naive T cells, down-regulated upon activation, and reexpressed in memory T cells (1, 3-5). KLF2-deficient T cells inefficiently exit from the thymus and thus accumulate there; this is thought to be a consequence of a decrease in sphingosin-1 phosphate receptor 1 (S1Pr1) expression, which is regulated by KLF2 (2, 6). In addition, KLF2 increases the expression of β 7 -integrin (gene symbol: Itgb7) and CD62L (L-selectin) on T cells (1, 2). However, other downstream effects of KLF2 expression are less clear, although they very likely contribute to the migratory behavior of T cells. For example, vav-cre-and lck-cre-mediated deletion of KLF2 in T cells resulted in the up-regulation of the inflammatory chemokine receptors CCR3 and CCR5 (7) on thymocytes, whereas CD4-cre-mediated deletion led to the up-regulation of only CXCR3 and spontaneous IL4 production in naive T cells (8).Although the role of KLF2 in T-cell migration has been studied extensively, its function in B cells is not fully understood. KLF2 expression at the RNA and protein level is induced by pre-B cell receptor (pre-BCR) signals (9). Analogous to T cells, KLF2 transcripts are abundant in resting mature B cells, down-regulated upon mitogenic activation, and reexpressed in plasma and memory B cells (10)(11)(12). Because KLF2 controls the expression of β 7 -integrin and CD62...