Cell circuits between B cell progenitors and IL-7+ mesenchymal progenitor cells control B cell development.

B cell progenitors require paracrine signals such as interleukin-7 (IL-7) provided by bone marrow stromal cells for proliferation and survival. Yet, how B cells regulate access to these signals in vivo remains unclear. Here we show that proB and IL-7+ cells form a cell circuit wired by IL-7R signaling, which controls CXCR4 and focal adhesion kinase (FAK) expression and restricts proB cell movement due to increased adhesion to IL-7+CXCL12Hi cells. PreBCR signaling breaks this circuit by switching the preB cell behavior into a fast-moving and lower-adhesion state via increased CXCR4 and reduced FAK/α4ß1 expression. This behavioral change reduces preB cell exposure to IL-7, thereby attenuating IL-7R signaling in vivo. Remarkably, IL-7 production is downregulated by signals provided by preB cells with unrepaired double-stranded DNA breaks and by preB acute lymphoblastic leukemic cells. Combined, these studies revealed that distinct cell circuits control the quality and homeostasis of B cell progenitors.

A Chemoattractant-Guided Walk Through Lymphopoiesis: From Hematopoietic Stem Cells to Mature B Lymphocytes.

B lymphocytes develop from hematopoietic stem cells (HSCs) in specialized bone marrow niches composed of rare mesenchymal lineage stem/progenitor cells (MSPCs) and sinusoidal endothelial cells. These niches are defined by function and location: MSPCs are mostly perisinusoidal cells that together with a small subset of sinusoidal endothelial cells express stem cell factor, interleukin-7 (IL-7), IL-15, and the highest amounts of CXCL12 in bone marrow. Though rare, MSPCs are morphologically heterogeneous, highly reticular, and form a vast cellular network in the bone marrow parenchyma capable of interacting with large numbers of hematopoietic cells. HSCs, downstream multipotent progenitor cells, and common lymphoid progenitor cells utilize CXCR4 to fine-tune access to critical short-range growth factors provided by MSPCs for their long-term maintenance and/or multilineage differentiation. In later stages, developing B lymphocytes use CXCR4 to navigate the bone marrow parenchyma, and predominantly cannabinoid receptor-2 for positioning within bone marrow sinusoids, prior to being released into peripheral blood circulation. In the final stages of differentiation, transitional B cells migrate to the spleen where they preferentially undergo further rounds of differentiation until selection into the mature B cell pool occurs. This bottleneck purges up to 97% of all developing B cells in a peripheral selection process that is heavily controlled not only by the intensity of BCR signaling and access to BAFF but also by the proper functioning of the B cell motility machinery.