In the bone marrow (BM), stromal cells constitute a supportive tissue indispensable for the generation of pro-B/pre-BI, pre-BII, and immature B lymphocytes. IL-7-producing stromal cells constitute a cellular niche for pro-B/pre-BI cells, but no specific stromal cell microenvironment was identified for pre-BII cells expressing a functional pre-B cell receptor (pre-BCR). However expression of the pre-BCR represents a crucial checkpoint during B-cell development. We recently demonstrated that the stromal cell derived-galectin1 (GAL1) is a ligand for the pre-BCR, involved in the proliferation and differentiation of normal mouse pre-BII cells. Here we show that nonhematopoietic osteoblasts and reticular cells in the BM express GAL1. We observed that pre-BII cells, unlike the other B-cell subsets, were specifically localized in close contact with GAL1 ؉ reticular cells. We also determined that IL-7 ؉ and GAL1 ؉ cells represent 2 distinct mesenchymal populations with different BM localization. These results demonstrate the existence of a pre-BII specific stromal cell niche and indicate that early B cells move from IL-7 ؉ to GAL1 ؉ supportive BM niches during their development. (Blood. 2011; 117(24):6552-6561) IntroductionThe bone marrow (BM) is the primary site of hematopoiesis after birth. Hematopoietic stem cells (HSCs) and hematopoietic progenitors are located in close contact with a special stromal microenvironment, known as niches, composed of mesenchymal cells, osteoblasts, endothelial cells, and adipocytes, indispensable for their differentiation. 1,2 Recent reports indicate that the spatial repartition of BM stromal cells and hematopoietic progenitors follows a coordinated regulation. 3 HSCs are in contact with osteoblasts on bone surfaces and endothelial cells lining sinusoids, and move near CXCL12 ϩ reticular stromal cells while they differentiate. In this tissue, B-cell differentiation is also a highly regulated process, which generates widely diversified immature B cells tolerant to self. Stage-specific cellular niches have been identified for the first steps of the B lymphopoiesis. 2 The earliest B220 ϩ cKit ϩ pro-B/pre-BI cells undergoing immunoglobulin H (IgH) gene rearrangements locate near IL-7-producing stromal cells. At the next step, when a functional Ig chain is produced and associated with the surrogate light chain (SLC) composed of 5 and VpreB and with the CD79a/CD79b signaling complex, the pre-B-cell receptor (pre-BCR) is formed and expressed at the surface of pre-BII cells. At this stage, cells leave IL-7-expressing stromal cells, but no specific pre-BII cellular niche has yet been identified. 3 Expression of the pre-BCR represents a crucial checkpoint during B-cell development as this receptor controls pre-BII-cell development and the selection of the Ig repertoire. 4 The galectin-1 (GAL1) lectin and heparan sulfates, produced by the BM microenvironment, have been reported to bind to the pre-BCR, [5][6][7] and GAL1 was shown to promote pre-BCR clustering and signaling. 5 GAL1 binds to the SLC ...

Activation of the pre-B-cell receptor (pre-BCR) in the bone marrow depends on both tonic and ligand-induced signaling and leads to pre-BII-cell proliferation and differentiation. Using normal mouse bone marrow pre-BII cells, we demonstrate that the ligand-induced pre-BCR activation depends on pre-BCR/galectin-1/integrin interactions leading to pre-BCR clustering at the pre-BII/stromal cell synapse. In contrast, heparan sulfates, shown to be pre-BCR ligands in mice, are not implicated in pre-BCR relocalization. Inhibition of pre-BCR/galectin-1/integrin interactions has functional consequences, since pre-BII-cell proliferation and differentiation are impaired in an in vitro B-cell differentiation assay, without affecting cellular apoptosis. Most strikingly, although galectin-1-deficient mice do not show an apparent B-cell phenotype, the kinetics of de novo B-cell reconstitution after hydroxyurea treatment indicates a specific delay in pre-BII-cell recovery due to a decrease in pre-BII-cell differentiation and proliferation. Thus, although it remains possible that the pre-BCR interacts with other ligands, these results highlight the role played by the stromal cell-derived galectin-1 for the efficient development of normal pre-BII cells and suggest the existence of pre-BII-specific stromal cell niches in normal bone marrow.