Coexpression of CCR7 and CXCR4 During B Cell Development Controls CXCR4 Responsiveness and Bone Marrow Homing

We have resolved B220+ IgM- B-lineage cells in mouse bone marrow into four fractions based on differential cell surface expression of determinants recognized by S7 (leukosialin, CD43), BP-1, and 30F1 (heat stable antigen). Functional differences among these fractions can be correlated with Ig gene rearrangement status. The largest fraction, lacking S7, consists of pre-B cells whereas the others, expressing S7, include B lineage cells before pre-B. These S7+ fractions, provisionally termed Fr. A, Fr. B, and Fr. C, can differentiate in a stromal layer culture system. Phenotypic alteration during such culture suggests an ordering of these stages from Fr. A to Fr. B to Fr. C and thence to S7- pre-B cells. Using polymerase chain reaction amplification with pairs of oligonucleotide primers for regions 5' of JH1, DFL16.1, and Jk1, we find that the Ig genes of Fr. A are in germline configuration, whereas Fr. B and C are pro-B cell stages with increasing D-J rearrangement, but no V-D-J. Finally, functional analysis demonstrates that the proliferative response to IL-7, an early B lineage growth factor, is restricted to S7+ stages and, furthermore, that an additional, cell contact-mediated signal is essential for survival of Fr. A.

B cell development is a highly regulated process whereby functional peripheral subsets are produced from hematopoietic stem cells, in the fetal liver before birth and in the bone marrow afterward. Here we review progress in understanding some aspects of this process in the mouse bone marrow, focusing on delineation of the earliest stages of commitment, on pre-B cell receptor selection, and B cell tolerance during the immature-to-mature B cell transition. Then we note some of the distinctions in hematopoiesis and pre-B selection between fetal liver and adult bone marrow, drawing a connection from fetal development to B-1/CD5(+) B cells. Finally, focusing on CD5(+) cells, we consider the forces that influence the generation and maintenance of this distinctive peripheral B cell population, enriched for natural autoreactive specificities that are encoded by particular germline V(H)-V(L) combinations.

Vascularization of organs generally occurs by remodelling of the preexisting vascular system during their differentiation and growth to enable them to perform their specific functions during development. The molecules required by early vascular systems, many of which are receptor tyrosine kinases and their ligands, have been defined by analysis of mutant mice. As most of these mice die during early gestation before many of their organs have developed, the molecules responsible for vascularization during organogenesis have not been identified. The cell-surface receptor CXCR4 is a seven-transmembrane-spanning, G-protein-coupled receptor for the CXC chemokine PBSF/SDF-1 (for pre-B-cell growth-stimulating factor/stromal-cell-derived factor), which is responsible for B-cell lymphopoiesis, bone-marrow myelopoiesis and cardiac ventricular septum formation. CXCR4 also functions as a co-receptor for T-cell-line tropic human immunodeficiency virus HIV-1. Here we report that CXCR4 is expressed in developing vascular endothelial cells, and that mice lacking CXCR4 or PBSF/SDF-1 have defective formation of the large vessels supplying the gastrointestinal tract. In addition, mice lacking CXCR4 die in utero and are defective in vascular development, haematopoiesis and cardiogenesis, like mice lacking PBSF/SDF-1, indicating that CXCR4 is a primary physiological receptor for PBSF/SDF-1. We conclude that PBSF/SDF-1 and CXCR4 define a new signalling system for organ vascularization.