Parrish YK , Baez I, Milford TA, Benitez A, Galloway N, Willeman-Rogerio J, Sahakian E, Wall NR, Dovat S and Payne KJ. "IL-7 in Human B Cell Development: Assembling the Puzzle Pieces," Program for the FASEB Summer Meeting: Molecular Mechanisms of Lymphocyte Differentiation: From Stem Cells to Effector Cells ( 6/2009 )
IL-7 plays a central role in murine B cell production, however its role in human B lymphopoiesis is controversial. Early studies of mouse B cell development showed that IL-7 increases in vitro B cell production by ~50–fold while the loss of IL-7R signaling in knockout mice results in a block in B cell development. In contrast, the addition of IL-7 to human cultures only increased B cell production by ~2-5 fold while patients with defects in IL-7 had normal numbers of peripheral blood B cells. This led to the conclusion that human B lymphopoiesis differs from that in mouse with respect to the requirement for IL-7. A more comprehensive analysis of IL-7 knockout mice showed that the production of B cells during the fetal/neonatal period was relatively normal, although these mice did indeed have a block in adult B cell production. More recent studies describe a “developmental switch” in murine B lymphopoiesis. In adult mice, signaling through the IL-7R is required for expression of the EBF transcription factor. EBF in turn upregulates a cascade of B lineage specific genes that are required for B cell development. During murine fetal and neonatal life, IL-7-independent expression of EBF allows for B lineage differentiation resulting in a developmental switch when this capacity is lost at ~two weeks of age. The above findings are important because most of the data concerning the role of IL-7 in human B cell development was obtained from fetal or neonatal hematopoietic sources. In addition, human studies that examine adult B lymphopoiesis have relied on co-cultures that include murine stromal cell lines. We and others have shown that these stromal cell lines produce high levels of murine IL-7 that induces human IL-7R signaling, thus making it difficult to interpret the effects of exogenous human IL-7. Here we describe a novel, human-only model of in vitro B cell development based on co-culturing hematopoietic stem cells (HSCs) from umbilical cord blood (CB) or adult human bone marrow (BM) on primary human BM stroma. This culture model produces a substantial population of CD19+cμ– B cell precursors at 3 weeks with 11-16% of B lineage progeny maturing to the IgM+ stage by 5 weeks of culture. The percentage of in vitro-generated B cell precursors that express IL-7Rα (~35%) mirrors that observed for in vivo-generated B cell precursors in adult BM, thus providing evidence of the physiological relevance of our human-only culture model. Using the human-only culture model we examined the role of IL-7 in human B lymphopoiesis from lineage marker negative (Lin–) CD34+ HSCs in adult BM and from their CB counterparts that give rise to B cells during the neonatal period. IL-7 increased human B cell production by >60-fold from both CB and adult BM HSCs. IL-7-induced increases were dose-dependent and specific to CD19+ cells. STAT5 phosphorylation and expression of the Ki-67 proliferation antigen indicate that IL-7 acts directly on CD19+ cells to increase proliferation at the CD34+ and CD34– pro-B cell stages. These data provide evidence that IL-7 plays a critical role in human B lymphopoiesis as it does in the mouse. While IL-7-independent B cell production is essentially absent in human-only cultures with adult BM, HSCs in CB give rise to a small but consistent population of CD19LO B lineage cells that express EBF and PAX-5 and respond to subsequent IL-7 stimulation. Flt3 ligand, but not thymic stromal-derived lymphopoietin (TSLP), was required for the IL-7-independent B cell production observed in human-only cultures with CB HSCs. These data provide the first evidence of a developmental switch in human B lymphopoiesis. As compared to CB, adult BM shows a B lymphoid-specific reduction of in vitro generative capacity that is progressively more profound in developmentally sequential precursor populations, resulting in a ~50-fold reduction in IL-7-dependent B lineage generative capacity. Indeed, the IL-7-dependent B cell generative capacity of adult BM is similar to the IL-7-independent B cell generative capacity of CB. These data suggest that a mechanism in addition to the developmental switch observed in the mouse model may be responsible for the IL-7 dependency that we observe in adult human B lymphopoiesis.