B-Cell Progenitors and Precursors Change Their Microenvironment in Fetal Liver During Early Development

Abstract

The microenvironments, in which B lymphocytes develop in fetal liver, are largely still unknown. Among the nonhematopoietic cells, we have identified and FACS-separated two subpopulations, CD45 2 TER119 2 VCAM-1 1 cells that are either CD105 high LYVE-1 high or CD105 low ALCAM high . Immunohistochemical analyses find three of four c-Kit 1 IL-7Ra 1 B220 low CD19 2 SLC 2 B progenitors in contact with vascular endothelial-type LYVE-1 high cells on embryonic day 13.5. One day later c-Kit 1 IL-7Ra 1 cells develop to CD19 2 and 1 , SLC-expressing, DHJH-rearranged pre/pro and pro/preB-I cells. Less than 10% are still in contact with LYVE-1 high cells, but half of them are now in contact with mesenchymally derived ALCAM high liver cells. All of these ALCAM high cells, but not the LYVE-1 high cells produce IL-7 and CXCL12, while both produce CXCL10. Progenitors and pro/preB-I cells are chemoattracted in vitro toward CXCL10 and 12, suggesting that lymphoid progenitors with Ig gene loci in germline configuration enter the developing fetal liver at E13.5 from vascular endothelium, attracted by CXCL10, and then migrate within a day to an ALCAM high liver cell microenvironment, differentiating to DHJH-rearranging, surrogate light chain-expressing pre/proB and pro/preB-I cells, attracted by CXCL10 and 12. Between E15.5 and E16.5 preB-I cells expand 10-fold in continued contact with ALCAM high cells and begin VH-to DHJH-rearrangements in further differentiated c-Kit 2 IL-7Ra 2 preBII cells.

The changing mouse embryo transcriptome at whole tissue and single-cell resolution

“…1c and Cluster 10 in Extended Data Fig. 2-10), while genes marking maturing B-cells [15][16][17][18] in Cluster 10 appear in liver, and then in tissues with developing lymphatics (Extended Data Fig. 3b).…”

Checkpoints that control B cell development

“…In fact, pHSCs and other hematopoietic progenitors have been seen in close contact with endothelium of the inside of embryonic blood vessels (15). Embryonic endothelium has been found to produce c-kit ligand (KIT-L), an essential cytokine for pHSCs and progenitors, as well as CXCL10, a chemokine that from E12.5 onward might attract these hematopoietic progenitors to transmigrate from blood into the developing primary organ, notably as the second B-lymphopoietic wave in fetal liver (11) and bone marrow.…”

“…The progenitors establish contact with mesenchymal and epithelial microenvironments, which produce the chemokines CXCL10, CXCL12, CCL7, CCL9, and CX3CL1 to attract the hematopoietic progenitors. They also produce KITLG, M-CSF, and IL-7 ( Figure 1 and (16), but it remains to be determined which cell type contributes to this part of B cell development, as mesenchymal stromal cells do not participate (11).…”

“…IL-7 inhibits apoptosis and further differentiation (19). Proliferation and differentiation of CLP, pro-B, and pre-B cells is seen between E14.5 and E16.5, when these cells have become resident in the mesenchymal microenvironment (11).…”

Chemokine polyreactivity of IL7Rα+CSF-1R+ lympho-myeloid progenitors in the developing fetal liver

“…In E14.5 fetal liver the two cytokines, IL7 and CSF-1 have previously been found to be produced by non-hematopoietic, VCAM + CD105 low ALCAM high mesenchymal stromal cells, but not by VCAM + CD105 high Lyve1 high endothelial cells 41 . Here, we extend this analysis of non-hematopoietic stromal cells for their function in support of homing and development of early progenitors on the branching point of the lympho-myeloid lineage decision in fetal liver at E13.5 and E16.5.…”