Liver sinusoidal endothelial cells promote B lymphopoiesis from primitive hematopoietic cells

Although the bone marrow (BM) microenvironment is the main inducer niche of early B lymphopoiesis during the adult life, other extramedullar microenvironments, such as the liver, may also have potential for supporting B-cell development. Previously, we reported that murine liver sinusoidal endothelial cells (LSECs) support in vitro and in vivo hematopoietic stem cell (HSC) proliferation and myeloid differentiation. In the present study, we investigated the capacity of LSEC to promote B lymphopoiesis from BM progenitor lineage-negative (Lin(-)) cells. Murine BM Lin(-) cells were co-cultured with LSEC, in the absence of exogenous cytokines. B cells were characterized by flow cytometry and cytokine expression by RT-PCR. We show that BM Lin(-) cells differentiated to early B-lymphoid progenitors (B220(+)) and subsequently to mature (CD19(+)) B cells. Functional studies showed the presence of a high number of non-adherent cells (NACs), collected from lipopolysaccharide (LPS)-treated Lin(-)/LSEC co-cultures, expressing IgM on their surface (sIgM). Colony formation from NAC was observed in the presence of IL-7 (CFU-IL-7). LSEC constitutively express IL-7, Flt-3L, and SCF at the mRNA level, and VCAM-1 on their surface, which may explain the capacity of these cells to promote B lymphopoiesis. These data demonstrate that LSEC promote all stages of B lymphopoiesis. To our knowledge, this is the first report that LSEC constitute an in vitro microenvironment for B lymphopoiesis. Further studies will establish whether LSEC can serve in vivo as a B-lymphopoietic niche under physiological or pathological condition, or when HSC are mobilized.