Fetal liver (FL) is the major embryonic hematopoietic organ and a site where circulating hematopoietic stem/progenitor cells (HSPCs) reside. However, HSPC migration/retention mechanisms in FL remain unclear. A chemokine screen revealed that the CCR4 ligands CCL17 and CCL22 are highly expressed in mouse embryonic day (E) 12.5 FL. Flow cytometric analysis confirmed CCR4 expression in FL HSPCs. To identify sources of CCL17 and CCL22, we fractionated FL into various cell types and found that Ccl17 and Ccl22 were predominantly expressed in HPCs/matured HCs. In vitro cell migration analysis confirmed enhanced HSPC migration in the presence of HPCs/matured HCs. Furthermore, exo-utero injection of anti-CCR4 neutralizing antibody into pregnant mice significantly reduced the number of FL HSPCs in embryos. These data demonstrate a paracrine mechanism by which HSPC migration/retention is regulated by CCL17 and CCL22 secreted from HPCs or matured HCs in FL.
Keywords: CCL17; CCL22; CCR4; Hematopoietic stem cell; Migration; Paracrine.
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