Introduction Hematopoietic stem cells HSCs in the
Introduction Hematopoietic stem PIK-75 (HSCs) in the adult bone marrow (BM) provide all types of blood cells throughout life, while also self-renewing to maintain blood homeostasis. Blood/marrow stem cell transplantation is the most advanced method to cure patients suffering from hematological malignancies and hematopoietic diseases. Consequently, tremendous effort has been dedicated to establishing a system to produce HSCs from pluripotent stem cells (PSCs) such as embryonic stem cells (ESCs) or inducible PSCs. Overexpressing the Hoxb4 gene (Kyba et al., 2002, Wang et al., 2005), teratoma formation in vivo (Amabile et al., 2013, Suzuki et al., 2013, Tsukada et al., 2017), and direct reprogramming by introducing multiple transcriptional factors into endothelial cells (ECs) (Sugimura et al., 2017) have demonstrated successful engraftment of PSC-derived hematopoietic cells. Similarly, a recent advance has reported that Hoxb4 expression combined with Delta-like 1 signaling enables mouse ESC-derived hematopoietic progenitor cells (HPCs) to engraft immunodeficient mice with a functional adaptive immune system (Lu et al., 2016). While these PSC-derived functional HSCs have been reported, low chimerism remains a persistent problem and it is still challenging to produce an HSC in vitro with equivalent properties of in vivo HSCs without gene manipulation. Although conventional ESC differentiation by embryoid body formation or OP9 co-culture produces erythromyeloid, B and T lymphoid cells, no transplantable HSCs are produced (Nakano et al., 1994, Schmitt et al., 2004, Yoshimoto et al., 2009). In this sense, conventional ESC differentiation reflects HSC-independent hematopoiesis and mimics yolk sac (YS) hematopoiesis before HSC emergence at the later stage (Irion et al., 2010, Lin et al., 2014, Yoshimoto, 2015). There are several waves of hematopoiesis in the YS before the detection of the first HSCs at embryonic day 11.5 (E11.5) in the aorta-gonado-mesonephros region that repopulate lethally irradiated adult mice (Hadland and Yoshimoto, 2017, Lin et al., 2014). These waves include primitive erythroid cells and primitive macrophages at around E7.5 in the YS and definitive (adult) type erythromyeloid progenitors from E8.5 to E9.5 YS. These waves have been considered transient, diminishing after birth. However, recent lineage tracing studies have revealed the presence of tissue-resident macrophages that are produced from early YS precursors independently of HSCs, persist into post-natal life, and are self-maintained without replenishment by BM progenitors (Ginhoux et al., 2010, Gomez Perdiguero et al., 2015, Schulz et al., 2012). These hematopoietic waves are recently recognized as HSC-independent hematopoiesis. Similarly, we and others have reported T and B lymphoid potential in the YS and/or para-aortic splanchnopleura (P-Sp) region prior to HSC emergence by co-culture with stromal cells (Cumano et al., 1996, Godin et al., 1995, Nishikawa et al., 1998, Yoshimoto et al., 2011, Yoshimoto et al., 2012). However, it is still controversial whether these T and B cells are produced independently of HSCs because the co-culture system also yields transplantable hematopoietic progenitor/stem cells from as early as E8.0 embryos, which makes the origin of early lymphoid cells unclear, whether it is derived from HSC-independent or -dependent precursors (Cumano et al., 2001, Matsuoka et al., 2001). We previously reported that the earliest B cells produced in vitro from YS/P-Sp at pre-HSC stages are B-1 cells (Yoshimoto et al., 2011). B-1 cells are unique innate-like B cells, residing mainly in the pleural and peritoneal cavities, and are segregated from conventional adaptive immune B-2 cells (Baumgarth, 2017). Two subtypes of B-1 cells are categorized; CD5+B-1a cells and CD5−B-1b cells. Among three subsets of B cells (B-1, B-2, and splenic marginal zone [MZ] B cells), B-1 and a part of MZ B cells are considered fetal derived. Especially, CD5+B-1a cells are derived exclusively from progenitors in the fetal liver (FL) and neonatal BM, not from adult HSCs based on the results of transplantation assays (Ghosn et al., 2012, Hardy and Hayakawa, 1991) and a conditional Rag2 knockout mouse model (Hao and Rajewsky, 2001). Our report demonstrating the presence of B-1-specific progenitors in the FL in HSC-deficient embryos supports the concept of HSC-independent lymphopoiesis (Kobayashi et al., 2014). In addition, the existence of HSC-independent T lymphopoiesis has been recently reported in a zebrafish model (Tian et al., 2017).