These insights into the HSC independent versus HSC

These insights into the HSC-independent versus HSC-dependent immune cell reconstitution pathway in mice can be expected to affect protocols used for HSC transplantation in clinical settings. Over the past 45 years, HSC transplantation has been widely used to treat patients with cancers such as leukemias, lymphomas, and myelomas, and to treat several immune-deficiency disorders, among other diseases (Liang and Zuniga-Pflucker, 2015; Pasquini and Zhu, 2014). In the United States alone, approximately 20,000 HSC transplantations are performed yearly (Pasquini and Zhu, 2014). The survival rate for most HSC transplantation used to treat malignant diseases is only ∼50% during the first few years after transplantation, and about 20% of all deaths after HSC transplantation are due to infectious diseases post transplant (Pasquini and Zhu, 2014). Thus, our data here call for further investigation on whether humans, like mice, generate immune cells independently of HSCs and whether these cells play critical roles that help protect HSC-transplanted patients from subsequent infectious diseases. As we have suggested, the failure to reconstitute B-1a may be a serious impediment, since B-1a were shown to be necessary, for example, to protect against viral, bacterial, and fungal pathogens, such as influenza virus (Choi and Baumgarth, 2008), Streptococcus pneumoniae (Haas et al., 2005; Weber et al., 2014), and Francisella tularensis (Yang et al., 2012), among others. Moreover, B-1a secrete most of the natural StemRegenin 1 detected in serum of unimmunized mice (Baumgarth et al., 2005; Gronwall et al., 2012), which is known to provide a first line of defense against viral (Ochsenbein et al., 1999) and bacterial (Haas et al., 2005; Ochsenbein et al., 1999) infections. A role for natural antibodies in protection from pneumonia has been appreciated for some time now (Briles et al., 1981). Knockout mice deficient in B-1a cells (CD19) lack natural antibodies and, hence, are more susceptible to infections caused by S. pneumoniae (Haas et al., 2005). Natural antibodies are also implicated in protection against pneumococci in humans (Carsetti et al., 2005). B-1a cells also generate innate response activator (IRA) B cells that produce both granulocyte-macrophage colony-stimulating factor and immunoglobulin M (Rauch et al., 2012). IRA B cells can protect against both microbial sepsis (Rauch et al., 2012) and pneumonia (Weber et al., 2014). Therefore, in therapeutic settings such as BM transplantation using highly purified HSCs (Czechowicz and Weissman, 2011), the lack of B-1a and natural antibodies may be detrimental. The human equivalent of B-1 has recently been proposed for human peripheral blood (Rothstein et al., 2013), but the definitive phenotype of human tissue B-1a is yet to be fully defined. In any event, it is reasonable to expect that the functions provided by B-1a in mice are performed by the human equivalent. In fact, as we show here, human fetal liver transplants readily reconstitute B cells in the peritoneal cavity of humanized mice. These human-derived peritoneal B cells are phenotypically similar to murine peritoneal B-1a. Importantly, given that B-1a is rarely detectable in blood, our findings here suggest that the human equivalent of B-1a are likely to be found to reside in peripheral tissues, rather than circulating in human blood. It remains to be determined whether, similar to mouse, LT-HSCs purified from human fetal liver fail to reconstitute the peritoneal (rather than blood) B cells we identified here. If so, the human equivalent of B-1a may also originate from a development pathway that is independent of the LT-HSC. Support for this hypothesis comes from several human B-cell development (Rechavi et al., 2015; Sanz et al., 2010), phenotyping (Rothstein et al., 2013; Sanz et al., 2010) and BCR sequencing (Rechavi et al., 2015) studies, which distinguish the B-cell development pathway in embryos and young children from the well-known pathway in adults.