SUMMARY The generation of distinct hematopoietic cell types, including tissue-resident immune cells, distinguishes fetal from adult hematopoiesis. However, the mechanisms underlying differential cell production to generate a layered immune system during hematopoietic development are unclear. Using an irreversible lineage tracing model, we identify a definitive hematopoietic stem cell (HSC) that supports long-term multilineage reconstitution upon transplantation into adult recipients, but does not persist into adulthood in situ. These HSCs are fully multipotent, yet display both higher lymphoid cell production and greater capacity to generate innate-like B and T lymphocytes as compared to coexisting fetal HSCs and adult HSCs. Thus, these developmentally restricted HSCs define the origin and generation of early lymphoid cells that play essential roles in establishing self-recognition and tolerance, with important implications for understanding autoimmune disease, allergy, and rejection of transplanted organs.

Purpose Fetal hydrops arises from multiple disease processes and can portend a grim prognosis. We reviewed our experience with hydropic fetuses to understand relevant antenatal anatomic and physiologic predictors of survival. Methods We reviewed fetal ultrasounds and echocardiograms of hydropic fetuses evaluated from 1996-2013. Results Overall neonatal survival in 167 fetuses was 44% (range, 0-75%) and was influenced by the underlying disease process. The anatomic distribution of fluid varied and was not significantly different between survivors and non-survivors. Univariate analysis indicated that resolution of hydrops and delivery at a later gestational age were predictive of survival (OR:5.7 (95% CI:2.5-13.2) and OR:1.3 (95% CI:1.1-1.4), respectively). Fetal intervention also improved survival in some diseases. Echocardiograms were reviewed to group fetuses with similar cardiac physiology and defined categories with high or low/normal cardiothoracic ratio (CTR). Among patients with a high CTR, the cardiovascular profile score was predictive of survival (p=0.009). Conclusion Survival in hydrops depends on the underlying disease, available fetal therapies to resolve hydrops, and the gestational age of delivery and not on the specific anatomic manifestations of hydrops. In hydropic fetuses with high CTRs, the cardiovascular profile score may be a useful prognostic indicator.

• In utero injection of an antibody against the c-Kit receptor can effectively deplete host HSCs in mice.• In utero depletion of host HSCs leads to significantly increased engraftment after neonatal congenic hematopoietic cell transplantation.Although in utero hematopoietic cell transplantation is a promising strategy to treat congenital hematopoietic disorders, levels of engraftment have not been therapeutic for diseases in which donor cells have no survival advantage. We used an antibody against the murine c-Kit receptor (ACK2) to deplete fetal host hematopoietic stem cells (HSCs) and increase space within the hematopoietic niche for donor cell engraftment. Fetal mice were injected with ACK2 on embryonic days 13.5 to 14.5 and surviving pups were transplanted with congenic hematopoietic cells on day of life 1. Low-dose ACK2 treatment effectively depleted HSCs within the bone marrow with minimal toxicity and the antibody was cleared from the serum before the neonatal transplantation. Chimerism levels were significantly higher in treated pups than in controls; both myeloid and lymphoid cell chimerism increased because of higher engraftment of HSCs in the bone marrow. To test the strategy of repeated HSC depletion and transplantation, some mice were treated with ACK2 postnatally, but the increase in engraftment was lower than that seen with prenatal treatment. We demonstrate a successful fetal conditioning strategy associated with minimal toxicity. Such strategies could be used to achieve clinically relevant levels of engraftment to treat congenital stem cell disorders. (Blood. 2014;124(6):973-980) IntroductionHematopoietic stem cell (HSC) transplantation is a promising strategy to treat many nonmalignant genetic disorders such as hemoglobinopathies, immunodeficiencies, and inborn errors of metabolism, 1 and may even provide tolerance for solid organ transplants.2 However, this approach often requires host myeloablation and immunosuppression, which carries significant morbidity. 3,4 Transplantation into the immunologically naive fetal environment to circumvent the host immune response is an attractive alternative strategy to achieve sustained engraftment and donor-specific tolerance. This approach of in utero hematopoietic cell transplantation (IUHCTx) has been successful in many animal models, but clinical applications remain hampered by low levels of engraftment that are not sufficient to ameliorate symptoms or cure most diseases (reviewed in Nijagal et al 5 ). Because the only clinical successes have been achieved in fetuses with severe combined immunodeficiency, [6][7][8] it has been suggested that success is limited by barriers such as rejection of the stem cell graft and lack of space within the hematopoietic niche (reviewed in Flake and Zanjani 9 ). We and others have previously explored the role of an immune response to donor cells and reported that the maternal immune system is a significant barrier to engraftment. 10,11 The fetal host can become tolerant to transplanted cells through clonal de...

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