Relative quiescence is a defining characteristic of hematopoietic stem cells, while their progeny have dramatic proliferative ability and inexorably move toward terminal differentiation. The quiescence of stem cells has been conjectured to be of critical biologic importance in protecting the stem cell compartment, which we directly assessed using mice engineered to be deficient in the G1 checkpoint regulator, cyclin-dependent kinase inhibitor, p21cip1/waf1 (p21). In the absence of p21, hematopoietic stem cell proliferation and absolute number were increased under normal homeostatic conditions. Exposing the animals to cell cycle-specific myelotoxic injury resulted in premature death due to hematopoietic cell depletion. Further, self-renewal of primitive cells was impaired in serially transplanted bone marrow from p21-/- mice, leading to hematopoietic failure. Therefore, p21 is the molecular switch governing the entry of stem cells into the cell cycle, and in its absence, increased cell cycling leads to stem cell exhaustion. Under conditions of stress, restricted cell cycling is crucial to prevent premature stem cell depletion and hematopoietic death.

The ability to generate lung and airway epithelial cells from human pluripotent stem cells (hPSCs) would have applications in regenerative medicine, drug screening and modeling of lung disease, and studies of human lung development. We established, based on developmental paradigms, a highly efficient method for directed differentiation of hPSCs into lung and airway epithelial cells. Long-term differentiation in vivo and in vitro yielded basal, goblet, Clara, ciliated, type I and type II alveolar epithelial cells. Type II alveolar epithelial cells generated were capable of surfactant protein-B uptake and stimulated surfactant release, providing evidence of specific function. Inhibiting or removing agonists to signaling pathways critical for early lung development in the mouse—retinoic acid, Wnt and BMP—recapitulated defects in corresponding genetic mouse knockouts. The capability of this protocol to generate most cell types of the respiratory system suggests its utility for deriving patient-specific therapeutic cells.

IntroductionHuman hematopoietic tissue and cell transplantation into immunodeficient mice, such as mice with the severe combined immunodeficiency (SCID) 1,2 mutation and their derivatives, including NOD/SCID, 3 NOD/SCID/␤2m null , 4 and NOD/SCID/ ␥c null mice, 5 has been widely used to study the function of human immune cells. However, only a few studies have shown the ability to achieve systemic in vivo immune responses in humanized mice. 6 In general, immunodeficient mice that received a transplant of human hematopoietic stem cells (HSCs) have poor human T-cell development. Although some human T-cell reconstitution was seen in NOD/SCID/␥c null mice after human umbilical cord blood (UCB) stem-cell transplantation, human thymopoiesis in the recipient thymus appeared inefficient, and the mouse thymus remained underdeveloped in these mice. 5 Recently, Traggiai et al 6 reported that intrahepatic injection of human CD34 ϩ UCB cells into newborn Rag2 Ϫ/ Ϫ␥c Ϫ/Ϫ mice results in improved human T-cell development in the mouse thymus. An important advancement in this model compared with UCB transplantation in adult mice was the ability of grafted mice to mediate in vivo antiviral immune responses. 6 However, a long period of time (16 weeks) was required to achieve significant peripheral human T-cell repopulation in these mice, and the levels of human T cells in the spleen of long-term surviving mice were relatively low. Furthermore, the MHC restriction and tolerance status of human T cells that were selected in the mouse thymus in these mice still remains undefined.A commonly used mouse model for the study of human For personal use only. on June 19, 2019. by guest www.bloodjournal.org From Materials and methods Animals and human fetal tissuesImmunodeficient nonobese diabetic/severe combined immunodeficient (NOD/SCID) mice were housed in a specific pathogen-free microisolator environment and used at 6 to 10 weeks of age. Human fetal thymus and liver tissues of gestational age of 17 to 20 weeks were obtained from Advanced Bioscience Resource (Alameda, CA). Inbred Massachusetts General Hospital miniature swine (kindly provided by David H. Sachs) 10 were used as porcine skin donors. Protocols involving the use of human tissues and animals were approved by the Massachusetts General Hospital Human Research Committee and Subcommittee on Research Animal Care, and all of the experiments were performed in accordance with the protocols. Human tissue implantationMice were conditioned with sublethal (2-3 Gy) whole-body irradiation. Human fetal Thy and Liv fragments measuring about 1 mm 3 were implanted under the recipient kidney capsule within 3 days after irradiation. Some mice also received CD34 ϩ FLCs (1-5 ϫ 10 5 /mouse, intravenously) purified from the same donor on the day of human Thy/Liv transplantation. CD34 ϩ FLCs were isolated by the magnetic-activated cell sorter (MACS) separation system using anti-CD34 microbeads (Miltenyi Biotec, Auburn, CA). Levels of human hematopoietic cells in the mice that underwent transplantation ...

SUMMARY Evaluation of the therapeutic potential of RNAi for HIV infection has been hampered by the challenges of siRNA delivery and lack of suitable animal models. Using a novel delivery method in humanized mice, we show that siRNA treatment can dramatically suppress HIV infection. A CD7-specific single-chain antibody was conjugated to oligo-9-arginine peptide (scFvCD7-9R) for T cell-specific siRNA delivery in NOD/SCIDIL2rγ−/− mice reconstituted with human lymphocytes (Hu-PBL) or CD34+ hematopoietic stem cells (Hu-HSC). In HIV-infected Hu-PBL mice, treatment with anti-CCR5 and antiviral siRNAs complexed to scFvCD7-9R controlled viral replication and prevented the disease-associated CD4 T cell loss. This treatment also suppressed endogenous virus and restored CD4 T cell counts in mice reconstituted with HIV+ PBMC. Moreover, scFvCD7-9R could deliver antiviral siRNAs to naïve T cells in Hu-HSC mice and effectively suppress viremia in infected mice. Thus, siRNA therapy for HIV infection appears to be feasible in a preclinical animal model.

The generation of humanized BLT mice by the cotransplantation of human fetal thymus and liver tissues and CD34؉ fetal liver cells into nonobese diabetic/severe combined immunodeficiency mice allows for the long-term reconstitution of a functional human immune system, with human T cells, B cells, dendritic cells, and monocytes/ macrophages repopulating mouse tissues. Here, we show that humanized BLT mice sustained high-level disseminated human immunodeficiency virus (HIV) infection, resulting in CD4 ؉ T-cell depletion and generalized immune activation. Following infection, HIV-specific humoral responses were present in all mice by 3 months, and HIVspecific CD4؉ and CD8 ؉ T-cell responses were detected in the majority of mice tested after 9 weeks of infection. Despite robust HIV-specific responses, however, viral loads remained elevated in infected BLT mice, raising the possibility that these responses are dysfunctional. The increased T-cell expression of the negative costimulator PD-1 recently has been postulated to contribute to T-cell dysfunction in chronic HIV infection. As seen in human infection, both CD4 ؉ and CD8 ؉ T cells demonstrated increased PD-1 expression in HIV-infected BLT mice, and PD-1 levels in these cells correlated positively with viral load and inversely with CD4 ؉ cell levels. The ability of humanized BLT mice to generate both cellular and humoral immune responses to HIV will allow the further investigation of human HIV-specific immune responses in vivo and suggests that these mice are able to provide a platform to assess candidate HIV vaccines and other immunotherapeutic strategies.

We have previously proven that human macrophages can phagocytose porcine cells even in the absence of Ab or complement opsonization, indicating that macrophages present a pivotal immunological obstacle to xenotransplantation. A recent report indicates that the signal regulatory protein (SIRP)␣ is a critical immune inhibitory receptor on macrophages, and its interaction with CD47, a ligand for SIRP␣, prevents autologous phagocytosis. Considering the limited compatibility (73%) in amino acid sequences between pig and human CD47, we hypothesized that the interspecies incompatibility of CD47 may contribute to the rejection of xenogeneic cells by macrophages. In the present study, we have demonstrated that porcine CD47 does not induce SIRP␣ tyrosine phosphorylation in human macrophage-like cell line, and soluble human CD47-Fc fusion protein inhibits the phagocytic activity of human macrophages toward porcine cells. In addition, we have verified that manipulation of porcine cells for expression of human CD47 radically reduces the susceptibility of the cells to phagocytosis by human macrophages. These results indicate that the interspecies incompatibility of CD47 significantly contributes to the rejection of xenogeneic cells by macrophages. Genetic induction of human CD47 on porcine cells could provide inhibitory signaling to SIRP␣ on human macrophages, providing a novel approach to preventing macrophage-mediated xenograft rejection.xenotransplantation ͉ phagocytosis ͉ reticuloendothelial system

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The breakthrough of induced pluripotent stem cell (iPSC) technology has raised the possibility that patient-specific iPSCs may become a renewable source of autologous cells for cell therapy without the concern of immune rejection. However, the immunogenicity of autologous human iPSC (hiPSC)-derived cells is not well understood. Using a humanized mouse model (denoted Hu-mice) reconstituted with a functional human immune system, we demonstrate that most teratomas formed by autologous integration-free hiPSCs exhibit local infiltration of antigen-specific T cells and associated tissue necrosis, indicating immune rejection of certain hiPSC-derived cells. In this context, autologous hiPSC-derived smooth muscle cells (SMCs) appear to be highly immunogenic, while autologous hiPSC-derived retinal pigment epithelial (RPE) cells are immune tolerated even in non-ocular locations. This differential immunogenicity is due in part to abnormal expression of immunogenic antigens in hiPSC-derived SMCs, but not in hiPSC-derived RPEs. These findings support the feasibility of developing hiPSC-derived RPEs for treating macular degeneration.