Functional CD47/signal regulatory protein alpha (SIRP(alpha)) interaction is required for optimal human T- and natural killer- (NK) cell homeostasis in vivo.

The homeostatic control mechanisms regulating human leukocyte numbers are poorly understood. Here, we assessed the role of phagocytes in this process using human immune system (HIS) BALB/c Rag2(-/-)IL-2Rγc(-/-) mice in which human leukocytes are generated from transplanted hematopoietic progenitor cells. Interactions between signal regulatory protein alpha (SIRPα; expressed on phagocytes) and CD47 (expressed on hematopoietic cells) negatively regulate phagocyte activity of macrophages and other phagocytic cells. We previously showed that B cells develop and survive robustly in HIS mice, whereas T and natural killer (NK) cells survive poorly. Because human CD47 does not interact with BALB/c mouse SIRPα, we introduced functional CD47/SIRPα interactions in HIS mice by transducing mouse CD47 into human progenitor cells. Here, we show that this procedure resulted in a dramatic and selective improvement of progenitor cell engraftment and human T- and NK-cell homeostasis in HIS mouse peripheral lymphoid organs. The amount of engrafted human B cells also increased but much less than that of T and NK cells, and total plasma IgM and IgG concentrations increased 68- and 35-fold, respectively. Whereas T cells exhibit an activated/memory phenotype in the absence of functional CD47/SIRPα interactions, human T cells accumulated as CD4(+) or CD8(+) single-positive, naive, resting T cells in the presence of functional CD47/SIRPα interactions. Thus, in addition to signals mediated by T cell receptor (TCR)/MHC and/or IL/IL receptor interactions, sensing of cell surface CD47 expression by phagocyte SIRPα is a critical determinant of T- and NK-cell homeostasis under steady-state conditions in vivo.

Monitoring the effect of gene silencing by RNA interference in human CD34+ cells injected into newborn RAG2-/- gammac-/- mice: functional inactivation of p53 in developing T cells.

Tumor suppressor p53 plays an important role in regulating cell cycle progression and apoptosis. Here we applied RNA interference to study the role of p53 in human hematopoietic development in vivo. An siRNA construct specifically targeting the human tumor-suppressor gene p53 was introduced into human CD34(+) progenitor cells by lentivirus-mediated gene transfer, which resulted in more than 95% knockdown of p53. We adapted the human-SCID mouse model to optimize the development of hematopoietic cells, particularly of T cells. This was achieved by the intraperitoneal injection of CD34(+) precursor cells into newborn Rag2(-/-) gammac(-/-) mice that lack T, B, and NK cells. Robust development of T cells was observed in these mice, with peripheral T-cell repopulation 8 weeks after injection of the precursor cells. Other lymphocyte and myeloid subsets also developed in these mice. Injecting p53 siRNA-transduced CD34(+) cells resulted in stable expression and down-modulation of p53 in the mature T-cell offspring. Inactivating p53 did not affect the development of CD34(+) cells into various mature leukocyte subsets, including T cells, but it conferred resistance to gamma-irradiation and other p53-dependent apoptotic stimuli to the T cells.

Human telomerase reverse transcriptase-transduced human cytotoxic T cells suppress the growth of human melanoma in immunodeficient mice.

Immunotherapy of melanoma by adoptive transfer of tumor-reactive T lymphocytes aims at increasing the number of activated effectors at the tumor site that can mediate tumor regression. The limited life span of human T lymphocytes, however, hampers obtaining sufficient cells for adoptive transfer therapy. We have shown previously that the life span of human T cells can be greatly extended by transduction with the human telomerase reverse transcriptase (hTERT) gene, without altering antigen specificity or effector function. We developed a murine model to evaluate the efficacy of hTERT-transduced human CTLs with antitumor reactivity to eradicate autologous tumor cells in vivo. We transplanted the human melanoma cell line melAKR or melAKR-Flu, transduced with a retrovirus encoding the influenza virus/HLA-A2 epitope, in RAG-2(-/-) IL-2Rgamma (-/-) double knockout mice. Adoptive transfer of the hTERT-transduced influenza virus-specific CTL clone INFA24 or clone INFA13 inhibited the growth of melAKR-Flu tumors in vivo and not of the parental melAKR melanoma cells. Furthermore, the hTERT-transduced CTL clone INFA13 inhibited tumor growth to the same extent in vivo as the untransduced CTL clone, as determined by in vivo imaging of luciferase gene-transduced melAKR-Flu tumors, indicating that hTERT did not affect the in vivo function of CTL. These results demonstrate that hTERT-transduced human CTLs are capable of mediating antitumor activity in vivo in an antigen-specific manner. hTERT-transduced MART-1-specific CTL clones AKR4D8 and AKR103 inhibited the growth of syngeneic melAKR tumors in vivo. Strikingly, melAKR-Flu cells were equally killed by the MART-1-specific CTL clones and influenza virus-specific CTL clones in vitro, but only influenza-specific CTLs were able to mediate tumor regression in vivo. The influenza-specific CTL clones were found to produce higher levels of IFNgamma on tumor cell recognition than the MART-1-specific CTL clones, which may result from the higher functional avidity of the influenza virus-specific CTL clones. Also, melAKR-Flu tumors were growing faster than melAKR tumors, which may have surpassed the relatively modest antitumor effect of the MART-1-specific CTL, as compared with the influenza virus-specific CTL. Taken together, the adoptive transfer model described here shows that hTERT-transduced T cells are functional in vivo, and allows us to evaluate the balance between functional activity of the CTL and tumor growth rate in vivo, which determines the efficacy of CTLs to eradicate tumors in adoptive transfer therapy.

Intrathymic and extrathymic development of human plasmacytoid dendritic cell precursors in vivo.

The development of plasmacytoid dendritic cells (pDC2) from human CD34(+) stem cells in vivo was studied in RAG-2(-/-) interleukin (IL)-2Rgamma(-/-) mice that lack functional T and B cells and natural killer cells. CD34(+) cells isolated from fetal liver or thymus were labeled with 5- and 6-carboxyfluorescein diacetate succinimidyl ester (CFSE) and were injected into a human thymus grafted subcutaneously in the RAG-2(-/-) IL-2Rgamma(-/-) mice. One to 4 weeks later the CFSE label was found not only in T cells but also in CD123(+/high) CD4(+)CD45RA(+) pDC2, indicating that the CD34(+) cells can develop into pDC2 within a thymus. In addition to pDC2, CFSE-labeled dendritic cells with a mature phenotype, determined by the cell surface markers CD11c, CD83, and CD80, were found in the injected human thymus graft. pDC2 was not found in the periphery of mice carrying a human thymic graft, indicating that the intrathymic pDC2 failed to emigrate from the thymus. We also demonstrate that pDC2 can develop outside the thymus because relatively high percentages of pDC2 were found in the periphery after the intravenous injection of CD34(+)CD38(-) fetal liver cells in RAG-2(-/-) IL-2Rgamma(-/-) mice without a human thymus graft. These data indicate that the thymus and the peripheral pDC2 develop independently of each other.