Embryonic endothelial progenitor cells armed with a suicide gene target hypoxic lung metastases after intravenous delivery.

We show that mouse embryonic endothelial progenitor cells (eEPCs) home preferentially to hypoxic lung metastases when administered intravenously. This specificity is inversely related to the degree of perfusion and vascular density in the metastasis and directly related to local levels of hypoxia and VEGF. Ex vivo expanded eEPCs that were genetically modified with a suicide gene specifically and efficiently eradicated lung metastases with scant patent blood vessels. eEPCs do not express MHC I proteins, are resistant to natural killer cell-mediated cytolysis, and can contribute to tumor vessel formation also in nonsyngeneic mice. These results indicate that eEPCs can be used in an allogeneic setting to treat hypoxic metastases that are known to be resistant to conventional therapeutic regimes.

Caspase-8L expression protects CD34+ hematopoietic progenitor cells and leukemic cells from CD95-mediated apoptosis.

Regulation of sensitivity or resistance for apoptosis by death receptor ligand systems is a key control mechanism in the hematopoietic system. Dysfunctional or deregulated apoptosis can potentially contribute to the development of immune deficiencies, autoimmune diseases, and leukemia. Control of homeostasis starts at the level of hematopoietic stem cells (HSC). To this end, we found that CD34+ hematopoietic progenitor cells are constitutively resistant to CD95-mediated apoptosis and cannot be sensitized during short-term culture to death receptor-mediated apoptosis by cytokines. Detailed analysis of the death machinery revealed that CD34+ cells do not express caspase-8a/b, a crucial constituent of the death-inducing signaling complex (DISC) of death receptors. Instead, we found a smaller splice variant termed caspase-8L to be present in HSC. Forced expression of caspase-8L using a recombinant lentiviral vector was able to protect hematopoietic cells from death receptor-induced apoptosis even in the presence of caspase-8a/b. Furthermore, we found that caspase-8L is recruited to the DISC after CD95 triggering, thereby preventing CD95 from connecting to the caspase cascade. These results demonstrate an antiapoptotic function of caspase-8L and suggest a critical role as apoptosis regulator in HSC. Similar to CD34+ HSC, stem cell-derived leukemic blasts from AML(M0) patients only expressed caspase-8L. Additionally we found, caspase-8L expression in several AML and ALL samples. Thus, caspase-8L expression might explain constitutive resistance to CD95-mediated apoptosis in CD34+ progenitor cells and might participate in the development of stem cell-derived and other leukemias by providing protection from regulatory apoptosis.

Induction of humoral and cellular immune responses in mice by HIV-derived infectious pseudovirions.

Infectious pseudovirions based on HIV show the morphology of the parent virus and a genome that is partially expressed in infected cells. The constructs are capable of a single round of infection. In this study, we generated vesicular stomatitis virus (VSV) glycoprotein (G) pseudotyped HIV-1-derived pseudovirions that contain a codonoptimized p17/p24 HIV-1 gag or the green fluorescent protein (GFP) gene as transgene. BALB/c mice were immunized in a DNA prime pseudovirion boost fashion. Immunization induced a Gag-specific antibody response, high titers of neutralizing antibodies directed against the VSV-G protein and a Gag-specific IFN-gamma-secreting cytotoxic T lymphocyte (CTL) response. CTL responses were induced by both structural proteins contained in the pseudovirion preparation and through expression of the transgene. Infection properties similar to those of live attenuated HIV and the immunogenicity observed make infectious pseudovirions valuable tools to further study the mechanism of immune stimulation in models of HIV infection.

Endothelial progenitor cell culture and differentiation in vitro: a methodological comparison using human umbilical cord blood.

OBJECTIVE: Endothelial progenitor cells (EPC) can contribute to vascular repair and targeted tumour therapy. Little is known about generating EPC from human umbilical cord blood. We therefore compared methods for purification of EPC from human umbilical cord blood. METHODS: Mononuclear cells were isolated from human umbilical cord blood by density gradient centrifugation and used either unselected or after CD34 preselection. Unselected mononuclear cells were cultured for 9 days. Culture-dish-adherent (CDAC) and non-adherent (CDNAC) CD34+ cells were cultured separately for 4 weeks. Surface markers were assessed by immunofluorescence staining and FACS analysis. RESULTS: In unselected mononuclear cells, VEGF-R2 and VE-cadherin expression increased up to day 6. They stained positive with UEA-1 and took up acetylated LDL. Expression of CD45 and CD14 decreased over time, but remained strong. CD133 and CD34 were not expressed. CD34+-CDNAC acquired an endothelial phenotype over time with an increase of VEGFR-2 and von Willebrand factor (vWF). CD45 and CD14 decreased, while CD34 and the progenitor-cell marker CD133 remained strongly expressed. CD34(+)-CDAC showed a strong increase in VEGFR-2, CD133, CD34 and vWF, while CD14 decreased, and CD45 did not change. CONCLUSION: Putative EPC can be obtained from human umbilical cord blood. When selected for CD34, cells can be differentiated in culture to express markers of mature endothelial cells, while keeping progenitor markers. In contrast, short-term culture of unselected mononuclear cells leads to an endothelioid-monocytoid phenotype devoid of progenitor markers. Thus, the outgrowth from CD34-selected cells appears to be superior to short-term culture of unselected mononuclear cells with regard to endothelial cell-lineage specific differentiation of cells with a progenitor marker profile.

Foamy virus envelope glycoprotein-mediated entry involves a pH-dependent fusion process.

In general, enveloped viruses use two different entry strategies and are classified accordingly into pH-dependent and pH-independent viruses. Different members of the retrovirus family use one or the other strategy. Little is known about the uptake of foamy viruses (FV), a special group of retroviruses, into the target cells. In this study, we examined the pH dependence of FV entry by analyzing FV envelope glycoprotein (Env)-mediated infection of target cells with murine leukemia virus or FV vector pseudotypes in the presence of various lysosomotropic agents. Similar to vesicular stomatitis virus glycoprotein G (VSV-G)-mediated uptake, FV Env-mediated entry was inhibited by various lysosomotropic agents, suggesting a pH-dependent endocytic pathway. However, in contrast to its effect on VSV-G pseudotypes, chloroquine failed to reduce the infectivity of FV Env pseudotypes, implying that the pathway is different from that of VSV-G. Glycoproteins of various other FV species showed inhibition profiles similar to that of the prototype FV (PFV) Env. Analysis of the pH dependence of the FV Env-mediated fusion process in a cell-to-cell fusion assay revealed an induction of syncytium formation by a short exposure to acidic pH, peaking around pH 5.5. Interestingly, of all FV Env species analyzed, only the PFV Env had a significant fusion activity at neutral pH. Taken together, these data suggest a pH-dependent endocytic pathway for infection of target cells by FV.

Improved primate foamy virus vectors and packaging constructs.

Foamy virus (FV) vectors that have minimal cis-acting sequences and are devoid of residual viral gene expression were constructed and analyzed by using a packaging system based on transient cotransfection of vector and different packaging plasmids. Previous studies indicated (i) that FV gag gene expression requires the presence of the R region of the long terminal repeat and (ii) that RNA from packaging constructs is efficiently incorporated into vector particles. Mutants with changes in major 5' splice donor (SD) site located in the R region identified this sequence element as responsible for regulating gag gene expression by an unidentified mechanism. Replacement of the FV 5' SD with heterologous splice sites enabled expression of the gag and pol genes. The incorporation of nonvector RNA into vector particles could be reduced to barely detectable levels with constructs in which the human immunodeficiency virus 5' SD or an unrelated intron sequence was substituted for the FV 5' untranslated region and in which gag expression and pol expression were separated on two different plasmids. By this strategy, efficient vector transfer was achieved with constructs that have minimal genetic overlap.