Kollek et al. show that transient inhibition of apoptosis by short-term BCL-XL overexpression increases the viability of hematopoietic stem cells (HSCs) during engraftment and improves the outcome of HSC transplantation without signs of adverse pathologies. This strategy represents a promising and novel therapeutic approach, particularly under conditions of limited donor stem cell availability.
Hematopoietic stem cell transplantation (HSCT) is the only curative treatment for many hematological and immunological diseases but is hampered by the risk of graft failure or delayed engraftment. Clinical experience has shown that these problems can be overcome by transplantation of higher numbers of donor stem cells. This can be achieved by more efficient collection strategies (i.e. mobilization regimens) or by ex vivoexpansion. Here, we aim to transiently inhibit apoptosis in donor hematopoietic stem and progenitor cells (HSPCs) prior to transplantation in order to increase their numbers and their fitness.
In previous studies we have identified two Bcl-2 proteins from the pro-apoptotic BH3-only subgroup, Bim and Bmf, to be central players in apoptosis induction of HSPCs during transplantation. Both proteins are efficiently repressed by the cytokines Flt3L and SCF and upregulated under cytokine deprivation in vitro. Lack of either protein or overexpression of their anti-apoptotic antagonists Bcl-2 or Bcl-xL strongly increased HSPC competitiveness during transplantation, both in murine transplantation and human xenotransplantation models. Moreover, less donor HSPCs were required for successful engraftment when Bim-mediated apoptosis was inhibited. In sum, our data indicated that modulation of Bim or Bmf levels inhibits apoptosis in murine and human HSPCs and that the resulting extended life span is beneficial during HSCT (Labi et al, 2013).
Inhibition of the intrinsic apoptosis pathway could serve as a novel therapeutic approach to increase resistance of human HSPCs to factor deprivation and other types of stress caused during HSCT. However, permanent apoptosis inhibition in HSPCs can trigger their malignant transformation over time, especially when occurring together with activation of oncogenes promoting cell proliferation. Thus, such apoptosis inhibition needs to be transient when used therapeutically.
Here we analyze whether transient apoptosis resistance lasting for only a limited time span is sufficient to increase competitiveness of HSPCs during HSCT. For overexpression of Bcl-xL we used adenoviral vectors known to act transiently. In proliferating murine HSPCs, adenoviral Bcl-xL persisted for 5-7 days, and for this period cells were protected from different stress stimuli engaging the intrinsic apoptosis pathway. Most importantly, adenoviral Bcl-xL overexpression increased the reconstitution potential of murine HSPCs in competitive transplantation experiments. Persistence of adenoviruses was excluded. As expected, transient Bcl-xL overexpression did not accelerate lymphomagenesis, neither on a wildtype nor on a premalignant murine background.
However, adenoviral infection was associated with a relevant amount of toxicity to murine HSPCs. We thus performed transfection of full length Bcl-xL protein coupled to a protein transduction domain. Fluorescence microscopy indicated its mitochondrial localization, and functional tests revealed protection from apoptosis. However, when compared to adenoviral overexpression, Bcl-xL protein transduction proved to be less efficient due to the short protein half-life. In vivoexperiments are ongoing and will show whether such short-term apoptosis inhibition is sufficient to increase efficacy of transplantation.
In sum, our studies will evaluate the benefit of therapeutic apoptosis inhibition in donor HSPCs during HSCT and contribute to on-going efforts aiming to improve transplantation medicine.
Disclosures
No relevant conflicts of interest to declare.
Cystinosis is a rare, autosomally and recessively inherited disorder of amino-acid metabolism with ocular involvement. Four patients with this disease are reported on. The clinical signs of the first patient, who had nephropathic cystinosis, are compared with the signs and symptoms of the benign phenotype (benign cystinosis). In both nephropathic and benign cystinosis, multiple crystals were found in the cornea and conjunctiva. In the second case a large number of crystals were demonstrated in bone-marrow smears and conjunctival sections; a chromatogram of the conjunctiva indicated the presence of cystine. Although the results of an analysis of cultured fibroblasts from this case were contradictory, an elevated cystine level appeared likely. Crystals were also found in conjunctival sections from the third case. In the fourth case it proved impossible to demonstrate crystals clearly in either the bone marrow or in conjunctival sections. There was no evidence of any other cause of the corneal or conjunctival crystals.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.