Mobilization-based engraftment of haematopoietic stem cells: a new perspective for chemotherapy-free gene therapy and transplantation

Abstract

Introduction: In haematopoietic stem cell transplantation (HSCT), haematopoietic stem cells (HSCs) from a healthy donor replace the patient's ones. Ex vivo HSC gene therapy (HSC-GT) is a form of HSCT in which HSCs, usually from an autologous source, are genetically modified before infusion, to generate a progeny of gene-modified cells. In HSCT and HSC-GT, chemotherapy is administered before infusion to free space in the bone marrow (BM) niche, which is required for the engraftment of infused cells. Here, we review alternative chemotherapy-free approaches to niche voidance that could replace conventional regimens and alleviate the morbidity of the procedure.

Sources of data: Literature was reviewed from PubMed-listed peer-reviewed articles. No new data are presented in this article.

Areas of agreement: Chemotherapy exerts short and long-term toxicity to haematopoietic and non-haematopoietic organs. Whenever chemotherapy is solely used to allow engraftment of donor HSCs, rather than eliminating malignant cells, as in the case of HSC-GT for inborn genetic diseases, non-genotoxic approaches sparing off-target tissues are highly desirable.

Areas of controversy: In principle, HSCs can be temporarily moved from the BM niches using mobilizing drugs or selectively cleared with targeted antibodies or immunotoxins to make space for the infused cells. However, translation of these principles into clinically relevant settings is only at the beginning, and whether therapeutically meaningful levels of chimerism can be safely established with these approaches remains to be determined.

Growing points: In pre-clinical models, mobilization of HSCs from the niche can be tailored to accommodate the exchange and engraftment of infused cells. Infused cells can be further endowed with a transient engraftment advantage.

Areas timely for developing research: Inter-individual efficiency and kinetics of HSC mobilization need to be carefully assessed. Investigations in large animal models of emerging non-genotoxic approaches will further strengthen the rationale and encourage application to the treatment of selected diseases.

Keywords: autologous stem cell transplantation; chemotherapy-free conditioning; gene therapy; haematopoietic stem and progenitor cells; mobilization.

Fig. 1

HSPC collection. HSPCs reside in the BM niche, where they are retained by molecular interactions (e.g. CXCR4-CXCL12). Mobilizing drugs like lenograstim and plerixafor disrupt these interactions, and HSPCs can leave the BM niche and enter the peripheral circulation. HSPCs are then collected by apheresis.

Fig. 2

Mobilization mechanism. HSPCs are retained in the BM niche by several interaction with stromal cells, as CXCL12-CXCR4, KITLG-KIT and VCAM1-ITGA4. G-CSF stimulates neutrophils to release proteases (MMP9, cathepsin G and neutrophil elastase) that cleave these receptors on both stromal cells and HSPCs, causing their release and egression into the blood stream. Plerixafor (AMD3100) acts as a competitive antagonist of CXCL12, disrupting the CXCR4-CXCL12 axis and causing the release of HSPCs. These two drugs can be used in combination to mobilize HSPCs.

Fig. 3

Strategies for HSPC genetic modification. In gene transfer approaches (left panel), HSPCs are transduced with a lentiviral vector encoding for a functional version of the disease-causing mutated gene. Upon infection, the viral genome is retro-transcribed and integrates semi-randomly in the cell genome. In gene-editing strategies (right panel), Cas9 nuclease introduces a targeted double-strand break in the cell genome, disrupting the mutated gene. Providing a DNA template, for example, with adeno-associated viral vectors or integrase-deficient lentivirus, it is possible to trigger the homology-directed repair mechanism, introducing a functional copy of the mutated gene in a specific region of the genome.

Fig. 4

Mobilization-based conditioning strategy. After administration of the mobilization regimen, HSPCs egress the BM niche, entering in the peripheral blood (panel 2). Careful timing of the transplant at the peak of mobilization (i.e. peak of BM depletion—panel 3), enables the competition of donor cells and mobilized resident cells for the repopulation of the BM niche and an exchange between donor and resident cells (panel 4).