Under the Microscope: Lentiviral-Gene corrected Hematopoietic Stem Cells
Begoña Díez Cabezas, PhD
María Fernández-García, PhD
Sala de producción celular CliniStem.
Disclaimer/Conflict of interest statement. None.
Lentiviral-gene corrected hematopoietic stem cells (HSC) are defined as modified self-renewing, multipotent cells that can generate hematopoietic progeny of all lineages carrying a specific therapeutic gene. These cells are being tested as a medicinal product for the treatment of certain life-threatening monogenic diseases, including primary immunodeficiencies, red blood disorders and bone marrow failure syndromes (1).
HSCs are the source of all mature blood cells that constitute a healthy hematopoietic system. For this reason, targeted gene modification and subsequent transplant of HSCs offers a source of self-renewing stem cells, with the promise of a life-long therapeutic effect, making these cells perfect candidates to treat the full complement of hematopoietic diseases, regardless of the specific hematopoietic cell type affected (2).
HSC was reported in the 60’s by Till and McCulloch
in a series of experiments to characterize radiation sensitivity, in which a transplant from donor adult bone marrow into syngeneic irradiated murine recipients was capable of protecting the recipients from lethal irradiation by regenerating the irradiation-ablated hematopoietic system (3). The first clinical trials using lentiviral vectors (LV) in HSC began in mid-2000 by Cartier et al (4) and Cavazzana-Calvo et al (5). The main driving force pushing the investigation, testing and release of this new category of medicinal products was found in Academia. However, the main limitations of these therapies are the logistics, as each disorder requires its specific vector and the entire pre-clinical and clinical drug development package.
The most widely used LV system, based on self-inactivated vectors, was first developed in 1996 by Naldini et al (6). Later, in 2009 lentiviral gene-corrected HSCs were used for the first time in a clinical trial for the treatment of a neurological disorder and in 2013 for a hematological disorder. Since then, LVs have become the most used vector for ex vivo
gene therapy clinical trials. In 2019, the European Medicines Agency (EMA
) gave conditional marketing authorization to the medicinal product Zynteglo
, based on gene-corrected HSC for patients with transfusion-dependent β-thalassaemia (7), although the marketing-authorization holder has withdrawn this authorization and the product is no longer available for these patients.
Nowadays, more than 60 clinical trials using LV modified HSCs are registered on the website ClinicalTrials.gov, most of them in Europe and USA. From these, one medicinal product received conditional marketing authorization for β-thalassaemia and another one a complete marketing authorization for the treatment of metachromatic leukodystrophy in Europe. None of them has been authorized in USA yet.
HSCs are usually isolated from mobilized peripheral blood using the presence of CD34 in their membrane by immunomagnetic separation, which enables 30- to 50-fold enrichment of HSCs. The main difficulty in producing this kind of product is the reduced number of the CD34-expressing progenitor cells and their propensity to differentiation in culture. As such, modification should be conducted in the shortest period of time possible. Moreover, as cells are modified using LVs, it is important to test each batch of viral vector production in order to adjust the multiplicity of infection (MOI) to target the desired number of cells and control the vector copy number introduced in each cell (8).
Did you know that… genomic modifications in HSCs using lentiviral vectors have been used not only to treat hematological disorders but also as delivery vehicles for the treatment of inherited neurometabolic disorders and acquired diseases. They have been used to treat certain lysosomal storage disorders such as metachromatic leukodystrophy, a disease in which myelinic nervous cells are very affected, leading to a decrease in neurocognitive and motor function. As a result of the therapeutic benefit observed in clinical trials conducted in this disease setting, the medicinal product Libmeldy was approved in October 2020 in Europe for treatment of metachromatic leukodystrophy. To date, this is the only lentiviral gene-correct HSC-based product commercially available in Europe.
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