Gene therapy using lentiviral vectors (LVs) has proven effective in ex vivo applications, where a patient’s stem cells are extracted, genetically modified, and then reinfused to achieve therapeutic benefits. This strategy, typically targeting hematopoietic stem and progenitor cells (HSPCs), has led to clinical successes—but it requires invasive procedures and substantial resources. For example, the sickle cell gene therapy Casgevy costs roughly $2.2 million per patient.
To overcome these limitations, researchers at the San Raffaele-Telethon Institute for Gene Therapy (SR-Tiget) explored an alternative approach: direct injection of LVs into the bloodstream. This strategy took advantage of a key observation the researchers made in mice: During the first two weeks of life, circulating HSPCs, the targets of LV gene therapy, are significantly more abundant than in adult animals—highlighting a potential therapeutic window.
Indeed, their findings, published in Nature, revealed that this early postnatal life was an optimal period for in vivo gene transfer in three genetic disorders.1 When the gene therapies were administered during this stage, the team noted a greater number of corrected stem cells and therapeutic benefits in newborn mice. They also observed a similar pattern of HSPC presence several months after birth in humans. These results suggest a comparable therapeutic window may exist in humans—opening the door to less invasive, early-life gene therapy approaches.
The team hypothesized that targeting a life stage with abundant circulating HSPCs would allow for more efficient gene transfer and a greater therapeutic effect. To determine this ideal timeframe, researchers first tracked HSPC levels in newborn mice, one-, two-, and eight-week-old adult mice; they found the highest concentration of hematopoietic stem cells within the first two weeks of life. The researchers then injected newborn mice with LVs tagged with a green fluorescent protein, confirming via clonal tracking that the treatment successfully modified long-term stem cells.
Noting enhanced gene transfer during this early window compared to adult mice, the team hypothesized that LVs targeted circulating HSPCs. To increase the numbers of these cells and potentially extend the treatment window, they treated the mice with granulocyte-colony stimulating factor and plerixafor, an immunostimulant, to mobilize stem cells from the bone marrow, thymus, and spleen into the bloodstream. They also optimized the LVs with a phagocytosis shield to improve their stability and uptake.
Then, they tested the in vivo LV-gene transfer treatment in mouse models of three genetic disorders: adenosine deaminase deficiency severe combined immunodeficiency (ADA-SCID), autosomal recessive osteopetrosis (ARO), and Fanconi anemia. Across all three models, the researchers observed therapeutic benefits. In ADA-SCID mice, treatment with LVs that targeted the ADA enzyme improved immune recovery by increasing lymphocyte counts. Then, in ARO mice, which have impaired bone cell remodeling, those treated with LVs had corrected cells capable of differentiating into functional osteoclasts. Lastly, the researchers observed that in LV-treated mice with Fanconi anemia, the corrected stem cells progressively repopulated the blood system and prevented bone marrow failure. Overall, the LV-treated mice had longer survival rates compared to untreated counterparts.
“This study provides proof of concept that in vivo lentiviral gene delivery to blood stem cells is feasible during a short but accessible period early in life as a gene therapy strategy for blood disorders,” remarked Alessio Cantore, a cell and molecular biologist at SR-Tiget, in a press release.
To explore a possible parallel in humans, the researchers examined the levels of HSPCs in newborns, 0–2 month-olds, 3–8 month-olds, and adults (>18 years). Indeed, the team observed a similar trend of elevated early-life HSPCs, suggesting a potential window for therapeutic intervention shortly after birth. However, the researchers noted that while the same drug treatment mobilized stem cells into the bloodstream, adult mice exhibited lower LV gene transfer efficiency. This suggests that additional interventions—such as transduction enhancers that block interferon-alpha—may be necessary to improve gene transfer effectiveness.
Cantore added, “While the efficiency currently remains limited as compared to established ex vivo treatments, it may suffice, if replicated in human babies, to benefit some genetic diseases such as severe immunodeficiencies or Fanconi anemia.”
- Milani M, et al. In vivo haemopoietic stem cell gene therapy enabled by postnatal trafficking. Nature. 2025.
