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E - Disease Models and Clinical Applications -> Hematologic and Immunologic Diseases

112: Transplantation without Myeloablation: Novel Conditioning Enables Robust Repopulation of Macrophage/Microglia Niches by Bone Marrow-Derived Cells

Type: Oral Abstract Session

Presentation Details
Session Title: Hematopoietic Stem Cell Gene Therapy
Location: Room 202
Start Time: 5/16/2022 17:15
End Time: 5/16/2022 17:30

Hematopoietic stem cell transplantation (HSCT) has been used for decades to treat multi-systemic diseases including several lysosomal and peroxisomal disorders. The therapeutic effect of HSCT for these diseases is mediated by bone marrow (BM)-derived cells that migrate to non-hematopoietic compartments as tissue-resident myeloid cells where they become a source of healthy cells and can cross-correct protein deficiencies. The engraftment and migration of BM-derived cells is partly influenced by the conditioning regimen, a pre-transplant treatment required to deplete host stem cells. However, despite its great promise, HSCT’s risk-benefit assessment is often seen unfavorably. Its morbidity is largely due to the need for pre-transplantation conditioning that can have severe toxicities. Furthermore, HSCT’s benefit is limited by the low efficiency of penetration of BM-derived cells in the central nervous system (CNS) and other non-hematopoietic tissues. Given the important therapeutic implications of HSCT for non-hematological diseases, we set out to establish conditioning regimens that act more specifically on the myeloid cell niches.
To this aim, we exploited the transient inhibition of the Colony-stimulating factor 1 receptor (CSF1R) which is specifically expressed in CNS microglia and tissue-resident myeloid cells, and whose genetic depletion results in robust CNS repopulation by BM-derived cells. We evaluated the effect of the CSF1R inhibitor PLX3397 (CSF1Ri) alone or in combination with irradiation or Busulfan in conditioning regimens prior to BM transplant (BMT) in C57BL/6 mice. Enhanced green fluorescent protein-positive (EGFP+) bone marrow cells, derived from CAG:GFP/GFP mice were used to quantify donor-derived cells in recipient mice.
We found that CSF1Ri combined with either irradiation or Busulfan dramatically increased the homing of BM-derived cells in the CNS, leading to superior engraftment of GFP+ microglia-like cells. Quantification of GFP+CD45+CD11b+ cells in the brain by flow cytometry showed that they represented the 6±2.4% (Busulfan), 33±2.3% (irradiation), 90±7% (CSF1Ri+irradiation) and 89±5% (CSF1Ri+Busulfan) of host microglia. A safer regimen combining CSF1Ri with non-myeloablative Busulfan (50 mg/kg) also provided superior engraftment of donor-derived cells in the CNS and other non-hematopoietic tissues compared to Busulfan-myeloablation alone. The addition of CSF1Ri did not impact BM engraftment (%GFP+/CD45+: 90±2.8% Busulfan, 92±0.6% irradiation, 92±0.8% CSF1Ri+Irradiation, 92±1% (CSF1Ri+Busulfan) or lineage reconstitution. Furthermore, comprehensive neurobehavioral phenotyping confirmed no added toxicities of the CSF1Ri+Busulfan regimen as compared to Busulfan alone.
In-depth characterization of endogenous and donor-derived CD45+CD11b+ cells from controls and transplanted brains by single-cell RNA-seq analyses showed a non-proinflammatory gene expression profile, supporting the therapeutic application of this novel conditioning regimen prior to HSCT for the treatment of CNS diseases. The CSF1Ri PLX3397 (Pexidartinib) is an attractive candidate as a CNS conditioning drug as it received FDA approval in 2019 for the treatment of diffuse-type tenosynovial giant cell tumors, has good CNS permeability and a favorable safety profile. The availability of more effective and specific conditioning regimens would make HSCT safer for the diseases for which HSCT is standard of care, de-risk it for diseases with a theoretical benefit, and ultimately establish a new treatment option for neurological and non-neurological diseases.

Pasqualina Colella1, Valentina Suarez-Nieto1, Jessica Arozqueta-Basurto2, Natalia Gomez-Ospina3

1Pediatrics, Stanford, Stanford, CA,2Institute for Stem Cell and Regenerative Medicine, Stanford, Stanford, CA,3Stanford, Stanford, CA
  N. Gomez-Ospina: 1; Commercial Interest i.e. Company X; Graphite Bio. 1; What was received? i.e. Honorarium; Ownership interest, Research Funding. 1; For what role? i.e. Speaker; Scientific Advisory Board. 2; Commercial Interest i.e. Company X; Codexis. 2; What was received? i.e. Honorarium; Consulting fees. 2; For what role? i.e. Speaker; Scientific Advisory Board.

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