Donor Bone Marrow Derived Macrophage Engraftment Significant in CNS of Allogeneic Transplant Patients

Engraftment from donor derived cells from a myeloablative transplant were at their highest following myelofrobrative conditioning, patients receiving multiple transplants, but at their lowest in non-myeloablative cases, according to results published in Blood.¹

In the largest study of bone marrow-derived macrophages in post hematopoietic stem cell transplantation (HSCT) patients with a hematologic malignancy, results showed that donor derived cells from the myeloablative transplants accounted for 4.1-25% of microglial cells. According to researchers, these cells were stable over time as the post-transplant period did not have what they deemed a major impact on the number of donor cells. Moreover, the strength of the conditioning therapy that would provide access to the central nervous system (CNS) and engraftment had a significant impact on the donor cell engraftment. The donor cells in myeloablative cases (> 1,000cGy) averaged 8% (4.2-14.9%) of microglial cells compared to non-myeloablative cases (< 300cGy) that averaged 1.3% (1.2-1.3%) of microglial cells.

“Recent studies have shown the role HSCT can have in gene therapy by providing long-lived genetically modified cells to treat a variety of human diseases,” explained the researchers. “It is well known that HSC and bone marrow-derived cells can differentiate into long-lived tissue macrophages and populate a wide spectrum of tissues including the brain.”

In this study, the researchers wanted report the detection and characterization of donor bone marrow-derived macrophages in the CNS, specifically the cerebral cortex, of allogeneic transplant patients. To determine the frequency of donor cell engraftment in this patient population they looked at 20 patients who underwent a sex-mismatched transplant. To then determine the identity of donor cells, select cases were stained with fluorescent tyramide based Iba1 IHC, imaged, then stained with XY fluorescent in situ hybridization (FISH) and re-imaged. Greater than 80% of the donor cells showed a strong expression of Iba1, which confirmed to the researchers that they were bone marrow derived.

Using tissue samples from male and female patients who underwent autologous transplants as controls, tissue sections were stained with XY FISH to identify the donor cells. These were imaged at 40x magnification on a TissueFAX system and scanned images were analyzed using TissueQuest, in blinded fashion. Parameters of the study were established and then applied to a larger area of 10,000-15,000 cells.

“In parallel Iba1 IHC studies we showed that microglial cells constitute (approximately) 12% of the scanned cell population. Thus, when computed as a percentage of the macrophage/microglial population, donor cells from myeloablative transplants range from 4.2-25%,” the researchers confirmed.

Two conditioning therapies looked at were busulfan (Myleran) and treosulfan (Trecondi) and both had similar levels of donor-derived cells as cases with total body irradiation myeloablative conditioning, which averaged 6.6% (4.4-8.3%) of microglial cells. However, a limited number of samples were available for analysis at the time, but the highest level of donor engraftment was observed in patients who had received 2 separate transplants that comprised of 16.3% (12.2-25.1%) microglial cells.

“Our studies document the magnitude of donor-derived macrophages in the CNS following a bone marrow transplant and serve as a basis for future gene therapy studies targeting neurodegenerative disorders,” the researchers concluded.

_References

_{Adams RC, Carter-Cusack D, Shaikh SN, et al. Donor bone marrow-derived macrophage MHC II drives neuroinflammation and altered behaviour during chronic GVHD in mice. Blood. 2021 Sep 27:blood.2021011671. doi: 10.1182/blood.2021011671}