Donor Bone Marrow Derived Macrophage Engraftment into the Central Nervous System of Allogeneic Transplant Patients

Abstract Hematopoietic stem cell transplantation is a well known treatment of hematologic malignancies wherein nascent stem cells provide a regenerating marrow and immunotherapy against the tumor. The progeny of hematopoietic stem cells also populate a wide spectrum of tissues, including the brain, as bone marrow derived macrophages similar to microglial cells. We developed a sensitive and novel combined IHC and XY FISH assay to detect, quantify and characterize donor cells in the cerebral cortex of 19 female allogeneic stem cell transplant patients. We show that the number of male donor cells ranged from 0.14-3.0% of total cells or 1.2-25% of microglial cells. Using tyramide based fluorescent IHC we found at least 80% of the donor cells express the microglial marker Iba1 consistent with being bone marrow derived macrophages. The percentage of donor cells was related to pretransplant conditioning; donor cells from radiation based myeloablative cases averaged 8.1% of microglial cells, while those from non-myeloablative cases averaged only 1.3%. The number of donor cells in patients conditioned with Busulfan or Treosulfan based myeloablation were similar to TBI based conditioning; donor cells averaged 6.8% of microglial cells. Notably, patients who received multiple transplants and those with the longest post-transplant survival had the highest level of donor engraftment, with donor cells averaging 16.3% of microglial cells. Our work represents the largest study characterizing bone marrow-derived macrophages in post-transplant patients. The efficiency of engraftment observed in our study warrants future research on microglial replacement as a therapeutic option for disorders of the central nervous system. Key Points We developed a sensitive Iba1 IHC and XY FISH co-stain assay to detect bone marrow derived donor cells in the brain of HSCT patients. Donor cells in the CNS express Iba1 and average 8% of microglia after myeloablative HSCT; double transplants increase engraftment up to 25%.