Effects of Myeloablation, Peripheral Chimerism, and Whole-Body Irradiation on the Entry of Bone Marrow-Derived Cells into the Brain

Abstract

Understanding how bone marrow-derived cells (BMDCs) enter the central nervous system (CNS) is critical for the development of therapies for brain-related disorders using hematopoietic stem cells. We investigated the brain damages and blood-brain barrier (BBB) modification following either whole-body irradiation or a myeloablative chemotherapy regimen in mice, and the capacity for these treatments to induce the entry of BMDCs into the CNS. Neither treatment had a lasting effect on brain integrity and both were equally efficient at achieving myeloablation. Injection of bone marrow cells from green fluorescent protein (GFP) transgenic mice was able to completely repopulate the hematopoietic niche in the circulation and in hematopoietic organs (thymus and spleen). However, GFP + cells only entered the brain following whole-body irradiation. We conclude that myeloablation, damages to the brain integrity, or the BBB and peripheral chimerism are not responsible for the entry of BMDCs into the CNS following irradiation.Key words: Microglia; Central nervous sysem (CNS); Neuroimmunology; Hematopoietic stem cell; Irradiation; Chemotherapy; Bone marrow-derived cells; Innate immunity INTRODUCTIONof high-grade glioma, severe combined immunodeficiency, multiple sclerosis, and other CNS pathologies (6). Despite the high therapeutic potential of BMDCs for Brain-specific macrophages called microglia are responsible for the immune defense of the brain through cerebral pathologies (13) and an inestimable tool to study microglial biology, the mechanisms by which they innate immunity processes (19). They are the only brain cell type deriving from bone marrow resident hematoenter the brain are poorly understood. Chimerism is the process by which injected bone poietic stem cells (18,22). The recruitment of bone marrow-derived cells (BMDCs) into the brain is highly marrow cells from a donor take over the hematopoietic system of a recipient. The use of the green fluorescent active throughout the embryonic life during which microglia populate the central nervous system (CNS) protein heterozygous (GFP +/− ) transgenic mouse model has eased the study of chimerism by facilitating the (5). It appears to be marginal during the adult life, as self-renewal of microglia could be sufficient to maintain tracking of injected GFP + cells in a GFP − background. Numerous models have been designed to induce chimethe microglial population (1), and more active in pathological conditions such as Alzheimer's disease (8). It rism in mouse models such as parabiosis, immunosuppression of the recipient mice, and irradiation of the was demonstrated that following whole-body irradiation, bone marrow transplantation was highly efficient to whole body, only the head, or the body with a protected head (24). It appears that only lethal doses of wholerestrict amyloid plaque formation and resolve the cognitive declines of mouse models of Alzheimer's disease body irradiation are able to create an efficient chimerism and to induce migrations of BMDCs to the brain. Con-...

Origin of Microglia: Current Concepts and Past Controversies

“…In conclusion, BM-derived microglia can engraft into the diseased brain and become an integral part of the cellular network in the CNS only under specific nonphysiological conditions. These specific conditions include irradiation and chemotherapeutic regimes, for example, the application of myeloablating agents, which all (1) alter the integrity of the BBB, and (2) induce local production of myeloattracting chemokines, such as CCL2 (Boettcher et al 2008;Lampron et al 2012;Kierdorf et al 2013b). Taken together, these data also clearly indicate that BM-derived nonmonocytic cells are able to permanently engraft to the diseased brain, whereas shortlived monocytes are only transiently recruited to the CNS.…”

Bone marrow-derived macrophages and the CNS: An update on the use of experimental chimeric mouse models and bone marrow transplantation in neurological disorders

“…These BMDMs probably originate both from circulating monocytes and from uncommitted hematopoietic progenitors that colonized the CNS after BMT and differentiated thereafter. Interestingly, mice preconditioned with BU + CY showed a very mild infiltration of BMDM, probably all originating from circulating monocytes [94]. Kierdof et al made a similar experiment where they compared a dose of 90 mg/kg of BU to an 11 Gy WBI regimen [96].…”

“…It also permits an important infiltration of myeloid cells under pathological conditions and is the best option to optimize the recruitment and study the therapeutic role of BMDC in CNS diseases. However, further work remains to be done to understand why BU treatment led to such discording results in term of microglial replacement/turnover [94][95][96][97][98] because treatments promoting microglia turnover are clinically promising for many rare genetic diseases [5,11,12,101,188]. Therefore, every conditioning regimen has their advantages and limitations and the best regimen should be chosen wisely depending on the purposes of the experiment and potential treatments (Fig.…”

“…Several studies in mouse came out lately comparing the effects of BU-and irradiation-based conditioning regarding microglial engraftment, BBB dysfunction and the neuroinflammatory response [94][95][96][97][98]. Our group compared a fully-myeloablative dose of 10 Gy WBI and a chemotherapeutic regimen consisting of 80 mg/kg of BU injected twice daily over four days (total 8 injections of 10 mg/kg) followed by two daily injection of 100 mg/kg of CY for a total of 200 mg/kg [94].…”

“…Our group compared a fully-myeloablative dose of 10 Gy WBI and a chemotherapeutic regimen consisting of 80 mg/kg of BU injected twice daily over four days (total 8 injections of 10 mg/kg) followed by two daily injection of 100 mg/kg of CY for a total of 200 mg/kg [94]. This regimen was based on the one used to induce myeloablation before the HSC transplantation (HSCT) in a clinical experiment that successfully treated two patients with adrenoleukodystrophy [11].…”