Bone Marrow Cell Recruitment to the Brain in the Absence of Irradiation or Parabiosis Bias

Abstract

The engraftment of bone marrow-derived cells has been described not only during diseases of the central nervous system (CNS) but also under healthy conditions. However, previous studies pointing to an ample bone marrow cell engraftment used irradiation-induced bone marrow chimeras that evoked severe alterations of the CNS micromilieu including disturbances of the blood brain barrier (BBB), damage of endothelial cells and local induction of proinflammatory cytokines. On the other hand, parabiosis experiments using temporarily joined circulatory systems generally yielded low levels of myeloid cell chimerism thereby potentially underestimating bone marrow cell turnover with the CNS. To avoid these drawbacks we established a protocol using the alkylating agent busulfan prior to allogenic bone marrow transplantation from CX3CR1GFP/+ donors. This regimen resulted in a stable and high peripheral myeloid chimerism, significantly reduced cytokine induction and preserved BBB integrity. Importantly, bone marrow cell recruitment to the CNS was strongly diminished under these conditions and only weakly enhanced during local neurodegeneration induced by facial nerve axotomy. These results underscore the requirement of local CNS conditioning for efficient recruitment of bone marrow cells, establish busulfan as an alternative treatment for studying bone marrow chimeras and suggest a critical re-evaluation of earlier chimeric studies involving irradiation or parabiosis regimens.

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

“…They show that BU induces an inflammatory response characterized by long-term upregulation of CCL2, while the inflammatory response induced by irradiation is characterized by long-term upregulation of IL-1α. Thereby, it contradicts Kierdof et al results regarding chemokine expression following WBI and BU [96]. Results concerning BMDC recruitment to the brain are therefore controversial.…”

“…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].…”

“…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]. They showed that BU treatment largely preserves BBB integrity while inducing a milder expression of cytokines and chemokines in the brain compared to irradiation.…”

Friend or Foe? Resident Microglia vs Bone Marrow-Derived Microglia and Their Roles in the Retinal Degeneration

“…Recently, busulfan treatment was used to induce the recruitment of BMderived cells. Enhanced engraftment of these cells was detected during local neurodegeneration induced by facial nerve axotomy [113]. Consistently, Capotondo et al demonstrated that hematopoietic stem cell transplantation in a non-disease state of the host mice resulted in a short-term of cell infiltration into the brain.…”

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.…”