Bone Marrow-Derived BRAFV600E-Mutated Cells Drive Neurodegeneration in a Mouse Model of Langerhans Cell Histiocytosis

Progressive neurodegeneration is a devastating and ultimately fatal complication in some survivors of multi-system high-risk Langerhans Cell Histiocytosis (LCH). Patients often develop LCH-ND years after the diagnosis and successful treatment of their systemic disease, and mechanisms that underlie LCH-ND remain largely undefined. BRAF^V600E+ myeloid cells have been identified in areas of active neurodegeneration. However, it is unclear if the cell of origin of this disease is mutated microglia or circulating peripheral LCH cells invading the brain tissue. Embryonic origin (e.g. microglia) versus hematopoietic origin (or both) is a critical question to be able to identify, prevent and cure LCH-ND. We therefore tested the ability of bone marrow (BM)-derived BRAF^V600E cells to migrate to the brain in a mouse model.

We used BM chimera to study the transition of BRAF^V600E-mutated cells to the brain as well as mechanisms of neurologic damage therein. Therefore, WT mice were conditioned with head-shielded Total Body Irradiation (TBI) and transplanted with BM from SCL^Cre-ER-TxBRAF^V600ExRosa26^YFP [LCH chimera] or SCL^Cre-ER-TxBRAF^WTxRosa26^YFP [control chimera]. Cre-recombination was induced after an engraftment period of 4 weeks. Mice were analyzed 4 months after Tamoxifen induction.

LCH chimera exhibit a parenchymal infiltration with BM-derived BRAF^V600E+ macrophages 4 months after Cre-recombination while control chimera do not have substantial numbers of reporter-tagged cells in the brain. The localization pattern of BRAF^V600E+ macrophages in the brain recapitulate the pattern observed in humans, i.e. in the Brain Stem and Cerebellum. BM-derived BRAF^V600E+ macrophages accumulate at the interface between the bloodstream and the central nervous system (CNS) parenchyma with 10-fold higher frequency than BM-derived BRAF^WT macrophages and are associated with signs of local inflammation, astrocyte activation and decline in Purkinje cell density. Finally, LCH chimera exhibit signs of neurologic impairment (e.g. ataxia, decreased strength) that was partially reversed by targeted treatment.

We conclude that BRAF^V600E+ BM-derived myeloid progenitors reproducibly invade brain parenchyma where they differentiate into macrophages and induce inflammation and CNS injury. These novel insights inform previously unrecognized pathogenic mechanisms for LCH, with clinical implications in predicting LCH-ND risk, surveillance strategies, and novel therapeutic opportunities. Further, these data demonstrate potential for MAPK activation in hematopoietic myeloid progenitors to drive CNS inflammation and neurodegeneration, which may impact a broad range of neuropathological conditions beyond LCH.