Neuropathology of CNS disease in Langerhans cell histiocytosis.

CNS involvement in Langerhans cell histiocytosis (LCH) is a rare but potentially devastating disorder. Different types of involvement have been described by MRI. CNS changes can have space-occupying or degenerative character. Little is known about the underlying neuropathology and pathophysiology. In our study we reviewed brain samples from 12 patients with LCH. The neuropathology findings were correlated with the MR morphology and the clinical presentation. By neuropathology, three types of lesions were distinguished. (i) Circumscribed granulomas within the brain's connective tissue space corresponded to tumorous lesions in the meninges or choroid plexus on MRI. They showed a composition similar to Langerhans granulomas in peripheral organs, with variable presence of CD1a-reactive cells and pronounced CD8-positive (+) T-cell infiltration. (ii) Granulomas occur within the brain's connective tissue spaces with partial infiltration of the surrounding CNS parenchyma by CD1a-reactive histiocytes. This was associated with profound T-cell-dominated inflammation and severe neurodegeneration, characterized by a nearly complete loss of neurons and axons, and gliosis. (iii) Neurodegenerative lesions lacking infiltration of CD1a+ cells, mainly affecting the cerebellum and brainstem, exhibited a profound inflammatory process dominated by CD8-reactive lymphocytes, associated with tissue degeneration, microglial activation and gliosis. Patients with such lesions showed different stages of neurological deterioration. This study indicates that neurodegeneration in LCH occurs on the background of a T-cell-dominated inflammatory process and is characterized by neuronal and axonal destruction with secondary demyelination, resembling paraneoplastic encephalitis.