Langerhans cell histiocytosis (LCH) is an inflammatory myeloid neoplasia characterized by granulomatous lesions containing pathological CD207+ dendritic cells (DCs) with persistent mitogen-activated protein kinase (MAPK) pathway activation. Standard of care chemotherapy strategies is inadequate for most patients with multisystem disease, and optimal strategies for relapsed and refractory disease are not defined. The mechanisms underlying development of inflammation in LCH lesions, the role of inflammation in pathogenesis and potential for immunotherapy are unknown. Analysis of the immune infiltrate in LCH lesions identified the most prominent immune cells as T lymphocytes. Both CD8+ and CD4+ T cells exhibited 'exhausted' phenotypes with high expression of the immune checkpoint receptors. LCH DCs showed robust expression of ligands to checkpoint receptors. Intra-lesional CD8+ T cells showed blunted expression of Tc1/Tc2 cytokines and impaired effector function. In contrast, intra-lesional regulatory T cells (Tregs) demonstrated intact suppressive activity. Treatment of BRAFV600ECD11c LCH mice with anti-PD-1 or MAPK inhibitor reduced lesion size, but with distinct responses: whereas MAPK inhibitor treatment resulted in reduction of the myeloid compartment, anti-PD-1 treatment was associated with reduction in the lymphoid compartment. Notably, combined treatment with MAPK inhibitor and anti-PD-1 significantly decreased both CD8+ T cell and myeloid LCH cells in a synergistic fashion. These results are consistent with a model that MAPK hyperactivation in myeloid LCH cells drives recruitment of functionally exhausted T cells within the LCH microenvironment and highlight combined MAPK and checkpoint inhibition as a potential therapeutic strategy.
Key Points
Transcriptional profile of LCH CD1a+CD207+ DCs is most closely related to that of CD1c+ mDCs in the blood. Lineage tracing with BRAFV600E and HLA-DQB2 expression supports CD1c+ mDCs as precursors to LCH CD1a+CD207+ DCs.
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