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Langerhans cell histiocytosis (LCH) is an inflammatory myeloid neoplasia characterized by granulomatous lesions containing pathological CD207+ dendritic cells (DCs) with persistent MAPK pathway activation. Standard-of-care chemotherapies are 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 the 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. Intralesional CD8+ T cells showed blunted expression of Tc1/Tc2 cytokines and impaired effector function. In contrast, intralesional regulatory T cells 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 cells 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 they highlight combined MAPK and checkpoint inhibition as a potential therapeutic strategy., (© 2021 by The American Society of Hematology.)

Simple Summary: Immune checkpoint blockers are important immunotherapies for the treatment of patients with Hodgkin lymphoma. The resistance mechanisms of these immunotherapies remain unknown. This pilot study aims to decipher the resistance mechanisms of immunotherapies in Hodgkin lymphoma. The main results show that immunotherapy-resistant Hodgkin lymphoma had CD8 lymphocytes depleted in microenvironment and overexpression of the LAG-3 molecule. This study proposes hypotheses for understanding the resistance to immunotherapies in patients with Hodgkin lymphoma. Background: Resistance to anti-PD-1 remains a considerable clinical challenge for the treatment of patients with classical Hodgkin lymphoma (cHL), and mechanisms of anti-PD-1 resistance remain unknown. This pilot study aims to investigate the tumor microenvironment in patients with cHL relapsing after anti-PD-1. Methods: This study investigated tumor samples of eight patients with cHL, including four patients exposed to anti-PD-1 with a paired longitudinal histological analysis before and after anti-PD-1, and four patients not exposed to anti-PD-1 who served as control for the cellular biological investigations. Fresh cells tumor microenvironment analysis included phenotypic characterization of their T cell surfaces immune checkpoint markers PD-1, PD-L1, ICOS, TIM-3, LAG-3, 41-BB and BTLA. Tumor tissues immunohistochemistry staining included CD30, CD4, CD8, CD68, CD163, PD-L1, PD-1, LAG-3 and TIM-3. Findings: Paired longitudinal tumor tissues analysis in the tumor microenvironment found a CD8+ lymphocytes tumor depletion and an increase of LAG-3 staining after anti-PD-1 exposure. The fresh cells analysis of the tumor microenvironment in patients exposed to anti-PD-1 found CD8+ lymphocyte depletion, with an elevated CD4+/CD8+ lymphocytes ratio (median ratio 9.77 in exposed anti-PD-1 versus 2.39 in not-exposed anti-PD-1 patients; p = 0.0943). On the cell surfaces of CD4+ lymphocytes, the median positive expression of LAG-3 was significantly higher in the samples exposed to anti-PD-1 compared to the controls (15.05 [IQR:17.91–10.65] versus 3.84 [IQR 1.87–6.57]; p = 0.0376). Interpretation: This pilot study proposes hypotheses for understanding the resistance to immunotherapies in patients with Hodgkin lymphoma. Hodgkin lymphoma exposed to anti-PD-1 correlated in tumor microenvironment with an immune depletion of CD8+ T lymphocytes and overexpression of LAG-3 on CD4+ helper T lymphocytes. [ABSTRACT FROM AUTHOR]