Effect of MAPK Inhibitors on Dendritic Cells in Pulmonary Langerhans Cell Histiocytosis (PLCH)

Abstract ID 98921 Poster Board 358 Background: PLCH occurs almost exclusively in cigarette smokers and has a median duration of survival from the time of diagnosis of 12.5 years. PLCH is characterized by bronchiolocentric histiocyte accumulation, inflammatory lesions, nodule formation, and cystic remodeling. A causative link between acquired BRAF (kinase in RAS/MAPK signaling pathway) mutations in the myeloid/monocyte lineage and the development of neoplasms has been reported, and a common, acquired mutation in BRAF (V600E) was found in lung lesions of more than half of PLCH patients. Although this causal link represents an important breakthrough, the mechanisms by which mutant histiocytes control the initiation and progression of PLCH are unknown. Objective: To address this critical gap, we developed preclinical models that recapitulate hallmark features of PLCH. We showed that cigarette smoke (CS) exposure of mice expressing an inducible BRAFV600E mutation in CD11c+ cells exhibit peribronchiolar inflammation, neoplasm formation, and airspace enlargement accompanied by cyst-like formations. These structural changes are accompanied by alterations in dendritic cell (DC) homeostasis, including increased expression of and responsiveness to the chemokine CCL20, and the production of the chemokine CCL7. Based on our recent characterization of the novel mouse model of PLCH and these preliminary findings, we hypothesize that PLCH pathogenesis is driven by a combination of RAS pathway mutations along with CS exposure to amplify CCL20 driven histiocyte accumulation and recruitment of inflammatory cells via CCL7. Methods:In vitro: WT or BRAFV600E DCs from mice were treated with or without LPS or BRAF inhibitor PLX4720 or both. Secreted CCL7 was measured by ELISA. In vivo: Mice with tamoxifen inducible BRAFV600E mutation or WT were fed tamoxifen chow and exposed to CS for 4 months followed by exposure to fresh air (FA) and normal or continued tamoxifen chow for 1 month (n = 4 mice per group). Serum CCL7 and bronchioalveolar lavage fluid (BAL) CCL20 levels were measured via ELISA. Results:In vitro, we found that pharmacological inhibition of BRAFV600E DCs with PLX4720 resulted in a 2-fold decrease in CCL7 production following stimulation with lipopolysaccharide (LPS) compared with those treated with LPS alone. Similarly, in vivo studies showed that elimination of the BRAFV600E mutation reduced serum CCL7 and BAL CCL20 levels by greater than 2-fold. Additionally, BRAFV600E elimination resulted in greater than 2-fold decrease in total absolute cell numbers, including DCs and T cells. Conclusions: These results demonstrate that inhibition of mutant BRAF-expressing dendritic cells ameliorates PLCH pathogenesis via decreased CCL7 and CCL20 expression, limiting inflammatory cell recruitment to the lungs. Taken together, this work provides a platform for therapeutic development to treat PLCH via inhibition of mutant BRAF and other RAS pathway molecules. This work is funded by support from the NIH (R01HL162662).