Molecular and Cellular Pathogenesis of Pulmonary Langerhans Cell Histiocytosis

Project Summary. Pulmonary Langerhans Cell Histiocytosis (PLCH) is a rare interstitial lung disease,
which occurs almost exclusively in cigarette smokers and has a median duration of survival from of of
12.5 years. PLCH is characterized by,Langerin+ dendritic cell (DC) accumulation, inflammatory lesions,
nodule formation and cystic remodeling. Recently, a causative link between acquired BRAF (kinase in
RAS pathway) mutations in the myeloid/monocyte lineage and the development of benign 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. To
address this critical knowledge gap, we have developed preclinical models that recapitulate hallmark
features of PLCH. We show that cigarette smoke (CS) exposure of mice expressing an inducible
BRAFV600E mutation in CD11c+ cells exhibit peribronchiolar inflammation, nodule formation, and
airspace enlargement accompanied by cyst-like formations. These structural changes are accompanied
by alterations in DC homeostasis, including increased expression of and responsiveness to the
chemokine CCL20. Further, our data suggest the BRAFV600E-dependent production of the pleiotropic
cytokine CCL7 is a potential driver of the inflammatory lesions in PLCH. Finally, we demonstrate that the
BRAF mutation induces DC senescence accompanied by upregulation of non-classical MHC ligands that
permit the DCs to evade immune-mediated clearance. Based on these novel preliminary data, we
hypothesize that PLCH pathogenesis is initiated by a combination of BRAF mutations along with CS
exposure to amplify CCL20 mediated histiocyte accumulation followed by the recruitment of inflammatory
cells/nodule development via CCL7 production and the amplification of this process is preserved by
BRAF-dependent expression of ligands for inhibitory receptors on cytotoxic lymphocytes. We will test this
hypothesis with three inter-related, yet independent, Specific Aims. We expect the completion of the
research will lead to new conceptual advances in PLCH. While PLCH is rare, we believe the preclinical
model will lead to insights into the role of BRAFV600E cells in PLCH. This model provides a compelling
platform for preclinical studies in the short term and evidence based clinical trials in the long term, and
should enable the development of treatments of a life threatening pulmonary disease. This goal may be
viewed as aggressive but the identification of an effective treatment for lymphanioleiomyomatosis by our
group and promising clinical trials underway by our group as part of the Rare Lung Disease Consortium,
are direct evidence that effective therapies can be found quickly when clues of nature are abundant,
molecular targets are known, biological plausibility is high and patient communities are well organized.

Grant Number: 5R01HL162662-03
NIH Institute/Center: NIH
Principal Investigator: Michael Borchers