Myeloid Progenitors Define Extent of Disease in Mouse Models of Histiocytic Disorders: BRAFV600E Expression in Monocyte Lineage Recapitulates Erdheim-Chester Phenotype

Langerhans cell histiocytosis (LCH) is a myeloid neoplastic disorder characterized by granulomatous lesions with CD207⁺ dendritic cells associated with a wide spectrum of clinical presentations. Activating MAPK pathway genes (most frequently BRAFV600E) are identified in lesion DCs as well as in precursor cells in bone marrow and peripheral blood in some cases. LCH typically arises in children, but can also occur de novo in adults. Erdheim-Chester disease (ECD) is a related histiocytic disorder characterized by CD163⁺ histiocytic lesions with activating MAPK mutations (most frequently BRAFV600E) arising primarily in adults. Unlike LCH, ECD lesion distribution frequently includes kidneys and heart. In order to test the impact of cell of origin on disease phenotype, we developed Cre-inducible mouse models in which BRAFV600E expression is enforced at distinct stages of differentiation: BRAFV600E^CD11c and BRAFV600E^MSaA3. ITGAX/CD11c is expressed across a range of myeloid dendritic cell and monocyte lineages, where MS4a3 is expressed exclusively in GMPs and GMP-derived cells (e.g. monocytes).

BRAFV600E expression was enforced through Cre-mediated recombination to create BRAFV600E^CD11c and BRAFV600E^MSaA3 mice. Mice were analyzed at weeks 5, 10, 25, and 52 for body and organ weights and histology. Tissues were characterized by immunohistochemistry, gene expression, Luminex, CYTOF and RNASeq.

BRAFV600E^CD11c mice exhibited a more aggressive phenotype than BRAFV600E^MS4A3 counterparts, though both developed systemic histiocytic infiltration. Kinetics of organomegaly, histiocytic infiltration, and systemic inflammation was much slower in the BRAFV600E^MS4a3 mice. Notably, BRAFV600E^MS4a3 demonstrated increased enrichment of CD207-negative macrophages in spleen, liver and bone marrow. BRAFV600E^MS4a3 mice readily survived past 52 weeks, while BRAFV600E^CD11c had 100% mortality by 16 weeks. Bulk RNASeq and Ingenuity Pathway Analysis identified relative decrease in macrophage functionality in BRAFV600E^MSaA3 mice. Additionally, macrophage infiltrate was observed in heart, major blood vessels and kidneys of BRAFV600E^MS4a3 where these organs were not affected in BRAFV600E^CD11c.

The same mutation that creates an aggressive LCH-like phenotype in CD11c-derived cells creates an indolent ECD-like phenotype in monocyte-derived cells. This study demonstrates a critical role for ontogeny in pathogenesis of histiocytic disorders. Further, the BRAFV600E^MS4a3 mouse provides an ECD model for pre-clinical testing of therapeutic strategies.

McClain:SOBI Corporation: Membership on an entity's Board of Directors or advisory committees. Allen:Electra Therapeutics: Consultancy; Sobi, Inc.: Consultancy.