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OP0179 Brafv600e promotes myeloid skewing in multisystemic langerhans cell histiocytosis humanized mouse model

Background Multisystemic Langerhans cell histiocytosis (mLCH) is an aggressive disease characterized by the accumulation of mononuclear phagocytes with immunohistochemical features of dendritic cell (histiocytes)1. Histiocytes infiltrate mostly the skin, bone marrow (BM), lung and spleen, and produce high levels of proinflammatory cyto/chemokines, leading to organ dysfunction2,3. In patients, around 10% of cells in lesions carries an oncogenic mutation in the MAPK pathway, mostly BRAFV600E (70% of cases). BRAFV600E can be detected also in monocytes and BM progenitors (HSPC) from these patients, whereas only a fraction of them carries a mutation in B cells and none in T cells4. Objectives To study the role of BRAFV600E in the pathogenesis of mLCH, we set up a humanized mouse model of mLCH based on the transplantation into immunodeficient mice (NSG) of human HSPC expressing BRAFV600E. Methods We isolated HSPC from human cord blood and transduced them at two different levels (50% and 20%) with lentiviral vectors that ubiquitously express BRAFV600E, BRAFWT or GFP. Results All BRAFV600E mice manifested severe weight loss within 7 weeks with a median of 3 and 5 weeks for 50% and 20% transduction groups, respectively. Mice showed dysplastic bone marrow (BM) with infiltration of histiocytes; lesions were present also in lungs, kidneys, CNS, spleen and liver. Immunophenotype of infiltrating histiocytes closely resembles mLCH, staining positive for CD14, CD68, S-100 and langerin. None of the control mice lost weight nor displayed organ alteration. Flow cytometry analyses of BM showed 5- to 7-fold reduction in engraftment of human cells in BRAFV600E vs GFP groups (p<0,001). Percentage of myeloid cells increased by 3- to 4-fold in BRAFV600E vs GFP groups (p<0,001). On the contrary, percentage of B cells was reduced by 3- to 6,5-fold in BRAFV600E vs GFP groups (p<0,001). Moreover, there was no difference in the percentage of GFP-positive cells between myeloid cells, whole BM and in vitro sample, suggesting a non-cell autonomous mechanism underlying this myeloid phenotype. Conclusions In summary, we generated for the first time a human xenogeneic transplantation mouse model of mLCH, showing that BRAFV600E in human HSPC promotes myeloid skewing rather than proliferation. References Wilejto et al. Curr Opin Rheumatol 2012. DOI: 10.1097/BOR.0b013e32834db53e. Donadieu et al. Histiocytic Disorders of Children and Adults 2005. Kannourakis et al. Br J Cancer 1994, Sep;23:S37–40. Berres et al. J Exp Med 2014, DOI: 10.1084/jem.20130977. Disclosure of Interest None declared