Vemurafenib acts as a molecular on-off switch governing systemic inflammation in Langerhans cell histiocytosis.

Langerhans cell histiocytosis (LCH) is a neoplasm marked by the accumulation of CD1A+CD207+ cells. It is most commonly driven by a somatic, activating mutation in the BRAF serine-threonine kinase (BRAFV600E). Multisystem disease with risk-organ involvement requires myelotoxic chemotherapy, making BRAF-inhibitors an attractive treatment option. Here, we present a comprehensive analysis of the course of an LCH patient treated with the combination of vemurafenib and salvage chemotherapy who achieved sustained clinical and molecular remission. We show that there is no relationship between peripheral blood BRAFV600E levels and clinical presentation during treatment with vemurafenib, but that vemurafenib leads to a fast, efficient, but reversible inhibition of clinical manifestations of systemic inflammation. In line, serum levels of inflammatory cytokines exactly mirror vemurafenib administration. Genotyping analysis identified the BRAFV600E mutation in multiple hematopoietic cell types, including NK cells and granulocytes. Single-cell transcriptome analyses of peripheral blood and bone marrow cells at time of diagnosis and during treatment indicate that RAF-inhibition abrogates the expression of inflammatory cytokines previously implicated in LCH such as IL1B and CXCL8. Together, our data suggest that while the CD1A+CD207+ histiocytes are the hallmark of LCH, other BRAF-mutated cell populations may contribute significantly to morbidity in patients with multisystem LCH.