Leukemia-associated activating mutation of Flt3 expands dendritic cells and alters T cell responses. Mus musculus

One of the most common genetic alterations in acute myeloid leukemia is the internal tandem duplication (ITD) in the FLT3 receptor for cytokine FLT3 ligand (FLT3L). The constitutively active FLT3-ITD promotes the expansion of transformed progenitors, but also has pleiotropic effects on normal hematopoiesis. We analyzed the effect of FLT3-ITD on dendritic cells (DCs), which express FLT3 and can be expanded by FLT3L administration. We report that young pre-leukemic mice with the Flt3ITD knock-in allele manifest an expansion of all DCs including classical (cDCs) and plasmacytoid (pDCs). The expansion originated in DC progenitors, occurred in a cell-intrinsic manner and was further enhanced in Flt3ITD/ITD mice. The mutation caused the downregulation of Flt3 on the surface of DCs and reduced their responsiveness to Flt3L. Flt3ITD mice showed enhanced capacity to support T cell proliferation, including a cell-extrinsic expansion of regulatory T cells (Tregs). Accordingly, these mice restricted alloreactive T cell responses during graft-versus-host reaction, but failed to control autoimmunity in the absence of Tregs. Thus, the FLT3-ITD mutation directly affects DC development, thereby indirectly modulating T cell homeostasis and supporting Treg expansion. This effect of FLT3-ITD may subvert immunosurveillance and promote leukemogenesis in a cell-extrinsic manner. Overall design: Sorted splenic dendritic cell subsets from either Flt3+/+ or Flt3ITD/+ mice were sequenced for mRNA profiling. For each subset per genotype contains 2-3 replicates, all from independent experiments.