WNK1 mediates M-CSF-induced macropinocytosis to enforce macrophage lineage fidelity

Tissue-resident macrophages (TRM) are critical for mammalian organismal development and homeostasis. Here we report that with-no-lysine 1 (WNK1) controls myeloid progenitor fate, with Csf1r^iCre-mediated Wnk1 deletion in mice (WNK1-deficient mice) resulting in loss of TRMs and causing perinatal mortality. Mechanistically, absence of WNK1 or inhibition of WNK kinase activity disrupts macrophage colony-stimulating factor (M-CSF)-stimulated macropinocytosis, thereby blocking mouse and human progenitor and monocyte differentiation into macrophages and skewing progenitor differentiation into neutrophils. Treatment with PMA rescues macropinocytosis but not macrophage differentiation of WNK-inhibited progenitors, implicating that M-CSF-stimulated, macropinocytosis-induced activation of WNK1 is required for macrophage differentiation. Finally, M-CSF-stimulated macropinocytosis triggers WNK1 nuclear translocation and concomitant increased protein expression of interferon regulatory factor (IRF)8, whereas inhibition of macropinocytosis or WNK kinase activity suppresses IRF8 expression. Our results thus suggest that WNK1 and downstream IRF8-regulated genes are important for M-CSF/macropinocytosis-mediated regulation of myeloid cell lineage commitment during TRM development and homeostasis.