Cutting edge: Type I IFN drives emergency myelopoiesis and peripheral myeloid expansion during chronic TLR7 signaling.

Mice overexpressing TLR7 (TLR7.1 mice) are a model of systemic lupus erythematosus pathogenesis and exhibit peripheral myeloid expansion. We show that TLR7.1 mice have a dramatic expansion of splenic cells that derive from granulocyte/macrophage progenitors (GMP) compared with wild-type mice. In the bone marrow, TLR7.1 mice exhibited hallmarks of emergency myelopoiesis and contained a discrete population of Sca-1(+) GMP, termed emergency GMP, which are more proliferative and superior myeloid precursors than classical Sca-1(-) GMP. The emergency myelopoiesis and peripheral myeloid expansion in TLR7.1 mice was dependent on type I IFN signaling. TLR7 agonist administration to nontransgenic mice also drove type I IFN-dependent emergency myelopoiesis. TLR7.1 plasmacytoid dendritic cells were cell-intrinsically activated by TLR7 overexpression and constitutively produced type I IFN mRNA. This study shows that type I IFN can act upon myeloid progenitors to promote the development of emergency GMP, which leads to an expansion of their progeny in the periphery.