Different Spatial and Temporal Roles of Monocytes and Monocyte-Derived Cells in the Pathogenesis of an Imiquimod Induced Lupus Model.

Mounting evidence indicates the importance of aberrant Toll-like receptor 7 (TLR7) signaling in the pathogenesis of systemic lupus erythematosus (SLE). However, the mechanism of disease progression remains unclear. An imiquimod (IMQ)-induced lupus model was used to analyze the lupus mechanism related to the aberrant TLR7 signals. C57BL/6 mice and NZB/NZW mice were treated with topical IMQ, and peripheral blood, draining lymph nodes, and kidneys were analyzed focusing on monocytes and monocyte-related cells. Monocytes expressed intermediate to high levels of TLR7, and the long-term application of IMQ increased Ly6C monocytes in the peripheral blood and Ly6C monocyte-like cells in the lymph nodes and kidneys, whereas Ly6C monocyte-like cell numbers were increased in lymph nodes. Ly6C monocyte-like cells in the kidneys of IMQ-induced lupus mice were supplied by bone marrow-derived cells as demonstrated using a bone marrow chimera. Ly6C monocytes obtained from IMQ-induced lupus mice had upregulated adhesion molecule-related genes, and after adoptive transfer, they showed greater infiltration into the kidneys compared with controls. RNA-seq and PCR analyses revealed Ly6C monocyte-like cells in the kidneys of IMQ-induced lupus mice had upregulated macrophage-related genes compared with peripheral blood Ly6C monocytes and downregulated genes compared with kidney macrophages (MF). Ly6C monocyte-like cells in the kidneys upregulated and chemoattracting genes including and . The higher expression of in Ly6C monocyte-like cells compared with MF suggested these cells were more inflammatory than MF. However, MF in IMQ-induced lupus mice were characterized by their high expression of . Genes of proinflammatory cytokines in Ly6C and Ly6C monocytes were upregulated by stimulation with IMQ but only Ly6C monocytes upregulated IFN-α genes upon stimulation with 2'3'-cyclic-GMP-AMP, an agonist of stimulator of interferon genes. Ly6C and Ly6C monocytes in IMQ-induced lupus mice had different features. Ly6C monocytes responded in the lymph nodes of locally stimulated sites and had a higher expression of IFN-α upon stimulation, whereas Ly6C monocytes were induced slowly and tended to infiltrate into the kidneys. Infiltrated monocytes in the kidneys likely followed a trajectory through inflammatory monocyte-like cells to MF, which were then involved in the development of nephritis.