Csf1r or Mer inhibition delays liver regeneration via suppression of Kupffer cells

INTRODUCTION: Murine Kupffer cells (KCs) comprise CD11b(hi) and F4/80(hi) subsets. Tissue-resident macrophages are known to express the tyrosine kinase receptors colony-stimulating factor 1 receptor (Csf1r) and Mer. However, the expression of Csf1r and Mer on KC subsets and the importance of these tyrosine kinases during liver regeneration (LR) are unknown. METHODS: KCs from wild-type and Csf1r-GFP mice were characterized by flow cytometry. Partial hepatectomy (PH) was performed in mice treated with clodronate liposomes, a Csf1r small molecule inhibitor or depleting antibody, or a small molecule Mer inhibitor. Sera and livers were analyzed. The function of sorted KC subsets was tested in vitro. RESULTS: Mer was specifically expressed on tissue-resident F4/80(hi) KCs, 55% of which also expressed Csf1r. Mer(+)Csf1r(+) and Mer(+)Csf1r(-) KCs had distinct expression of macrophage markers. Csf1r inhibition in mice reduced F4/80(hi) KCs by approximately 50%, but did not affect CD11b(hi) KCs. Clodronate liposomes depleted F4/80(hi) KCs, but also altered levels of other intrahepatic leukocytes. Csf1r inhibition delayed LR, as demonstrated by a 20% reduction in liver-to-body weight ratios 7 days after PH. At 36h after PH, Csf1r inhibition increased serum ALT and histological liver injury, and decreased liver cell proliferation. A small molecule inhibitor of Mer did not alter the percentage of KCs or their proliferation and just modestly delayed LR. In vitro, Csf1r or Mer inhibition did not decrease KC viability, but did attenuate their cytokine response to stimulation. CONCLUSIONS: F4/80(hi) KCs are Mer(+) and can be subdivided based on Csf1r expression. Csf1r or Mer inhibition each reduces KC cytokine production and delays LR.