Depletion of Myeloid Cells Exacerbates Hepatitis and Induces an Aberrant Increase in Histone H3 in Mouse Serum

Tissue-resident macrophages and bone marrow (BM)-derived monocytes play a crucial role in the maintenance of tissue homeostasis; however, their contribution to recovery from acute tissue injury is not fully understood. To address this issue, we generated an acute murine liver injury model using hepatocyte-specific Cflar-deficient (Cflar(Hep-low)) mice. Cellular FLICE-inhibitory protein expression was down-regulated in Cflar-deficient hepatocytes, which thereby increased susceptibility of hepatocytes to death receptor-induced apoptosis. Cflar(Hep-low) mice developed acute hepatitis and recovered with clearance of apoptotic hepatocytes at 24 hours after injection of low doses of tumor necrosis factor a (TNF alpha), which could not induce hepatitis in wild-type (WT) mice. Depletion of Kupffer cells (KCs) by clodronate liposomes did not impair clearance of dying hepatocytes or exacerbate hepatitis in Cflar(Hep-low) mice. To elucidate the roles of BM-derived monocytes and neutrophils in clearance of apoptotic hepatocytes, we examined the effect of depletion of these cells on TNF alpha-induced hepatitis in Cflar(Hep-low) mice. We reconstituted Cflar(Hep-low) mice with BM cells from transgenic mice in which human diphtheria toxin receptor (DTR) was expressed under control of the lysozyme M (LysM) promoter. TNF alpha-induced infiltration of myeloid cells, including monocytes and neutrophils, was completely ablated in LysM-DTR BM-reconstituted Cflar(Hep-low) mice pretreated with diphtheria toxin, whereas KCs remained present in the livers. Under these experimental conditions, LysM-DTR BM-reconstituted Cflar(Hep-low) mice rapidly developed severe hepatitis and succumbed within several hours of TNF alpha injection. We found that serum interleukin-6 (IL-6), TNF alpha, and histone H3 were aberrantly increased in LysM-DTR BM-reconstituted, but not in WT BM-reconstituted, Cflar(Hep-low) mice following TNF alpha injection. Conclusion: These findings indicate an unexpected role of myeloid cells in decreasing serum IL-6, TNF alpha, and histone H3 levels via the suppression of TNF alpha-induced hepatocyte apoptosis.