The innate immune system has been implicated in the pathogenesis of alcoholic liver disease. Although innate immunity is usually considered an early response to injury, previous work implicating innate immunity in ethanol-induced liver injury focuses primarily on long-term ethanol exposure. We investigated the early period of ethanol exposure to determine whether there were temporal associations between activation of innate immune responses and known correlates of liver injury. Female C57BL/6 mice were allowed free access to an ethanol-containing Lieber-DeCarli diet or were pair-fed a control diet. Within 4 days of ethanol exposure, we observed a striking spike in expression of hepatic proinflammatory cytokines-including tumor necrosis factor alpha (TNF-alpha), interleukin-6, and interferon-gamma-prior to hepatic triglyceride accumulation or increased plasma alanine aminotransferase activities, as well as before the induction of cytochrome P450 2E1 or oxidative stress. This early spike in inflammatory cytokines coincided with deposition of C3b-iC3b/C3c (C3b) in the liver. This deposition, resulting from the cleavage of the third component of the complement system (C3), is evidence for activation of complement in response to ethanol. C3(-/-) mice were protected from the early, ethanol-induced increase in hepatic TNF-alpha expression. Ethanol increased C3b deposition in mice deficient in C3a receptor or C5a receptor, as well as in wild-type mice depleted of hepatic macrophages; however, there was no increase in hepatic TNF-alpha in the absence of C3a receptor, C5a receptor, or hepatic macrophages. In contrast, the absence of Toll-like receptor 4 (TLR-4) had no effect on the early, ethanol-induced increase in either C3b or TNF-alpha.
Conclusion: We have identified a complement- and macrophage-dependent, but TLR-4 independent, phase in the pathogenesis of ethanol-induced liver injury.