Activation of the innate immune system and alcoholic liver disease: Effects of ethanol per se or enhanced intestinal translocation of bacterial toxins induced by ethanol?

It is generally accepted that activation of the innate immune system and increased release of proinflammatory cytokines and other mediators plays an important role in the development of alcoholic liver disease (ALD). The mechanisms involved in the ethanol-induced activation of monocytes/macrophages (including Kupffer cells) are however, still a matter of debate. The brief review will summarize the published data from the literature on the two main pathomechanisms discussed until now: I) Gut-derived bacterial toxins, specially endotoxin; and II) metabolic changes induced by alcohol oxidation (independent of mechanism I). For pathomechanism I, clear evidence has been published from numerous groups: Alcohol induces mucosal injury in the upper gastrointestinal tract and leads to marked increase in the permeability of the gut mucosa to macromolecules such as endotoxin. The resulting endotoxemia then leads to activation of Kupffer cells and other macrophages. The increased release of pro-inflammatory mediators (e.g., TNF-alpha, II-1, reacting oxygen species) and infiltration of other inflammatory cells (e.g., neutrophils) finally causes liver damage. Regarding the second pathomechanism it has repeatedly been argued that the metabolic alterations which are induced by chronic administration of ethanol to rats or mice might increase the sensitivity of monocytes/macrophages to secrete TNF-alpha and other pro-inflammatory mediators thereby increasing the susceptibility to ethanol-induced liver injury. However, in all feeding experiments the effect of ethanol on intestinal permeability and enhanced translocation of bacterial toxins (endotoxin) is likely to occur (or at least cannot be excluded). The latter holds true also for experiments using isolated macrophages/Kupffer cells from ethanol fed animals. Therefore, to clarify whether or not alterations related to ethanol metabolism ("direct" effects of ethanol) contribute to the activation of the innate immune system studies using germ-free animals are needed to exclude the "indirect" effect of ethanol via gut-derived bacterial toxins.