Hepatic injury induced by various toxic agents, including acetaminophen (APAP), has been attributed, in part, to the production of proinflammatory cytokines and other mediators by resident Kupffer cells within the liver. However, recent evidence from our laboratory has demonstrated that hepato-protective factors, such as interleukin (IL)-10 and cyclooxygenase-derived mediators, are also upregulated in response to hepatic damage to help protect against exacerbated injury, and Kupffer cells have been suggested to be a source of these modulatory factors. In other models, Kupffer cells also serve important regulatory functions in pathophysiological states of the liver. Therefore, we reevaluated the role of Kupffer cells in a murine model of APAP-induced liver injury using liposome-entrapped clodronate (liposome/clodronate) as an effective Kupffer cell-depleting agent. We show that in contrast to pretreatment of mice with a widely used macrophage inhibitor, gadolinium chloride, which did not deplete Kupffer cells but moderately protected against APAP-induced hepatotoxicity as reported previously, the intravenous injection of liposome/clodronate caused nearly complete elimination of Kupffer cells and significantly increased susceptibility to APAP-induced liver injury as compared with mice pretreated with empty liposomes. This increased susceptibility was apparently unrelated to the metabolism of APAP since liposome/clodronate pretreatment did not alter APAP-protein adduct levels. Instead, Kupffer cell depletion by liposome/clodronate led to significant decreases in the levels of hepatic mRNA expression of several hepato-regulatory cytokines and mediators, including IL-6, IL-10, IL-18 binding protein and complement 1q, suggesting that Kupffer cells are a significant source for production of these mediators in this model. Our findings indicate that, in addition to their protoxicant activities, Kupffer cells can also have an important protective function in the liver through the production of a variety of modulatory factors which may counteract inflammatory responses and/or stimulate liver regeneration.

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Mechanistic study of idiosyncratic drug-induced hepatitis (DIH) continues to be a challenging problem because of the lack of animal models. The inability to produce this type of hepatotoxicity in animals, and its relative rarity in humans, may be linked to the production of antiinflammatory factors that prevent drug-protein adducts from causing liver injury by immune and nonimmune mechanisms. We tested this hypothesis by using a model of acetaminophen (APAP)-induced liver injury in mice. After APAP treatment, a significant increase was observed in serum levels of interleukin (IL)-4, IL-10, and IL-13, cytokines that regulate inflammatory mediator production and cell-mediated autoimmunity. When IL-10 knockout (KO) mice were treated with APAP, most of these mice died within 24 to 48 hours from liver injury. This increased susceptibility to APAP-induced liver injury appeared to correlate with an elevated expression of liver proinflammatory cytokines, tumor necrosis factor (TNF)-␣, and IL-1, as well as inducible nitric oxide synthase (iNOS). In this regard, mice lacking both IL-10 and iNOS genes were protected from APAP-induced liver injury and lethality when compared with IL-10 KO mice. All strains, including wild-type animals, generated similar amounts of liver APAPprotein adducts, indicating that the increased susceptibility of IL-10 KO mice to APAP hepatotoxicity was not caused by an enhanced formation of APAP-protein adducts. Epidemiologic studies reveal that as many as 25% of fulminant hepatitis cases in intensive care units are caused by DIH. 1 It is also the major reason for removal of new drugs from clinical development and widespread use. 2 Although DIH can be very severe, the incidence for any given drug is usually low. It has been hypothesized that this idiosyncratic nature of DIH is either caused by a metabolic idiosyncrasy, in which patients who develop toxicity metabolize the drug to reactive metabolites to a greater extent than most others, 3 or an immunologic idiosyncrasy (druginduced allergic hepatitis [DIAH]), in which the immune system of patients who develop DIH react against protein adducts of drugs or other neoantigens formed in the liver. 3,4 These 2 mechanisms are not mutually exclusive and might both contribute to the initiation of DIH. 3 However, the demonstration of specific antibodies and sensitized T cells that react with drugs, their metabolites, or specific protein neoantigens and/or autoantigens supports the immunologic basis of DIH. [2][3][4][5][6] The host-dependent, idiosyncratic nature of DIH is poorly understood, but may be related to a variety of factors that in most individuals either fail to promote or, in fact, protect against DIH. For example, genetic polymorphisms in the specific repertoire of major histocompatibility complex class I and II antigens and/or Band T-cell receptors may render the immune cells from most individuals incapable of satisfactorily binding or presenting protein adducts of drugs or their derived peptides and, therefore, unable to lead to immunopatholog...