Dimethyl sulfoxide (DMSO) is commonly used in biological studies to dissolve drugs and enzyme inhibitors with low solubility. Although DMSO is generally thought of as being relatively inert, it can induce biological effects that are often overlooked. An example that highlights this potential problem is found in a recent report demonstrating a pathogenic role for natural killer T (NKT) and natural killer (NK) cells in acetaminophen-induced liver injury (AILI) in C57Bl/6 mice in which DMSO was used to facilitate acetaminophen (APAP) dissolution. We report that NKT and NK cells do not play a pathologic role in AILI in D rug-induced liver injury (DILI) is a serious, often fatal side effect of drug treatment representing a major impediment to drug development. 1,2 Attempts to identify hepatotoxic drugs early in development have been hindered in part by a poor understanding of the underlying mechanism of DILI. Current evidence suggests that reactive metabolites, drug-protein adducts, and glutathione depletion might be common events involved in the initiation of DILI; however, it is currently not possible to predict hepatotoxicity based on these criteria alone. 3 Therefore, there is a great deal of interest in studying the downstream events of these initiating factors in hope of identifying potential markers of hepatotoxicity and pathways leading to DILI. 4,5 The most extensively used model for uncovering fundamental mechanisms of DILI has been acetaminopheninduced liver injury (AILI) in mice. 4,6 In this model, the preponderance of evidence indicates that N-acetyl-p-benzoquinone imine (NAPQI), the reactive metabolite of acetaminophen (APAP); NAPQI-protein adducts; glutathione depletion; oxidative stress; and mitochondria damage all play a role in AILI. 7 More recently, evidence suggests that the innate immune system can contribute to the severity of AILI through the production of proinflammatory cytokines and other protoxicant factors subsequent to the early metabolic events initiated by NAPQI

Current evidence suggests that drug-induced liver disease can be caused by an allergic response (drug-induced allergic hepatitis, DIAH) induced by hepatic drug-protein adducts. The relatively low incidence of these reactions has led us to hypothesize that tolerogenic mechanisms prevent DIAH from occurring in most people. Here, we present evidence for the existence of one of these regulatory pathways. Following a hepatotoxic dose of acetaminophen in C57Bl/6 mice, lymphocyte loss that appeared to be due at least in part to apoptosis was noted in the spleen, thymus, and draining lymph nodes of the liver. There was no observable lymphocyte loss in the absence of hepatotoxicity. Acetaminophen-induced liver injury (AILI) also led to a functional suppression of the immune system as determined by the inhibition of a delayed-type hypersensitivity response to dinitrochlorobenzene. Further studies with adrenalectomized mice suggested a role for corticosterone in the depletion of lymphocytes following APAP-induced liver injury. In conclusion, these findings suggest that lymphocyte loss and immunosuppression following AILI may prevent subsequent occurrences of allergic hepatitis and possibly other forms of APAP-induced allergies induced by hepatic drug-protein adducts. Similar regulatory pathways may inhibit other hepatotoxic drugs from causing allergic reactions.