Sickness behaviors in chronic cholestasis: An immune-mediated process?
Authors: Alexander I. Aspinall, David H. Adams
Journal: Hepatology (2005)

Sickness behaviors are frequently seen in patients with chronic liver disease and include symptoms of anorexia, listlessness, malaise, poor concentration, and fatigue. Such behaviors probably evolved to allow sick animals to conserve energy and reduce the chance of spreading infection within a herd. In primary biliary cirrhosis (PBC) fatigue is a common and often debilitating symptom which affects 50%-85% of patients and in some cases may be an indication for liver transplantation.1, 2 Clinicians are as vexed by the difficulty in treating fatigue as they are by the lack of understanding of the pathophysiology underlying sickness behavior. The study by Kerfoot and colleagues in this issue of HEPATOLOGY3 provides a novel mechanism to link cholestasis, inflammation and sickness behavior and is a potentially important advance in understanding this poorly characterized aspect of chronic cholestasis. PBC, primary biliary cirrhosis; CNS, central nervous system; BDR, bile duct resected; SLE, systemic lupus erythematosus; MS, multiple sclerosis; BBB, blood–brain barrier. Several hypotheses have been proposed to explain the pathophysiology of sickness behaviors and the most compelling of these point to a central nervous system (CNS) etiology. For example, stimulation of the hepatic vagal nerve and other peritoneum-derived/brainstem-terminating vagal afferents can reproduce sickness behaviors4, 5 and abnormalities in the hypothalamic-pituitary-adrenal axis have been identified in the bile duct resected (BDR) rat, a model in which cholestasis is associated with sickness behaviours.6 A recent clinical study showed CNS accumulation of the neurotoxic heavy metal manganese to correlate with symptoms of fatigue in patients with early stages of PBC.7 The fact that fatigue does not correlate with the degree of cholestasis or the stage of disease in PBC2 has lead to the hypothesis that sickness behavior is an immune-mediated extra-hepatic manifestation of PBC. Other autoimmune diseases are associated with fatigue and sickness behavior including rheumatoid arthritis and systemic lupus erythematosus (SLE)8 and perhaps most notably multiple sclerosis (MS).9 Fatigue is the most common symptom in MS and a major factor in the reduced quality of life associated with the disease. Similarly to PBC there is no association between MS-related fatigue and the level of disability or clinical disease subtype. In SLE severity of fatigue does not correlate with inflammatory markers8 but this does not exclude immune activation as a cause because sickness behavior can be induced in animals with very small doses of cytokines. For example significant fatigue can be induced in bile duct ligated rats by doses of IL-1β that have no effect on control animals.10 Furthermore, a recent study has shown that vaccination of volunteers with a salmonella typhi vaccine causes modest increases in IL-6 levels in blood which correlate directly with vaccination induced mood changes.11 Cirrhosis and cholestasis result in increased circulating levels of several other substances and cytokines that have been associated with sickness behavior, including TNFα.12 Intuitively one might hypothesize that these cytokines gain access to the CNS through regions of the brain lacking a blood–brain barrier (BBB), such as the circumventricular organs,13 or perhaps in a region where the BBB has been made porous through an uncharacterized humoral factor associated with cholestasis. However, previous work has clearly shown that the permeability of the BBB is decreased rather than increased in chronic cholestasis.14 Thus, if the theory is correct there must be another mechanism to explain the immune effects on the brain. Kerfoot and colleagues noted that several of the circulating factors in cirrhosis and cholestasis, particularly TNFα, can activate endothelium15 and this led them to the novel hypothesis that increased leukocyte recruitment into the brain, as a consequence of activation of brain endothelium, might be the critical factor in delivering high local levels of cytokines in sickness behaviour.12 They tested their hypothesis using the BDR cholestatic model and were able to show that cholestasis was associated with an increase in circulating, activated monocytes (as defined by increased TNFα expression) and a selective increase in monocyte recruitment into the brain. Furthermore, recruitment was associated with increased TNFα expression in both the transmigrated monocytes and the local resident macrophages (microglial cells) suggesting that the recruitment of circulating monocytes establishes a local network in which enhanced TNFα expression is sustained in the brain. They investigated the mechanism of monocyte recruitment and showed that cerebral endothelium displays increased VCAM-1 expression which together with P-selectin promotes monocyte recruitment through cerebral vessels. Analysis of the cell infiltrate showed increased monocyte numbers in BDR mice brains supporting the hypothesis of increased recruitment over the activated endothelium. In future studies it will be important to characterize the nature and location of the infiltrate not only in the brains of BDR animals but also in brains of patients dying with chronic cholestasis. The fact that monocytes in the present study were found in similar locations to the distribution seen in inflammatory brain disease (i.e., in perivascular cuffs and infiltrating the parenchyma itself) supports the likelihood that they are able to mediate significant pathological effects.16 While this research is a significant step forward, there are unresolved issues to our complete understanding of the sickness behaviors of chronic cholestasis. The BDR model results in acute cholestasis, and it is unknown how much can be extrapolated from this acute animal model to chronic cholestasis in humans. Furthermore, previous work in the BDR model has shown decreased neutrophil adhesion and migration in mesenteric vessels and in the present study, lymphocyte levels in BDR brains were not increased. So how does cholestasis selectively activate the monocyte arm of the immune system? Is the cerebral endothelial activation seen in this study a generalized phenomenon of blood vessels supplying the brain, or are some sites more affected than others? The total count of monocytes migrating into brain was increased, but while there was increased rolling in the cerebral vessels studied by intravital microscopy, adhesion was not increased implying that monocytes are gaining access to the CNS at other sites. This might be consistent with the earlier work implicating the hypothalamic–pituitary–adrenal axis and the brainstem. The fact that leukocyte rolling was maintained in BDR TNFα and TLR4 knockout mice is perplexing and as the authors suggest warrants further investigation. It is likely that several inflammatory mediators are involved in the endothelial activation and increased monocyte recruitment and this may result in redundancy of TNFα. Alternatively, TNFα may be a marker of monocyte activation but not the important mediator. It would have been interesting to complement the knockout studies with experiments in which TNFα was inhibited in WT animals to see if this produced an effect. Interestingly recent studies in an experimental autoimmune encephalitis model of sickness behaviour using pharmacological treatments demonstrated critical roles for PGE2 synthesis and IL-1 whereas pharmacological inhibition of TNFα and gene-targeted deletion of type 1 TNF receptors had no effect.17 A key message from these knockout studies is that the CNS events mediating sickness behaviors are likely to be multifactorial. Identifying the mediators involved is important not only to complete our understanding of the pathogenesis of sickness behaviors but also to inform the development of appropriate therapeutic targets. The effects on endothelium reported in this study have parallels with other studies showing that negative mood states are associated with dysregulated cardiovascular function and an increased risk of atherosclerosis and coronary artery disease.18-20 How increased CNS levels of TNFα cause fatigue is also an unresolved issue. The DNA binding protein HMGB-1 has been proposed as a critical factor linking the effects of endotoxin and TNFα in the brain and intracerebral injection of HMGB-1 results in local TNFα expression and sickness behaviour.21 Previous work from Swain's group22 and others23 (reviewed24) implicates an impairment in serotonergic neurotransmission in causing cholestasis-associated fatigue. However the tantalizing link between monocyte migration into the brain and decreased serotonergic neural tone remains unclear although it is tempting to speculate that such a link exists driven by the local release of inflammatory mediators. As the authors argue, there seem to be at least three pathways of communication between the cholestatic liver and the CNS (Fig. 1). If and how these pathways converge is, as yet, unknown. Pathways from the cholestatic liver to the CNS that mediate sickness behaviors. Previous work has demonstrated how brainstem-terminating vagal afferents (Signal 1) and IL-1β (Signal 2) cause sickness behaviors. The study by Kerfoot and colleagues3 demonstrates a third pathway (Signal 3). In an acute setting of cholestasis, monocytes were shown to be recruited to the brain via cerebral vessels in a P-selectin and α4 integrin process. Once recruited into the CNS the monocytes secrete TNFα which together with increased cytokine secretion by resident microglial cells might trigger sickness behavior. Could it be that these monocytes and microglial cells initiate an immune response that is subsequently propagated by other immune cells as the fatigue of chronic cholestasis? The steps between TNFα elaboration and sickness behaviors remain to be elucidated, but might involve a serotonin-dependent pathway. The fact that monocytes in the present study were found in perivascular cuffs and within the parenchyma, a similar distribution to that seen in inflammatory brain disease16 is intriguing. The brains response to injury includes activation of microglia (microgliosis) and astrocytes (astrocytosis) to promote defense against invading pathogens and repair of damage but if this process is prolonged profound changes in neuroglia and neuronal damage can result.16 Thus a persistent infiltration of the parenchyma might be expected to promote permanent damage and neurodegeneration. It is usually assumed that the neuropsychological complications of cholestasis are reversible but if a similar mechanism applies in chronic cholestasis in humans it is possible that long-term sequelae could develop as a consequence of the persistent inflammation. While our understanding of the pathophysiology of sickness behaviors in chronic cholestasis in substantially advanced by the identification of this new pathway of communication between diseased liver and the CNS, the clinician is no closer to providing new or durable therapy for the PBC patient afflicted by debilitating fatigue. It is likely that the risks of anti-TNFα therapy (i.e., infliximab or similar agents)25 would outweigh any potential benefit in cholestasis-associated fatigue, particularly given that anti-TNFα therapy can worsen inflammatory brain disease and act as a trigger for multiple sclerosis.26 Thalidomide has been used as a TNFα inhibitor and its ability to decrease circulating TNFα levels and inhibit chemotaxis and monocyte phagocytosis would make it an attractive candidate. However this drug was originally developed as a sedative and causes significant somnolence in up to 38% of patients.27 Therapeutic targeting of leukocytes through α4 integrins is also fraught with life-threatening CNS complications including progressive multifocal leukoencephalopathy.28 It is possible that the beneficial response to steroids sometimes seen in symptomatic patients with PBC may be due to anti-inflammatory effects in the brain although the use of corticosteroids in patients with cholestasis is associated with an increased risk of osteoporosis. Targeting the downstream events may be more attractive at the moment but the use of the 5-HT3 antagonist ondansetron to modulate serotonergic neurotransmission yielded disappointing results in a recent randomized controlled trial.29 Although the observation that monocytes are recruited to the brain in cholestasis is an important novel observation it is a midpoint of the pathophysiology. We currently lack understanding of both the upstream triggers that lead to endothelial activation, monocyte recruitment and TNFα production as well as the downstream cellular and molecular events that link monocyte recruitment with aberrant behavior. It is to be hoped that a better understanding of these processes may lead to the development of more effective and rational therapies for this disabling symptom of cholestatic liver disease.