Destruction of the hepatocyte junction by intercellular invasion of Leptospira causes jaundice in a hamster model of Weil's disease

Leptospirosis is a zoonotic disease of global importance caused by pathogenic Leptospira. Weil's disease is the most severe form of human leptospirosis, which is characterized by jaundice, haemorrhage and renal failure (Levett 2001; Bharti et al. 2003). More than 50,000 cases of severe leptospirosis are reported each year with a case fatality rate higher than 10% (WHO 1999). Although jaundice has been one of the well-known symptoms of leptospirosis since its discovery, its mechanism still remains unknown. Jaundice refers to the yellow discoloration of the sclerae, mucous membranes and skin and results from either enhanced bilirubin production or impaired bilirubin handling (uptake, conjugation or biliary excretion) by the liver (Roche & Kobos 2004). As jaundice is caused by dysfunctions at any steps of bilirubin metabolism, in some cases, it is difficult to elucidate its main mechanism. Several kinds of bacterial infection are known to cause jaundice through a variety of mechanisms (Trauner et al. 1998; Chand & Sanyal 2007). Leptospira are spiral-shaped, extremely thin bacteria. They can bore through a gel-like medium in a cork-screw-like manner (Goldstein & Charon 1988). They are able to penetrate abraded skin and mucous membrane and migrate in host tissue (Bharti et al. 2003). Several functional and histopathological studies on leptospirosis-related jaundice have been made on human patients (Ramos-Morales et al. 1959; Arean 1962a; Kobayashi 2001; De Brito et al. 2006) and experimental animals (Arean 1962b; Miller & Wilson 1966; Higgin & Cousineau 1977; Van den Ingh & Hartman 1986; Nally et al. 2004). In human autopsies and animal models of acute Leptospira infection, hepatic cord disarray and dilation of intercellular space are commonly reported (Arean 1962a; Miller & Wilson 1966; De Brito et al. 2006). Degenerative lesions, inflammation and necrosis are minimal (Nally et al. 2004; De Brito et al. 2006). Serum biochemical studies on human patients have demonstrated that concentrations of aspartate transaminase (AST) and alanine transaminase (ALT) are moderately increased, accompanied by minor increase in alkaline phosphatase (ALP) concentration (Ramos-Morales et al. 1959; Kobayashi 2001). From these findings, most researchers have reported that impairment of bile excretion, intrahepatic cholestasis, is related to jaundice (Ramos-Morales et al. 1959; Higgin & Cousineau 1977; Kobayashi 2001; Bharti et al. 2003), but the mechanism is still unclear. Moreover, there are few reports that correlated histopathological findings of liver with serum biochemical studies and localization of the organisms in leptospirosis or that examine the time course of histopathological changes and its association with leptospiral localization. In this study, we examined the association between serum biochemical, histopathological changes and leptospiral localization during the development of jaundice in a hamster model of Weil's disease. Hamsters inoculated with virulent Leptospira strains are used as animal models because their clinical manifestations and histopathological changes are similar to those of human leptospirosis (Haake 2006; Silva et al. 2008). For scanning electron microscope (SEM) observation, freeze-cracking and cross-cutting methods were used to obtain three-dimensional images of the liver. To investigate direct interactions between hepatocytes and the organism, hepatocyte couplets isolated from hamsters were co-cultured with either pathogenic or non-pathogenic leptospires.