Abstract
Background: Con A administration leads to T cell-mediated hepatitis in mice, the mechanism of which involves the cytokines interferon (IFN)-γ and tumour necrosis factor (TNF)-α. Other nonparenchymal liver cells (NPLC) are also believed to be involved in Con A-induced hepatitis. Both T cells and the NPLC express TLR4. Although lipopolysaccharide (LPS)/TLR4 signalling pathway is involved in several forms of liver injury, its role in T cell-mediated hepatitis has not been elucidated. In the current study the impact of removing the LPS/TLR4 signalling pathway in Con A-induced liver injury was investigated.
Methods: The ability of Con A to elicit liver injury was examined in three groups of mice. Group 1: C3H/HeN mice (functional TLR4) and C3H/HeJ mice (defective TLR4); group 2: mice with no TLR4 (TLR4-/- mice) and their wild-type counterparts (C57BL/6); and group 3: C57BL/6 mice with TLR4 blocked with antibody. All groups of mice were injected with Con A. Plasma ALT was measured to assess liver injury. Proinflammatory cytokine levels were measured using Bioplex suspension array. TLR4 expression on T cells and Kupffer cells from C57BL/6 mice was examined using FACS analysis. The requirement for gut-derived LPS was investigated by pre-treatment of C57BL/6 mice with antibiotics prior to Con A administration and assessing liver injury. The effect of Con A on gut permeability was assessed by oral administration of FITC and measurement of FITC in the systemic circulation. The effect of Con A on the microcirculation of C57BL/6 mice was measured by Laser Doppler Flowmetry (LDF) and by intravital fluorescence microscopy (IVFM).
Results: Wild-type mice developed severe injury following Con A administration, indicated by elevated plasma ALT and confirmed by histology. However, no ALT increase was seen in mice lacking a functional LPS/TLR4 pathway. Wild-type mice pre-treated with antibiotics to eliminate gut-derived LPS did not display increased ALT or histological signs of liver injury in response to Con A administration. Plasma proinflammatory cytokines, TNF-α and IFN-γ, were significantly elevated rapidly following Con A administration in wild-type mice (C3H/HeN and C57BL/6) but not in mice without a functional TLR4 pathway. FACS analysis showed an increase in Kupffer cells expressing TLR4 (F4/80+, TLR4+ cells) and an increase in T cells expressing TLR4 (CD3+, TLR4+ cells) following Con A administration. Con A administration was followed by an increase in gut permeability. Con A administration also induced a fall in lobular perfusion and red blood cell velocity, which was partially attenuated by antibody blockade of TLR4. Furthermore, Con A administration induced an increase in leukocyte recruitment, which was partially attenuated by antibody blockade of TLR4.
Discussion: Our results show that Con A causes liver injury and cytokine production in mice with functional TLR4. In the absence of a functional TLR4 pathway, cytokine production is attenuated and injury prevented. This thesis has demonstrated that Con A-induced injury is dependent on gut-derived LPS and is accompanied by an increase in gut permeability. We have also shown increased TLR4 expression by T cell and Kupffer cells following Con A administration. Moreover, Con A has been shown to have profound effects on the microcirculation, which are at least partially dependent on TLR4. Thus we have demonstrated that LPS/TLR4 signalling plays an essential role in Con A hepatitis, probably mediated by cytokines produced by both T cells and Kupffer cells.