The membrane-bound bile acid receptor (TGR5) controls the immune response against Listeria monocytogenes

Introduction: TGR5 is a G-protein-coupled bile acid receptor found in many cell types of the liver including Kupffer cells. Activation of TGR5 in macrophages or Kupffer cells by bile acids inhibits production of proinflammatory cytokines, chemokines and attenuates the development of atherosclerosis (Kawamata et al., 2003; Keitel et al., 2008; Wang et al., 2011; Pols et al., 2011). Aim of the present study was to determine the anti-inflammatory properties of TGR5 in an animal model for pathogen defense.

Methods: Male, 8 – 12 week old TGR5 knockout (KO) and wildtype (WT) mice were injected intravenously (i.v.) with 8 × 104 CFU/ml Listeria monocytogenes (L.m.) and observed for 7 days. At different time points (1 – 4 d) after infection with 2.5 × 104 CFU/ml L.m. bacterial titers were determined in spleen and liver. Serum levels of AST and ALT were analyzed using Spotchem-biochemical analyzer. Cytokine levels were analyzed in serum using Luminex cytometric bead assay. Flow cytometry has been used to examine immune cells in non-lymphoid tissues. mRNA expression in liver and spleen was quantified by realtime PCR in relation to an endogenous control (HPRT1 or SDHA). Serum bile acids were analyzed by UHPLC-MS/MS.

Results: Following L. monocytogenes (L.m.) infection TGR5 KO displayed a 50% mortality rate within a week of inoculation, while 90% of the TGR5 WT animals survived the infection. In addition, three days after infection higher L.m. titers were detected in liver and spleen of TGR5 KO as compared to WT mice. Moreover, the levels of AST and ALT were significant higher in serum from TGR5 KO animals as compared to the WT littermates. Flow cytometry staining revealed an increase in inflammatory cells in livers from TGR5 KO mice. Furthermore, TGR5 KO mice had increased cytokine levels after L.m. infection as compared to WT littermates. TGR5 mRNA expression was significantly up-regulated in WT mice at day 3 and 4 after L.m. infection. Serum bile acid composition was more hydrophobic and thus more cytotoxic h in TGR5 KO following L.m. infection as compared to their WT littermates.

Conclusion: TGR5 KO mice were more susceptible to L.m. infection as demonstrated by increased mortality rates and elevated serum transaminases. Furthermore, TGR5 KO mice showed, raised Listeria titers, increased liver inflammation, elevated serum inflammatory cytokines and a more hydrophobic serum bile acid composition after L.m. infection. The TGR5 mRNA expression was significantly up-regulated in WT mice following L.m. infection, indicating that TGR5 may confer protective effects in the wildtype animals.