effect of the acid sphingomyelinase/ceramidase system on bacterial induced colitisThe

Abstract

Sphingolipids are complex structures, which are shown to be involved in the maintenance of the intestinal integrity. Increasing evidence implicates a function of sphingolipids in intestinal diseases such as inflammatory bowel disease (IBD). Interestingly, the activation of various phospholipases and sphingomyelinases were shown to be altered through inflammatory cytokine secretion. However, little is known about the role of sphingolipids, such as ceramide, and sphingomyelin in the context of bacterial induced inflammation. In the present study, we determined the function of the acid sphingomyelinase (Asm) and acid ceramidase (Ac) during the bacterial induced colitis using the Citrobacter (C.) rodentium infection model. C. rodentium is a natural mouse gram-negative mucosal pathogen that induces colonic inflammation. Importantly, sphingomyelin and ceramide concentrations were significantly decreased in the colon of C. rodentium infected mice. However, treatment with amitriptyline to inhibit Asm and Ac activity during infection as well as infection of Asm KO or Ac cKO mice led to an increase in sphingomyelin and ceramide in the colon within 6 to 10 days post infection. This increase was accompanied by an enhanced histopathological score. Interestingly, Asm KO, as well as Ac cKO mice showed enhanced bacterial translocation of the normally non-invasive C. rodentium into the liver and the spleen compared to infected wildtype mice. Flow cytometry analysis revealed that neither the frequencies of macrophages, as part of the first line of defence from the innate immune system, nor the MHCII expression by macrophages were impaired in wildtype compared to amitriptyline treated mice. Furthermore, uptake capability of macrophages generated from bone marrow of ASM KO or Ac cKO, as well as killing activity was not dysregulated compared to BMDMs isolated from wildtype littermates. Intriguingly, increased cell infiltration of adaptive immune cells, such as Th1 and Th17, into lamina propria (LP) was detected during C. rodentium infection in mice lacking Asm and Ac activity due to amitriptyline treatment compared to wildtype littermates. In contrast, frequencies of Tregs were reduced in colonic tissue at the peak of infection in amitriptyline treated mice compared to wildtype littermates. However, the differentiation capacity of Th1 and Th17 cells in vitro was not altered after loss of Asm. \nIn summary, loss of Asm and Ac leads to a dysregulated immune response during C. rodentium infection. Reduced frequencies of colonic Tregs in Asm/Ac inhibited mice allowed the uncontrolled expansion of Th1 and Th17 cells, thereby inducing severe pathology in the colon. \nWe showed for the first time that Asm and Ac possess a protective function during bacterial induced colitis. Intriguingly, Asm and Ac shape the adaptive immune response to effectively fight invading C. rodentium and protect the mice against severe intestinal pathology.