Unfolded protein response is a critical mediator in Campylobacter jejuni pathogenesis and host defence
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Campylobacter jejuni is the major bacterial cause of foodborne gastroenteritis worldwide. How this pathogen interacts with the host defence machinery of human intestinal epithelial cells (IECs) and is involved in pathogenesis remains elusive. Bacterial pathogens utilise strategies to gain access to the eukaryotic cell machinery that can involve subversion of biological processes in host. Unfolded protein response (UPR) is a highly conserved host cell stress response to the accumulation of misfolded proteins in the endoplasmic reticulum (ER) and is a conserved evolutionary response against invading pathogens. Several bacterial pathogens can induce the UPR for their own survival and thus design a dual scenario where UPR can both protect and facilitate pathogen evasion.
Herein, we investigated whether UPR represents a virulence mechanism exploited by C. jejuni during bacterial invasion in human IECs. Our data show that following C. jejuni infection, we observe upregulation of protein kinase R-like ER kinase (PERK), inositol-requiring enzyme 1α (IRE1α) and activating transcription factor 6 (ATF6) pathways of the UPR, albeit differentially in human T84 and Caco-2 cells. Chemical induction of UPR by thapsigargin in host cells reduced intracellular survival of C. jejuni while conversely pretreatment with UPR inhibitors increased intracellular survival of C. jejuni and attenuated IL-8 release. Finally, we show that C. jejuni mutant C. jejuni capsular polysaccharide and flagella contribute to UPR activation in IECs. Collectively, these findings provide mechanistic insights into how C. jejuni infection leads to UPR activation and inflammation, potentially contributing to downstream C. jejuni -mediated damage.
Author Summary
Campylobacter jejuni is one of the most prevalent bacterial causes of foodborne human gastroenteritis worldwide. The common symptoms of campylobacteriosis include diarrhoea, abdominal pain and fever. C. jejuni -induced diarrhoea is associated with disruption of tight junctions and adherens junctions, downregulation of epithelial sodium channels and disruption of bile reabsorption. In addition, C. jejuni -induced diarrhoea in part result from intestinal inflammation caused by increased C. jejuni colonisation and invasion due to dysfunctional intestinal barrier. C. jejuni was reported to cause an inflammatory signalling cascade via ligand recognition by Toll-like receptor 2 (TLR2), TLR4 and nucleotide-binding oligomerisation domain (NOD) in IECs. However, other potential mechanisms of C. jejuni -induced inflammation, with respect to the IEC responses remains unknown. In this study, we demonstrate UPR activation following C. jejuni infection. The UPR is a conserved eukaryotic regulatory response which is initiated to relieve ER stress and recover ER homeostasis, however several bacteria have evolved to exploit UPR pathways for their own survival and replication, creating a dual scenario where UPR can both protect and facilitate pathogen evasion. This study shows that the activation of the PERK pathway of host UPR is activated in the presence of live C. jejuni or its structural components, flagella and a capsular polysaccharide. Furthermore, we propose that although activation of the UPR is a conserved evolutionary response against invading pathogens, UPR induction induces inflammation which potentially contributes to C. jejuni -mediated damage.
