Thursday 21 April 2016
Campylobacter jejuni & inflammation grilling the pathogen
Promotor: Prof. dr. J.P.M. van Putten
Defence: 21 April 2016, 14.30 h
Campylobacter jejuni, a Gram- negative bacteria, is the most common cause of bacterial foodborne disease. The bacterium displays commensal behavior in chicken and other animals, but for unknown reason causes intestinal infections almost exclusively in humans. C. jejuni causes a mild to serious enterocolitis characterized by acute inflammation, abdominal pain, fever and (bloody) diarrhea. The infection is usually self-limiting but in a small percentage of cases secondary complications can arise that can manifest as the paralyzing auto-immune based Guillain-Barré syndrome. Research on the pathogenesis of C. jejuni has been hampered by its highly variable in vitro behavior and the lack of a suitable animal model of infection. However, it is generally accepted that C. jejuni lacks the classical virulence factors present in many other enteropathogens, whereas the clinical pathology points to invasive bacterial behavior and the induction of potent pro-inflammatory factors during human infection.
Our goal was to was to better understand Campylobacter pathogenesis by focusing on the molecular events that drive the direct interaction of the pathogen with epithelial cells and macrophages, and its interaction with the innate immune system. Previously our lab discovered that C. jejuni displays highly invasive behavior under conditions of nutrient limitation and that C. jejuni is capable to elicit a powerful innate immune response. The noted invasive phenotype of C. jejuni involves a process in which the pathogen first forces its way underneath epithelial cells (referred to as subvasion) before efficiently invading the cultured cells from the basal side. The attention for the innate immune system followed our previous work showing that C. jejuni interacts with several Toll-like receptors (TLRs) and the recent discoveries of the inflammasome as innate defense mechanism for intracellular bacteria.
During our studies we discovered that C. jejuni invades epithelial cells via a novel mechanism independent of the known actin filament or microtubules-mediated cellular uptake processes. After this highly efficient invasion process C. jejuni enters the endolysosomal pathway were the majority of the bacteria die within 48 hours. Next, we discovered a novel species-specific pathogen-associated molecular patterns (PAMPs) secreted by C. jejuni that induces a potent inflammatory immune response in epithelial cells. Activation was independent of TLRs and Nod-like receptors (NLRs) and could be of great importance in C. jejuni pathogenesis. The discovery of a C. jejuni-specific innate immune agonist fits with the emerging concept of the existence of bacterial species-specific immune stimuli that contribute to the innate immune response. Finally, we demonstrated for the first time that C. jejuni activates the NLRP3 inflammasome in macrophages. Inflammasomes are large multiprotein complexes in the cytosol of eukaryotic cells that respond to (non-)microbial or endogenous danger signals. This activation proceeded via a potassium efflux-dependent process and required ongoing bacterial protein synthesis. Activation of the inflammasome resulted in the secretion of the potent inflammatory cytokine IL-1β and thus may provide a molecular basis for the presence of this cytokine in patient sera during natural C. jejuni infection.
Overall our results support the scenario that C. jejuni is capable to breach the human epithelial barrier, to invade epithelial cells and that the host senses the pathogen through different innate immune response mechanism, which generate the potent pro-inflammatory environment typically present during natural symptomatic infection.