Protective and Aggravating Effects of Nlrp3 Inflammasome Activation in IBD Models: Influence of Genetic and Environmental Factors

Abstract

<b><i>Background:</i></b> Inflammatory bowel disease (IBD) is characterized by chronic intestinal inflammation due to dysregulation of the mucosal immune system. The cytokines IL-1β and IL-18 appear early in intestinal inflammation and their pro-forms are processed via the caspase-1-activating multiprotein complex, the Nlrp3 inflammasome. Previously, we reported that the uptake of dextran sodium sulfate (DSS) by macrophages activates the Nlrp3 inflammasome and that Nlrp3<sup>–/–</sup> mice are protected in the acute DSS colitis model. Of note, other groups have reported opposing effects in regards to DSS susceptibility in Nlrp3<sup>–/–</sup> mice. Recently, mice lacking inflammasomes were found to develop a distinct intestinal microflora. <b><i>Methods:</i></b> To reconcile the contradicting observations, we investigated the role of Nlrp3 deficiency in two different IBD models: acute DSS colitis and TNBS (2,4,6-trinitrobenzene sulfonic acid)-induced colitis. In addition, we investigated the impact of the intestinal flora on disease severity by performing cohousing experiments of wild-type and Nlrp3<sup>–/–</sup> mice, as well as by antibiotic treatment. <b><i>Results:</i></b> Nlrp3<sup>–/–</sup> mice treated with either DSS or TNBS exhibited attenuated colitis and lower mortality. This protective effect correlated with an increased frequency of CD103+ lamina propria dendritic cells expressing a tolerogenic phenotype in Nlrp3<sup>–/–</sup> mice in steady state conditions. Interestingly, after cohousing, Nlrp3<sup>–/–</sup> mice were as susceptible as wild-type mice, indicating that transmission of endogenous bacterial flora between the two mouse strains might increase susceptibility of Nlrp3<sup>–/–</sup> mice towards DSS-induced colitis. Accordingly, treatment with antibiotics almost completely prevented colitis in the DSS model. <b><i>Conclusions:</i></b> The composition of the intestinal microflora significantly influences disease severity in IBD models comparing wild-type and Nlrp3<sup>–/–</sup> mice. This observation may – at least in part – explain contradictory results concerning the role of the inflammasome in different labs. Further studies are required to define the role of the Nlrp3 inflammasome in noninflamed mucosa under steady state conditions and in IBD.

Loss-of-function CARD8 mutation causes NLRP3 inflammasome activation and Crohn’s disease

“…Studies of murine models of colitis addressing this question, usually of dextran sulfate sodium-induced colitis (DSS-colitis), are inconclusive, since this form of colitis phenocopies human IBD rather poorly, particularly with regard to its dependence on loss of epithelial cell integrity. In any case, in utilizing these models, it has usually been shown that loss-of-function abnormalities of the NLRP3 inflammasome, the main source of IL-1β, are associated with more rather than less intestinal inflammation (20)(21)(22)(23)(24). One possible explanation of this somewhat paradoxical result is that the NLRP3 inflammasome also generates protective factors, such as antiinflammatory cytokines and IL-18, the latter a cytokine that helps maintain epithelial cell integrity and whose absence therefore leads to increased colitis, especially in the DSS-colitis model ly to anti-TNF-α and/or immunosuppressive treatment, but did respond to anakinra or anti-IL-1β antibody administration.…”

Nlrp3-dependent IL-1β inhibits CD103+ dendritic cell differentiation in the gut

“…Considering the peculiarities of biochemical inhibitor studies, including off-target effects versus genetic approaches with altered microbiome, most disease models currently available are susceptible to microbial diversity. In line with this, we have previously reported that cohousing led to comparable susceptibility to colitis-inducing agents in Nlrp3 -/-and WT mice, and that treatment with antibiotics significantly reduced disease severity (30). Intestinal DC comprise multifaceted cell populations with antigen presenting activity, effector T cell stimulation, and induction of Treg differentiation, depending on the local stimulus (31)(32)(33)(34) …”

“…Recently, we have reported that the frequency of CD103 + DC in the LP was higher in Nlrp3 -/-mice compared with WT mice at steady state (30). In the present study, we investigated the molecular basis for this observation and elucidated how Nlrp3 links innate to adaptive immune responses in intestinal DC using a T cell transfer colitis model.…”

“…We recently reported an increased frequency of CD103 + DC in the LP of Nlrp3 -/-mice. As CD103 + DC are considered to exert tolerogenic functions, we hypothesized that this finding might explain reduced colitis severity in Nlrp3 -/-mice, as observed in DSS-and TNBS-induced colitis models (19,30). To elucidate the relevance of our finding, we analyzed immune cells from LP, mesenteric lymph node, and spleen of WT and Nlrp3 -/-mice in steady state.…”

“…Clinical and histological scoring. Scoring of disease activity was done as previously described (19,30). Briefly, combined diarrhea and hematochezia (clinical) scoring was done in a blinded fashion.…”