Loss of TLR2 Worsens Spontaneous Colitis in MDR1A Deficiency through Commensally Induced Pyroptosis

Abstract

Variants of the multidrug resistance gene (MDR1/ABCB1) have been associated with increased susceptibility to severe ulcerative colitis (UC). In this study, we investigated the role of TLR/IL-1R signaling pathways including the common adaptor MyD88 in the pathogenesis of chronic colonic inflammation in MDR1A deficiency. Double- or triple-null mice lacking TLR2, MD-2, MyD88, and MDR1A were generated in the FVB/N background. Deletion of TLR2 in MDR1A deficiency resulted in fulminant pancolitis with early expansion of CD11b+ myeloid cells and rapid shift toward TH1-dominant immune responses in the lamina propria. Colitis exacerbation in TLR2/MDR1A double-knockout mice required the unaltered commensal microbiota and the LPS coreceptor MD-2. Blockade of IL-1β activity by treatment with IL-1R antagonist (IL-1Ra; Anakinra) inhibited colitis acceleration in TLR2/MDR1A double deficiency; intestinal CD11b+Ly6C+-derived IL-1β production and inflammation entirely depended on MyD88. TLR2/MDR1A double-knockout CD11b+ myeloid cells expressed MD-2/TLR4 and hyperresponded to nonpathogenic Escherichia coli or LPS with reactive oxygen species production and caspase-1 activation, leading to excessive cell death and release of proinflammatory IL-1β, consistent with pyroptosis. Inhibition of reactive oxygen species–mediated lysosome degradation suppressed LPS hyperresponsiveness. Finally, active UC in patients carrying the TLR2-R753Q and MDR1-C3435T polymorphisms was associated with increased nuclear expression of caspase-1 protein and cell death in areas of acute inflammation, compared with active UC patients without these variants. In conclusion, we show that the combined defect of two UC susceptibility genes, MDR1A and TLR2, sets the stage for spontaneous and uncontrolled colitis progression through MD-2 and IL-1R signaling via MyD88, and we identify commensally induced pyroptosis as a potential innate immune effector in severe UC pathogenesis.

TLR2-Dependent Activation of β-Catenin Pathway in Dendritic Cells Induces Regulatory Responses and Attenuates Autoimmune Inflammation

“…Consistent with a regulatory role for the TLR2-signaling pathway, recent studies have shown TLR2-ligands or microbial activation of TLR2 can suppress inflammation and limit autoimmunity in mice in different disease settings (19, 21, 39, 47-49). Further, treatment of mice with zymosan at low doses limits EAE and type I diabetes, and these responses were dependent on TLR2.…”

“…We and others have shown that zymosan and other TLR2-ligand treatment can suppress inflammation and limit autoimmunity in different experimental mouse models such as EAE (21, 39), diabetes (19, 47), airway hyperresponsiveness (AHR) (48) and colitis (49). So, we next determined the importance of zymosan-TLR2-mediated activation of β-catenin pathway in DCs on EAE outcome.…”

“…What evolutionary benefit might accrue to the microbe or to the host, from activating the β-catenin/TCF pathway? Recent studies have shown that sensing of commensals via TLR2 induces regulatory responses and limits inflammatory responses in the gut (49, 65-67) and skin (68). In the face of a persistent immune response against a persistent microbe such as yeasts, virus and commensal bacteria, activating β-catenin pathway could represent a mechanism to dampen excessive inflammation and limit collateral damage to host.…”

Reduction of MDR1, MRP1 and BCRP expression in Crohn’s disease: a role in disease pathogenesis?

“…The decrease in ABC transporter expression could have a direct implication on the disease pathology, as mdr1a -/-mice develop spontaneous intestinal inflammation triggered by the normal bacterial flora that resembles human IBD (13). Moreover, double knock-out mdr1 -/-and tlr2 -/-mice develop extensive colitis due to inefficient control of the nonpathogenic bacteria, confirming a protective function of MDR1 in antibacterial defense (14,15). Studies on genetic polymorphisms of MDR1 in humans also implicate its involvement in the IBD pathogenesis (16).…”

The Ground State of Innate Immune Responsiveness Is Determined at the Interface of Genetic, Epigenetic, and Environmental Influences

“…Other PRRs, including TLR1, TLR3, and TLR4, are not expressed in human intestinal macrophages (44). In contrast, loss of related receptor Tlr2 in mouse intestinal macrophages actually worsens inflammatory outcomes during experimental colitis (45), indicating that a restricted TLR-signaling network is itself important in maintaining healthy intestinal homeostasis. Interestingly, "training" mouse or human hematopoietic progenitor cells by exposing them to a Tlr2 ligand impacts on the responsiveness of subsequently differentiated monocytes and macrophages by reducing, but not ablating, the inflammatory cytokine production (46).…”