Induction of innate immune memory via microRNA targeting of chromatin remodelling factors.

Prolonged exposure to microbial products such as lipopolysaccharide can induce a form of innate immune memory that blunts subsequent responses to unrelated pathogens, known as lipopolysaccharide tolerance. Sepsis is a dysregulated systemic immune response to disseminated infection that has a high mortality rate. In some patients, sepsis results in a period of immunosuppression (known as 'immunoparalysis')1 characterized by reduced inflammatory cytokine output2, increased secondary infection3 and an increased risk of organ failure and mortality4. Lipopolysaccharide tolerance recapitulates several key features of sepsis-associated immunosuppression5. Although various epigenetic changes have previously been observed in tolerized macrophages6-8, the molecular basis of tolerance, immunoparalysis and other forms of innate immune memory has remained unclear. Here we perform a screen for tolerance-associated microRNAs and identify miR-221 and miR-222 as regulators of the functional reprogramming of macrophages during lipopolysaccharide tolerization. Prolonged stimulation with lipopolysaccharide in mice leads to increased expression of miR-221 and mir-222, both of which regulate brahma-related gene 1 (Brg1, also known as Smarca4). This increased expression causes the transcriptional silencing of a subset of inflammatory genes that depend on chromatin remodelling mediated by SWI/SNF (switch/sucrose non-fermentable) and STAT (signal transducer and activator of transcription), which in turn promotes tolerance. In patients with sepsis, increased expression of miR-221 and miR-222 correlates with immunoparalysis and increased organ damage. Our results show that specific microRNAs can regulate macrophage tolerization and may serve as biomarkers of immunoparalysis and poor prognosis in patients with sepsis.

The Alternative NF-κB Pathway in Regulatory T Cell Homeostasis and Suppressive Function.

CD4+Foxp3+ regulatory T cells (Tregs) are essential regulators of immune responses. Perturbation of Treg homeostasis or function can lead to uncontrolled inflammation and autoimmunity. Therefore, understanding the molecular mechanisms involved in Treg biology remains an active area of investigation. It has been shown previously that the NF-κB family of transcription factors, in particular, the canonical pathway subunits, c-Rel and p65, are crucial for the development, maintenance, and function of Tregs. However, the role of the alternative NF-κB pathway components, p100 and RelB, in Treg biology remains unclear. In this article, we show that conditional deletion of the p100 gene, nfkb2, in Tregs, resulted in massive inflammation because of impaired suppressive function of nfkb2-deficient Tregs. Surprisingly, mice lacking RelB in Tregs did not exhibit the same phenotype. Instead, deletion of both relb and nfkb2 rescued the inflammatory phenotype, demonstrating an essential role for p100 as an inhibitor of RelB in Tregs. Our data therefore illustrate a new role for the alternative NF-κB signaling pathway in Tregs that has implications for the understanding of molecular pathways driving tolerance and immunity.

An Essential Role for ECSIT in Mitochondrial Complex I Assembly and Mitophagy in Macrophages.

ECSIT is a mitochondrial complex I (CI)-associated protein that has been shown to regulate the production of mitochondrial reactive oxygen species (mROS) following engagement of Toll-like receptors (TLRs). We have generated an Ecsit conditional knockout (CKO) mouse strain to study the in vivo role of ECSIT. ECSIT deletion results in profound alteration of macrophage metabolism, leading to a striking shift to reliance on glycolysis, complete disruption of CI activity, and loss of the CI holoenzyme and multiple subassemblies. An increase in constitutive mROS production in ECSIT-deleted macrophages prevents further TLR-induced mROS production. Surprisingly, ECSIT-deleted cells accumulate damaged mitochondria because of defective mitophagy. ECSIT associates with the mitophagy regulator PINK1 and exhibits Parkin-dependent ubiquitination. However, upon ECSIT deletion, we observed increased mitochondrial Parkin without the expected increase in mitophagy. Taken together, these results demonstrate a key role of ECSIT in CI function, mROS production, and mitophagy-dependent mitochondrial quality control.

Molecular mechanisms of innate memory and tolerance to LPS.

LPS is a potent trigger of macrophage-mediated inflammation. However, prolonged exposure to LPS induces a state of tolerance that reprograms the inflammatory response, resulting in reduced inflammatory cytokine production in vitro and in vivo. Recent evidence suggests that LPS tolerance also increases the expression of a subset of genes that may protect animals from systemic infection while they are in the tolerized state. However, a molecular basis for these selective changes in inflammatory gene expression during LPS tolerance has remained elusive. In this review, we discuss the molecular mechanisms that may account for these effects, focusing on changes in LPS signaling, epigenetic markers, and chromatin remodeling that may be responsible for cellular memory and physiologic changes that comprise the LPS tolerance phenomenon.

PDK1 Is a Regulator of Epidermal Differentiation that Activates and Organizes Asymmetric Cell Division.

Asymmetric cell division (ACD) in a perpendicular orientation promotes cell differentiation and organizes the stratified epithelium. However, the upstream cues regulating ACD have not been identified. Here, we report that phosphoinositide-dependent kinase 1 (PDK1) plays a critical role in establishing ACD in the epithelium. Production of phosphatidyl inositol triphosphate (PIP3) is localized to the apical side of basal cells. Asymmetric recruitment of atypical protein kinase C (aPKC) and partitioning defective (PAR) 3 is impaired in PDK1 conditional knockout (CKO) epidermis. PDK1(CKO) keratinocytes do not undergo calcium-induced activation of aPKC or IGF1-induced activation of AKT and fail to differentiate. PDK1(CKO) epidermis shows decreased expression of Notch, a downstream effector of ACD, and restoration of Notch rescues defective expression of differentiation-induced Notch targets in vitro. We therefore propose that PDK1 signaling regulates the basal-to-suprabasal switch in developing epidermis by acting as both an activator and organizer of ACD and the Notch-dependent differentiation program.

Tolerization of inflammatory gene expression.

Lipopolysaccharide (LPS) is a potent inducer of inflammation. However, in a phenomenon known as LPS tolerance, prolonged exposure to LPS reprograms the host response to subsequent LPS reexposure. Proinflammatory cytokine production is suppressed, while production of antimicrobial genes is increased. In vivo models suggest that LPS tolerance dramatically reduces susceptibility to septic shock, while keeping the capacity for clearance of certain pathogens intact. These observations imply that artificial induction of tolerance may be an attractive means to intervene in the progression of sepsis and other inflammatory diseases. However, the underlying mechanisms that govern the tolerogenic response remain poorly understood, hindering efforts to evaluate LPS tolerance induction as a therapeutic approach. Recent advances indicate that chromatin modifications and microRNA mediators may be particularly important in the tolerogenic response. In this review, we discuss possible mechanisms to account for the phenomenon of LPS tolerance, with particular emphasis on the role of newly identified mediators.