IRAK4 turns IL‐10⁺ phospho‐FOXO⁺ monocytes into pro‐inflammatory cells by suppression of protein kinase B

Abstract

IRAK4, a serine/threonine kinase is a central adaptor protein in TLR signaling. To better understand the clinical significance of IRAK4 deficiency we examined the impact of IRAK4 on bacterial recognition in human monocytes. We show that IRAK4 knockdown modulates monocyte-derived cytokine secretion in response to Staphylococcus aureus and Streptococcus pneumoniae, resulting in decreased IL-12 and elevated IL-10 production, a finding also reproducible with ligands for TLR2 and TLR4. In contrast, silencing of MyD88 leads to a complete loss of cytokine secretion, indicating that IRAK4 acts as a differential regulator of bacteria/TLR-induced cytokine secretion downstream of MyD88. Further analysis revealed that this modulatory function results from IRAK4-mediated suppression of protein kinase B (PKB/Akt). Release of suppression upon IRAK4 silencing (but not MyD88 knockdown) increases phosphorylation of PKB/Akt, counteracts NF-κB activation and finally results in a monocyte phenotype with tolerogenic features, thus unleashing Akt-and mTOR-dependent release of IL-10, along with concomitant phosphorylation of FOXO transcription factors. In line with these observations IRAK4-deficient monocytes failed to induce allogeneic CD8 + and CD4 + T-cell responses, an effect reverted by neutralization of IL-10. Taken together, our data highlight an unexpected role of IRAK4, Akt, and mTOR in the regulation of tolerance in human monocytes.Keywords: IRAK4 r monocytes r PKB/Akt r TLR Additional supporting information may be found in the online version of this article at the publisher's web-site IntroductionMonocytes are among the first to encounter bacterial pathogens in infections of the bloodstream. They account for 10% of the human peripheral blood leukocytes, making monocytes one of the most Correspondence: Dr. Isabelle Bekeredjian-Ding e-mail: bekeredjian-ding@uni-bonn.de abundant antigen-presenting cell subsets in the circulation and a very potent source for cytokines [1,2]. Their ability to produce high levels of cytokines and thereby shape the systemic immune response is thought to be important in determining the outcome of sepsis and balancing pro-inflammatory versus compensatory anti-inflammatory responses [3].Human blood monocytes are a heterogeneous cell population and functionally defined subsets can be distinguished based on their cytokine and receptor expression profiles. Classical monocytes express high levels of CD14, but no CD16 (FcγRIII) C 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.eji-journal.eu Eur. J. Immunol. 2013. 43: 1630-1642 Innate immunity 1631 and account for ∼90% of blood monocytes. The nonclassical subset is characterized by the expression of CD16 and low CD14 levels. While the classical CD14 + subset is characterized by the preferential production of anti-inflammatory IL-10 rather than pro-inflammatory cytokines after TLR stimulation, the nonclassical subset produces high amounts of TNF and low levels of IL-10 in response to TLR ligands, and is, therefore, referred to as proinflammatory [4][5][...

Porphyromonas gingivalis Outer Membrane Vesicles Induce Selective Tumor Necrosis Factor Tolerance in a Toll-Like Receptor 4- and mTOR-Dependent Manner

“…3). Thus, similarly to our previous study, our data identify IL-10 as an important mediator of tolerance (26). It has further been suggested that increased TNF levels in monocytes and macrophages after rapamycin treatment reflect loss of IL-10 (32, 34).…”

“…IL-10 production, which suppresses IL-12 expression and the T cell response to alloantigen (26). This tolerogenic response is mediated in a PI3K/PKB/Akt/mTOR-dependent pathway and counterregulated by IRAK4 (26). Similarly, in the study described in reference 29), the PI3K/Akt pathway reciprocally regulated P. gingivalis LPS-induced IL-10 and IL-12 synthesis, notably, in a TLR2-dependent manner.…”

“…Previous work indicated that TLR2-and TLR4-dependent induction of IL-10 release in monocytes is mediated via phosphatidylinositol 3-kinase (PI3K)/PKB/Akt/ mTOR-dependent signaling associated with a tolerogenic response (26). In view of the strong TLR2 activity in OMV (see Fig.…”

“…Lower panel: **, P ϭ 0.00222; ***, P ϭ 0.00083. IL-10 production, which suppresses IL-12 expression and the T cell response to alloantigen (26). This tolerogenic response is mediated in a PI3K/PKB/Akt/mTOR-dependent pathway and counterregulated by IRAK4 (26).…”

“…4). Notably, abrogated IL-12 production desensitizes T cells to alloantigen and prevents the development of protective Th1 and Th17 responses in the mucosa (26,40).…”