CD13 Restricts TLR4 Endocytic Signal Transduction in Inflammation

Abstract

Dysregulation of the innate immune response underlies numerous pathological conditions. The Toll-like Receptor 4 (TLR4) is the prototypical sensor of infection or injury that orchestrates the innate response via sequential activation of both cell-surface and endocytic signaling pathways that trigger distinct downstream consequences. CD14 binds and delivers LPS to TLR4 and has been identified as a positive regulator of TLR4 signal transduction. It is logical that negative regulators of this process also exist to maintain the critical balance required for fighting infection, healing damaged tissue and resolving inflammation. We showed that CD13 negatively modulates receptor-mediated Ag uptake in dendritic cells to control T-cell activation in adaptive immunity. Here we report that myeloid CD13 governs internalization of TLR4 and subsequent innate signaling cascades, activating IRF-3 independently of CD14. CD13 is co-internalized with TLR4, CD14 and dynamin into Rab5+ early endosomes upon LPS treatment. Importantly, in response to TLR4 ligands HMGB1 and LPS, pIRF-3 activation and transcription of its target genes is enhanced in CD13KO DCs while TLR4 surface signaling remains unaffected, resulting in a skewed inflammatory response. This finding is physiologically relevant as ischemic injury in vivo provoked identical TLR4 responses. Finally, CD13KO mice showed significantly enhanced IFNβ-mediated signal transduction via JAK-STAT, escalating iNOS transcription levels and promoting accumulation of oxidative stress mediators and tissue injury. Mechanistically, inflammatory activation of macrophages upregulates CD13 expression and CD13 and TLR4 co-immunoprecipitate. Therefore, CD13 negatively regulates TLR4 signaling, thereby balancing the innate response by maintaining the inflammatory equilibrium critical to innate immune regulation.

A mucin-like peptide from Fasciola hepatica instructs dendritic cells with parasite specific Th1-polarizing activity

“…Since Fhmuc enhanced the pro-inflammatory effect of LPS on DCs, but did not induce DC maturation itself, we sought to evaluate whether Fhmuc could induce an increase in the expression of TLR4, the main receptor responsible for mediating LPS recognition and engagement of pro-inflammatory signaling pathways leading to activation of the NF-κB transcription factor43334. Thus, we treated BMDCs with Fhmuc followed by LPS, or with Fhmuc or LPS alone and evaluated the kinetics of TLR4 expression by flow cytometry.…”

Early innate immune responses to bacterial LPS

“…CD13 is upregulated during LPS stimulation and functions to restrict TRIF signaling, but not myddosome signaling, in DCs and macrophages (Fig. 1A) [27]. CD13-deficient DCs exhibit enhanced TLR4 endocytosis, suggesting its involvement in regulating the endocytic process.…”

“…Exactly how CD13, which binds directly to TLR4, inhibits TLR4 endocytosis remains an open question. The mechanism is not likely to be through inhibition of known TLR4 endocytic determinants such as MD-2 or CD14, as myddosome signaling remains intact in CD13-deficient cells and CD13 does not affect CD14 endocytosis or consistently co-localize with CD14 in endosomes [27]. Importantly, the effects of CD13 on myeloid cells may have consequences in vivo as dysregulation of TRIF signaling results in increased iNOS expression and potentially limits tissue repair during ischemic injury in CD13-deficient mice.…”

“…Importantly, the effects of CD13 on myeloid cells may have consequences in vivo as dysregulation of TRIF signaling results in increased iNOS expression and potentially limits tissue repair during ischemic injury in CD13-deficient mice. However, as CD13 appears to have multiple functions [27], future research will be necessary to dissect its important roles in vivo .…”

Antithrombin: An anticoagulant, anti‐inflammatory and antibacterial serpin

“…The SPR data also revealed that β‐AT binds to both CD13 and CD300f with higher affinity than α‐AT. Both of these receptors are known to be negative regulators of inflammatory signaling on monocytes with CD13 playing a role in internalization of Toll‐like receptor 4 (TLR4) and CD300f being involved in blocking both MyD88 and TRIF‐mediated TLR signaling through activation of Src homology region 2 domain‐containing phosphatase 1 (SHP‐1) . Based on these observations, it has been hypothesized that β‐AT contributes to modulation of innate immune responses during infection.…”