Thoroughly Remold the Localization and Signaling Pathway of TLR22.

Jianfei Ji,Jianzhong Shao,Jianguo Su,Wenqian Li,Zhen Xu,Youliang Rao,Chunrong Yang,Gailing Yuan,Hao Feng,Zhiwei Liao

doi:10.3389/fimmu.2019.03003

Abstract

TLR22 exists in nearly all the poikilothermic vertebrates and plays a central role in the initiation of innate immunity and activation of adaptive immunity. TLR22 signaling pathway has been characterized in detail in fugu (Takifugu rubripes). Here, we thoroughly remold the localization and signaling pathways of TLR22. We characterized TLR22a and TLR22b in grass carp (Ctenopharyngodon idella), designated as CiTLR22a and CiTLR22b, and explored the ligand(s), adaptor(s), and signaling pathway(s). Results show that both CiTLR22a and CiTLR22b localize to lysosome, acidic compartment. Correspondingly, CiTLR22a and CiTLR22b directly bind and respond to dsRNA analog poly(I:C) at pH 5, but not at pH 7.4, the physiological pH. Moreover, CiTLR22a and CiTLR22b exhibit antagonistic function in signal transmission, wherein CiTLR22a facilitates the protein and phosphorylation levels of IRF7 and enhances the promoter activities of major IFNs and NF-κBs, while CiTLR22b downregulates IRF7 phosphorylation and IRF3 protein level and suppresses the IFN and NF-κB pathways. Further investigations revealed that CiTLR22a restrains grass carp reovirus (GCRV) replication and protects cells from GCRV infection, whereas CiTLR22b plays a negative role in response to GCRV infection. This is the first time to systematically clarify the signaling pathways of two isotype TLR22s; especially, subcellular localization and adaptor are different from previous TLR22 report, which results from technical limitations. The results will serve the antiviral immune mechanisms in poikilothermic vertebrates and evolutionary immunology.

Highlights

TLRs, a major family of pattern recognition receptors, play crucial roles in initiating innate immunity and triggering adaptive immunity by recognizing pathogen-associated molecular patterns (PAMPs) or danger-associated molecular patterns [1, 2]
The primary test showed that both CiTLR22aeGFP and CiTLR22b-enhanced GFP (eGFP) were detected inside the cells, while DrCD58-eGFP was localized to cell surface (Figure 1A), which was consistent with a previous report [35], indicating that eGFP tag did not affect the localization of them as DrCD58-eGFP localized to cell surface
Our results showed that both CiTLR22a and CiTLR22b localize to lysosome, recruit adaptor molecule MyD88, facilitate or inhibit the phosphorylation level of IRF7, and regulate the IFN and NF-κB pathways

Summary

Introduction

TLRs, a major family of pattern recognition receptors, play crucial roles in initiating innate immunity and triggering adaptive immunity by recognizing pathogen-associated molecular patterns (PAMPs) or danger-associated molecular patterns [1, 2]. TLRs are type I transmembrane proteins consisting of an extracellular leucine-rich repeat (LRR) domain, a transmembrane domain, and an intracellular Toll/interleukin-1 receptor (TIR) domain [3, 4]. The ectodomain functions as a PAMP recognition domain, whereas the endodomain engages in the downstream signaling pathways [5,6,7]. TLRs are activated, initiating signaling cascades that lead to NF-κB and IFN transcription. In the MyD88-dependent pathway, MyD88 recruits the members of IL-1 receptor-associated kinase (IRAK) family, in which IRAK and IRAK4 are sequentially phosphorylated to dissociate from the receptor complex, and associate with TRAF6, while IRAK3 ( called IRAK-M) prevents the progress and acts as a negative regulator [10]

Methods

Results

Conclusion