Treponemapallidum Dysregulates Monocytes and Promotes the Expression of IL-1β and Migration in Monocytes Through the mTOR Signaling Pathway.

Wen-Na Liu,Xiao-Han Sun,Yong Lin,Li-Li Liu,Kai-Xuan Wu,Yan-Zhu Xu,Xiao-Yong Jiang,Xin-Lin Hu,Li-Rong Lin,Man-Li Tong

doi:10.3389/fcimb.2020.592864

Abstract

Monocytes are widely involved in the body’s defense response, and abnormally regulated monocyte subsets are closely related to the pathogenesis of various diseases. It is unclear whether Treponema pallidum (Tp) dysregulates monocyte subsets and impacts the functions of monocytes. This study aims to analyze the distribution of monocyte subsets in syphilis patients and the effect of Tp on monocyte functions to explore the pathogenesis of syphilis. Flow cytometry was employed to detect monocyte subsets. With or without pre-treatment with rapamycin, THP-1 cell migration stimulated by Tp was investigated by a Transwell migration assay, and THP-1 cell phagocytosis was studied using fluorescent microspheres. IL-1β and TNF-α expression was quantified by PCR and flow cytometry, while LC3 and mTOR were investigated in Tp-exposed THP-1 cells using western blotting. Tp infection led to an increase in the proportion of CD14++CD16+ monocytes and a decrease in the proportion of CD14++CD16- monocytes. In addition, Tp promoted monocyte (THP-1) CD14 and CD16 expression in vitro, induced the expression of IL-1β and TNF-α in a dose-dependent manner and promoted the migration and autophagy of monocytes. Furthermore, mTOR phosphorylation on monocytes was stimulated by Tp, and the levels peaked at 30 min. Pre-treatment with rapamycin (mTOR inhibitor) attenuated the expression of IL-1β and migration in Tp-exposed THP-1 cells. Tp abnormally regulates monocyte subsets and promotes migration, autophagy, and the expression of IL-1β and TNF-α in THP-1 cells. Meanwhile, the mTOR affected the expression of IL-1β and migration in Tp-exposed THP-1 cells. This study is important as it sheds light on the mechanism by which monocytes interact with Tp during infection.

Highlights

Syphilis caused by Treponema pallidum (Tp) continues to be a prevalent disease, which raises public health concerns
Our study found that syphilis patients demonstrate changes in monocyte subpopulations, of which intermediate monocytes were significantly increased
Tp could increase the expression of CD14 and CD16 in a human monocyte cell line (THP-1) in vitro and promote the transformation of monocytes into intermediate monocytes (CD14++CD16+ monocytes)

Summary

Introduction

Syphilis caused by Treponema pallidum (Tp) continues to be a prevalent disease, which raises public health concerns. Based on the expression patterns of FcgRIII, known as CD16, and the LPS receptor CD14, circulating human monocytes are classified into three subsets: classical monocytes (CD14++CD16− monocytes), non-classical monocytes (CD14+CD16++ monocytes) and intermediate monocytes (CD14++CD16+ monocytes). These different monocyte subpopulations can exhibit some distinct functional roles in a range of homeostatic and pathological conditions. Classical monocytes (CD14++CD16− monocytes) were primed for phagocytosis, innate sensing/immune responses and migration using a novel single-cell PCR gene expression analysis tool (Gren et al, 2015); Brucella infection led to an increase in the proportion of classical monocytes, which showed downregulation of immune responses (Wang Y. et al, 2017). To the best of our knowledge, no detailed analysis of the Abbreviations: FSC, forward scatter; SSC, side scatter; HC, healthy controls; SP, syphilis patients; Tp, Treponema pallidum; HIV, human immunodeficiency virus; HBV, hepatitis B virus; HCV, hepatitis C virus; RPR, rapid plasma regain; TPPA, treponema pallidum gelatin; agglutination; ANOVA, one-way analysis of variance; MOI, multiplicity of infection; Rapa, rapamycin; mTOR, mammalian target of rapamycin

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