Dual-Faced Implication of CD4 <sup>+</sup>Foxp3 <sup>+</sup> Regulatory T Cells Expanded by Acute Dengue Infection via TLR2/MyD88 Pathway

Abstract

Background: Dengue infection causes dengue fever (DF) and fatal dengue hemorrhagic fever/dengue shock syndrome (DHF/DSS). CD4+Foxp3+ Tregs are expanded in human patients during dengue infection, and appear to be closely associated with clinical severity. However, the molecular pathways involved in Treg proliferation and the reason for their insufficient control of severe clinical disease in dengue infection are poorly understood. Methods: We investigated Treg expansion and their role in DF and DHF/DSS with elaborated immunological analysis using several genetically engineered mouse strains. Results: Here we found that dengue infection induced the proliferation of fully functional CD4+Foxp3+ Tregs via TLR2/MyD88 pathway. Also, surface TLR2 on Tregs was responsible for their proliferation, and dengue-expanded Tregs subverted in vivo differentiation of effector CD8+ T cells. A more interesting finding was that dengue-infected hosts displayed changed levels of susceptibility to other diseases in a TLR2-dependent manner. This change included enhanced susceptibility to tumors and bacterial infection, but highly enhanced resistance to viral infection. Further, transfer of dengue-proliferated Tregs protected the recipients from DF and DHF/DSS caused by dengue infection and LPS-induced sepsis. In contrast, hosts that developed sepsis were more susceptible to dengue infection, an effect attributed to early TLR2-dependent production of proinflammatory cytokines. Interpretation: These facts may explain the reason why in some patients, dengue-proliferated Tregs is insufficient to control DF and DHF/DSS. Our observations lead to new insights into Treg responses activated by dengue infection in a TLR2-dependent manner, which could differentially act on subsequent exposure to other disease-producing situations. Funding: National Research Foundation of Korea (2018R1A2B2001873, 2019R1A6A1A03033084). Declaration of Interest: The authors declare no competing financial interests. Ethical Approval: All animal experiments described in the present study were conducted at Chonbuk National University according to the guidelines set by the Institutional Animal Care and Use Committee (IACUC) of Chonbuk National University, and were pre-approved by the Ethics Committee for Animal Experiments of Chonbuk National University (approval number: 2013-0028). The animal research protocol used in this study followed the guidelines set by the nationally recognized Korea Association for Laboratory Animal Sciences (KALAS). All experimental protocols requiring biosafety were approved by the Institutional. Biosafety Committee (IBC) of Chonbuk National University.