Role of the TIM and TAM receptors during dengue virus infection – TIMTAMDEN

Dengue disease, caused by four dengue virus (DV) serotypes, has emerged as the most important mosquito-borne viral disease of humans and is now a major global health concern. The molecular bases of DV-host cell interactions leading to virus entry are poorly understood, hampering the discovery of new targets for antiviral intervention. DV infect a wide range of cell types and no cellular receptor directly binding the viral envelope glycoprotein has yet been identified, except for DC-SIGN in dendritic cells. We recently discovered that the TIM and TAM proteins, two receptor families that mediate the phosphatidylserine (PtdSer)-dependent phagocytic removal of apoptotic cells, are DV entry factors. TIM and TAM receptors mediate DV infection by interacting with PtdSer on the surface of DV particles through a mechanism that mimics the recognition and engulfment of apoptotic cells by phagocytes.
The general objective of the TIMTAMDEN project is to establish a detailed understanding of the molecular interactions that occur between DV and TIM and TAM receptors during infection. We will mainly focus our study on two members of the TIM and TAM families, respectively TIM-1 and AXL, which massively enhance DV infection and are expressed on DV natural target cells.

Our specific aims are:

1) To decipher molecular and cellular mechanisms of TIM and AXL-mediated enhancement of DV internalization: The mechanisms by which TIM and AXL receptors mediate DV entry are currently unknown. We will investigate whether TIM-1 and AXL directly promote DV endocytosis or recruit host receptor(s) that ultimately trigger virus uptake. Because TIM-1 and AXL-mediated DV entry is dependent on PtDSer, we will investigate how this lipid is incorporated at the viral membrane and how it becomes accessible for TIM-1 and AXL interactions.

2) To gain insight into the role of AXL-mediated signaling in DV infection: Our preliminary data indicate that AXL ligation by virus during entry initiates signal transduction events that inhibit interferon response. We propose that, rather than acting merely as virus docking sites, AXL receptor might be directly activated by ligand-coated virus particles to shut down the host immune responses and promote infection. We will define the role of AXL receptor engagement in the modulation of antiviral and immunomodulatory genes in primary cells.

3) To characterize the role of Mer during DV entry: TAM receptor family includes three members, TYRO3, AXL and Mer. We showed that TYRO3 and AXL mediate DV infection. The role of Mer, the dominant TAM receptor for the phagocytosis of apoptotic cells, is not known. We will perform a series of gain- and loss-of-function studies to further characterize its role during DV infection in primary cells.
4) To explore the role of TIM-1 and AXL during DV transmission: Our preliminary data show that TIM-1 and AXL receptors are highly expressed in human skin, the anatomical site of the mosquito bite. We hypothesize that exploitation of TIM-1 and AXL by DV could play a critical role in infection of skin cells and represent important host determinant for viral dissemination. We will investigate whether TIM-1 and AXL usage impacts DV infection of skin cells by using a skin explant model that closely reproduces virus transmission.
The TIMTAMDEN project, which was selected by ANR in the complementary list in 2013, is based on collaboration between three partners, INSERM, Institut Pasteur and the Institut de la Recherche et du Dévelopment (IRD) that have complementary expertise on dengue and innate immunity research and have long-standing fruitful scientific exchanges. We expect that our findings will provide major insights into DV pathogenesis and will uncover important host factors that may be targeted prophylactically or therapeutically for efficient control of DV.