Rôle des cellules endothéliales dans l’immunité innée précoce induite lors d’infections par des coronavirus murins

Endothelial cells (EC) act as a physical barrier against invasion by pleiotropic blood borne viruses but their contribution in innate antiviral defense is poorly known. Dysfunctions in blood-brain barrier EC (BMECs) and liver sinusoidal EC (LSECs) have been reported in viral neuropathologies and hepatitis, suggesting that loss of ECs integrity may contribute to the pathogenesis. Mouse hepatitis coronaviruses (MHV), differing in their ability to induce severe to subclinical hepatitis and neurological diseases and / or their tropism for ECs, are relevant viral models to study the consequences of EC infection in viral pathogenesis. Following hematogenous infection, the MHV3 serotype, the most virulent MHV, induces fulminant hepatitis, characterized by severe inflammatory response, followed by neurological damage whereas the less virulent MHV-A59 serotype induces milder hepatitis but does not invade the central nervous system (CNS). In addition, MHV3, in contrast to MHV-A59, shows ability to induce TLR2-dependent cytokine response. The attenuated MHV3 variants, 51.6-MHV3 and YAC-MHV3, are characterized by a weak tropism for LSECs and induce moderated and subclinical hepatitis respectively. Given the importance of LSECs in hepatic tolerance and the elimination of circulating pathogens, it has been postulated that the severity of hepatitis and inflammatory response induced by MHVs correlates with infection and alterations in vascular and tolerogenic properties of LSECs. Hepatic inflammatory disorders may result from differential activation of TLR2, rather than other TLRs and helicases, according to serotypes. Moreover, given the role of BMECs in preventing CNS infections, it has been postulated that secondary cerebral invasion by coronaviruses is related to infection of BMECs and subsequent breakdown of the blood-brain barrier (BBB). Through in vitro and in vivo infections of isolated BMECs, LSECs or mice with the different MHVs, we demonstrated, first, that in vitro productive infection of LSECs by the highly virulent MHV3 serotype, in contrast to 51.6- et YAC-MHV3 variants, altered their production of vasoactive factors and overthrew their intrinsic tolerogenic properties by promoting inflammatory cytokines and chemokines production. These disturbances were reflected in vivo by an uncontrolled inflammatory response and a deregulation of intrahepatic leukocyte recruitment, favoring viral replication and liver damages. We demonstrated, using TLR2 KO mice and LSECs treated with siRNA for TLR2 that the abnormal inflammatory response induced by MHV3 depended in part on preferential induction and activation of TLR2 by the virus on the surface of hepatic cells. Moreover, the severity of the primary viral replication in the liver and disorders in intrahepatic leucocyte recruitment induced by MHV3, but not by MHV-A59 and 51.6-MHV3, correlated with a subsequent brain invasion at the BBB level. Such invasion was related to productive infection of BMECs and subsequent IFN--dependent disruption of tight junction proteins occludin, VE-cadherin and ZO-1, resulting in an increase of BBB permeability and further viral entry into the CNS.  Overall, the results of this study highlight the importance of structural and functional integrity of LSECs and BMECs during acute viral infections by MHVs to limit liver damages associated with viral-induced exacerbation of inflammatory response and prevent brain invasion by MHVs following viral spread through the bloodstream. They also reveal a new worsening role for TLR2 in the evolution of acute viral hepatitis paving the way for new therapies targeting TLR2-induced inflammatory activity.

Keyphrases: Coronavirus, Inflammation, TLR2, Cytokines, Innate immune response, Mouse hepatitis viruses (MHV), Chemokines, Acute hepatitis, Liver sinusoidal endothelial cells, Brain microvascular endothelial cells, Interferon-beta