ACE2 is necessary for SARS-CoV-2 infection and sensing by macrophages but not sufficient for productive viral replication (preprint)

Macrophages are a major source of pro-inflammatory cytokines in COVID-19. How macrophages sense the causative virus, SARS-CoV-2, to drive cytokine release is, however, unclear. Here, we show that human macrophages do not directly sense and respond to infectious SARS-CoV-2 virions because they lack sufficient ACE2 expression to support virus entry and replication. Over-expression of ACE2 in human macrophages permits SARS-CoV-2 entry and early-stage replication and facilitates macrophage pro-inflammatory and anti-viral responses. ACE2 over-expression does not, however, permit the release of newly synthesised virions from SARS-CoV-2-infected macrophages, consistent with abortive replication. Release of new, infectious SARS-CoV-2 virions from ACE2 over-expressing macrophages only occurred if anti-viral mediator induction was also blocked, indicating that macrophages restrict SARS-CoV-2 infection at two stages of the viral life cycle. These findings resolve the current controversy over macrophage-SARS-CoV-2 interactions and identify a signalling circuit that directly links macrophage recognition of SARS-CoV-2 to restriction of viral replication.

Endothelial cells elicit a pro-inflammatory response to SARS-COV-2 without productive viral infection (preprint)

Thrombotic and microvascular complications are frequently seen in deceased COVID-19 patients, suggesting that vascular pathology is a major driver of severe disease. However, whether this is caused by direct viral infection of the endothelium or inflammation-induced endothelial activation remains highly contentious. What role the endothelium plays in viral amplification and inflammation thus remains a key unresolved question in the pathogenesis of SARS-CoV-2. Here, we use patient autopsy samples, primary human endothelial cells and an in vitro model of the pulmonary epithelial-endothelial cell barrier to show that primary human endothelial cells express the SARS-CoV-2 receptor ACE2 and the protease TMPRSS2, albeit at low levels. Accordingly, when present in a sufficiently high concentration, SARS-CoV-2 can enter primary human endothelial cells from either the apical or basolateral surface. Whilst inducing an inflammatory response, this is not a productive infection. We further demonstrate that in a co-culture model of the pulmonary epithelial-endothelial barrier, endothelial cells are not infected with SARS-CoV-2. They do however, sense and respond to an infection in the adjacent epithelial cells, resulting in the induction of a pro-inflammatory response. Taken together, these data suggest that in vivo, endothelial cells are unlikely to be infected with SARSCoV-2 and that infection is only likely to occur if the adjacent pulmonary epithelium is denuded (basolateral infection) or a high viral load is present in the blood (apical infection). In such a scenario, whilst SARS-CoV-2 infection of the endothelium can occur, it does not contribute to viral amplification. However, endothelial cells are still likely to play a key role in SARS-CoV-2 pathogenesis by sensing and mounting a pro-inflammatory response to SARS-CoV-2.