Cell surface GRP78 functions as an alternative virus entry receptor on monocytes during SARS-CoV-2 infection

Abstract

It has been widely acknowledged that severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infects host cells via the angiotensin-converting enzyme 2 (ACE2) and transmembrane serine protease 2 (TMPRSS2) entry mechanism. However, ACE2 and TMPRSS2 cannot explain the Toll-like receptor driven response of monocytes since there is no ACE2 expressed on monocytes, suggesting alternative receptor(s) on these cells. Here, we report cell surface glucose-regulated protein 78 (csGRP78) which is abundantly expressed on monocytes to function as an alternative receptor for SARS-CoV-2 internalization. Our results show that csGRP78 is upregulated on the monocyte of COVID-19 patients. Moreover, in vitro cell culture experiments revealed that&nbsp;GRP78 over-expressing THP-1 cells and stimulation of wtTHP-1 cells with the relevant cytokines IL-1β, IL-6, TNF and IFN-γ induces similar csGRP78 and activation marker upregulation on cell surface as found on patients’ monocytes. <i>In vitro</i> spike protein and GRP78 interaction tests (SPR assay, GST-pull down and Co-IP), confirmed direct binding of spike protein and GRP78. Finally, pseudo-typed virus expressing spike protein was used to infect mock or GRP78 over-expressing THP-1 cells. We found that pseudo-typed virus entered GRP78 over-expressing THP-1 cells but not control THP-1 cells. Our results demonstrate that csGRP78 acts as a potential&nbsp;functional receptor for SARS-CoV-2 spike protein and mediates ACE2 independent SARS-CoV-2 entry into monocytes. These findings provide insight into role of monocytes in the pathophysiology of COVID-19, and suggest a new therapeutic target candidate for anti-SARS-CoV-2 treatment.