SARS-CoV-2 variants of concern remain dependent on IFITM2 for efficient replication in human lung cells

Abstract

It has recently been shown that an early SARS-CoV-2 isolate (NL-02-2020) hijacks interferon-induced transmembrane proteins (IFITMs) for efficient replication in human cells. To date, several "Variants of Concern" (VOCs) showing increased infectivity and resistance to neutralization have emerged and globally replaced the early viral strains. Here, we determined whether the four SARS-CoV-2 VOCs (Alpha, Beta, Gamma and Delta) maintained the dependency on IFITM proteins for efficient replication. We found that depletion of IFITM2 strongly reduces viral RNA production by all four VOCs in the human epithelial lung cancer cell line Calu-3. Silencing of IFITM1 had little effect, while knock-down of IFITM3 resulted in an intermediate phenotype. Strikingly, depletion of IFITM2 generally reduced infectious virus production by more than four orders of magnitude. In addition, an antibody directed against the N-terminus of IFITM2 inhibited SARS-CoV-2 VOC replication in iPSC-derived alveolar epithelial type II cells thought to represent major viral target cells in the lung. In conclusion, endogenously expressed IFITM proteins (especially IFITM2) are critical cofactors for efficient replication of genuine SARS-CoV-2 VOCs, including the currently dominating Delta variant. IMPORTANCERecent results showed that an early SARS-CoV-2 isolate requires endogenously expressed IFITM proteins for efficient infection. However, whether IFITMs are also important cofactors for infection of emerging SARS-CoV-2 VOCs that out-competed the original strains and currently dominate the pandemic remained to be determined. Here, we demonstrate that depletion of endogenous IFITM2 expression almost entirely prevents the production of infectious Alpha, Beta, Gamma and Delta VOC SARS-CoV-2 virions in a human lung cell line. In comparison, silencing of IFITM1 had little impact, while knock-down of IFITM3 had intermediate effects on viral replication. Finally, an antibody targeting the N-terminus of IFITM2 inhibited SARS-CoV-2 VOC replication in iPSC-derived alveolar epithelial type II cells. Our results show that SARS-CoV-2 VOCs including the currently dominant Delta variant are dependent on IFITM2 for efficient replication suggesting that IFITM proteins play a key role in viral transmission and pathogenicity.