The P681H mutation in the Spike glycoprotein escapes IFITM restriction and is necessary for type I interferon resistance in the SARS-CoV-2 alpha variant

Abstract

The appearance of new dominant variants of concern (VOCs) of severe acute respiratory syndrome coronavirus type 2 (SARS-CoV-2) threatens the global response to the COVID-19 pandemic. Of these, the alpha variant (also known as B.1.1.7) that appeared initially in the UK became the dominant variant in much of Europe and North America in the first half of 2021. The Spike (S) glycoprotein of alpha acquired seven mutations and two deletions compared to the ancestral virus, including the P681H mutation in the polybasic cleavage site that has been suggested to enhance S cleavage. Here, we show that the alpha S protein confers a level of resistance to the effects of interferon-{beta} (IFN{beta}) in human lung epithelial cells. This correlates with resistance to an entry restriction mediated by interferon-induced transmembrane protein 2 (IFITM2) and a pronounced infection enhancement by IFITM3. Furthermore, the P681H mutation is essential for resistance to IFN{beta} and context-dependent resistance to IFITMs in the alpha S. However, while this appears to confer changes in sensitivity to endosomal protease inhibition consistent with enhanced cell-surface entry, its reversion does not reduce cleaved S incorporation into particles, indicating a role downstream of furin cleavage. Overall, we suggest that, in addition to adaptive immune escape, mutations associated with VOCs may well also confer replication and/or transmission advantage through adaptation to resist innate immune mechanisms. IMPORTANCEThe emergence of Variants of Concern of SARS-CoV-2 has been a key challenge in the global response to the COVID-19 pandemic. Accumulating evidence suggests VOCs are being selected to evade the human immune response, with much interest focussed on mutations in the Spike protein that escape from neutralizing antibody responses. However, resistance to the innate immune response is essential for efficient viral replication and transmission. Here we show that the alpha (B.1.1.7) VOC of SARS-CoV-2 is substantially more resistant to type-1 interferons than the parental Wuhan-like virus. This correlates with resistance to the antiviral protein IFITM2, and enhancement by its paralogue IFITM3, that block virus entry into target cells. The key determinant of this is a proline to histidine change at position 681 in S adjacent to the furin-cleavage site that we have shown previously modulates IFITM2 sensitivity. Unlike other VOCs, in the context of the alpha spike, P681H modulates cell entry pathways of SARS-CoV-2, further reducing its dependence one endosomal proteases. Reversion of position 681 to a proline in viruses bearing the alpha spike is sufficient to restore interferon and IFITM2 sensitivity without reducing furin-mediated spike cleavage, suggesting post cleavage conformational changes in S are changing the viral entry pathway and therefore sensitivity to interferon. These data highlight the dynamic nature of the SARS CoV-2 S as it adapts to both innate and adaptive immunity in the human population.