The ORF8 protein of SARS-CoV-2 mediates immune evasion through down-regulating MHC-Ι

Abstract

COVID-19, caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), has become a global pandemic and has claimed over 2 million lives worldwide. Although the genetic sequences of SARS-CoV and SARS-CoV-2 have high homology, the clinical and pathological characteristics of COVID-19 differ significantly from those of SARS. How and whether SARS-CoV-2 evades (cellular) immune surveillance requires further elucidation. In this study, we show that SARS-CoV-2 infection leads to major histocompability complex class Ι (MHC-Ι) down-regulation both in vitro and in vivo. The viral protein encoded by open reading frame 8 (ORF8) of SARS-CoV-2, which shares the least homology with SARS-CoV among all viral proteins, directly interacts with MHC-Ι molecules and mediates their down-regulation. In ORF8-expressing cells, MHC-Ι molecules are selectively targeted for lysosomal degradation via autophagy. Thus, SARS-CoV-2–infected cells are much less sensitive to lysis by cytotoxic T lymphocytes. Because ORF8 protein impairs the antigen presentation system, inhibition of ORF8 could be a strategy to improve immune surveillance.

Mutations in SARS-CoV-2 ORF8 Altered the Bonding Network With Interferon Regulatory Factor 3 to Evade Host Immune System

“…Even the double-mutant V62L/L84S did not significantly affect its binding with IRF3. Our results are in line with the previous studies in which L84S substitution did not enhance the IFNß antagonism and the downregulation of MHC-I compared to ORF8 WT (Rashid et al, 2021;Zhang et al, 2021). We have found that the nuclear translocations of IRF3 by either overexpressing ORF8 WT or L84S in HEK293T cells were the same (Rashid et al, 2021).…”

“…So far, these mutations occurred in structural, non-structural, and accessory proteins of SARS-CoV-2 (Nagy et al, 2021). The accessory protein, i.e., the ORF8 protein sequence of SARS-CoV-2, has the least homology with that of SARS-CoV (Zhang et al, 2021). In the current study, we adapted structural and biophysical analysis approaches to explore the impact of various mutations of ORF8, such as S24L, W45L, V62L, and L84S, on its ability to bind IRF3 and to evade the host immune system.…”

“…The ORF8 protein is one of the fast evolving viral proteins in beta coronaviruses (Flower et al, 2021). Some of the functions attributed to ORF8 include inhibition of IFN-1 signaling and downregulation of MHC-I in cells (Li et al, 2020;Rashid et al, 2021;Zhang et al, 2020bZhang et al, , 2021. Moreover, ORF8 can also stimulate the immune system to produce strong humoral and cellular immune responses upon virus infection, which are the biomarkers for SARS-CoV-2 infection (Flower et al, 2021).…”

“…In the WT of SARS-CoV-2 ORF8 protein, the L84 residue is flanked by disulfides Cys83 and Pro85, which are highly conserved among ORF8 orthologs, indicating their indispensable roles for ORF8 (Flower et al, 2021). ORF8 was found to be involved in IFNß antagonism and MHC-I downregulation (Rashid et al, 2021;Zhang et al, 2021). However, the biological function of the residue 84 remains to be elucidated.…”

“…This protein has 366 nucleotides and 121 amino acids (Pereira, 2020). Recently, several studies showed that ORF8 could downregulate the major histocompatibility complex class I (MHC-I; Zhang et al, 2021) and the type I IFN signaling pathway to evade the host immune system by decreasing the nuclear translocation of IRF3 (Li et al, 2020;Rashid et al, 2021;Flower et al, 2021). Several mutations in SARS-CoV-2 ORF8 protein have been identified, i.e., ORF8 L (Leucine) and ORF8 S (Serine) at amino acid 84 (Ceraolo and Giorgi, 2020;Tang et al, 2020); ORF8 V (Valine) and ORF8 L at amino acid 62; and ORF8 S and ORF8 L at amino acid 24 (Laha et al, 2020).…”