SARS-CoV-2 ORF8: A Rapidly Evolving Immune and Viral Modulator in COVID-19

Abstract

The COVID-19 pandemic has resulted in upwards of 6.8 million deaths over the past three years, and the frequent emergence of variants continues to strain global health. Although vaccines have greatly helped mitigate disease severity, SARS-CoV-2 is likely to remain endemic, making it critical to understand its viral mechanisms contributing to pathogenesis and discover new antiviral therapeutics. To efficiently infect, this virus uses a diverse set of strategies to evade host immunity, accounting for its high pathogenicity and rapid spread throughout the COVID-19 pandemic. Behind some of these critical host evasion strategies is the accessory protein Open Reading Frame 8 (ORF8), which has gained recognition in SARS-CoV-2 pathogenesis due to its hypervariability, secretory property, and unique structure. This review discusses the current knowledge on SARS-CoV-2 ORF8 and proposes actualized functional models describing its pivotal roles in both viral replication and immune evasion. A better understanding of ORF8's interactions with host and viral factors is expected to reveal essential pathogenic strategies utilized by SARS-CoV-2 and inspire the development of novel therapeutics to improve COVID-19 disease outcomes.

Keywords: COVID-19; ORF8; SARS-CoV-2; accessory protein; immune evasion; viral mimicry.

Figure 1

Structure of SARS-CoV-2 ORF8 gene and encoded ORF8 protein. (A) ORF8 location within the SARS-CoV-2 genome (NCBI reference NC_045512.2, Gene ID 43740577), with nucleotide positions indicated. Created in BioRender.com, accessed on 9 March 2023 (B) ORF8 RNA secondary structure adapted from Pereira, F. [20], generated in ViennaRNA Web Services. The coding region of ORF8 is indicated. Also shown is the Δ382 deletion. (C) ORF8 amino acid sequence. The N-terminal signal peptide, dimerization residue C20, 73YIDI76 motif, and glycosylation residue N78 are highlighted in green, yellow, cyan, and blue, respectively. β-sheets are represented by blue arrows. Intramolecular disulfide bridges are shown in maroon. Three frequently emerging mutations S24L, V62L, and L84S are shown. (D) Cartoon representation of the dimerized SARS-CoV-2 ORF8 crystal structure (PDB: 7JTL) generated in PyMOL 2.5.4. Disulfide bridges are highlighted in yellow, 73YIDI76 is highlighted in cyan, and N78 is highlighted in blue. TRS; transcription regulatory sequence.

Figure 2

Modulation of cellular pathways promoting viral replication by SARS-CoV-2 ORF8. Replication cycle of SARS-CoV-2 is illustrated from virus entry, viral RNA replication, and assembly, to virus particle release. Viral and cellular events known to be modulated by ORF8 include ER stress responses, histone post-translation modifications, and spike expression. Reduced spike expression by ORF8 is represented as dimmed spike protein. Processes modulated by ORF8 are highlighted in high contrast, while steps of the SARS-CoV-2 life cycle unaffected by ORF8 (entry, replication, and translation) have been dimmed. M, Membrane; E, Envelope; N, Nucleocapsid; ER, Endoplasmic reticulum; ERGIC, ER intermediate compartment. Created in BioRender.com, accessed on 9 March 2023.

Figure 3

Modulation of host immunity by SARS-CoV-2 ORF8. ORF8 modulates the type-I IFN response, MHC-I antigen presentation and the CTL response, IL-17 signaling, and monocyte functions such as ADCC. Created in BioRender.com, accessed on 9 March 2023.