# Mammalian antiviral proteins ZAP and KHNYN can independently restrict CpG-enriched avian viruses

## TL;DR

Mammalian proteins ZAP and KHNYN can block avian viruses with high CpG content, offering a defense against zoonotic influenza transmission.

## Contribution

Discovery that mammalian ZAP and KHNYN independently restrict CpG-enriched avian viruses, including a potent platypus KHNYN variant.

## Key findings

- Human ZAP and KHNYN independently restrict CpG-enriched IAV in chicken and human cells.
- Platypus KHNYN can restrict multiple diverse viruses despite being evolutionarily distant.
- Avian species lack KHNYN, potentially explaining susceptibility to CpG-enriched viruses.

## Abstract

Zoonotic viruses are an omnipresent threat to global health. Influenza A virus (IAV) transmits between birds, livestock, and humans. Proviral host factors involved in the cross-species interface are well known. Less is known about antiviral mechanisms that suppress IAV zoonoses. We observed CpG dinucleotide depletion in human IAV relative to avian IAV. Notably, human ZAP selectively depletes CpG-enriched viral RNAs with its cofactor KHNYN. ZAP is conserved in tetrapods, but we uncovered that avian species lack KHNYN. We found that chicken ZAP may not affect IAV (PR8) or CpG-enriched IAV (PR8CG). Human ZAP or KHNYN independently restricted CpG-enriched IAV PR8CG by overexpression in chicken cells and by combined knockout in human cells. Additionally, mammalian ZAP-L and KHNYN also independently restricted an avian retrovirus (ROSV). Curiously, platypus KHNYN, the most divergent from eutherian mammals, was also capable of independent restriction of multiple diverse viruses. We suggest that some mammalian KHNYN can act as a bona fide restriction factor with cell-autonomous activity. Furthermore, we speculate that through repeated contact between avian viruses and mammalian hosts, protein changes may accompany CpG-biased mutations or reassortment to evade mammalian ZAP and KHNYN.

Zoonotic influenza A viruses pose a persistent threat, yet the antiviral barriers to cross-species transmission remain unclear. This study shows that CpG-enriched avian viruses are restricted by several mammalian proteins, including human homologues and a potent platypus KHNYN.