RNA's First Nucleotide Influences Antiviral Immune Response

A subtle difference at the beginning of an RNA molecule can significantly impact how strongly a cell activates its innate immune antiviral responses. Researchers at the International Institute of Molecular and Cell Biology (IIMCB) in Warsaw found that RNA molecules starting with adenosine (A) can trigger a stronger immune response than similar RNA molecules starting with guanosine (G).

The study focused on the RIG-I protein. RIG-I is a sensor in the innate immune system. This system is the body's first line of defense against infection. RIG-I detects suspicious RNA molecules, including those linked to viral infections. It then helps activate type I interferons, which are alarm signals that warn the cell and its surroundings of danger.

The team compared highly similar RNA molecules. These molecules differed only in their first nucleotide, either A or G. Double-stranded RNAs beginning with A activated the RIG-I/type I interferon pathway more strongly. This indicates that a single nucleotide can influence how intensely a cell perceives RNA as a potential threat.

This difference was not fully explained by RNA binding to purified RIG-I in a simplified biochemical system. This suggests that immune sensing in living cells is shaped by more than just the receptor. It is also influenced by proteins that gather around the RNA. The team discovered that RNAs starting with G preferentially recruit guanosine triphosphate (GTP)-binding proteins.

GTP-binding proteins may gather around G-starting RNA. This can partly reduce its recognition by RIG-I. This link between the first nucleotide and the immune response means the initial nucleotide affects the RNA molecule itself and the proteins that associate with it inside the cell. When G is present at the beginning, the RNA may become partially shielded from the cellular alarm system.

This research provides a missing piece in understanding how the early antiviral alarm system functions. It explains why some RNAs trigger a stronger response than others. It also sheds light on why cellular RNAs are not recognized by this system and why some viruses can evade immune detection for extended periods.