Clues about how the immune system detects membrane fusion, the earliest event when a virus infects a cell, are reported in a mouse study published in Nature Immunology.

Soren Paludan and colleagues found that cells exposed to virus-like particles (VLPs) or artificially created liposomes lacking any genetic material or known stimulatory molecules could still elicit a robust immune response in mice in vitro and in vivo. Although unable to trigger any conventional pathogen recognition pathways, these agents are able to fuse with cell membranes. Recognition of membrane fusion seemed to be exclusively dependent on STING - an intracellular molecule more commonly associated with responding to pathogen-derived DNA material. Although membrane fusion could elicit an immune response on its own, it could also synergize with other immune-stimulatory signals and may therefore represent a generalized and important first step in pathogen recognition.