Immune cells take in unknown enemies

MHC class I molecules normally present endogenously synthesized antigens and thus activate cytotoxic T lymphocytes (CTLs). In antigen- presenting cells (APCs), however, some seemingly extracellular antigens are cross-presented: they are loaded onto the class I pathway in addition to their usual presentation by MHC class II. For example, APCs in mice were recently shown to activate antipoliovirus CTLs, despite lacking a receptor for the virus, presumably by cross-presenting poliovirus antigens on class I. But now Stefan Freigang, Rolf Zinkernagel, and colleagues (University of Zurich, Zurich, Switzerland) show that the missing receptor does not stop viral uptake in APCs, suggesting that cross-presentation is not needed as an explanation.

“Even in a situation where everybody would suspect that cross-presentation is going on, we see that APCs are taking in the virus,” says Freigang. His...