We thank M. M. Mohiuddin (Cardiac Xenotransplantation Program, University of Maryland School of Medicine, Baltimore, Maryland, USA) and J. Denner (Institute of Virology, Free University of Berlin, Germany) for critical review of the manuscript before submission. Our xenotransplantation studies are supported by the Deutsche Forschungsgemeinschaft (CRC-TR 127) and the Swiss National Science Foundation (SNSF; Sinergia grant CRSII5_198577/1).