Prior infection with PyV.OVA-II significantly enhances alloimmunization to a subsequent HOD RBC transfusion. C57BL/6 mice were infected with wild-type polyoma virus (PyV.WT) or polyoma virus expressing (OVA)_323-339 (PyV.OVA-II). Additional control mice were uninfected. All groups received subsequent transfusion with HOD RBCs. Alloimmunization was assessed 2 weeks later by anti-HEL ELISA. (A) A representative experiment with 5 mice/group is shown, with mean ± SEM shown at sera dilutions of 1:50 to 1:50 000. (B) Enhancement was also evident by flow cytometric cross-matching with FVB or HOD RBC targets. These experiments have been reproduced 3 times (5 mice/group per experiment) with similar results.