hGPA or mHEL RBCs are nonimmunogenic (or weakly immunogenic) when transfused into murine recipients in their baseline states. (A) Anti-hGPA response by flow cross-matching in FVB recipients transfused once with 1 “unit” of hGPA RBCs (100 μL of packed RBCs) in the presence or absence of poly(I:C); compilation of 3 representative experiments, 30 recipients total. Adjusted mean fluorescence intensity (MFI) was 0 for the hGPA group and 262 for the poly(I:C) hGPA group (P < .0001). (B) Anti-hGPA response by flow cross-matching in FVB recipients (10 total) after each of 3 hGPA transfusions given 3 weeks apart in the presence or absence of poly(I:C). Mean adjusted MFI for hGPA group after 1, 2, and 3 transfusions: 0. Mean adjusted MFI for poly(I:C) hGPA group after 1, 2, and 3 transfusions: 477.2, 1141, and 1887, respectively. SD depicted by error bars. (C) Anti-HEL response by ELISA in B10.BR recipients transfused once with 1 “unit” of leukoreduced mHEL RBCs in the presence or absence of poly(I:C). Representative experiment with 13 mice total; mean optical density 0.03 for the mHEL group and 0.77 for the poly(I:C) mHEL group (P < .005). This anti-HEL response in poly(I:C) mHEL–immunized recipients was not boosted by additional poly(I:C) mHEL transfusions. OD 415 indicates optical density at 415 nm.