Poly(I:C) causes breakthrough anti-KEL alloimmunization despite KELIg immunoprophylaxis. (A) General experimental design: recipients were passively immunized with KELIg, followed by treatment with poly(I:C) or a saline control, followed by transfusion with murine RBCs expressing the human KEL glycoprotein. Alloimmune responses were measured longitudinally in serum posttransfusion. (B) Total anti-KEL IgG measured in the serum of recipients from day 0 to day 28 posttransfusion. (C) Experiment altering the timing of poly(I:C) administration; in this instance, poly(I:C) was administered 4 hours before or 36 hours following KEL RBC transfusion. (D) Animals depleted of CD4⁺ T cells using GK1.5 antibody were treated with KELIg and transfused in the presence or absence of poly(I:C), with anti-KEL IgG measured longitudinally. These data are representative of 2 to 3 independent experiments, with 3 to 6 mice per group per experiment (in total, 13 and 15 mice were studied across 3 experiments for panel A; 6 and 6 mice were studied across 2 experiments for panel B; 11 and 10 mice were studied across 3 experiments for panel C). *P < .05; error bars indicate standard deviation between mice.