Antibody production in the platelet-transfused mice. (A) Percentage of WT (•) and CD8-depleted (○) mice positive for IgG antiplatelet antibodies during the transfusion protocol. Donor platelets were incubated with a 1:50 dilution of sera obtained from recipient mice after each transfusion, labeled with FITC-goat antimouse IgG antibody, and analyzed by flow cytometry. Results shown are the percent antidonor antibody-positive mice (n = 20) from each group. All the immune sera were also reactive with donor leukocytes. (B) Titres of IgG antidonor antibodies in the sera of WT (•), CD8-depleted (○), and asialo GM1-depleted (□) mice at the fifth platelet transfusion. Donor leukocytes were incubated with the indicated titrations of recipient sera obtained after the fifth platelet transfusion, labeled with FITC-goat antimouse IgG antibody, and analyzed by flow cytometry. Fold increase was calculated by the following formula: percentage of donor cells positive with test serum/percentage of donor cells positive with pretransfusion test serum. The results are expressed as the mean (± SD) of the fold increases in 20 mice from each group. No detectible antidonor antibodies were observed in any of the pretransfusion sera. Significance values between WT and CD8-depleted mice are indicated (^*P < .01; ^**P < .001).