HOD reactive CD4⁺ T cells render recipients responsive to GPA following (HOD × GPA) RBC exposure. Where indicated, B6 recipients were adoptively transferred with OTII splenocytes. 24 hours following adoptive transfer, all recipients were transfused with HOD RBCs, GPA RBCs, (HOD × GPA) RBCs, or a mixture of HOD RBCs and GPA RBCs (HOD + GPA). (A-B) Anti-GPA IgM (A) and anti-GPA IgG (B) antibody formation is shown for transfused recipients 5 days (D5 for IgM) and 14 days (D14 for IgG) after transfusion. Serological analysis for anti-GPA alloantibodies was examined by indirect immunofluorescence staining using GPA and B6 RBCs. The MFI depicted was generated by normalizing the MFI of serum samples incubated with GPA RBCs to the MFI of serum samples incubated with background control B6 RBCs. Error bars represent mean ± SEM. The mean of each group is depicted as a horizontal line. Statistics were generated using a 1-way ANOVA with a Tukey’s post-test. All panels show combinatorial data from experiments reproduced 2 times, with 5 mice per group. ***P < .001; ****P < .0001.