Transfer of CD4⁺ T cells from PIC/KEL-primed donors enhances HOD alloimmunization following exposure to (HOD × KEL) RBCs. (A) Representative gating strategy of CD4⁺ T-cell enrichment. CD4⁺ T cells enriched from naive or PIC/KEL-primed B6 recipients were adoptively transferred 24 hours prior to transfusion of HOD RBCs alone or (HOD × KEL) RBCs. Anti-HOD IgG (B) was evaluated 14 days (D14) after transfusion by indirect immunofluorescence staining using HOD and B6 RBC targets. The MFI in panel B was derived from normalizing the MFI of HOD RBCs to the MFI of 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. Panel B shows combinatorial data from 2 experiments, with at least 5 mice per group. *P < .05; **** P < .0001.