Comparison of CD4 T cell responses against two HLA-DR4 epitopes between DR4 and ABabDR4 mice. ABabDR4 and DR4 mice were immunized twice with hemagglutinin peptide 307–319 (HA) or PTPN11G503A peptide 492–506 (PTPN11_mut). At 2 wk after the second immunization, CD4 T cell responses were detected with peptide restimulation of draining LN cells, followed by intracellular IFN-γ staining. Percentages of IFN-γ⁺ CD4 T cells were measured by flow cytometry. Gates were set on live CD3⁺ lymphocytes. Each dot represents one mouse. Results are from two experiments combined and shown as mean ± SD. ***, P < 0.001 (Mann–Whitney test, two-tailed).