CD4⁺ T-cell response in an HLA-DRB1*03–positive patient with acquired TTP. (A) Analysis of PBMCs from an HLA-DRB1*03–positive patient in the acute phase of the disease. Incubations of PBMCs with AIM-V (medium control), SEB (positive control), ADAMTS13 (10 µg/mL), ASYILIRD peptide (10 µg/mL), CP_ASYILIRD peptide (10 µg/mL) with modified anchor residues (negative control). CD40L is plotted on the y-axis whereas CD4 is plotted on the x-axis. The percentage of CD4⁺ T cells expressing CD40L is depicted in the upper right quadrant. (B) Analysis of PBMCs obtained from an HLA-DRB1*03–positive healthy volunteer. PBMCs were incubated as described for the sample depicted in the panel A column. PBMCs of this HLA-DRB1*03–positive healthy volunteer only respond to SEB stimulation and not to stimulation with ADAMTS13, ASYILIRD, or CP_ASYILIRD peptides. We acquired at least 0.5 × 10⁶ events for each sample analyzed.