CD4⁺ T-cell response in an HLA-DRB1*11–positive patient with acquired TTP. (A) Analysis of PBMCs from an HLA-DRB1*11 patient in the acute phase of the disease. Incubations of PBMCs with AIM-V (medium control), SEB (positive control which activates a subset of CD4⁺ T cells by crosslinking MHC-II to TCRs with specific Vβ chains [Vβ12, 14, 15, and 17]),¹⁹  ADAMTS13 (10 µg/mL), FINVAPHAR-peptide (10 µg/mL), CP_FINVAPHAR peptide (10 µg/mL) with modified MHC-II 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*11 healthy volunteer. PBMCs were incubated as described for the sample depicted in the panel A column. PBMCs of this HLA-DRB1*11–positive healthy volunteer only respond to SEB stimulation and not to stimulation with ADAMTS13, FINVAPHAR, or CP_FINVAPHAR peptides. (C) Analysis of PBMCs derived of the same HLA-DRB1*11 TTP patient 2 years after splenectomy revealed no FINVAPHAR- or ADAMTS13-reactive CD4⁺ T cells. We acquired at least 0.5 × 10⁶ events for each sample analyzed.