Abstract Abstract 489 Autoantibodies directed against ADAMTS13 prohibit the processing of VWF multimers initiating a rare and life-threatening disorder called acquired thrombotic thrombocytopenic purpura (TTP). Recently, HLA-DRB1*11 has been identified as a risk factor for the development of acquired TTP. Here, we identified ADAMTS13-derived peptides presented on MHC class II alleles. Dendritic cells from a panel of both HLA-DRB1*11 positive and negative donors were pulsed with ADAMTS13 and the HLA-DR-presented peptide repertoire was analyzed by mass spectrometry. Interestingly, pulsing of dendritic cells of HLA-DRB1*11 positive donors with 100 nM ADAMTS13 resulted in presentation of a single CUB2 derived ADAMTS13 peptide. This peptide was not presented by HLA-DRB1*11 negative donors. Pulsing of HLA-DRB1*0301 positive cells with 100 nM ADAMTS13 resulted in presentation of a different CUB2 domain derived peptide in 2 out of 3 donors analyzed. Pulsing of dendritic cells employing higher concentrations of ADAMTS13 (500 nM) resulted in increased presentation of ADAMTS13 derived peptides by both HLA-DRB1*11 positive and negative donors. In DRB1*11 negative donors peptides derived of multiple domains were presented which included spacer, TSR2-8, metalloprotease and disintegrin domains. Interestingly, the CUB2 domain peptide (specific for DRB1*11 when dendritic cells were pulsed with 100 nM ADAMTS13) was also presented under these conditions. Apparently, this peptide can be presented by multiple MHC class II alleles although higher concentrations of ADAMTS13 are required for its presentation on DCs derived of non-DRB1*11 positive donors. Interestingly, the diversity of ADAMTS13 derived peptides presented by iDCs of donors HLA-DRB1*11 was not affected by pulsing of iDCs with higher concentration of ADAMTS13. Also, under these conditions the only peptides that were presented were derivatives of the CUB2 domain derived peptide that was also presented at lower concentrations of ADAMTS13. Our results clearly demonstrate that this peptide is efficiently presented when compared to other ADAMTS13-derived peptides. Therefore, we hypothesize that efficient presentation of this CUB2 domain derived peptide on DRB1*11 may provoke proliferation of low affinity self-reactive CD4+ T cells that have escaped negative selection in the thymus and contribute to the onset of acquired TTP. Together these findings may provide further insight in the initiation of the autoimmune reactivity against ADAMTS13 in patients affected by TTP. Disclosures: No relevant conflicts of interest to declare.