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glycan-containing peptides derived of ADAMTS13 are presented on MHC-II.40
Identification of the immunogenic determinants of ADAMTS13 represents a major challenge for the under- standing of TTP pathogenesis. Our study shows that there is an overlap, but also differences, in the repertoire of ADAMTS13-derived peptides that are presented on HLA- DQ and HLA-DR. This is illustrated by the peptide with core sequence FINVAPHAR derived from the CUB2 domain that was identified exclusively in the case of HLA-DR. In contrast with the study of Sorvillo et al., only 2 out of the 3 HLA-DRB1*11 donors presented the FIN- VAPHAR peptide. In addition, this peptide was also iden- tified in 2 non-HLA-DRB1*11 donors, confirming that the presentation of FINVAPHAR is not restricted to HLA- DRB1*11.19 Interestingly, several peptides were identified associated with HLA-DQ only. Especially, two peptides derived from the cysteine-rich domain were identified exclusively on HLA-DQ. Lastly, we identified sets of pep- tides that were presented on both HLA-DR and HLA-DQ, as exemplified by the CUB1-derived peptide with core- sequence CAVAIGRPL.
Whether the novel peptides identified in this study play a role in the onset of acquired TTP remains to be deter- mined. A previous study of Verbij et al. showed that CUB2 domain-derived peptides FINVAPHAR and ASYILIRD are able to activate CD4+ T cells from an HLA-DRB1*11 and an HLA-DRB1*03-positive acquired TTP patient, respec- tively.20 A recent study by Gilardin et al. did not identify CD4+ T cells responding to peptide ADAMTS131239-1253 con- taining the FINVAPHAR core-sequence in HLA-DRB1*11- positive patients.21 In their hands, another CUB2 domain- derived peptide ADAMTS131239-1253 was identified as an immunodominant T-cell epitope for both DRB1*01 and DRB1*11-positive acquired TTP patients.21 Surprisingly, the ADAMTS131239-1253 peptide was not identified to be presented on HLA-DR or DQ in our current study, despite the fact that several HLA-DRB1*01 and HLA-DRB1*11 positive donors were included in our cohort. Two pep- tides containing the same core-sequence were, however,
identified in a DRB1*0401/DRB1*1301 positive donor in a previous study from our group.19
In silico approaches (EpiMatrix) identified a majority of ADAMTS13 peptides (70%) with significant EpiMatrix scores in the CUB1/2 domains. Six of the eluted ADAMTS13 peptides with low cross-conservation with the human proteome (based on JanusMatrix analysis) were considered to be more likely to elicit an effector T- cell response. These predicted immunogenic peptide sequences included ADAMTS131322-1341 (FINVAPHAR) and ADAMTS131355-1373 (ASYILIRD); CD4+ T cells targeting these epitopes have indeed been found in patients with acquired TTP.20 In contrast, four of the peptides had TCR facing residues that were highly conserved with multiple peptides found in the human proteome and putatively restricted by the same HLA, and, therefore, could poten- tially be tolerated or actively tolerogenic in patients with acquired TTP. These peptides include peptide ADAMTS131238-1252, containing a minimal core sequence recognized by the ADAMTS13-specific DRB1*01:01- restricted T-cell hybridomas as described by Gilardin et al.21 CD4+ T cells targeting different epitopes may develop in patients with acquired TTP. Phenotypic profiling of CD4+ T-cell responses in large numbers of patients with acquired TTP will be needed to define whether multiple immune-dominant and potential immunoregulatory CD4+ T-cell epitopes contribute to the etiology of acquired TTP.
Acknowledgments
The authors would like to thank Erik Mul and Mark Hoogeboezem for elutriation of the buffy coats. We are grateful to William Martin and Frances Terry (EpiVax) for their help with the in silico data analysis.
Funding
This study was supported by grants from the Landsteiner Foundation of Blood Transfusion Research and Horizon 2020 Framework Programme for Research and Innovation of the European Union (Grant Agreement No. 675746).
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