Identification of promiscuous FVIII-specific CD4+ T-cell epitopes in a humanized mouse model for hemophilia A

It is generally accepted that B cells require cognate interactions with CD4+ T cells to develop high affinity antibodies against proteins. CD4+ T cells recognize peptides (epitopes) presented by MHC-class II molecules that are expressed on antigen-presenting cells. Structural features of both the MHC-class II molecule and the peptide determine the specificity of CD4+ T cells that can bind to the MHC-class II-peptide complex. We used a new humanized hemophilic mouse model to study the regulation of immune responses against factor VIIII (FVIII) by immunodominant FVIII peptides presented by HLA-DRB1*15:01. This new mouse model carries a knockout of all murine MHC-class II molecules and expresses a chimeric murine-human MHC-class II complex that contains the peptide binding sites of the human HLA-DRB1*15:01. Using T-cell hybridoma technology, we identified 8 FVIII peptide regions that contained CD4+ T-cell epitopes presented by HLA-DRB1*15:01 to CD4+ T cells during immune responses against FVIII. Most of the 8 peptide regions contained promiscuous epitopes that bound to several different HLA-DR proteins in in vitro binding assays. Despite their promiscuous binding behaviour, the kinetics of peptide binding to the HLA-DR proteins investigated differed substantially.
Katharina Steinitz, PhD, is a scientist in the Department of Immunology within the division of Hemophilia and Hematology at Baxter Innovations GmbH. She received her first degree in Molecular Biology from the University of Vienna, Austria, and studied for her PhD in Immunology at the Technical University of Vienna. Her current research focuses on the immunogenicity of therapeutic proteins and the search for new approaches to prevent these unwanted immune responses.