Not All Mismatches Were Created Equal: Empirical Identification of Regions Within HLA-DQ Molecules That Harbor the Highest Immunogenicity

Background. HLA-DQ mismatches between donor and recipient confer the highest risk for generation of donor-specific antibodies leading to poor allograft survival. Therefore, understanding hierarchical immunogenicity of HLA-DQ mismatches is needed to guide risk stratification. Methods. Adsorption/elution experiments were optimized using homozygous cell lines as targets. Serum samples from transplant recipients studied herein were limited to those obtained at first documentation of de novo HLA-DQ donor-specific antibodies. Results. The use of different HLA-DQ targets in optimized adsorption/elution experiments allowed for separation of sera exhibiting polyclonal antibody reactivity into individual antibodies recognizing epitopes with monospecificity. Our heterogeneous cohort demonstrated only a small repertoire of amino acids, or regions of interest, associated with the initial HLA-DQ humoral response. Most of the mismatches listed in the HLA Eplet Registry did not participate in these first-tier responses. Adsorption/elution studies were instrumental in documenting epitope spreading in patients with broadening of HLA-DQ antibody repertoire, highlighting the occurence of a first-tier response before triggering additional immune responses. Finally, first-tier regions of interest correlate with first-field (low-resolution) HLA typing, namely conventional serologic HLA-DQ antigen families, enriched with DQα chains specificity. Conclusions. These findings clearly demonstrate that some “molecular mismatches” are more immunogenic than others. They further support our previous observations documenting increased risk of HLA-DQα*05 heterodimer mismatches and provide a tool to continue interrogating differential immunogenicity. Finally, we provide guidance to identify high- and low-risk HLA-DQ mismatches. Taken together, our study supports a different conceptual and experimental framework to study HLA-DQ immunogenicity, and an immunological rationale to guide risk stratification.