Complex Linkage Disequilibrium Effects in HLA-DPB1 Expression and Molecular Mismatch Analyses of Transplantation Outcomes.

BACKGROUND: HLA molecular mismatch (MM) is a risk factor for de novo donor-specific antibody (dnDSA) development in solid organ transplantation. HLA expression differences have also been associated with adverse outcomes in hematopoietic cell transplantation. We sought to study both MM and expression in assessing dnDSA risk. METHODS: One hundred three HLA-DP-mismatched solid organ transplantation pairs were retrospectively analyzed. MM was computed using amino acids (aa), eplets, and, supplementarily, Grantham/Epstein scores. DPB1 alleles were classified as rs9277534-A (low-expression) or rs9277534-G (high-expression) linked. To determine the associations between risk factors and dnDSA, logistic regression, linkage disequilibrium (LD), and population-based analyses were performed. RESULTS: A high-risk AA:GX (recipient:donor) expression combination (X = A or G) demonstrated strong association with HLA-DP dnDSA (P = 0.001). MM was also associated with HLA-DP dnDSA when evaluated by itself (eplet P = 0.007, aa P = 0.003, Grantham P = 0.005, Epstein P = 0.004). When attempting to determine the relative individual effects of the risk factors in multivariable analysis, only AA:GX expression status retained a strong association (relative risk = 18.6, P = 0.007 with eplet; relative risk = 15.8, P = 0.02 with aa), while MM was no longer significant (eplet P = 0.56, aa P = 0.51). Importantly, these risk factors are correlated, due to LD between the expression-tagging single-nucleotide polymorphism and polymorphisms along HLA-DPB1. CONCLUSIONS: The MM and expression risk factors each appear to be strong predictors of HLA-DP dnDSA and to possess clinical utility; however, these two risk factors are closely correlated. These metrics may represent distinct ways of characterizing a common overlapping dnDSA risk profile, but they are not independent. Further, we demonstrate the importance and detailed implications of LD effects in dnDSA risk assessment and possibly transplantation overall.

Generation of Full-Length Class I Human Leukocyte Antigen Gene Consensus Sequences for Novel Allele Characterization.

BACKGROUND: Routine, high-resolution human leukocyte antigen (HLA) genotyping by next generation sequencing within clinical immunogenetics laboratories can now provide the full-length gene sequence characterization of fully phased HLA alleles. This powerful technique provides insights into HLA variation beyond the traditionally characterized antigen recognition domain, providing sequence annotation across the entire gene including untranslated and intronic regions and may be used to characterize novel alleles from massively parallel sequencing runs. METHODS: We evaluated the utility of the Omixon Holotype HLA assay to generate credible, fully phased full-length gene consensus sequences for 50 individuals at major histocompatibility complex, class I, A (HLA-A), HLA-B, and HLA-C loci (300 genotyped alleles in total) to identify and characterize novel class I HLA alleles using our downstream analytical pipeline. RESULTS: Our analysis revealed that 7.7% (23/300) of genotyped class I HLA alleles contain novel polymorphisms. Interestingly, all of the novel alleles identified by our analysis were found to harbor sequence variations within intronic regions of the respective locus. In total our analysis identified 17 unique novel class I HLA alleles from 23 of the 300 genotyped alleles and generated full-length gene sequence annotations for 9 previously incompletely annotated HLA class I allele sequences derived from 14 of the 300 genotyped alleles. CONCLUSIONS: The demonstrated utility of the Omixon Holotype HLA assay in combination with our downstream analytical framework to generate fully phased, full-length gene consensus sequences for the identification and characterization of novel HLA alleles, facilitates the study of HLA polymorphism beyond the antigen recognition domain in human health and disease.