Perspective: HLA functional elements outside the antigen recognition domains

Novel HLA-DQB1, DQA1, DPA1 and DPB1 alleles identified in the Australian New South Wales tissue typing laboratory

Introduction of NGS into clinical histocompatibility laboratories has greatly increased the frequency of novel HLA allele discovery. We identified 9 DQA1, 7 DQB1, 8 DPA1 and 2 DPB1 novel alleles over the last 2 years. 92% (24 of 26) differed from their closest allele by single nucleotide substitutions detected in coding regions and 84% (22 of 26) contained polymorphism outside of exon 2. Identification of novel alleles in non-coding intronic regions were restricted to splice sites. Two novel alleles arising from nucleotide substitution affecting splicing were identified and named with questionable expression status. Novel alleles with polymorphism detected only in non-coding regions (introns and UTRs) were not reported. 21 additional DQA1, DQB1 and DPA1 alleles detected in multiple individuals as novel at the time of testing, were also detected in other laboratories. The number of novel alleles detected further emphasises the role of NGS in novel allele discovery.

Next-generation sequencing and clinical histocompatibility testing

Histocompatibility testing is essential for donor identification and risk assessment in solid organ and hematopoietic stem cell transplant. Additionally, it is useful for identifying donor specific alleles for monitoring donor specific antibodies in post-transplant patients. Next-generation sequence (NGS) based human leukocyte antigen (HLA) typing has improved many aspects of histocompatibility testing in hematopoietic stem cell and solid organ transplant. HLA disease association testing and research has also benefited from the advent of NGS technologies. In this review we discuss the current impact and future applications of NGS typing on clinical histocompatibility testing for transplant and non-transplant purposes.

Factors affecting HLA expression: A review

The detection and semiquantitative measurement of circulating human leucocyte antigen (HLA)-specific antibodies is essential for the management of patients before and after transplantation. In addition, the pretransplant cross-match to assess the reactivity of recipient HLA antibody against donor lymphocytes has long been the gold standard to prevent hyperacute rejection. Whilst both of these tests assume that recipient HLA-specific antibody is the only variable in the assessment of transplant risk, this is not the case. Transplant immunologists recognize that some HLA antigens are expressed at levels a magnitude lower than others (e.g., HLA-C, HLA-DQ), but within loci, and between different cell types there are many factors that influence HLA expression in both resting and activated cells. HLA is not usually expressed without the specific promoter proteins NLRC5, for HLA class I, and CIITA, for class II. The quantity of HLA protein production is then affected by factors including promoter region polymorphisms, alternative exon splice sites, methylation and microRNA-directed degradation. Different loci are influenced by multiple combinations of these control mechanisms making prediction of HLA regulation difficult, but an ability to measure the cellular expression of each HLA antigen, in conjunction with knowledge of circulating HLA-specific antibody, would lead to a more informed algorithm to assess transplant risk.