An In-Depth Review of the Genetics of the Non-Classical HLA Class I Gene HLA-E and Its Effects on Haematopoietic Cell Transplant Outcomes.

HLA-E is a non-classical HLA class I gene with limited reported genetic variability and few published studies into full-gene sequencing or population allele frequencies. Two protein variants, HLA-E*01:01 and HLA-E*01:03, are very common, accounting for 94%-100% of observed alleles in most studies performed to date. Frequently utilised exon-based sequencing strategies have led to the assumption of HLA-E being a near bi-allelic gene; however, recent full-gene sequencing studies have shown a greater degree of genetic variability than initially imagined. We carried out a literature review of HLA-E genotype and ethnicity data, which suggested HLA-E*01:03 is more common in Asian and, in particular, East Asian populations. Furthermore, HLA-E*01:03:02 is more frequently observed than HLA-E*01:03:01 in European and American populations, whereas HLA-E*01:03:01 is found at higher frequencies in Asian populations. It has been proposed that HLA-E may have a role in Haematopoietic Cell Transplantation (HCT) due to its interaction with NK and CD8 T cells and its non-canonical peptide binding repertoire. Here we also review published literature into the effects of HLA-E genetics on HCT outcomes. Heterogeneity between cohorts muddies the waters; hence, studies report confounding effects of HLA-E genotype and matching on HCT outcomes. The need for further HLA-E sequencing of larger cohorts is evident to gain useful insight into the true genetic variability of HLA-E and its impact on HCT.