Allelic Variation within the NKG2DL COMPLEX Influences Event Free Survival Following Stem Cell Transplantation.

NKG2D, an important activating receptor on NK cells, is capable of transmitting costimulatory signals by certain subsets of T cells. Viruses and tumours have evolved strategies to evade the NKG2D-mediated immune recognition thus illustrating the importance of NKG2D in mediating anti-tumour and anti-viral responses. Although NKG2D is a highly conserved molecule, it interacts with several structurally diverse ligands that are upregulated by cellular stress. The currently identified Ligands (NKG2DL) are the MHC class I-chain related proteins A and B (MICA and -B) and the ULBP/RAET1 families. MICA and MICB are structurally similar to MHC class I proteins, but share only 27% identity and do not associate with β2-microglobulin or bind peptides. MICA and MICB are very polymorphic with more than 50 MICA and 13 MICB alleles so far reported. Some of their alleles have been shown to be associated with auto-immune diseases such as Behcet’s disease and psoriasis vulgaris. Furthermore, there are currently six identified ULBPs and RAET genes, which are related to MHC class I in their α1 and α 2 domains, but lack the α3 domain and are located in a linked cluster on the long arm of chromosome 6. These proteins are closely related and polymorphic; however, the functional significance of this polymorphism remains unclear. We developed a novel polymerase chain reaction sequence specific primer (PCR-SSP) method for typing NKG2DL using genomic DNA and newly designed primers. First, we established the population genetics of 29 SNPs of NKG2DL, both in the promotor and coding regions in a control population of 223 Healthy controls and confirmed the presence of 24 of these SNPs. For our clinical studies we have limited the analysis to 10 NKG2DL SNPs with allele frequencies of at least 10%, which were used in subsequent analysis of association with clinical outcome of allogeneic stem cell transplantation. The allele and haplotype frequencies were determined in 211 patient and donor pairs who had undergone sibling stem cell transplantation (SSCT) for a range of diseases. Interestingly, a strong association between event free survival (EFS) and a specific SNP within this group of ligands was observed, which resulted in greater than a 25% difference in Event Free Survival at 6 years between its two allelic forms, (p=0.001). Initial analysis of the effect of matching of the SNPs between donor and recipient does not indicate any clear association with SSCT outcome. This is the first study of genetic variation within NKG2D ligands and effect on stem cell transplantation outcome and the findings of the significant associations of their SNPs with EFS indicates a critical role for the NKG2D-ligand interactions with transplant outcome.