A GPI anchor explains the unique biological features of the common NKG2D-ligand allele MICA*008

The human MICA (MHC I-related chain A) gene, encoding a ligand for the NKG2D (NKG2-D type II integral membrane protein) receptor, is highly polymorphic. A group of MICA alleles, named MICA 5.1 (prototype, MICA*008), produce a truncated protein due to a nucleotide insertion in the transmembrane domain. These alleles are very frequent in all of the human populations studied and they have different biological properties, compared with full-length alleles, e.g. recruitment into exosomes, which makes them very potent for down-modulating the NKG2D receptor in effector immune cells. Moreover, MICA*008 is not affected by viral immune evasion mechanisms that target other MICA alleles. In the present study, we demonstrate that MICA*008 acquires a GPI (glycosylphosphatidylinositol) anchor and that this modification is responsible for many of the distinct biological features of the truncated MICA alleles, including recruitment of the protein to exosomes. MICA*008 processing is also unusual as it is observed in the endoplasmic reticulum as a Triton (TM) X-114 soluble protein, partially undergoing GPI modification while the rest is exocytosed, suggesting a new model for MICA*008 release. This is the first report of a GPI-anchored MICA allele. The finding that this modification occurs in both families of human NKG2D ligands, as well as in the murine system, suggests positive pressure to maintain this biochemical feature.