Codominant expression of the polymorphic MICA alloantigens encoded by genes in the HLA region

The HLA-related, polymorphic MHC class I-related chain A (MICA) gene encodes a 383-amino acid polypeptide, with three extracellular domains (α1, α2 and α3), a transmembrane region and a cytoplasmic tail. We have previously shown that freshly isolated endothelial cells, fibroblasts, keratinocytes and monocytes express MICA, while peripheral blood CD4⁺, CD8⁺ or CD19⁺ lymphocytes do not. This polymorphic MICA molecule is a target for specific alloantibodies in sera from kidney, heart and lung transplant recipients, although its possible role during graft rejection remains to be demonstrated. In this study we investigated whether there is codominance in the expression of MICA. We isolated RNA from a heterozygous cell line (HCT116), previously shown by sequencing-based typing to be MICA*001/MICA*00902, as well as 12 clones derived from it. Thereafter, we retrotranscribed the RNA into cDNA, and performed a molecular typing using MICA-sequence specific oligonucleotides (SSOP). Using this approach, we detected the RNA encoding MICA*001 and MICA*00902 in all the clones and in the parental cell line, indicating that MICA is codominantly expressed. This codominant expression was further confirmed by cloning and sequencing plasmids encoding these two alleles produced from the same HCT116 RNA preparation. We also produced the two recombinant MICA proteins (MICA*001 and MICA*00902). They reacted with rabbit anti-MICA polyclonal antibodies by ELISA and Western blot, indicating that the plasmids carrying the cDNA inserts probably encode functional MICA proteins. This strongly suggests that, like the HLA class I and class II proteins, MICA is codominantly expressed. The codominant expression of the polymorphic, HLA-like MICA alloantigens may have implications for the immune response elicited by the allograft in organ transplantation.