The role of tapasin in MHC class I protein trafficking in embryos and T cells.

Preimplantation mouse embryos express both classical (class Ia) and nonclassical (class Ib) MHC class I proteins, and yet are not rejected by the maternal immune system. Although the function of the embryonic MHC class Ia proteins is unknown, one MHC class Ib protein, Qa-2, the product of the preimplantation embryo development (Ped) gene, actually enhances reproductive success. Similar in structure to MHC class Ia proteins, Qa-2 protein is a trimer of the alpha (heavy) chain, beta(2) microglobulin and a bound peptide. Studies on the folding, assembly and trafficking of MHC class Ia molecules to the cell surface have revealed this process to be dependent on multiple protein chaperone molecules, but information on the role of chaperone molecules in Qa-2 expression is incomplete. Here, we report the detection of mRNA for four chaperone molecules (TAP1, TAP2, calnexin and tapasin) in preimplantation embryos. We then focused on the role of the MHC-dedicated chaperone, tapasin, on Qa-2 protein expression. First, we demonstrated that tapasin protein is expressed by preimplantation embryos. Then, we used tapasin knockout mice to evaluate the role of tapasin in Qa-2 protein expression on both T cells and preimplantation embryos. We report here that optimal cell surface expression of Qa-2 is dependent on tapasin in both T cells and preimplantation embryos. Identification of the molecules involved in regulation of MHC class I protein expression in early embryos is an important first step in gaining insight into mechanisms of escape of embryos from destruction by the maternal immune system.

Evidence that HLA-G is the functional homolog of mouse Qa-2, the Ped gene product.

Qa-2, a murine class Ib major histocompatibility complex (MHC) molecule, is a possible functional homolog of human leukocyte antigen G (HLA-G). Both molecules have been implicated in immunoregulation and embryonic development and both occur in membrane-bound and soluble isoforms that arise by alternative splicing. Soluble splice variants have been implicated in the reproductive functions of HLA-G. While soluble variants of Qa-2 have been previously detected in T lymphocytes, we now demonstrate the presence of mRNA for one of the two known soluble forms of Qa-2 in eight-cell embryos and in blastocysts. Qa-2 is glycosylphosphatidylinositol (GPI) linked in the outer leaflet of the cell membrane and is found in lipid raft microdomains where other raft-associated proteins transduce signals into the cell. In contrast, HLA-G has a truncated six amino acid cytoplasmic tail. By fluorescence co-localization in JEG-3 cells, using fluorescent cholera toxin beta subunit (a lipid raft marker) and anti-HLA-G antibody, we have demonstrated that membrane-bound HLA-G also localizes to lipid rafts, consistent with functional homology between the two molecules. Finally, our experiments in which we have purified Qa-2 and transferred it via a process known as protein painting to Qa-2 negative cells represent a model for potential therapy involving HLA-G.