Mhc Class I Proteins


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 Mhc Class I Proteins Background

The Major Histocompatibility Complex (MHC) is the genetic region that encodes the transplantation antigens or MHC class I (MHC-I) and MHC class II (MHC-II) molecules, which are primarily involved in protecting the body against pathogens.

Class I proteins comprise an alpha (α) polypeptide chain which is a large subunit of 44 kilodaltons (D) molecular weight (MW) that is non-covalently linked with a 12 kD-light chain called β2 microglobulin (β2m) to form a stable cell surface protein. The gene for the β2m is not located in the MHC region. MHC-II proteins are heterodimers composed of an alpha (α) and a beta (β) polypeptide chain. Class II molecules are expressed on professional antigen presenting cells (APCs) such as dendritic cells (DCs), macrophages and B cells. Class II molecules are not expressed on placental trophoblast cells and, therefore, will not be described in detail here.

 

Classical (MHC-Ia) and Non-classical (MHC-Ib) MHC-I Proteins

MHC-Ia proteins show ubiquitous expression. They are absent from mature erythrocytes of larger mammals such as humans and pigs but in rodents they are present on erythrocytes at low density. The MHC-Ia molecules are extremely polymorphic with a large number of alleles present in the population. These proteins are transmembrane glycoproteins and play an important role in immune regulation.

MHC-Ib genes are monomorphic or oligomorphic and often possess premature stop codons, and/or putative non-classical amino acid motifs (IPI and VPI) in the transmembrane domain. Similar to class Ia genes, most class Ib genes have eight exons which encode the heavy chain. Exon 1 encodes the signal sequence that is cleaved after the newly synthesized protein is targeted into the endoplasmic reticulum (ER). Exons 2, 3 and 4 encode the α1, α2 and α3 domains, which form the extracellular portion of the protein. The transmembrane domain is encoded by exon 5. Exons 6, 7, and sometimes part of exon 8 encode the cytoplasmic domain. In contrast to MHC-Ia proteins, nonclassical class I proteins (MHC-Ib) are expressed in specific tissues and under specific conditions. These proteins often have a truncated cytoplasmic domain. As a result of alternative splicing, these proteins are produced as transmembrane and soluble isoforms. The process of alternative splicing determines the secreted or lipid-linked nature of some class Ib molecules such as HLA-G (HLA-G2) and Qa-2. Alternative splicing of HLA-G or Qa-2 transcripts that eliminates or splices out exon 5, results in only secreted isoforms. In contrast to class Ia molecules which require binding with a light chain or β2-microglobulin (β2m) for their cell-surface expression, MHC-Ib proteins do not always require a light chain for their expression. Membrane HLA-G1 and soluble HLA-G5 associate with a light chain whereas membrane HLA-G2 and –G3 and soluble HLA-G6 do not associate with a light chain or β2m.

 

Peptide Binding to MHC I Glycoproteins

MHC class I proteins bind peptides from intracellular pathogens and the animal’s own protein-derived peptides. The protein complexes are digested in the cytosol by proteasomes into 8-10 amino acid long peptides. These peptides are accommodated in the peptide-binding cleft formed by the α1 and α2 domains of MHC-I proteins. The N-termini of freshly synthesized MHC-I glycoproteins contain N-linked glycan. N-linked glycans are trimmed by Glucosidases I and II (GlsI/II) to a single terminal glucose residue, which allows the MHC-I protein to interact with chaperon proteins. After this, the first interaction is with calnexin (CNX), which allows β2m to bind with the MHC-I heavy chain. Calreticulin (CRT) then recruits the MHC- protein to the peptide loading complex (PLC). The PLC is formed by the transporter associated with antigen processing (TAP) heterodimer together with other proteins and chaperon molecules. Tapasin, Erp 57, and CRT in association with other chaperons in the PLC help to locate the MHC-I protein to the PLC. Peptides longer than 8-10 amino acids are trimmed by ER aminopeptidases known as “ERAAP/ERAP1 and ERAP2.” Finally, tapasin-mediated editing results in preferential binding of approximately sized peptides in the peptidebinding groove. The MHC-I-peptide complexes then move to the cell surface for recognition by T cell-receptors on CD8+ T lymphocytes.

 

The Role of MHC Class I Proteins in Cattle Reproduction

In cattle, trophoblast cells in interplacentomal, arcade, and villous/crypt regions possess unique MHC-I expression patterns. In normal pregnancy, cattle trophoblasts do not show MHC-I expression before 120 days of pregnancy. However, MHC-I expression by interplacentomal and arcade trophoblast cells during the last trimester in cattle is normal. Cows carrying MHC-compatible pregnancies at term have reduced levels of immunoreactive interleukin-2 (IL-2), less apoptosis, less tumor necrosis factor-alpha (TNF-α) in macrophages, and reduced degranulation of binucleate trophoblast cells, which is similar to the conditions seen with retained placenta. In the third trimester of pregnancy, MHC class Ia and class Ib proteins are expressed in the interplacentomal and arcade regions. The ratio of class Ia to class Ib gene expression varies extensively among pregnancies. About 34-79% of transcripts from interplacentomal trophoblast cells are encoded by class Ib genes. Gene sequence analysis led to the discovery of four bovine non-classical loci: BoLA-NC1, BoLA-NC2, BoLA-NC3, and BoLA-NC4.

Normally, cattle have a greater number of lymphocytes in the non-gravid uterine horn than in the gravid horn. In early pregnancy, the endometrial lymphocytes decrease in number in sheep, pigs, and cattle. In 34-63 day old SCNT pregnancies, trophoblast MHC-I expression was widespread and accompanied by endometrial infiltration of CD3+ T lymphocyte that formed aggregates as compared to normal pregnancies. This maternal lymphocytic response involved 80% CD4+ helper T cells, with the remaining cells comprised of equal numbers of CD8+ cytotoxic T cells and B cells with a minimal number of γ/δ-T lymphocytes (Davies unpublished). The large number of CD4+ T helper cells suggests that allogeneic trophoblast MHC-I proteins are processed by maternal antigen presenting cells (APCs) and that MHC-I derived fetal peptides are presented on maternal MHC class II proteins. This is an indirect recognition pathway, where recognition is restricted by the host MHC class II molecules that have bound peptides derived from an allogeneic MHC molecule. Fetal antigens are probably carried towards the uterine endometrial epithelium by binucleate cells and released with the secretory granules after the BNC fuse with the uterine epithelial cells. Presentation of peptides derived from fetal MHC-Ia antigens triggers recruitment of Th1 cells and initiation of an inflammatory response involving release of tumor necrosis factor (TNF)-α, interleukin (IL)-1β, IL-12, and IFN-γ, which interferes with placental attachment and causes embryonic death. The success rate of nuclear transfer in cattle ranges from 0% to 10%.

These findings suggest that abnormal trophoblast MHC-I expression accompanied by lymphocytic infiltration of the endometrium causes early embryonic mortality of cloned fetuses. Therefore, appropriate expression of fetal MHC-I antigens at the bovine fetal-maternal interface is critically important for immunological acceptance of the allogeneic fetus by its mother.