Expression of major histocompatibility complex (MHC) class 1 molecules on early trophoblast.

This research study focused on investigating how the Major Histocompatibility Complex (MHC) class I molecules are expressed in early embryonic development of different types of equine pregnancies – horses, donkeys, mules and donkey-in-horse transfers. The results showed these molecules’ expression varied throughout gestation and potentially could be linked to the direct contact with other tissues such as the endometrial epithelium for their functionality.

Understanding the Research

• In the context of this study, trophoblasts are the cells forming the outer layer of a blastocyst, providing nutrients to the embryo and developing into a large part of the placenta. The Major Histocompatibility Complex (MHC) is a set of genes that code for proteins found on the surfaces of cells that help the immune system recognize foreign substances. MHC class I is one type of these proteins.
• The researchers used a technique known as a peroxidase anti-peroxidase immunohistochemical technique with a rat monoclonal antibody (MAC 291) to examine the expression of MHC Class I molecules by early trophoblasts.

Key Findings

• The researchers found that most non-invasive allantochorion (a part of the developing placenta) did not express MHC Class I molecules at any stage of gestation in horse, donkey, and mule pregnancies. There were exceptions with small patches of trophoblast cells next to gland openings.
• In horse pregnancies, MHC Class I molecules were strongly expressed in the chorionic girdle cells (cells that eventually form the placenta) at days 33 and 34 of gestation, just before they start to invade other tissues. However, this expression was reduced when these cells matured and started to secrete gonadotropin, a hormone that plays a crucial role in sexual development and reproduction.
• All endometrial cups (placental structures in pregnant mares) from intraspecific donkey pregnancies and interspecific mule pregnancies were negative for MHC Class I molecules.
• The researchers also examined seven pregnancies involving the transfer of embryonic donkey to horse. They found that MHC Class I molecule expression was strong in the chorionic girdle at day 35 of gestation. However, most of the allantochorion was stained for MHC class I molecules, and many lymphocytes (white blood cells) were present in adjacent endometrial stroma (connective tissue) later on.

Implications

• The study suggests that the mechanisms that normally suppress the expression of MHC class I molecules by the epithelial trophoblast layer of the equine placenta can only function if the cells are in close and stable contact with other tissues, such as endometrial epithelium.
• The findings raise the possibility that abnormal MHC molecule expression could be a sign of complications in pregnancy, and could serve as a diagnostic tool in the future.