Major histocompatibility complex class I in the horse (Equus caballus) placenta during pregnancy and parturition

Major histocompatibility protein class I (MHC-I) is believed to be expressed in the horse allantochorion only in limited areas at limited times. However, its expression has only been investigated in early pregnancy with non-quantitative techniques that cannot reliably detect small amounts of protein.

To quantify the relative expression of MHC-I in the allantochorion and endometrium during days 90-240 of pregnancy (PREG), parturition with physiological delivery of fetal membranes (PHYS), and parturition with retention of these membranes (FMR). Also, to visualize protein expression and determine whether classical or non-classical MHC-I mRNA is expressed.

Heavy draft horses.

PREG horses (n = 12) were sampled postmortem at a slaughterhouse. PHYS (n = 6) and FMR (n = 5) horses were sampled at farms in the vicinity of Olsztyn, Poland.

For relative quantification of MHC-I, western blotting with densitometry was used. To visualize MHC-I, immunohistochemistry was used. For mRNA identification, RT-PCR was performed.

Although the quantity of MHC-I was lower during PREG than parturition, it was present in the allantochorion and endometrium during PREG. During parturition, MHC-I expression was upregulated in the allantochorion (PHYS vs. PREG: 2.7-times higher, 95% confidence interval, 1.3- to 5.7-times higher; FMR vs. PREG: 3.2-times higher, 95% confidence interval, 1.5- to 6.7-times higher). At parturition, staining for MHC-I was detected in the microcotyledons. Classical and non-classical MHC-I were expressed in both tissues during PREG, PHYS, and FMR.

MHC-I protein is present in the horse allantochorion and endometrium for at least the first two-thirds of pregnancy and at parturition.