21,88 The transplanted trophoblasts undergo autonomous terminal differentiation in ectopic sites independent of the physiological state of pregnancy. They stimulate maternal antibody responses and attract T cells to the sites of transplantation and yet evade immediate destruction by the immune system of the recipients. The trophoblasts also maintain their endocrine capacity

and produce eCG.88 In addition to the characteristics that make the horse unique as a species in the study of pregnancy immunology, many advantages offered by commonly used animal models apply. The MHC of the horse has been well characterized using functional and genetic studies.89–94 Angiogenesis inhibitor Horses have been selectively bred for homozygosity at the MHC region, enabling the establishment of MHC-compatible and MHC-incompatible pregnancies to investigate the role of paternal antigens in maternal immune recognition.21 Advanced assisted reproductive techniques, such as artificial insemination and embryo transfer, are routinely used in horse breeding. Notably, embryo transfer is performed in thousands of horses

every year worldwide with high success rates,95 suggesting that the insemination-induced tolerance that plays a role in pregnancy in some species96 may be less important in others. Other more advanced techniques such check details as oocyte transfer, intracytoplasmic sperm injection, and nuclear transfer (cloning) are also successfully used in horse reproduction.97 These techniques are primarily used to generate genetically desirable offspring, but they can also be useful tools in understanding early reproductive events such as fertilization and conception. Recent advances in equine genomics and immunology have expanded opportunities for the study Exoribonuclease of pregnancy immunology at the mechanistic level. A 6.8X sequence of the equine genome has been determined

and extensively annotated.98 Multiple horse-specific expression microarrays have been developed and validated, allowing researchers to investigate the expression of thousands of genes simultaneously.99–102 Molecular advances have also facilitated the development of new horse-specific monoclonal antibodies103–106 and immune assay technologies.107 Our understanding of the mare’s immune responses during pregnancy has progressed substantially, but several critical questions still remain. Firstly, why do the chorionic girdle trophoblasts express such high levels of paternal MHC class I while invading the maternal endometrium? The horse is not unique in this respect – MHC class I expression can be observed in trophoblast populations of other species at various stages of placentation. However, the horse demonstrates the clearest evidence for maternal immune recognition of paternal alloantigens expressed by trophoblast. A proposed role for the expression of HLA molecules by human invasive extravillous trophoblasts is to confer protection from cytotoxic natural killer (NK) cells.