Table 1 Comparison of nonprimate and primate placental MHC biology

bovine:

• trophoblasts lack MHC class I expression [5]

• Trophoblasts from cloned fetuses express MHC class I, cow has altered endometrial immune cells [5]

horse:

• trophoblasts express classical MHC class I [6]

• mare raises an alloresponse to paternal MHC [7]

mice:

• no trophoblast MHC class I [8]

• pregnancy is successful despite transgenic MHC expression (or β2 m knockout) [8, 9]

• disruption of T cell or complement regulation detrimental to pregnancy success [10, 11]

nonhuman primate:

• lack of functional HLA-G locus [28]

• nonclassical MHC class I expression: Mamu-AG [29], Mamu-E [30]

• novel MHC class I expression, Mamu-I [31]

human:

• extravillous trophoblast HLA-G expression [12, 13]

• abundant placental nonclassical MHC expression (HLA-E, HLA-F) [15,22]

• placental HLA-C expression in some cytotrophoblasts [14]

HLA-G homologs have not been identified in nonprimate species