Several lines of evidence suggest that the human leukocyte antigen (HLA)-G play a key role in the regulation of human pregnancy. A sub-population of cells highly represented at the decidua belong to the myeloid-derived monocyte/macrophage lineage, which potentially interact with HLA-G expressing cells. It is proposed that HLA-G protects decidual trophoblasts from lysis by blocking the effector function of decidual monocyte/macrophages. The interaction between HLA-G and monocyte/macrophages may therefore contribute to a successful pregnancy. Here we examine existing knowledge on the convergent role of HLA-G and monocyte/macrophages in pregnancy and define the synergy that exists between these two elements in the decidua. Key features of the HLA-G gene product are discussed followed by the main characteristics of decidual monocyte/macrophages. A hypothetical model for the interaction between HLA-G and monocyte/macrophage cells at the fetal-maternal interface is proposed.
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