Alloantigen recognition in utero: dual advantage for the fetus?

The relatively low frequency of vertical transmission of HIV-1 suggests that natural protective mechanisms block viral infection during gestation and at parturition. Findings of independent observations raise the possibility that HLA alloantigen recognition in utero between the maternal and fetal immune systems can activate or enhance soluble factors, such as leukemia inhibitory factor, human chorionic gonadotropin, stromal cell-derived factor-1 and ribonucleases (RNases), all of which have all been reported to inhibit HIV-1 replication. We propose that these factors, which can be activated by MHC allorecognition, promote survival of the fetus and neonate by reducing the incidence of vertical transmission of certain pathogenic viruses. It might be relevant that some of these factors are also important for embryo implantation and fetal development, and might therefore have a dual role at this crucial point in the survival of mammalian species.