Modulation of the HLA class II antigen at a molecular level by maternal serum.

The recent immunological literature has described the existence of a retroplacental serum factor being responsible for the downregulation of the MHC Class II antigen expression. In this report, the same property has been found in peripheral maternal serum. The HLA Class II modulating activity is however diminished in the presence of peripheral maternal serum as compared to retroplacental serum suggesting that the IA like inhibiting factor is released at the fetomaternal interface. After a three-day incubation period of unrelated lymphocytes in a maternal serum pool, it was shown that the HLA Dr., the HLA Dp., and more significantly, the HLA Dq. molecules were modulated. When third party lymphocytes were stimulated by Candidine or unrelated mononuclear cells in the presence of retroplacental serum, only the cellular subpopulation belonging to the CD4+ subset showed an HLA Dq. downregulation. The molecular constituents of the MHC Class II antigen expression characterizing cells belonging to other subsets remained unchanged. When the same stimulation assay was performed in the presence of a control medium (nulliparous serum), no changes concerning the MHC Class II molecular constituents were observed. When unrelated mononuclear cells were PHA stimulated in the presence of maternal and nulliparous serums, the HLA Dq. expression of the CD8+ subset showed a significant downregulation in the maternal serum mediated stimulation assay as compared to the control stimulation test. The molecular expression of the HLA Class II antigen related to the other subpopulations (CD4+, CD3+) stimulated by a mitogenic lectin remained unchanged. It is suggested that these molecular MHC Class II modulations are due to a factor included in the maternal IgG reaction. Retroplacental IgG contains the highest concentration of this factor.