Co-localization of beta 2-microglobulin and IgG in human placental syncytiotrophoblasts

The fetal syncytiotrophoblast cells in close contact with maternal blood circulation apparently lack surface expression of HLA molecules, including the HLA light chain beta 2-microglobulin. This is thought to contribute significantly to a successful pregnancy. We find that syncytiotrophoblasts do express beta 2-microglobulin. Beta 2-microglobulin is primarily localized intracellularly in apical granules, and co-localize with human IgG. The origin and function of syncytiotrophoblast beta 2-microglobulin is unknown, but its localization in the syncytiotrophoblasts may implicate beta 2-microglobulin in the transplacental transport of IgG in conjunction with a recently identified class I HLA-like receptor for IgG/Fc. Alternatively, beta 2-microglobulin may associate with a hitherto unidentified class I HLA molecule.