CALRETICULIN ASSOCIATES WITH NON-HLA-A,-B CLASS-I PROTEINS IN THE HUMAN CHORIOCARCINOMA CELL-LINES JEG-3 AND BEWO

Human placental trophoblast expresses an unusual repertoire of major h istocompatibility complex (MHC) class I products that appears to refle ct the unique role of this epithelium in mediating fete-maternal relat ions during pregnancy. Trophoblast is devoid of human leucocyte antige n (HLA) -A,-B antigens but can express one or more non-HLA-A,-B class I proteins. The human choriocarcinoma cell lines JEG-3, BeWo and JAR a re widely used as models to study trophoblast. During attempts to isol ate non-HLA-A,-B class I from JEG-3 and BeWo by immunoaffinity chromat ography using a monoclonal antibody to beta(2)-microglobulin we observ ed a 55000 MW protein co-purifying with class I. N-terminal amino acid sequencing and immunoblotting using a specific antiserum identified t his product as calreticulin, a molecule recently shown to be involved in the assembly of classical class I in human B-lymphoblastoid cells. In our hands JEG-3 and BeWo were Found to express 45 000 MW non-HLA-A, -B class I proteins while the 40 000 MW HLA-G product was identified o nly in JEG-3. Our data suggest that calreticulin associates with non-H LA-A,-B class I heterodimers and with free 45 000 MW non-HLA-A,-B clas s I H chains in JEG-3. JAR was found to be devoid of detectable class I H chains but contained beta(2)-microglobulin and calreticulin. Howev er, calreticulin-beta(2)-microglobulin complexes were not detected in JAR. Calreticulin and class I were apparently co-localized within the endoplasmic reticulum of JEG-3 cells whereas only class I was expresse d at the cell surface. These studies demonstrate that calreticulin is associated with non-HLA-A,-B class I products in human choriocarcinoma cells.