MLR-blocking antibodies are directed against alloantigens expressed on syncytiotrophoblasts

In this study, the reactivity of mixed lymphocyte reaction-blocking antibodies toward trophoblasts was investigated. Nineteen sera that had a significant level of MLR-blocking effect were obtained from normal pregnant women and from primary habitual aborters after immunotherapy. The blocking effects of these sera were studied before and after absorption with syncytiotrophoblast plasma membrane (STPM) from the respective mothers' placenta (autologous) as an experimental group and from unrelated mothers' placenta (allogeneic) as a control. The HLA types of cord blood lymphocytes corresponding to ten of these STPM were then determined. In the study with the 11 normal pregnancy sera, the blocking effect of every serum was partially removed by autologous STPM absorption. This revealed a significant difference between the preabsorption and postabsorption MLR-blocking effect. Nevertheless, absorption by the allogeneic STPM either partially removed or did not remove the blocking effect. Furthermore, there was a statistically significant difference in the elimination of the MLR-blocking effect between the autologous STPM and the allogeneic STPM. Studies with the sera from the immunized aborters revealed similar results. There was no statistically significant difference in the elimination of the antibodies between absorption with autologous and with allogeneic STPM. In these habitual aborters' sera, comparison of paternal stimulator and cord blood lymphocytes HLA types showed that the positive allogeneic STPM absorptions could not be explained sufficiently by coincidence of HLA types. Further studies are needed to determine whether the alloantigens are HLA-linked or not.