C4d deposition is considered to be evidence of antibody-mediated rejection. This study was conducted to compare C4d immunoreactivity between villitis of unknown etiology (VUE) and cytomegaloviral placentitis. C4d immunohistochemistry was performed in cases with VUE (n = 16) and cytomegaloviral placentitis (n = 5). Distinct, linear C4d immunoreactivity along the syncytiotrophoblast was found in all VUE cases. In cytomegaloviral placentitis, the intensity of C4d immunoreactivity along the syncytiotrophoblast was not prominent, but cytoplasmic C4d immunoreactivity of villous cytotrophoblasts was frequently observed. Further screening of the cases with placental infarcts (n = 5) demonstrated prominent C4d immunoreactivity in the chorionic villi adjacent to the infarct. We report the characteristic co-localization of VUE and C4d immunoreactivity. The overall findings in this study strongly suggest that the complement activation is a common mechanism of diverse placental injuries associated with rejection, infection, and ischemia.
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