ABCB1 and viral infectious disease: We have previously demonstrated that LPS treatment down-regulated P-gp and ABCB1 expression in first trimester human placental explants, while in third trimester placentas, ABCB1 expression was down-regulated by polyinosinic:polycytidylic acid (polyI:C) treatment (which models viral infection) but not by LPS [11].