The signalling of the receptor tyrosine kinase Met is critical in promoting trophoblast cell invasion, and the deficiency in HGF/Met signalling is associated with preeclampsia. The semaphorin family member semaphorin4D (sema4D) and its receptor Plexin-B1 have been reported to control tumour cell invasion by coupling with Met. We hypothesized that sema4D/Plexin-B1 may promote trophoblast invasion by activating Met, and downregulation of sema4D/Plexin-B1 may account for the deficiency in Met signalling in preeclamptic placenta.In this study, Met and Erk activation and the expression of sema4D/Plexin-B1 in normal and preeclamptic placentas were comparably measured. The role of sema4D in trophoblast cell invasion and tubulogenesis was examined in vitro using the Transwell invasion assay and tube formation assay in trophoblast-endothelial cell co-culture model.Met, sema4D and Plexin-B1 co-localized in various subtypes of human trophoblast cells, including villous trophoblasts and extravillous trophoblasts (EVTs). In early-onset preeclampsia (E-PE) placentas, the phosphorylated Met and Erk as well as sema4D and Plexin-B1 were much lower than those in gestational week-matched preterm-labour (PTL) placentas. In human trophoblast HTR8/SVneo cell line, sema4D could promote Met and Erk phosphorylation as well as enhance trophoblast cell invasion and tubulogenesis with endothelial cells. Moreover, the effect of sema4D on HTR8/SVneo could be blocked by knocking down Met with specific siRNA.The crosstalk between sema4D and Met could transactivate Met to promote trophoblast cell invasion and differentiation, and decreased expression of sema4D and Plexin-B1 may be responsible for the deficiency in Met signalling and the development of preeclampsia.