Determination of the Number of Circulating Small Extracellular Vesicles in Pregnancy Using the Novel Marker CD9

Small extracellular vesicles (small EVs) play pivotal roles in intercellular communication and pregnancy maintenance, but their clinical significance in preeclampsia (PE) remains unclear. We obtained plasma samples from non-pregnant women, healthy pregnant women, and patients with early-onset (EoPE) and late-onset PE (LoPE). Small EVs were isolated using ultracentrifugation and validated using transmission electron microscopy and nanoparticle tracking analysis; in addition, Western blotting was performed to identify suitable surface markers for plasma-derived small EVs. In our analysis, we consistently detected cluster of differentiation 9 (CD9), whereas classical markers such as cluster of differentiation 63 (CD63) and tumor susceptibility gene 101 (TSG101) were absent. In a prospective, nested case–control study, we analyzed first-trimester samples by using a CD9-based ELISA for small-EV quantification. The number of small EVs did not significantly differ between non-pregnant and healthy pregnant women regardless of the gestational age. However, EVs were significantly elevated in both EoPE (3.5-fold) and LoPE (1.5-fold) compared with matched controls. First-trimester EV levels did not show differences between women who later developed PE and normal controls. These findings indicate that CD9 is a promising marker for plasma-derived small EVs and that an elevated number of small EVs is associated with established PE but has limited predictive value in early pregnancy. Further studies are required to elucidate the cellular origin and clinical implications of small EVs in PE.