Rarity of fetal cells in exocervical samples for noninvasive prenatal diagnosis

The possibility to isolate fetal cells from pregnant women cervical samples has been discussed for five decades but is not currently applied in clinical practice. This study aimed at offering prenatal genetic diagnosis from fetal cells obtained through noninvasive exocervical sampling and immuno-sorted based on expression of HLA-G.

We first developed and validated robust protocols for cell detection and isolation on control cell lines expressing (JEG-3) or not (JAR) the HLA-G antigen, a specific marker for extravillous trophoblasts. We then applied these protocols to noninvasive exocervical samples collected from pregnant women between 6 and 14 weeks of gestational age. Sampling was performed through insertion and rotation of a brush at the ectocervix close to the external os of the endocervical canal. Finally, we attempted to detect and quantify trophoblasts in exocervical samples from pregnant women by ddPCR targeting the male SRY locus.

For immunohistochemistry, a strong specific signal for HLA-G was observed in the positive control cell line and for rare cells in exocervical samples, but only in non-fixative conditions. HLA-G positive cells diluted in HLA-G negative cells were isolated by flow cytometry or magnetic cell sorting. However, no HLA-G positive cells could be recovered from exocervical samples. SRY gene was detected by ddPCR in exocervical samples from male (50%) but also female (27%) pregnancies.

Our data suggest that trophoblasts are too rarely and inconstantly present in noninvasive exocervical samples to be reliably retrieved by standard immunoisolation techniques and therefore cannot replace the current practice for prenatal screening and diagnosis.