sFlt1, a key anti-angiogenic factor central to pre-eclamptic pathophysiology, has  recently been found to have multiple isoforms. The sFlt1-e15a variant appears to be the predominant isoform specifically produced by the human placenta. In contrast, the well-described sFlt1-i13 isoform is the main isoform of the maternal endothelium. Therefore, the placental specific sFlt1-e15a splice variant may be chiefly responsible for the pathogenesis of pre-eclampsia, however its protein expression has never been characterized.

To investigate the expression of sFlt1-e15a within both normal and pre-eclamptic placenta we developed polyclonal antibodies targeting a unique peptide sequence in the C-terminal region of sFlt1-e15a. The specificity of these antibodies was then established using western analysis. Immunofluorescence confirmed expression of sFlt1-e15a principally within the syncytiotrophoblast layer of the placenta, with expression appearing increased in pre-eclampsia compared to preterm controls. We then developed an sFlt1-e15a specific capture ELISA. Initial assessment of sFlt1-e15a serum levels across normal pregnancy, using serially collected bloods from 6 to 36 weeks’ gestation (n=27), confirmed a gradual increase in sFlt1-e15a expression with a significant increase at 36 weeks’ gestation (p<0.05). The diagnostic potential for this ELISA compared to commercially available tests in a cohort of severe early-onset pre-eclamptics (n=14) was then explored. Notably, serum sFlt1-e15a in severe preterm preeclampsia was elevated 28.5 fold compared to a cohort of 50 women with normal pregnancies (median (IQR) 129 (100-188) ug/ml vs 4.5 (2.2-8.1) ug/ml; p<0.0001). In contrast, there was only an 11.7 fold increase in total sFlt1 in the preeclampsia cohort, when measured with a commercial sFlt1 ELISA.

Our data indicates that sFlt1-e15a protein expression is localised to the syncytiotrophoblast, is low in normal pregnancies and highly elevated in severe preterm preeclampsia. Furthermore, an sFlt1-e15a ELISA may provide better diagnostic/predictive performance to detect pre-eclampsia than current sFlt1 ELISA’s as it specifically detects only placentally derived sFlt1.