Interpretation
To explore and compare the levels of sHLA-G during pregnancy in mothers delivering SGA and those delivering AGA, we chose a homogenous population of mothers delivering singleton live term SGA and AGA infants A univariate analysis was carried out to compare the maternal and neonatal characteristics between the two groups. Our analysis revealed a statistically significant difference in maternal height, maternal BMI and parity between cases and controls. Short stature (<145cm)20, BMI<18kg/m2 21 and nulliparity22 have been reported as risk factors for SGA births in previously published reports. Our results reflect the same with higher frequency of nulliparous and shorter mother with lower BMI in cases as compared to control. As there are no associations reported between aberrant levels of sHLA-G and maternal height, low BMI or parity, these variables were not adjusted for in the analysis.
Soluble HLA-G protein produced in the human placenta has been reported to enter maternal circulation10 and in corroboration to these studies effect of sHLA-G on cytotoxic T cells and cytokine stimulation has been described to be dose-dependent23. In line with these observations, we wanted to explore if the differential levels of sHLA-G protein in the pregnant mothers is associated with SGA births. sHLA-G levels measured in our study population across pregnancy were found to be highest at the start of gestation and revealed a decrease as pregnancy progressed and finally diminished at delivery in both cases and control pregnancies. This finding corroborates with the earlier studies conducted in Spanish and Turkish pregnant females where it was found that in healthy control pregnancies HLA-G levels were high at early gestation and start decreasing as pregnancy progressed to term24, 25. A drop in the levels of sHLA-G was observed at 26-28 weeks of gestation because the invasion of HLA-G expressing EVT into the maternal uterine spiral arteries is completed by 18-20 weeks of gestation26, 27 and hence the need of high levels of sHLA-G. Invasion of EVTs into the maternal decidua is needed for successful establishment of pregnancy and also for remodeling of maternal spiral arteries from high resistance high pressure to low resistance low pressure arteries28. The levels of sHLA-G drop further at term for parturition to occur.
No significant difference was observed in the sHLA-G trajectories during pregnancy in mothers delivering SGA as compared to those delivering AGA. A trend towards a higher sHLA-G levels at the first trimester of pregnancy (< 14weeks of gestation) was observed in mothers delivering SGA infants as compared to those delivering AGA infants. A higher level of sHLA-G at the first trimester of pregnancy in mothers delivering SGA as compared to those delivering AGA could be due to interplay of several immune mechanisms that operate in the placenta and help in establishment and maintenance of a healthy pregnancy. A perturbation in the immune mechanisms could have triggered the increase in expression of sHLA-G in the placenta as a compensatory mechanism in SGA fetuses so as to protect the fetus from being an adverse event at the initial stage in pregnancy. This is being reflected by the higher sHLA-G levels in the first trimester of pregnancy in the peripheral circulation of the mother. This mechanism is not sturdy enough to counter-balance the disturbed immune status and ultimately results in the delivery of an SGA infant. It has been demonstrated that HLA-G plays an important role in shifting Th1/Th2 balance towards Th2 polarization in the decidual tissues and this shift is crucial for establishment and maintenance of pregnancy29. It is also known that IL-10, a crucial anti-inflammatory cytokine of pregnancy, is released by extra villous trophoblast cells and induces HLA-G expression in EVTs. IL-10 also induces differentiation of CD4+ T cells into Th2 cells30. In line with this literature, we could speculate that in case of pregnancies complicated by SGA, there is a disruption in Th2 polarization and in order to achieve Th2-skewing state, more of IL-10 is released by the Tregs, EVTs and other decidual cells. Increased levels of IL-10 may upregulate the expression of HLA-G in these pregnancies. A concurrent upregulation of IL-10 and sHLA-G has been also been observed in cervical cancer patients, suggesting that IL-10 induces an immunosuppressive environment in the cancer cells by up regulating HLA-G expression31. In case of pregnancies with SGA fetuses, these compensatory mechanisms are not sufficient to restore the immune balance and ultimately results in the birth of an SGA neonate, but albeit protecting it from other mortal adverse outcomes such as recurrent pregnancy loss. The reduced levels of sHLA-G have been associated with recurrent pregnancy loss32.
Another explanation for higher sHLA-G levels at the first trimester in mothers delivering SGA could be that apart from the fetal derived sHLA-G, maternal-derived sHLA-G is also being expressed at the maternal-fetal interface to support implantation of SGA fetuses and hence the sHLA-G that is being detected in the maternal circulation in the first trimester is a mix of fetal-derived and maternal derived sHLA-G. This speculation could be supported by the study in which it has been reported that those women who had successful pregnancies after IVF expressed higher sHLA-G in the pre-ovulatory phase as compared to those with failed IVF33. Maternal sHLA-G was able to protect the fetus from rejection but could not save it from being born SGA.