Pique-Regi R, Romero R, Tarca AL, et al. Does the human placenta express the canonical cell entry mediators for SARS CoV-2? eLife. 2020;9:e58716.
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Maternal infection with the novel SARS-CoV-2 virus has been associated with severe maternal morbidity and mortality causing adverse pregnancy outcomes, such as preterm birth and, potentially, stillbirth, with vertical transmission of the virus to the fetus possible. 1,2
Uniquely, maternal physiology supports both pro- and anti-inflammatory states within pregnancy—a system that not only must protect the mother but also must tolerate a semi-allogenic fetus. Studies demonstrate that the first and third trimesters are pro-inflammatory, while the second trimester is thought to be anti-inflammatory. 3 Since the discovery of the SARS-CoV-2 virus, the question surrounding vertical transmission (infectivity from mother to fetus via the placenta) has occupied the imagination of physicians, scientists, and pregnant women. Importantly, the virus is transmitted to human cells via the ACE2 (angiotensin-converting enzyme 2) receptor, which aids in viral cell attachment. ACE2 receptors are expressed in placental stromal cells, perivascular cells of decidua, cytotrophoblast and syncytiotrophoblast, 4 as well as blood vessel endothelium and vascular smooth muscle from both primary and secondary villi.
Details of the study
In their recent study, Pique-Regi and colleagues used single-cell RNA sequencing data to investigate whether the receptors responsible for SARS-CoV-2 infection are expressed in the human placenta. 5 Their findings suggest that TMPRSS2 is present in insufficient quantity in the placenta to make vertical transmission possible and/or clinically relevant. Thus, despite the presence of ACE2 receptors in placental tissue, without the enzymatic assistance of a helper protein like TMPRSS2 (transmembrane protease, serine 2), vertical transmission is highly unlikely. The researchers found that there was negligible co-transcription for ACE2 and TMPRSS2 in the placenta and that placental tissue lacks the mRNA necessary to produce the enzyme; they concluded that the likelihood of vertical transmission to the fetus was therefore unlikely.
As a caveat to their research, the authors noted that:
- transcription levels do not always correlate with protein expression
- it is possible that a noncanonical cell-entry mediator facilitates entry
- individuals with complications related to the renin-angiotensin-aldosterone system (such as hypertensive disease) may have alterations to the expression of ACE2.
Study strengths and limitations
Methods for this study reveal that the researchers examined 32 placentas, all taken in the third trimester (32.9-39.1 weeks), with a median gestational age of 36.9 weeks. Notably, 81.3% of placentas were from Black women, 6.2% from White women, and 12.5% from Other women. The median maternal age was 25 years, median body mass index was 27.8 kg/m 2, and 84.4% of women were multiparous. While this sample was not representative of race, gestational age, or parity, it is difficult to know whether those selection biases would have changed the researchers' findings.
The question regarding vertical transmission is one not answered solely on the basis of RNA sequencing data. Clinically, we know that neonates of mothers infected with SARS-CoV-2 have been born with immunoglobulin M antibodies, indicating antenatal exposure to the virus. 6,7 In addition, infants have tested positive immediately after birth for coronavirus disease 2019 (COVID-19) via nasopharyngeal swab and amniotic fluid, and there are ample cases of histologic and polymerase chain reaction evidence of placental infection. 8,9 We also know that inflammatory damage to the placenta could possibly break down the placental barrier. 10