Protein expression of angiotensin-converting enzyme 2, a SARS-CoV-2-specific receptor, in fetal and placental tissues throughout gestation: new insight for perinatal counseling

Abstract

Objective: Pregnant women can be infected by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), yet the incidence of perinatal infection is low. We hypothesized that this could be related to low expression of the membrane receptor for SARS-CoV-2, angiotensin-converting enzyme 2 (ACE2), in the fetoplacental unit. We evaluated protein expression of ACE2 at various gestational ages in both placentae and fetal organs from pregnancies not infected with SARS-CoV-2.

Methods: In May 2020, using samples from a registered biobank, we performed immunohistochemical analysis for ACE2 in tissue samples from fetal organs and placentae from five cases of second- or third-trimester medical termination of pregnancy in healthy women (performed between 15 and 38 weeks' gestation), as well as a further two placentae, one from a 7-week spontaneous miscarriage in a non-infected woman and one from a symptomatic pregnant woman positive for SARS-CoV-2 delivered by Cesarean section at 34 weeks. Samples were paraffin-embedded and organ tissues included kidney, brain, lung, intestinal tract, heart and testis. Matching tissues (kidney, intestinal tract, lung and testis) from autopsies of four 8-year-old children were tested as controls. Tissue sections were incubated with rabbit monoclonal anti-ACE2, and protein expression of ACE2 was detected by immunohistochemistry.

Results: ACE2 expression was detected in fetal kidney, rectum and ileum samples from 15 weeks onwards and in the pediatric controls. It was barely detectable in fetal lung samples at 15 + 5 weeks' gestation and not detectable thereafter, and, in the pediatric controls, ACE2 was detectable only in type-2 pneumocytes. No ACE2 expression was found in the cerebral ependymal or parenchymal tissues or in cardiac tissues. ACE2 was expressed in placental syncytiotrophoblast and cytotrophoblast samples, but not in the amnion, from 7 weeks onwards. The intensity and distribution of ACE2 staining in the placenta from the symptomatic SARS-CoV-2 woman was similar to that in the non-infected placentae.

Conclusions: Marked placental expression of ACE2 provides a rationale for vertical transmission at the cellular level. Absence of ACE2 expression in the fetal brain and heart is reassuring regarding the risk of congenital malformation. Clinical follow-up of infected pregnant women and their children is needed to validate these observations. © 2020 International Society of Ultrasound in Obstetrics and Gynecology.

Keywords: ACE2; COVID-19; SARS-CoV-2; fetal organs; placenta; protein expression; vertical transmission.

Figure 1

Expression of SARS‐CoV‐2‐specific receptor, angiotensin‐converting enzyme 2 (ACE2), in fetal kidney (a), intestinal tract (b), lung (c) and testis (d), compared with those of 8‐year‐old control patients, as detected by immunohistochemistry using anti‐ACE2 antibodies. (a) Light microscopy (original magnification, ×200) showing strong ACE2 expression across gestation in kidney, within proximal tubule (black arrows) and glomeruli (red arrows), with parietal cell and podocyte expression. (b) Light microscopy (×40, ×40, ×400, ×200, ×40) showing ACE2 expression across gestation in intestinal tract (rectum and ileum). Red arrow indicates goblet cell with no ACE2 expression surrounded by enterocytes positive for ACE2 expression. (c) Light microscopy (×200, ×400, ×400, ×400) showing ACE2 expression in lung. 8‐year‐old control patient showed positive ACE2 staining in type‐2 pneumocyte. 15 + 5‐week fetus showed weak apical expression in apical membrane in alveolar epithelium (arrows), and there was no clear ACE2 expression after this gestational age. (d) Light microscopy (×400) showing weak expression within fetal testis at 38 + 1 weeks.

Figure 2

Expression of SARS‐CoV‐2‐specific receptor, angiotensin‐converting enzyme 2 (ACE2), in fetal brain (a), heart (b), eye (c) and olfactory mucosa (d), as detected by immunohistochemistry using anti‐ACE2 antibodies. (a) Fetal brain at 20 + 1 weeks. Top left: light microscopy (original magnification, ×100) revealed no ACE2 expression in frontal cortex. Top right: light microscopy (×400) revealed no ACE2 expression in brain ependyma. Bottom left and right: light microscopy (×40 and ×400) showed ACE2 expression in choroid plexus. (b) Heart at 38 + 1 weeks. Light microscopy (×400) showed no expression of ACE2 in heart or coronary artery. (c) Eye at 38 + 1 weeks. Light microscopy (×400) showed ACE2 expression in apical membrane in cornea and bulbar conjunctiva. (d) Olfactory mucosa at 29 + 4 weeks. Light microscopy (×400, ×200) showed staining in Bowman's gland.

Figure 3

Comparison of expression of SARS‐CoV‐2‐specific receptor, angiotensin‐converting enzyme 2 (ACE2), in placental tissue of non‐infected (a) and SARS‐CoV‐2‐infected (b) patients, as detected by immunohistochemistry using anti‐ACE2 antibodies. (a) Placentae of non‐infected patients. Top row: light microscopy (original magnification, ×100) of basal plate revealed ACE2 expression in intermediate trophoblasts. Bottom row: light microscopy (×400) of placental villi showed ACE2 expression in cytotrophoblast (black arrow) and syncytiotrophoblast (red arrow) tissue. (b) ACE2 expression in placenta of SARS‐CoV‐2‐infected patient, showing similar staining to that in placentae of non‐infected patients.