key: cord-1044895-w1cs7680
authors: Zhu, Guo‐hua; Liu, Lu; Huang, Xi‐xi; Li, Da‐jin; Zhu, Yi‐zhun; Lu, Xin; Du, Mei‐rong
title: The risk of intrauterine exposure to SARS‐CoV‐2 in female COVID‐19 patients: A comprehensive review
date: 2022-02-25
journal: Am J Reprod Immunol
DOI: 10.1111/aji.13528
sha: b031848d100236326a7722aae5c489c30c566773
doc_id: 1044895
cord_uid: w1cs7680

Severe acute respiratory syndrome coronavirus 2 (SARS‐CoV‐2) is a new type of coronavirus that has caused fatal infectious diseases and global spread. This novel coronavirus attacks target cells through the interaction of spike protein and angiotensin‐converting enzyme II (ACE2), leading to different clinical symptoms. However, for a successful pregnancy, a well‐established in‐uterine environment includes a specific immune environment, and multi‐interactions between specific cell types are prerequisites. The immune‐related changes in patients infected with novel coronavirus could interfere with the immune microenvironment in the uterus, leading to fetal loss. We first reviewed the intrauterine environment in the normal development process and the possible pregnancy outcome in the infection state. Then, we summarized the immune response induced by SARS‐CoV‐2 in patients and analyzed the changes in ACE2 expression in the female reproductive system. Finally, the present observational evidence of infection in pregnant women was also reviewed.

It had been more than 1 year since China first reported a few cases of pneumonia of unknown cause. The pathogen was later identified as a new type of coronavirus, called severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). 1 The coronavirus is spreading rapidly, and there is no sign of ending. SARS-CoV-2 infects and lysis the host cells by binding with angiotensin-converting enzyme II (ACE2), which is widely expressed in the ovary, uterus, vagina, and placenta. [2] [3] [4] [5] However, whether SARS-CoV-2 would cause intrauterine infection or affect female fertility is still a mystery, which should not be neglected.

Researchers mainly pay attention to women in the second or third trimester of pregnancy and paid more attention to the evidence of the vertical transmission of SARS-CoV-2. 6, 7 At present, there is little evidence that SARS-CoV-2 would spread vertically during the third trimester of pregnancy. [8] [9] [10] It was proved that most of the clinical symptoms of pregnant women infected with SARS-CoV-2 were not obvious. [10] [11] [12] However, the effect of SARS-CoV-2 on early pregnancy still lacks clinical evidence. Because the successful blastocyst implantation is closely related to the state of the endometrium and its acceptance time window, the establishment and maintenance of a normal pregnancy require sufficient crosstalk between the endometrium and the embryo and the embryo best microenvironment in uterus. [13] [14] [15] In addition, abnormal hormone and inflammatory environments may disturb the dialog of immune-endocrine interaction between the decidua and trophoblast during embryo implantation, leading to pregnancy complications. [16] [17] [18] [19] In this review, we would like to explain the risk of intrauterine exposure to the SARS-CoV-2 virus in female patients with COVID-19 and whether it could interrupt the development of pregnancy.

Endometrial epithelial cells express estrogen and progesterone receptors, which can sense hormone changes and send signals to endometrial stromal cells and immune cells. 20 Endometrial stromal cells also express estrogen and progesterone receptors and experience decidualization under the stimulation of hormones. They can secrete cytokines and regulate the activity of immune cells. [20] [21] [22] During pregnancy, trophoblast cells and decidual cells secrete different chemokines to recruit immune cells from peripheral blood. 23, 24 Decidual immune cells (DICs) participated in the formation of uterine spiral arteries. 25 the directional migration of trophoblast cells, 26 and protection of the fetus from infection. 27 Because the fetus is similar to a semiallograft, maintaining normal pregnancy requires DICs to form an immune tolerance microenvironment in the maternal interface to avoid the fetus's maternal immune rejection. For example, decidua natural killer Cells (dNK cells) show low-toxicity CD16 dim CD56 high phenotype. 28 Moreover, our previous studies confirmed that the decid-ual CD8 + T cells highly expressed inhibitory receptors, such as PD-1 and Tim-3, and showed a highly exhausted phenotype, with low cytotoxicity. 29 In addition, indoleamine-pyrrole 2,3-dioxygenase (IDO) derived from monocytes and FasL on the trophoblast surface could inhibit T cells' activity via different mechanisms. [30] [31] [32] Progesterone, which was sustained at a high plateau during pregnancy, can also inhibit the cytotoxicity of CD8 + T cells. 33 Therefore, immune adaptability is required to adapt to the fetus. The special immune-microenvironment of the fetal-maternal interfaces may relate to the increased risk for acquiring infections, which have demonstrated that certain conditions disproportionately affect pregnant women. 34 The dysfunction of the endometrium is one of the main causes of decreasing fertility. It was reported that about 60% of pregnancy loss occurs at 3-4 weeks during embryo implantation, and 10% occurs at 4-12 weeks of pregnancy. 35 Abnormal maternal-fetal interaction may cause spontaneous abortion in the first trimester of pregnancy, resulting in fetal growth restriction or eclampsia in the second or third trimester. The study of infection during pregnancy shows that various pathogens such as cytomegalovirus (CMV), dengue fever, acquired immunodeficiency syndrome, and rubella infections in early pregnancy would induce abortion. 36 Infection can lead to abnormal maternalfetal interactions through various mechanisms. TNF-α is a critical effector to induced pregnancy loss because blockade of TNF-α or TNF-α deficiency is a protective effect against the antiphospholipid syndrome. 37 While pregnant mice were challenged with LPS, a cell wall component from gram-negative bacteria, the decidua can synthesize and secret TNF-α, which results in fetal resorption. 38 Injection of polyinosinic-polycytidylic acid (Poly(I: C)), a viral mimic, can lead to fetal death through NKG2D-dependent manner by uterine NK cells or through IFNAR expression in pregnant mice. 39, 40 Meanwhile, we cannot neglect that pathogens could directly attack the trophoblast leading to fetal death. For example, CMV can target trophoblast cells, induce apoptosis of trophoblast cells, and activate TNF-α, leading to cell death and pregnancy failure. 41 

SARS-CoV-2 mainly encodes four structural proteins named spike protein (S protein, composed of S1 subunit and S2 subunit), membrane protein, envelope protein, and nucleocapsid, respectively. 42 SARS-CoV-2 initiates the fusion of the viral envelope and cell membrane through the interaction between S protein and ACE2 receptor, and the viral genome is subsequently released to target cells. 43 Subsequently, the virus started to replicate and translate into the host cells. Singlestranded (ss) RNA-sensing Toll-like receptors (TLR), such as TLR3 and TLR7 in the endosome, could recognize SARS-CoV-2 virions. And then, the recognition could activate the interferon regulatory factor (IRF) 3, IRF7, and nuclear factor (NF) -κB, inducing fast production of IFN-I, IFN-III, and proinflammatory cytokine. 44 IFN-signaling plays a critical role in antiviral immune response. 45 However, the function of more than a dozen of kinds of proteins encoded by SARS-CoV-2 is to escape immune recognition of host cells and killing. [46] [47] [48] For example, nonstructural protein 1 (NSP1) protein could inhibit host protein, thus inhibiting IFN-β production. 46 Based on immune escape mechanisms, severe patients usually show a delayed IFN reaction and prolonged virus replication. 49 Cytokine storm followed with impaired IFN response was another feature in coronavirus disease 2019 (COVID- 19) clinical manifestation. 50, 51 After transcription factors such as NF-κB, c-Jun, and the down- Among men infected with SARS-CoV-2, different degrees of inflammatory changes can be found in testicular tissue, which expresses ACE2 in various types of cells with impaired sperm quality (Figure 1. A-B) . [62] [63] [64] Recently, a preprint article revealed that the SARS-CoV-2 viral RNA was detected in 42.5% of reproductive tissues. 65 These pieces of evidence suggest that SARS-CoV-2 could target the reproductive system that expresses ACE2. However, there is insufficient research to provide substantial evidence for the harmful effects of this new virus on the reproductive system. Whether SARS-CoV-2 could also impair the female reproductive system was still a puzzle. 66 

It is reported that before pregnancy, the endometrial state of the women of childbearing age will change with the menstrual cycle. were high, TMPRSS2 was medium, and ACE2 was low. 67 However, the expression of ACE2, TMPRSS4, CTSB, CTSL, and MX1 increased dur-ing the implantation window. In addition, the mRNA levels of ACE2, TMPRSS4, CTSB, CTSL, and MX1 genes increase with age. 67 It corresponded to the earlier research. 4 In our unpublished data, the expression level of ACE2 protein in decidual was higher than that in the proliferation stage. In addition, studies found that the RAS system was expressed in large quantities in the uterus, mainly containing target cells that encode ACE2. 4 A recent multicenter prospective study showed that 46.2% of women had abdominal or pelvic pain. 12 68 The results showed that ACE2 was highly expressed in the stromal cells, peridecidual cells, the placenta cytotrophoblast (CT) and syncytiotrophoblasts (SCT), while it was lower expressed in extravillous trophoblasts (EVT) and gradually increased with the development of embryo. 68 However, another team analyzed the published single cells data of early pregnancy and concluded that the maternal-fetal interface showed little susceptibility to infections of SARS-CoV-2 because compared to the expression levels of AXL, a receptor of Zika virus (ZIKV) which can be vertically transmitted from an infected mother to the developing fetus in utero, the expression level of ACE2 was lower. 69 In addition, the expression of ACE2 in placental gradually decreased with pregnancy. 70 These researches may partly explain why SARS-CoV-2 infections in the third trimester scarcely caused vertical transmission. 11 

In the early period of the epidemic, due to the lack of a clear understanding of the pathogenic mechanism of SARS-CoV-2, most pregnant women chose cesarean section for delivery. By performing nucleic acid tests on the fetus, delivery placenta, and amniotic fluid of women with SARS-CoV-2 infection, researchers found little evidence of virus intrauterine infection. 10, 71 A cohort study in the United Kingdom showed that about 5% of newborn babies were detected SARS-CoV-2 mRNA positive. Six were found positive for SARS-CoV-2 within 12 h after birth, which could not exclude the possibility of vertical transmission. 11 It is worth noting that compared with nonpregnant women, the proportion of pregnant women suffering from serious diseases is relatively higher. 71 According to a case report, a 22-week pregnant woman diagnosed with COVID-19 had aborted, and the placenta was positive for the SARS-CoV-2 test (Figure 1. C-D) . 72 Specifically, the placenta (3 × 10 7 virus copies/mg) and umbilical cord (2 × 10 3 virus copies/mg) were show the relationship between the virus, placental pathology, maternal and perinatal outcomes. 8 The Lancet recently published an article reviewing the maternal and perinatal outcomes of the COVID-19 epidemic. 80 They found significant increases in stillbirth and maternal death during verses before the pandemic. However, there was no sig-nificant change in the rate of premature delivery, while the incidence of ectopic pregnancies was increased. 80 Besides, there was no significant effect on pregnancy complications, such as maternal gestational diabetes, pregnancy-induced hypertension, and premature delivery. 80 Although there was no vertical transmission, an article reported that IFN-stimulating gene (ISG) and major histocompatibility complex gene were up-regulated in the umbilical blood mononuclear cells of the fetuses whose mothers were diagnosed as COVID-19. 81 In conclusion, though the transmissibility of SARS-CoV-2 is horrific, the fetal development in the mother's uterus was relatively secure. It might be owed to the maternal immunologic barriers.

In summary, considering the expression of ACE2 and the discovery of virus particles in the placenta, the reproductive system, including the uterus and ovary, is more likely to be infected with SARS-CoV- 

There is no conflict of interest to declare.

Data derived from public domain resources.

Da-jin Li https://orcid.org/0000-0003-2280-9727

Mei-rong Du https://orcid.org/0000-0001-9998-1379

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