Immune System and Psychological State of Pregnant Women during COVID-19 Pandemic: Are Micronutrients Able to Support Pregnancy?

The immune system is highly dynamic and susceptible to many alterations throughout pregnancy. Since December 2019, a pandemic caused by coronavirus disease 19 (COVID-19) has swept the globe. To contain the spread of COVID-19, immediate measures such as quarantine and isolation were implemented. These containment measures have contributed to exacerbate situations of anxiety and stress, especially in pregnant women, who are already particularly anxious about their condition. Alterations in the psychological state of pregnant women are related to alterations in the immune system, which is more vulnerable under stress. COVID-19 could therefore find fertile soil in these individuals and risk more severe forms. Normally a controlled dietary regimen is followed during pregnancy, but the use of particular vitamins and micronutrients can help counteract depressive-anxiety states and stress, can improve the immune system, and provide an additional weapon in the defense against COVID-19 to bring the pregnancy to fruition. This review aims to gather data on the impact of COVID-19 on the immune system and psychological condition of pregnant women and to assess whether some micronutrients can improve their psychophysical symptoms.


Introduction
Pregnancy is a unique and particular immune state [1]. The maternal immune system must tolerate the paternal antigens present in the fetus and at the same time maintain its ability to defend against possible pathogens. Physiological pregnancy consists of three stages: first pro-inflammatory phase (I trimester), anti-inflammatory phase (II and III trimesters), and second pro-inflammatory phase (late III trimester and childbirth) [2]. During pregnancy, the immune system and the body itself are highly dynamic and subject to several changes [2].
The uterus expands by increasing pressure in the abdominal cavity, lifting the diaphragm. The sub-costal angle of the rib cage increases and the ribs expand outward, decreasing chest compliance by about 35-40% [3,4]. This natural mechanism explains the Figure 1. Summary of progesterone immunomodulatory mechanism. P4 directly influences T cell activation and differentiation by modulating TCR signal transduction or indirectly by generating tolerant antigen-presenting cells (APCs) such as dendritic cells (DC) that inhibit T cell activation during TCR interaction. P4 can also inhibit cellular cytotoxicity. P4 can promote placental tissue growth and invasion at the maternal-fetal interface by inducing immune-tolerant phenotypes of macrophages, natural killer (NK), and T regulatory (Treg) cells, as well as exhausting activated CD4 and CD8 T cells that have interacted with placental-derived fetal-paternal antigens. Chemoattractant molecules produced on placental tissue help these tolerant leukocytes migrate to maternal-fetal contact. Created with BioRender.com.
Concentrations of P4 similar to those present in the lutein phase conferred protection against influenza A-induced pneumonia in mice, decreasing the inflammatory state, improving lung function, and stimulating cell proliferation [69]. Scientific evidence shows that P4 also has an antiviral activity in VeroE6 cells infected with SARS-CoV-2 [70]. P4 and human chorionic gonadotropin may act by inhibiting Th1, containing cytokine storm, and limiting the evolution of severe forms of COVID-19 [71].
Pregnant women during the first and third trimesters show a pro-inflammatory state that could facilitate the cytokine storm induced by SARS-CoV-2, causing more severe pathological conditions in these patients [72]. The transition during pregnancy from a proinflammatory state to an anti-inflammatory phase may depend on estrogens and progesterone, to better assist the fetal tolerance mechanism [73]. Pregnant women appear to be relatively protected from serious COVID-19-induced outcomes [74,75]. During pregnancy, there are several adaptive changes in the respiratory and immune systems that increase morbidity in pregnant women [76]. In the third trimester of pregnancy, there is an intensification in circulating phagocytes and dendritic cells, immune cells that are evolutionarily conserved with the role of antigen-presenting cells (APC) [77][78][79], capable of producing IFN type I, essential for the antiviral response. In addition, a decrease in T, B, Figure 1. Summary of progesterone immunomodulatory mechanism. P4 directly influences T cell activation and differentiation by modulating TCR signal transduction or indirectly by generating tolerant antigen-presenting cells (APCs) such as dendritic cells (DC) that inhibit T cell activation during TCR interaction. P4 can also inhibit cellular cytotoxicity. P4 can promote placental tissue growth and invasion at the maternal-fetal interface by inducing immune-tolerant phenotypes of macrophages, natural killer (NK), and T regulatory (Treg) cells, as well as exhausting activated CD4 and CD8 T cells that have interacted with placental-derived fetal-paternal antigens. Chemoattractant molecules produced on placental tissue help these tolerant leukocytes migrate to maternal-fetal contact. Created with BioRender.com.
Concentrations of P4 similar to those present in the lutein phase conferred protection against influenza A-induced pneumonia in mice, decreasing the inflammatory state, improving lung function, and stimulating cell proliferation [69]. Scientific evidence shows that P4 also has an antiviral activity in VeroE6 cells infected with SARS-CoV-2 [70]. P4 and human chorionic gonadotropin may act by inhibiting Th1, containing cytokine storm, and limiting the evolution of severe forms of COVID-19 [71].
Pregnant women during the first and third trimesters show a pro-inflammatory state that could facilitate the cytokine storm induced by SARS-CoV-2, causing more severe pathological conditions in these patients [72]. The transition during pregnancy from a pro-inflammatory state to an anti-inflammatory phase may depend on estrogens and progesterone, to better assist the fetal tolerance mechanism [73]. Pregnant women appear to be relatively protected from serious COVID-19-induced outcomes [74,75]. During pregnancy, there are several adaptive changes in the respiratory and immune systems that increase morbidity in pregnant women [76]. In the third trimester of pregnancy, there is an intensification in circulating phagocytes and dendritic cells, immune cells that are evolutionarily conserved with the role of antigen-presenting cells (APC) [77][78][79], capable of producing IFN type I, essential for the antiviral response. In addition, a decrease in T, B, and NK cells is evident [80]. A possible release of viral RNA in cells activates Toll-like receptors (TLRs), a group of phylogenetically conserved receptors [81][82][83][84][85][86][87], which induce the release of pro-inflammatory cytokines. These activate epithelial cells that recruit innate immune cells (NK cells and neutrophils). In addition, dendritic cells exhibit antigen by activating mediated CD4+ and CD8+ T responses [88]. Alterations due to COVID-19 infections can lead to preeclampsia, preterm birth, and emergency cesarean surgery [89]. The immune profile of pregnant and non-pregnant women is almost similar, differing only in lower blood white cell counts in pregnant women COVID-19 positive [15,90].
One of the consequences of COVID-19 infection is lymphopenia, accompanied by a major release of cytokines, called a "cytokine storm", which leads to a very severe form of bilateral pneumonia, with extensive widespread tissue damage [91]. The susceptibility to this infection varies from individual to individual and is closely related to immune status. Immune dysfunction can therefore play an important role in the evolution of the disease [92,93]. Patients healed from COVID-19 showed a decrease in immature B cells, CD4+ memory T cells, and CD8+ T cells [94], while they had high plasma cell levels [95]. These data were compared with those of pregnant women who were COVID-19 positive after healing. Decreasing Th17 cells, belonging to memory cells and specific NK-virus cells (CD3-CD56+NKP46+) were found. NKP46+ is an NK cell activation receptor involved in interactions against pathogens [91]. Lymphopenia occurs during normal pregnancy. However, in pregnant women with SARS-CoV-2, no major changes were found in this particular condition. Marked infiltration of CD68+ macrophages has been observed in the placenta, which had slight hypoxic characteristics, but it did not cause any particular problem during childbirth [91].

Psychological Profile of Pregnant Women during COVID-19
Pregnancy involves a series of changes both hormonal and psychological, altering the emotional and sensitive state, due to the uncertainty, unpredictability, and novelty of the situation [96,97]. This stage of life brings women into a particularly vulnerable psychopathological state. Between 10% and 16% of pregnant women suffer from depression [98], while about 8% suffer from anxiety [99], with serious effects on fetal and maternal health, such as preterm birth, low birth weight, postpartum depression, and hypothalamic-pituitary-adrenal dysregulation in newborns [100,101].
The COVID-19 pandemic, especially in its early stages, has had a very strong psychological impact, with an increasing prevalence and severity of mental health problems in the general population [102], especially in pregnant women, due to the implementation of health precautions such as quarantine, wearing masks, and social distancing [103]. Immunological functions, physiological changes, and susceptibility to infection in pregnant women lead to increased experiences of stress, anxiety, and depression. A cross-sectional study conducted in China between February and March 2020 examined a total of 560 pregnant women with a mean age of 25.8 years, who were given a questionnaire with questions about attitudes and mental health toward COVID-19. The psychological impact caused by this disease has been significantly associated with the trimester of pregnancy in which these women were found. There was more evidence of psychological distress during the second trimester of pregnancy, which also harmed health (work stress, stress at home, apprehension, and helplessness), although pregnant women paid more attention to their mental health and performed relaxation activities [104,105]. A similar result emerged in Italian pregnant women from a recent systematic review on the same subject [106].
During the first year of the pandemic, symptoms of depression, anxiety, and posttraumatic stress were detected. Among the major risk factors for the development of these psychopathologies are increased concern for the health of the other person, previous mental disorders, and lack of social support. Women who have had critical antenatal and postnatal experiences during the dissemination of COVID-19 tend to focus on the other, decentralizing, ignoring, and replacing personal needs to take care of the newborn [107].
A study by Ravaldi et al. (2020), also conducted in Italy, focused on expectations and concerns regarding childbirth of pregnant women during the pandemic. While in the pre-pandemic period birth was accompanied by joy and excitement, after the spread of COVID-19, birth was associated with fear and sadness in more than half of the women interviewed. Other reasons related to fear were: loneliness, anguish, incapacity, and constraint [108]. In addition, isolation and separation from the family in the vicinity of childbirth have had a traumatic effect on COVID-19 positive women who, feeling lonely in the hospital setting, lacked social support, which is crucial in the perinatal period [109].
In the United States, a recent study (2021) found that 36.4% of pregnant women had clinically significant levels of depression, higher than the generally accepted prevalence of 20% [110]. One in five women also had clinical thresholds for generalized anxiety, and one in ten reported significant levels of post-traumatic stress disorder (PTSD). Women with a pre-existing diagnosis of mental health problems were also 1.5 to almost 4 times more likely to develop symptoms above the clinical threshold for anxiety, depression, and PTSD [111].
It is therefore necessary to provide psychological and social support for pregnant women during this period of uncertainty to avoid and minimize serious and invalidating perinatal and postnatal consequences for parents and the newborn [112]. Individuals with less social support experience greater mental health problems, especially at high levels of negative cognitive assessment. It is therefore considered necessary to raise awareness among health professionals of the increased risk to which pregnant women are exposed to mental health problems and, at the same time, to draw up clear guidelines to provide the best possible care for these patients, even in the event of a pandemic, using e.g., electronic interventions, as suggested by several studies [113,114]. Psychological therapies of choice include Cognitive Behavioral Therapy (CBT) and mindfulness-based approaches: both manage to reduce anxiety and depression in perinatal women through a structured treatment plan that trains people to internalize their attention to regularize their emotions and attention [112,115].

Micronutrients Helping Pregnancy
A correct intake of micronutrients can help the success of a physiological pregnancy and enforce the immune system of the mother, through diet or with additional dietary supplements [116]. These micronutrients, such as iron (Fe), zinc (Zn), selenium (Se), and vitamins play a key role in strengthening innate and specific immunity and preventing possible risks of pregnancy [117], while a deficiency of these nutrients can induce alterations in the immune system and make the organism most susceptible to infections [118,119]. A summary of the micronutrients herein described, with recommended intake and further suggestions can be found in Table 1.
Iron (Fe). Iron is an essential micronutrient and strengthens several immune functions. It is involved in the proliferation and differentiation of T cells and is a fundamental component of some enzymes linked to immune cells [120]. In addition, iron promotes the intrauterine growth of the fetus [121]. A deficiency, which in pregnant women is estimated for 50% of cases globally, reduces pro-and anti-inflammatory cytokines and may lead to anemia, morbidity, and prenatal mortality [122,123] and, in childhood, may contribute to the development of cognitive and behavioral problems [124,125]. In addition, low blood levels of Fe are related to altered functions of macrophages, NK cells, neutrophils, T and B cells [126]. Iron deficiency is associated with higher levels of prenatal depression [127,128]. Its integration can reduce the severity of respiratory viral infections and combat them [129].
Selenium (Se). Selenium is an essential micronutrient closely linked to the function of the immune system [124]. It shows immunostimulant effects on the proliferation of T and B cells and the activity of macrophages and NK cells [130,131]. Selenoproteins support the body's defense system by influencing the functions of leukocytes [116]. Selenium deficiency is related to increased susceptibility to viral infections and, in pregnant women, may contribute to spontaneous abortion and preeclampsia [132]. The integration of this micronutrient increases resistance to viral infections, reduces the risk of prenatal mortality [133], and prevents hypertension [134]. Selenium intake also inhibits the development of postpartum depression, preventing its symptoms [135].

Zinc (Zn).
Zinc plays a key role in modulating the immune system by promoting the development and function of macrophages, neutrophils, NK cells, inducing CD8+ T-cells proliferation, and assisting in the inflammatory process [120]. This micronutrient shows important antiviral properties, counteracting the replication of RNA viruses and inhibiting the replication of SARS-CoV at low concentrations [136]. Zinc deficiency is related to an altered phagocytic function, and reduced production of macrophages, DCs, and B and T cells [137]. Zn is essential for embryogenesis and fetal development. It is estimated that about 18% of pregnant women have a zinc deficiency. This can lead to premature birth, slowed fetal growth, increased risk of infection and also dwarfism [138], and alterations in brain development [139]. Integration of this micronutrient improves immune functions, protects against respiratory viral infections, reduces the risk of preterm birth, and lowers the incidence of gestational hypertension [140]. Zinc shows anxiolytic and antidepressant properties and its intake in higher quantities can better buffer the impact of stress on the development of symptoms of prenatal depression [141] and postpartum anxiety and depression [142].
Vitamin A. Vitamin A is a fat-soluble vitamin also called anti-inflammatory vitamin [116]. It plays an important role in the production, maturation, regulation, and function of macrophages, NK cells, neutrophils, innate lymphoid cells (ILCs), dendritic cells (DCs), T and B cells [143]. Its deficiency is related to respiratory infections [144], congenital fetal defects [145], gestational and diabetes mellitus [146], and schizophrenia [147]. Proper supplementation of this vitamin can reduce the risk of anemia [148] and shows to decrease the morbidity and mortality of some infectious diseases such as HIV [149]. Birth abnormalities have not been linked to vitamin A intakes of less than 10,000 IU per day during pregnancy. However, there are contradictory findings for daily doses ranging from 10,000 to 30,000 IU. Vitamin A intakes of more than 10,000 IU per day are not suggested for pregnant women who are well-nourished [150]. Moreover, excessive doses of vitamin A may be related to an increased risk of developing anxiety, depression, and mood disorders [151].
Vitamin C. Vitamin C, or ascorbic acid, is a water-soluble vitamin [152], which has high antioxidant activity and has the ability to fight infection. It modulates the migration of neutrophils to the site of infection and regulates phagocyte cells and NK cells [116]. In addition, it can mitigate the symptoms of respiratory infections [153]. Vitamin C is also able to promote intrauterine growth and childbirth [154]. Its integration contrasts preeclampsia, hypertension, and gestational diabetes [155]. With an average intake of 2000 IU/day of this vitamin, the risk of pregnancy-related complications, such as premature rupture of membranes, is decreased [156]. Vitamin C plays a key role in the biosynthesis of neurotransmitters such as serotonin and norepinephrine, lower levels of this vitamin have been found in patients with depression. Its intake also prevents schizophrenic and depressive symptoms [157,158].
Vitamin D. Vitamin D is a fat-soluble vitamin capable of regulating calcium homeostasis in bones [116]; moreover, it has powerful anti-inflammatory and immunomodulatory activities on innate and adaptive immunity, stimulating the differentiation of monocytesmacrophages and inhibiting the production of cytokines [159,160] (Figure 2). Calcium in extremely important for S100 protein expressed by immune cells, in particular macrophages, in immune response [161]. Vitamin D is related to a reduction in viral replication and viral infectivity [162,163]. Vitamin D in pregnant women has shown preventive effects against preeclampsia [164,165], and preterminal birth [166], playing a role in angiogenesis, placental implantation, and endothelial functions [167]. Vitamin D can regulate the immune system through its intracellular receptors (VDRs) present in monocytes, macrophages, B cells, T cells, DCs, and NKs [168]. It also modulates the synthesis of antimicrobial peptides, such as cathelicidin and defensins [169]. Vitamin D is also able to control and regulate the differentiation and function of NKs and DCs, promoting the presentation of antigens to lymphocytes [168]. Moreover, by reducing the secretion of IL-2, it leads to the suppression of Th lymphocytes, inhibiting the secretion of inflammatory cytokines such as IFNγ and IL-17 [168], helping to maintain a tolerogenic state [170,171].
Vitamin D modulates the proliferation of CD4+ lymphocytes [172] and promotes high levels of Treg cells, responsible for fetal tolerance [173]. Women with low vitamin D intake show a higher risk of developing anxiety and depressive symptoms early in pregnancy [174], with a higher probability of developing depression even after childbirth [175]. Calcium in extremely important for S100 protein expressed by immune cells, in particular macrophages, in immune response [161]. Vitamin D is related to a reduction in viral replication and viral infectivity [162,163]. Vitamin D in pregnant women has shown preventive effects against preeclampsia [164,165], and preterminal birth [166], playing a role in angiogenesis, placental implantation, and endothelial functions [167]. Vitamin D can regulate the immune system through its intracellular receptors (VDRs) present in monocytes, macrophages, B cells, T cells, DCs, and NKs [168]. It also modulates the synthesis of antimicrobial peptides, such as cathelicidin and defensins [169]. Vitamin D is also able to control and regulate the differentiation and function of NKs and DCs, promoting the presentation of antigens to lymphocytes [168]. Moreover, by reducing the secretion of IL-2, it leads to the suppression of Th lymphocytes, inhibiting the secretion of inflammatory cytokines such as IFNγ and IL-17 [168], helping to maintain a tolerogenic state [170,171]. Vitamin D modulates the proliferation of CD4+ lymphocytes [172] and promotes high levels of Treg cells, responsible for fetal tolerance [173]. Women with low vit- are depicted in this diagram. Different actors in the innate (red square) and adaptive immune (blue square) compartments are targeted by 1,25(OH)2D3. Vitamin D has been found to promote innate immune responses by increasing chemotaxis, antimicrobial peptides, and macrophage differentiation. Vitamin D also helps to enhance adaptive immune responses. For example, at the level of antigen-presenting cells, such as dendritic cells, vitamin D inhibits the surface expression of antigen complexed with MHCII, costimulatory molecules, and the production of cytokines IL12 and IL23, causing an indirect change in the Th1 and T-cell polarization. drives the Th17 phenotype towards a Th2 phenotype. Abbreviation: Th-T helper; NK-natural killer; DC-dendritic cells. Created with BioRender.com. Vitamin E. Vitamin E is a fat-soluble vitamin with high antioxidant power and a high immunoregulatory capacity [176]. It modulates macrophages, NK cells, T cells, maturation and function of DCs, improves the humoral response, and reduces oxygen free radicals (ROS), nitrogen-free radicals (RNS), and prostaglandins [177,178]. Its integration improves the protection against viral infections and reduces the viral load of influenza [176,179]. In addition, this vitamin is capable to reduce and counteract oxidative stress during pregnancy, lowering the risk of preeclampsia and preterm birth [180]. Vitamin E levels in maternal blood are also correlated with improved fetal growth [181]. However, high doses of vitamin E (≥400 IU/day) are associated with an apparent decrease in birth weight, and its administration is suggested during the first trimester of pregnancy [182]. Low levels of vitamin E are also associated with an increased risk of depression, and its intake in the prenatal period could represent a protective factor against the development of depressive symptoms [183]. Improves immune functions, protects against respiratory viral infections, reduces the risk of preterm birth, lowers the incidence of gestational hypertension, has antidepressant effect and may prevent teratogenesis. [138,141] Vitamin A ≤10,000 IU/day Helpful for fetal development and for the regulation of the immune cells, but teratogenic at high doses. [143,150] Vitamin C 2000 IU/day Mitigates the effects of respiratory syndromes, helps in the biosynthesis of serotonin and norepinephrine, protects from pregnancy-related complications. [116,156] Vitamin D 600-1500 IU/day Implicated in the innate and adaptive immunity, protects against preeclampsia and preterminal birth. [160,166] Vitamin E 32.8-44.7 IU/day Improves humoral response and protects against viral infections. Better if administrated at suggested doses during the first trimester of pregnancy. [177,182]

Conclusions
Pregnancy involves changes in women's immune system and psycho-physical status, which may make them more vulnerable to infection. During COVID-19, altered immune response and stress may contribute to facilitating SARS-CoV-2 infection, exacerbating the symptoms. The use of vitamins and micronutrients, usually taken during pregnancy, can help strengthen the immune system, mood, and psycho-physical state of pregnant women. In addition, they can counteract the severe symptoms of COVID-19, helping to prevent infection.

Conflicts of Interest:
The authors declare no conflict of interest.