Male and female fetuses respond differently to COVID-19 virus

Maternal SARS-CoV-2 infection elicits sexually dimorphic placental immune responses. Evan A Bordt et al. Sciece Translational Medicine, Oct 19 2021. DOI: 10.1126/scitranslmed.abi7428

Abstract: There is a persistent bias toward higher prevalence and increased severity of coronavirus disease 2019 (COVID-19) in males. Underlying mechanisms accounting for this sex difference remain incompletely understood. Interferon responses have been implicated as a modulator of COVID-19 disease in adults, and play a key role in the placental antiviral response. Moreover, the interferon response has been shown to alter Fc receptor expression, and therefore may impact placental antibody transfer. Here we examined the intersection of maternal-fetal antibody transfer, viral-induced placental interferon responses, and fetal sex in pregnant women infected with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). Placental Fc receptor abundance, interferon stimulated gene (ISG) expression, and SARS-CoV-2 antibody transfer were interrogated in 68 human pregnancies. Sexually dimorphic expression of placental Fc receptors, ISGs and proteins, and interleukin-10 was observed following maternal SARS-CoV-2 infection, with up-regulation of these features in placental tissue of pregnant individuals with male fetuses. Reduced maternal SARS-CoV-2-specific antibody titers and impaired placental antibody transfer were also observed in pregnancies with a male fetus. These results demonstrate fetal sex-specific maternal and placental adaptive and innate immune responses to SARS-CoV-2.

Popular version: https://news.harvard.edu/gazette/story/2021/10/male-and-female-fetuses-respond-differently-to-covid-19/

DISCUSSION

Our results demonstrate the impact of fetal sex on the maternal and placental immune response to SARS-CoV-2, and the potential consequences for neonatal antibody-mediated immunity. We show that maternal SARS-CoV-2 infection is associated with reduced maternal SARS-CoV-2-specific IgG titers in the setting of a male fetus. SARS-CoV-2-specific placental antibody transfer to the male fetus was reduced despite up-regulation of placental Fc receptors in SARS-CoV-2-exposed male placentas; males were unable to overcome the reduced maternal titers and the highly fucosylated glycan profile of the spike protein-specific antibodies. Mirroring Fc receptor expression, placental expression of interferon stimulated genes and proteins was also sexually dimorphic, with notable up-regulation noted in male placentas in the setting of maternal SARS-CoV-2 infection. Collectively these findings provide evidence of maternal-placental-fetal immune crosstalk in the setting of maternal viral infection, with fetal sex playing a key role in modifying maternal humoral responses and placental innate and adaptive immune responses.

Epidemiologic data point to a persistent male bias in the development and severity of COVID-19 disease in adults, children, and infants (6, 8, 9, 82, 83). Male COVID-19 patients are three times as likely to require admission to intensive care units and have higher odds of death than females (84). This male-biased vulnerability to maternal SARS-CoV-2 infection mirrors the male-biased risk of mortality and morbidity across the perinatal period (1–3). Our findings of sexually dimorphic placental innate immune responses to infection, coupled with sex differences in transfer of maternal humoral immunity, may provide insight into increased vulnerability of male infants to morbidity and mortality.

Although the impact of fetal sex is not consistently evaluated in studies of placental function (85), sex-specific alterations in the placental transcriptome have been described in both normal and pathologic pregnancies (86–89). Sex differences in the placental immune response to prenatal infections and other immune stressors have been described in human and animal models (54, 90–93), but have not been examined in SARS-CoV-2 infection. Here we report that maternal SARS-CoV-2 infection induces a sexually dimorphic placental antiviral innate immune response, with up-regulation of ISGs in male, but not female, placentas. Male-specific stimulation of placental ISGs following SARS-CoV-2 exposure is consistent with the heightened male immune responses reported in SARS-CoV-2-infected adult and pediatric cohorts (5, 6, 8–10, 42, 94). Interestingly, although we did not see evidence of maternal viremia nor placental, cord blood, or neonatal SARS-CoV-2 infection (28, 95), and the majority of maternal infections represent mild or moderate disease, there is still evidence of altered placental gene expression and an antiviral response in the placentas of male pregnancies. This indicates that even a mild maternal infection in the absence of placental or fetal infection has the potential to affect placental function and fetal development.

Due to their immature immune system, newborns rely on the passive transplacental transfer of maternal antibodies for initial protection against infectious pathogens (15, 18, 96). Although previous reports in adults have noted sex differences in the production of SARS-CoV-2-specific antibodies (42, 97), sex-biased maternal production and transplacental transfer of SARS-CoV-2-specific antibodies has not been well-described. We previously reported impaired placental transfer of maternal SARS-CoV-2-specific antibodies in the setting of maternal COVID-19 (28, 29). Although there are known sex differences in adult antibody production in response to SARS-CoV-2 infection (42, 94), little is known about sex differences in maternal titers or transplacental antibody transfer (98, 99) in the setting of maternal SARS-CoV-2 infection. Our finding of decreased maternal antibody titers against all measured SARS-CoV-2-specific antigens (S, S1, S2, RBD, N) when the fetus was male versus female was a difference not observed for influenza or pertussis-specific antibodies. Reduced maternal SARS-CoV-2-specific IgG titer in male pregnancies was undoubtedly a driver of the reduced transplacental transfer noted in male fetuses (15). This finding of impaired placental transfer of SARS-CoV-2-specific antibodies, more pronounced in males, is consistent with the male-specific reduction of placental transfer of maternal IgG reported in a non-human primate model of maternal stress (98). Reduced maternal antibody titers in the setting of a male fetus were likely attributable to suppressed maternal pro-inflammatory responses in the setting of a male fetus, which have been described in prior studies and may function to improve tolerance of the fetal allograft (13, 14). The direct correlation between pro-inflammatory response and increased antibody production noted in COVID-19 infection (66, 67) suggests that blunted maternal inflammatory responses in the setting of a male fetus may limit maternal antibody production in the setting of acute infection. Whether the male-biased impairment in placental SARS-CoV-2-specific antibody transfer renders male infants more vulnerable to early-life SARS-CoV-2 infection remains unclear, as the amount of antibody necessary for protection against SARS-CoV-2 infection is unknown and there are few sex-disaggregated reports of neonatal (100, 101) or infant infection (8).

Although the up-regulation of Fc receptor expression in male placentas may represent a compensatory placental response driven by reduced maternal antibody titer and transplacental transfer of SARS-CoV-2 antibodies (22, 102, 103), this response was likely reinforced by the increased IFN signaling in males versus females. IFN-stimulated signaling may impact placental antibody transfer via alteration in Fc receptor expression and function (104–106); for example, Type I IFN signaling is known to up-regulate Fcγ receptor expression on monocytes (107). Hofbauer cells, tissue-resident macrophages of the placenta, express FcγRI, II and III (23). The male-specific Hofbauer cell hyperplasia in placentas exposed to maternal SARS-CoV-2 infection could therefore also be contributing to increased placental FcγRI and FcγRIII expression in males.

Although the low maternal antibody titers in male pregnancies may have driven a compensatory up-regulation of Fc receptors in the male placenta, the up-regulation of FcγRIII and co-localization of FcγRIII with FcRn in the male placenta likely impeded placental transfer of SARS-CoV-2-specific antibodies, given their distinct Fc-glycan profile. Our Fc-glycan analysis demonstrated that SARS-CoV-2-specific antibodies were highly fucosylated in both male and female pregnancies, a post-translational modification that lowers antibody affinity for FcγRIII (69–71). The male-specific placental increase in FcγRIII expression and co-localization of FcγRIII with FcRn might therefore present an additional impediment to transferring the already-low maternal titers of SARS-CoV-2-specific antibodies to the fetus. Males instead preferentially transferred bisected (afucosylated) and agalactosylated (G0), afucosylated spike protein-specific antibodies, as afucosylated antibodies are more easily transferred by FcγRIII. Given the inflammatory nature of G0 and B antibodies (72, 108, 109), their preferential transfer might promote a more inflammatory immune response in male fetuses.

Innate immune sensing of SARS-CoV-2 involves the activation of type I and type III interferons and up-regulation of ISGs in target cells (110). Given the relative paucity of SARS-CoV-2 placental infection (28) in comparison to other pandemic infections such as Zika virus (ZIKV) (111), the increased ISG production and up-regulated IL10 expression in exposed male placentas may be a protective mechanism to prevent direct placental infection and pathology. Indeed, high IFN concentrations during pregnancy have proven protective against placental herpes simplex virus infection (112) and type III IFNs impair ZIKV transplacental transmission (113). Induction of ISGs is likely not universally protective, however. Whereas type III IFNs primarily serve a barrier defense role, type I and type II IFNs can serve a more classical immune activating or inflammatory role (31, 45). Animal models of viral infection in pregnancy implicate type I and type II interferons and ISGs in impaired placental development and fetal growth restriction (46, 50, 51, 114), conditions which can have both short- and long-term impact on fetal and offspring health. We demonstrated increased expression of IFN-γ, initiator of Type II interferon signaling, in male SARS-CoV-2-exposed placentas. IFN-γ and the Type II IFN response have been implicated in placental spiral artery remodeling, and may mediate fetal growth restriction and fetal demise in malarial and Toxoplasma gondii infection in pregnancy (50, 51, 115, 116). A transcriptomic analysis of SARS-CoV-2 response genes demonstrated that IFN-γ was an upstream regulator of host viral response in the setting of SARS-CoV-2 infection (117), with higher IFN-γ abundance associated with increased risk for SARS-CoV-2 viral entry (52) and increased mortality in moderate and severe COVID-19 illness (53). Thus, it remains unclear if the male-specific up-regulation of ISGs described here is potentially beneficial (protection from viral infection) versus harmful (increased placental inflammation, increased risk for fetal growth restriction or poor placental function). It was noteworthy that female placentas from SARS-CoV-2-negative control pregnancies generally had higher expression of interferon-stimulated genes and proteins than did male SARS-CoV-2-negative placentas. The potential for a baseline female “antiviral placental advantage” is consistent with the established increased vulnerability of the male fetus to in utero insults, including viral and bacterial infection (92, 118), and observed sex differences in baseline innate immunity described in non-placental cells and tissues (12, 119). These findings highlight the necessity of future studies assessing baseline differences in male and female placental immune responses. The long-term consequences of SARS-CoV-2-associated placental induction of Type I, II, and III IFN responses for fetal development and in utero programming of later life metabolic and neurodevelopmental outcomes remains to be determined.

A limitation of our study is the infection of participants primarily in the third trimester, because these samples were collected during the initial wave of the SARS-CoV-2 pandemic in Boston. Whether maternal SARS-CoV-2 infection in the first and second trimester alters ISG and Fc receptor expression, and how such altered expression might durably impact placental immune function, is a question that remains to be answered in future studies. It remains unclear whether the reduced SARS-CoV-2-specific maternal antibody titers, highly fucosylated glycan profile of spike protein-specific antibodies, and attenuated male-specific transplacental antibody transfer are unique to SARS-CoV-2 biology, or whether these phenomena instead reflect a common response to de novo infection during pregnancy. Future studies should assess the effect of fetal sex on maternal SARS-CoV-2 antibody titers and transplacental transfer in women infected prior to pregnancy, and the effect of fetal sex on maternal antibody responses to other de novo infections during pregnancy. In addition, although we found no association between disease severity and placental gene expression or antibody transfer, such examinations were limited by the relatively small number of women with severe or critical illness. Although our results demonstrate male-specific up-regulation of Type I and II IFNs (IFN-α and IFN-γ), and interferon-stimulated genes and proteins downstream of Type I-III signaling cascades, this study did not assess protein expression of Type III IFN-λ. Finally, although our regression models did not find time from infection to delivery to be a substantial contributor to the antibody transfer ratios, we cannot entirely rule out any contribution of timing of maternal infection to the reduced antibody transfer noted in males. However, our robust sexually dimorphic gene and protein expression results, with up-regulation of both placental ISGs and Fc receptors in males, demonstrate placental factors are a stronger driver of antibody transfer than any time-from-infection effect.

In conclusion, our comprehensive evaluation of the impact of fetal sex on placental gene expression and transplacental antibody transfer in maternal SARS-CoV-2 infection provides insight into sexually dimorphic or sex-specific placental innate and adaptive immune responses to maternal SARS-CoV-2 infection. The increased impact of maternal SARS-CoV-2 infection on male placental and neonatal immunity highlights the importance of evaluating fetal sex in future studies of placental pathology and infant outcomes in SARS-CoV-2, as well as the critical importance of disaggregating sex data in follow-up studies of offspring neurodevelopmental and metabolic outcomes. These findings may have broader implications for understanding placental immune response, male vulnerability, and passive transfer of maternal antibody in other viral infections. Studies investigating SARS-CoV-2 vaccine safety and efficacy in pregnant women should also evaluate placental immune response and antibody-transfer effects, in addition to neonatal infection rates, and report these data in a sex-disaggregated fashion (120).