In this manuscript, we report the pathological findings observed in six consecutive placentas delivered from five women affected by COVID–19. Differently from the limited data reported so far,15 we describe for the first time pathological abnormalities potentially related to SARS-CoV–2 infection which could help understand the incoming reports suggesting higher rates of obstetric complications in these patients.
In 4 cases, we found microscopic signs of inflammation involving the decidua and the chorial villi: three out of four cases showed a CD8-positive T cell lymphocytic infiltrate, while in the remaining case an intervillous histiocytic infiltrate was identified.
The presence of an inflammatory lymphocytic infiltrate (cytotoxic CD8+ T cell lymphocytes in particular, as observed in our series) and of villous damage are consistent with the diagnosis of chronic villitis.22, 23 Of note, none of these three patients had comorbidities that could justify the presence of placental inflammation. Chronic villitis etiology is often related to an undergoing infection, although in most cases it is not possible to identify the specific etiopathogenetic agent: these cases are defined as “villitis of unknown etiology” (VUE) and can be found in 2–33.8% of placentas.24 Within the group of chronic villitis secondary to maternal infections, viruses are the most frequently involved pathogens and many specific agents, such as varicella zoster, herpes simplex virus, cytomegalovirus, and influenza A/H1N1, have been found to be associated with this condition.25, 26, 27, 28 Little is known about the association between placental pathology and coronavirus infections. Seven placentas from coronavirus-infected women during the 2002–2004 SARS outbreak have been previously described:29 histological examination found no abnormalities in 2/7, an increase in subchorionic and intervillous fibrin in 3/7, and massive thrombotic vasculopathy associated with intra-uterine growth restriction (IUGR) in the remaining 2/7 cases. No signs of chronic villitis were detected in any case and the same is true for MERS infections.30 Based on present knowledge, infection by SARS-CoV–2 is characterized by an extensive immune response with activation of both CD4+ and CD8+ lymphocytes18, 31, 32, thus findings consistent with chronic villitis could be expected in the presence of placental involvement, also considering the high expression of ACE2 in decidual cells, villous cytotrophoblast and syncytiotrophoblast cells.17
Moreover, experiences derived from influenza A/H1N1 infections in pregnancy suggest that activation of cellular immune response with release of inflammatory cytokines can lead to indirect placental damage.28 Considering this evidence, the placenta could be potentially harmed because of intrinsic active virus replication and/or through indirect activity of inflammatory cytokines.33 Viral in situ assessment (by immunohistochemistry for viral proteins and/or in situ hybridization of viral RNA) will be important to define which specific mechanisms are involved in this setting.
In the fourth case with inflammatory abnormalities, features consistent with chronic histiocytic intervillositis were identified. This rare entity is usually associated with maternal immunologic conditions like systemic lupus erythematosus, lupus anticoagulant and antiphospholipid syndrome.34, 35 Despite the different type of inflammatory infiltrate (histiocytic versus lymphocytic) and of etiologies (chronic histiocytic intervillositis is usually not related to viral infections), chronic histiocytic villositis is frequently associated with chronic villitis (30% to 50% of cases), supporting a potential overlap in terms of involved immunologic pathways.36, 37 Moreover, alveolar histiocytic infiltration seems to be a frequent autoptic hallmark of SARS-CoV–2 infection.38 Lastly, it has to be noted that this patient’s significant comorbidities (chronic hypertension, severe obesity, and gestational diabetes) are not usually associated with chronic histiocytic intervillositis.
Our data open up an interesting perspective: the three patients with chronic villitis had a shorter/milder history of COVID–19 (<14 days, estimated time for IgG appearance in patients serum)39, while chronic histiocytic intervillositis was found in a patient with long-standing and more severe symptoms (Figure 1). Could placental alterations mirror the stage of immune response to SARS-CoV–2 and/or infection severity? Should this prove to be true, placenta could be used as a representative tissue to investigate SARS-CoV–2 pathogenetic mechanisms helping avoid the biases of autoptic samples. Perspective and integrated studies (i.e. including seriated viral and serological testing) on larger series are needed to characterize these clinico- pathological correlations and address this hypothesis.
Vascular alterations represent the second type of abnormalities observed in our series. Thrombo- hemorrhagic alterations were mainly found in association with chronic villitis (Cases 1, 2 and 3): in these samples, thrombi involving the arterial microcirculation (<1 mm in diameter) were present in association with an inflammatory infiltrate, similarly to the alterations observed in the lungs of patients who died of COVID–19.38 The relationship between COVID–19 and systemic coagulopathy with a thrombo- inflammatory state has been well documented40, 41 and placental thromboses were also described in two cases of SARS-CoV during the previous outbreak as previously listed. In Case 4 of our series, an extensive hemorrhagic placental abruption was observed, but this finding is probably unrelated to SARS-CoV–2 infection.
Regarding our knowledge about the clinical outcomes of pregnancy in COVID–19 patients, it is based on limited series and systematic reviews. Chen et al.6 reported the outcomes of a relatively large series of pregnant women affected by COVID–19 observing a high rate (14/68, 21%) of preterm delivery. A systematic review evaluating 41 pregnancies affected by COVID–19 confirmed this observation, reporting preterm birth (< 37 weeks) in above 40% of pregnancies.9
Could the poorer obstetrics outcomes reported in women with COVID–19 be related to SARS-CoV–2- induced inflammatory and/or vascular abnormalities? The findings of our study provide an initial pathological basis for these clinical findings: chronic villitis/VUE as well as vascular abnormalities are associated with several obstetric and neonatal adverse outcomes including preterm birth.22, 23, 27, 42, 43, 44 Moreover, it is well known that an inflammatory/infectious reaction can lead to preterm delivery by increasing prostaglandins secretion or by impairing the fetal immune tolerance.
Negative results of SARS-CoV–2 placental swabs observed in our series should not be taken as a conclusive evidence of SARS-CoV–2 absence in these tissues. This finding could rather depend on compartmentalization of virus replication within specific cell types. This hypothesis is supported by the differential expression of ACE2 and TMPRSS2, proteins required for SARS-CoV–2 cell entry, observed in the maternal-fetal interface tissues.17, 45 This open question could also be efficaciously addressed by in situ viral assays which will hopefully become more widely available in the coming weeks.
The main limitation of our observations is that our pathological findings are shared by multiple conditions and etiologies, which could have been pre-existent at time of SARS-CoV–2 infection. Nevertheless, some considerations support this relationship: i) the observed rate of abnormalities is higher compared to what could have been expected in a consecutive unselected series of placentas; ii) findings are consistent with other viral infections and with our current knowledge about SARS-CoV–2 pathogenetic mechanisms; iii) patients’ histories did not include comorbidities justifying the identified pathological hallmarks. Analysis of larger series of cases and in situ viral assays will be needed to definitively prove and characterize the causative role of SARS-CoV–2. Nevertheless, our findings are important to shed a light on the reported associations between COVID–19 infection and poorer obstetrics outcomes.
Finally, can therapeutic implications be derived from our data? As prophylactic low-molecular-weight heparin is recommended in pregnant women with high thrombotic risk,46, 47 our findings provide a pathological basis to consider heparin use in COVID–19 affected pregnant patients to prevent inflammatory and vascular thrombotic complications. This suggestion is in line with the recent proposal of The Society of Thrombosis and Haemostasis which recommends (if not otherwise contraindicated) the use of prophylactic low molecular weight heparin in all patients who require hospitalization for COVID–19, regardless of illness severity.48