Chronic Villitis as a Distinctive Feature of Placental Injury in Maternal SARS-CoV-2 Infection

Background SARS-CoV-2 infection during pregnancy is associated with increased risk of stillbirth, preeclampsia and preterm birth. However, this does not appear to be due to intrauterine fetal infection, as vertical transmission is rarely reported. There is a paucity of data regarding associated placental SARS-CoV-2 histopathology and their relationship to timing and severity of infection. Objective To determine if maternal SARS-CoV-2 infection is associated with specific patterns of placental injury and whether these findings differ by gestational age at time of infection or disease severity. Study Design A retrospective cohort study was performed at University of California San Diego between 03/2020 to 02/2021. Placentas from pregnancies with a positive SARS-CoV-2 test were matched to two sets of controls: one set was time-matched by delivery date and sent to pathology for routine clinical indications, and the other was chosen from a cohort of placentas previously collected for research purposes without clinical indications for pathologic examination prior to the SARS-CoV-2 outbreak. Placental pathologic lesions were defined based on standard criteria and included maternal and fetal vascular malperfusion, and acute and chronic inflammatory lesions. Bivariate analysis was performed with independent Student’s T test and Pearson’s chi-square. Logistic regression was used to control for relevant covariates. Regions of SARS-CoV-2-associated villitis were further investigated using protein-based digital spatial profiling (DSP) assays on the GeoMx platform, validated by immunohistochemistry, and compared to cases of infectious villitis and villitis of unknown etiology. Differential expression analysis was performed to identify protein expression differences between these groups of villitis. Results We included 272 SARS-CoV-2 positive cases, 272 time-matched controls, and 272 historical controls. The mean age of SARS-CoV-2 affected subjects was 30.1 ± 5.5 years and the majority were Hispanic (53.7%) and parous (65.7%). SARS-CoV-2 placentas demonstrated a higher frequency of the four major patterns of placental injury (all p<.001), compared to historical controls. SARS-CoV-2 placentas also showed a higher frequency of chronic villitis (CV) and severe CV (p=.03 for both) compared to time-matched controls, which remained significant after controlling for gestational age at delivery (aOR 1.52, 95% CI 1.01-2.28; aOR 2.12, 95% CI 1.16-3.88, respectively). DSP revealed programmed death-ligand 1 (PD-L1) to be increased in villitis-positive regions of the SARS-CoV-2 (logFC 0.47, adj p-value=0.002) and VUE (logFC 0.58, adj p-value=0.003) cases, but conversely decreased in villitis-positive regions of the infectious villitis group (log FC -1.40, adj p-value<0.001). Conclusions CV appears to the most specific histopathological finding associated with SARS-CoV-2 maternal infection. CV involves damage to the vasculosyncytial membrane of the chorionic villi, involved in gas/nutrient exchange, suggesting potential mechanisms of placental (and perhaps neonatal) injury, even in the absence of vertical transmission. Surprisingly, changes in protein expression in SARS-CoV-2-associated villitis appear to be more similar to VUE rather than infectious villitis.