Elucidating the placental histopathological and molecular biological markers of preterm birth

Preterm birth (PTB) remains a major global burden; its heterogeneous nature results in challenges in prediction and prevention. Inflammation contributes to increased PTB risk, yet limited data exist on the placenta’s role in PTB and the contribution of placental inflammation or dysfunction on birth timing. Thus, there remains a need for histopathological characterisation of preterm placentas and comparison with term placentas. Additionally, there are limited molecular biomarkers available that provide information on placental health during pregnancy or predict the presence of PTB-associated inflammatory or malperfusion lesions. Placentas were histopathologically examined following spontaneous preterm (n=47) or term (n=51) birth and gross morphology recorded before dissection and sampling according to the Amsterdam Criteria to determine macroscopic and histologic variations between cohorts. To investigate the contribution of viral placentitis to PTB, immunohistochemical staining was performed to quantify and localise cytomegalovirus (CMV), herpes simplex virus-1/2 (HSV-1/2) and severe acute respiratory virus coronavirus-2 (SARS-CoV-2). Hofbauer cells, and maternal macrophages were assessed using immunofluorescence to determine changes in distribution, quantity and phenotype between preterm and term cohorts and placentas with histologically-diagnosed inflammatory responses. Finally, ELISAs were performed to ascertain differences in preterm and term placental expression levels of a panel of antimicrobial and inflammation-associated proteins implicated in PTB. Distinct morphological and histopathological characteristics were defined in preterm placentas, the severity of reported inflammatory and non-inflammatory lesions negatively correlated with gestational age and distinct histopathological placental phenotypes were identified for PTB subgroups. CMV and HSV-1/2 were undetectable in the cohort, but one positive SARS-CoV-2 case was observed. Three potential biomarkers of inflammation-associated PTB, Interleukin-6, secretory leukocyte protease inhibitor and soluble CD163, were identified through analysis of macrophages and target proteins. These preliminary data hold promise for providing additional information on placental pathology reports which may benefit neonatal health, and aid in developing novel biomarkers of PIR-associated PTB.