Background/Purpose: Systemic Lupus Erythematosus (SLE) predominantly affects women of childbearing age and is associated with adverse pregnancy outcomes including pre-eclampsia, intrauterine fetal growth restriction (IUGR), placental abnormalities and maternal and fetal mortality, particularly during active disease or in the presence of antiphospholipid antibodies (aPL). Pro-inflammatory factors rather than thrombosis are now thought to be the main contributor to placental and fetal damage in women with APS. Pathogenic mechanisms that damage the fetal-maternal unit and cause abnormal placental development, however, remain poorly understood. The endosomal RNA sensor Toll-like receptor 8 (TLR8) that is predominantly expressed in macrophages and neutrophils was recently hypothesized to play a role in antiphospholipid syndrome (APS) and aPL antibody-induced placental and fetal damage. The role of TLR8 in systemic autoimmunity remains elusive as TLR8 function differs from mouse to man with respect to recognition of RNA. We are currently studying the functional consequences of one or two copies of human (huTLR8) in murine SLE, and hypothesize that the presence of huTLR8 and aPLs will adversely affect pregnancy outcomes in SLE.

Methods: Sle1 mice expressing huTLR8 as a BAC transgene (huTR8tg) were generated and followed clinically. HuTLR8 DNA copy number and mRNA expression was confirmed by qDigital and qRT-PCR respectively. Pregnancies were closely followed and terminal C-sections were performed in pregnant females with dead pups or exhibiting signs of dystocia or prolonged end-stage pregnancy. Pup and placental weight was assessed. Placental tissues were histologically characterized by H&E and immunohistochemical staining for neutrophil GR1-Ly6 and the granular protein Myeloperoxidase (MPO) to distinguish intact from netting neutrophils. Additionally, placental tissue was stored for further mRNA and microRNA analysis. Auto-antibody status in serum from pregnant females was assessed.

Results: Female huTLR8tg Sle1 mice manifested splenomegaly [mean 0.323g, n=16] compared to their non-huTLR8 counterparts [mean 0.123g, n=18] (p=0.0013) and developed anti-cardiolipin antibodies at a younger age than their wild type controls. We observed 26/55=47% loss of litters due to fetal resorption/dystocia resulting in fetal and maternal death, compared with 0% in n=20 Sle1 controls. Pup weights were significantly lower (p<0.0001) for stuck/dead pups [median 1.212g, n=31] compared to normal pups [median 1.649g, n=55]. Abnormal placental morphology was observed, with dense tissue structure, defective vasculature and increased GR1-Ly6 neutrophil infiltration. An increase in netting neutrophils was identified by MPO as decondensed non-intact nuclei.

Conclusion: We observed 47% loss of litters in huTLR8tg Sle1 pregnancies due to fetal resorption/dystocia resulting in both fetal and maternal death. This was due to placental developmental abnormalities, including vascular remodeling and inflammation in the presence of aPLs. Here we identify a potential new model to study adverse pregnancy complications in SLE and APS and further define the mechanistic role of TLR8 in promoting pregnancy loss.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/human-toll-like-receptor-8-adversely-affects-placental-development-and-pregnancy-outcomes-in-a-mouse-model-of-systemic-lupus-erythematosus/