DNA-mediated inflammation in pregnancy
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Date
27/07/2020Item status
Restricted AccessEmbargo end date
27/07/2021Author
van Boeckel, Sara Raquel
Metadata
Abstract
During pregnancy, the maternal immune system must adapt to allow for the growth of a semiallogenic fetus whilst maintaining the ability to respond to pathogens. Maternal immune
system adaptions are imperfect. Pregnant women are more susceptible to infections and have
a higher morbidity from infections than non-pregnant women. An inappropriate inflammatory
response to the developing fetus can result in miscarriage or preterm birth.
The maternal immune system is exposed to circulating cell-free fetal DNA (cff-DNA) in
pregnancy, produced by the cell death of placental syncytiotrophoblasts. Circulating DNA is
pro-inflammatory through various cellular pathways, including through intracellular toll-like
receptor 9 (TLR9). Fetal DNA can elicit inflammation both in vivo, resulting in fetal resorption
in mouse models, and in vitro in peripheral blood mononuclear cells (PBMCs) from pregnant
women. As such, cff-DNA has potential to be involved in the pathogenesis of preterm birth.
When compared to microbial DNA, it is more difficult for mammalian DNA (such as cff-DNA)
to reach intracellular TLR9. Cathelidicin, a circulating host defence peptide, facilitates DNA
entry to cells, and can enhance TLR9-mediated inflammation. It is not known whether
cathelicidin can mediate inflammation in pregnancy. Indeed, little is known about microbial
and cff-DNA sensing mechanisms in pregnancy.
This thesis was based on the following hypotheses: (1) cff-DNA is pro-inflammatory in
pregnancy; (2) cathelicidin modulates cff-DNA induced inflammatory response in pregnancy;
and (3) the pathways of DNA-induced inflammation are altered in pregnancy. To investigate
these hypotheses, I aimed to: (1) assess the pro-inflammatory potential of cff-DNA (2) explore
the role of cathelicidin in cff-DNA-mediated inflammation in pregnancy, (3) to determine if
PBMC responses to DNA-stimuli differ between pregnant and non-pregnant women.
To assess the pro-inflammatory potential of cff-DNA, cff-DNA was generated using human
placental explants. cff-DNA was not pro-inflammatory to PBMCs from pregnant women and
in an in vivo model of preterm birth, injection of mouse placental DNA did not decrease time
to delivery. These data suggested that cff-DNA and placental DNA alone are not proinflammatory, possibly due to the inability of cff-DNA to reach intra-cellular TLR9. When
cathelicidin was added, cff-DNA successfully induced inflammation in the form of chemokine
interferon-γ inducible protein ϭϬ ;CXCLϭϬͿ production from PBMCs from pregnant women. In a lipopolysaccharide (LPS) ʹ induced inflammatory preterm birth model, cathelicidin deficient
mice had decreased rates of preterm birth compared to wildtype mice. Cathelicidin deficient
mice also had significantly less circulating interleukin 6, suggesting that cathelicidin
contributes to inflammation in LPS-induced inflammatory preterm birth.
To examine whether pregnancy results in an altered inflammatory response to DNA-stimuli,
the inflammatory responses of PBMCs from pregnant and non-pregnant women to CpG-ODN
(a synthetic DNA TLR9 agonist) were compared. PBMCs from pregnant women produced a
significantly smaller type 1 interferon-driven response (at protein level) and an overall lesser
inflammatory response (at transcriptional level) compared to PBMCs from non-pregnant
women. PBMCs from pregnant women also had a lower proportion of TLR9-positive
plasmacytoid dendritic cells (pDCs), providing a possible mechanism for the reduced
inflammatory response to DNA-stimuli in pregnancy. The placenta produces large amounts of
progesterone in pregnancy, and progesterone can affect TLR9 responsiveness. Therefore, the
ability for progesterone to alter TLR9 expression and function on PBMCs from non-pregnant
women was explored. Progesterone pre-treatment significantly decreased the CXCL10
response of PBMCs from non-pregnant women to CpG-ODN, and decreased TLR9 expression
in pDCs from non-pregnant women. Together, this suggested that DNA-mediated
inflammation from PBMCs is altered during pregnancy, potentially due to progesterone.
The altered inflammatory response to DNA-stimuli in pregnancy described in this thesis is a
novel finding. It represents a mechanism that contributes to the altered immune response in
pregnancy. This mechanism may be a protective physiological response of the maternal
immune system against increasing cff-DNA levels during pregnancy. It may, however, also
contribute to the known vulnerability of pregnant women to infections. Together, the findings
of this thesis provide an avenue for the future investigation of the altered maternal immune
system and of the ability of cathelicidin to modulate its response to DNA-stimuli.