ePoster Presentation 49th Annual Scientific Meeting of the Australian and New Zealand Society for Immunology 2021

Preterm birth (PTB), the leading cause of child mortality in developed countries, is characterised by uncontrolled inflammation and is linked with maternal immune dysfunction. Preterm labour is associated with a premature decline in immune tolerance in favour of pro-inflammatory T cell activation. CD4⁺Foxp3⁺ regulatory T (Treg) cells mediate maternal tolerance in early pregnancy, however their influence at birth is ill-defined. Given that unstable Treg cells drive pathology in inflammatory diseases, we hypothesised that maternal Treg cells exhibit phenotypic plasticity prior to preterm labour that may contribute to PTB pathophysiology. To investigate this, we characterised uterine-draining lymph node Treg cells by flow cytometry in BALB/c-mated C57Bl/6 dams prior to inflammation-induced PTB using either lipopolysaccharide (LPS) to mimic infection-driven PTB or interleukin-1 beta (IL1B) to mimic sterile inflammation-induced PTB. In both models, 24h following treatment we observed an increase in total and Nrp1⁺ thymic-derived Foxp3⁺ Treg cells. Notably, Treg cells expressing inflammatory cytokine interleukin-17 (IL17) were expanded prior to LPS- or IL1B-induced PTB. This was associated with an increase in IL17 and CCR6 MFI in Treg cells from IL1B-treated dams (P<0.05). Strikingly, CD4⁺Foxp3^-IL17⁺ T effector cells were not expanded, highlighting the Treg-specific effect. As Treg cell plasticity is linked with instability and loss of Foxp3 expression, we utilised Foxp3^GFPCrexRosa26^RFP mice to measure Treg cell fate, revealing GFP⁺RFP⁺ Treg cells are expanded in normal late gestation and retain stability, with GFP^-RFP⁺ exTreg abundance remaining unchanged. We also found IL17⁺GFP⁺RFP⁺ Treg cells expand 3-fold from early to late pregnancy (P<0.05). We are now assessing the significance of Treg-derived IL17 expression in term and preterm labour and whether Treg instability occurs during PTB. Preliminary evidence suggests exTregs expand prior to PTB. These findings are relevant to understanding the role of Treg plasticity in PTB and their potential value as therapeutic targets to mitigate PTB susceptibility.