Toxoplasma gondii-induced adverse pregnancy outcomes: insight into the inhibitory role of Trem2 on TLR4/TRAF6/JNK signaling pathway.

Background: Decidual macrophages (dMφs) are not only essential for maintaining normal pregnancy but also serve as crucial immune defenders against infections, including Toxoplasma gondii. Triggering receptor expressed on myeloid cells 2 (Trem2), as a critical immunoregulatory receptor on dMφs, can counteract inflammation and defend against pathogen infection. However, the mechanisms underlying the Trem2 downstream pathways during T. gondii infection-particularly their impact on adverse pregnancy outcomes (APOs)-remain elusive.

Methods: The interaction between Trem2 and Toll-like receptor 4 (TLR4) was initially predicted through molecular docking models and subsequently confirmed by co-immunoprecipitation, using both animal models and cellular systems to examine the impact of Trem2 knockout, overexpression, and TLR4-blocking antibody treatment on downstream signaling molecules as well as cytokine production.

Results: The interaction between Trem2 and TLR4 was validated. Trem2 downregulation during T. gondii infection coincided with increased TLR4, tumor necrosis factor (TNF) receptor-associated factor 6 (TRAF6), and c-Jun N-terminal kinase (JNK) activation, while Trem2 knockout further enhanced TLR4/TRAF6/JNK signaling in mice and macrophages. Conversely, Trem2 overexpression suppressed this signaling cascade and reversed T. gondii-induced activation. Treatment with a TLR4-blocking antibody inhibited TRAF6 and P-JNK activation in macrophages but did not affect Trem2 expression. Additionally, Trem2-deficient bone marrow-derived macrophages (BMDMs) exhibited elevated transcription of TNF-α and interferon-γ (IFN-γ) upon T. gondii antigen stimulation.

Conclusions: Trem2 deficiency in pregnant mice promotes the TLR4/TRAF6/JNK signaling cascade following T. gondii infection. This study demonstrates that Trem2 acts as a pregnancy-specific inhibitor of TLR4/TRAF6/JNK signaling, providing novel mechanistic insights into T. gondii-induced APOs.