🧬 biology

KLF6 Downregulation-Mediated Loss of LILRB4 Impairs Decidual Macrophage Tolerance During Toxoplasma gondii Infection

This study reveals that *Toxoplasma gondii* infection impairs maternal-fetal tolerance and causes adverse pregnancy outcomes by suppressing the pSTAT3/KLF6 axis, which downregulates LILRB4 on decidual macrophages and disrupts the IDO/iNOS immunoregulatory balance.

Original authors: Xuemei Hu, Zhidan Li, Fan Zhang, Rutao Mou, Tianyu Jing, Zhendong Zhang, Xianbing Liu, Liqin Ren, Yuzhu Jiang, Haixia Zhang

Published 2026-07-01

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CC BY 4.0

Original authors: Xuemei Hu, Zhidan Li, Fan Zhang, Rutao Mou, Tianyu Jing, Zhendong Zhang, Xianbing Liu, Liqin Ren, Yuzhu Jiang, Haixia Zhang

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ⚕️ This is an AI-generated explanation of a preprint that has not been peer-reviewed. It is not medical advice. Do not make health decisions based on this content. Read full disclaimer

The Big Picture: A Broken Peace Treaty

Imagine a pregnancy as a delicate peace treaty between a mother and her baby. The baby is technically "foreign" to the mother's body (like a guest with a different passport), so the mother's immune system usually wants to attack it. To prevent this, the mother's body has a special "Peacekeeping Force" stationed at the border (the uterus).

One of the most important officers in this force is a cell called a Decidual Macrophage (let's call it the Peacekeeper). Its job is to tell the rest of the immune system, "Stand down, this baby is safe."

The paper investigates what happens when a parasite called Toxoplasma gondii (the Invader) infects a pregnant woman. The researchers found that the Invader tricks the Peacekeepers into dropping their guard, leading to a failed peace treaty (miscarriage or stillbirth).

The Chain of Events: How the Invader Wins

Here is the step-by-step story of how the infection causes the problem, broken down into three acts:

Act 1: The Signal Jammer (STAT3 and KLF6)

Inside the Peacekeeper cell, there is a control room.

The General (STAT3): This is a signal protein that usually stays active, sending orders to keep the peace.
The Architect (KLF6): This is a worker who builds the "Peace Sign" (a protein called LILRB4) on the surface of the Peacekeeper. The Peace Sign is what tells other immune cells to stop attacking.

What the Invader does:
When Toxoplasma gondii attacks, it acts like a jammer. It shuts down the General (STAT3). Without the General's orders, the Architect (KLF6) stops working and disappears.

Act 2: The Missing Peace Sign (Loss of LILRB4)

Because the Architect is gone, the Peacekeeper stops building the Peace Sign (LILRB4).

Normal State: The Peacekeeper is covered in Peace Signs, telling the immune system, "We are tolerant. Do not attack."
Infected State: The Peace Signs vanish. The Peacekeeper looks like a normal soldier again, and the immune system gets confused and starts thinking, "Wait, is this a threat? Let's attack!"

Act 3: The Double-Edged Sword (IDO vs. iNOS)

Once the Peace Sign is gone, the Peacekeeper's internal machinery goes haywire in two opposite directions:

The "Stop" Button Breaks (Loss of IDO):
- IDO is like a "Stop" button that produces a calming chemical to keep the immune system quiet.
- Without the Peace Sign, the "Stop" button breaks. The cell stops making the calming chemical.
- Analogy: It's like a traffic cop taking down the "Stop" sign at a busy intersection. Chaos ensues.
The "Go" Button Gets Stuck (Rise of iNOS):
- iNOS is like a "Go" button that produces inflammatory chemicals (like fire).
- Without the Peace Sign, this button gets stuck in the "ON" position. The cell starts pumping out inflammatory chemicals.
- Analogy: The traffic cop is now blowing a whistle and waving everyone forward into a collision.

The Conclusion: Why This Matters

The paper concludes that the Toxoplasma parasite doesn't just attack the baby directly; it hijacks the mother's own security system.

It jams the signal (STAT3).
It fires the architect (KLF6).
It removes the Peace Sign (LILRB4).
This causes the Peacekeeper to stop calming the immune system and start attacking it.

The researchers identified LILRB4 (the Peace Sign) as the critical weak link. If they can figure out how to keep the Peace Sign on the cell even when the parasite is there, they might be able to prevent the pregnancy complications caused by this infection.

In short: The parasite breaks the "Peace Sign" on the mother's immune cells, turning a protective guard into an attacker, which leads to pregnancy loss. The study maps out exactly which switches the parasite flips to cause this disaster.

Technical Summary: KLF6 Downregulation-Mediated Loss of LILRB4 Impairs Decidual Macrophage Tolerance During Toxoplasma gondii Infection

Problem Statement
Toxoplasma gondii (T. gondii) infection during pregnancy is a known cause of adverse pregnancy outcomes (APOs), including spontaneous abortion, stillbirth, and congenital toxoplasmosis. While the immune microenvironment at the maternal-fetal interface, particularly decidual macrophages (dMφ), is critical for maintaining maternal-fetal tolerance, the molecular mechanisms by which T. gondii disrupts dMφ function remain incompletely understood. Previous work established that T. gondii infection downregulates the inhibitory receptor Leukocyte Immunoglobulin-like Receptor B4 (LILRB4) on dMφ, correlating with APOs. However, the upstream transcriptional regulators mediating this LILRB4 suppression and the downstream signaling pathways leading to immune dysfunction were not elucidated.

Methodology
The study employed a multi-faceted approach combining in vitro and in vivo models:

Models: Primary human dMφ isolated from elective abortion tissues (8–10 weeks gestation), a human dMφ-like cell line (MPA-pTHP-1), and a murine T. gondii-infected pregnancy model using C57BL/6 and LILRB4-deficient (LILRB4-/-) mice.
Infection Protocol: T. gondii tachyzoites were cultured in HEp-2 cells. Mice were infected intraperitoneally at gestational day (gd) 8; human and mouse dMφ were infected in vitro at a 3:1 parasite-to-cell ratio.
Interventions:
- Inhibitors: Stattic (STAT3 inhibitor), TPI-1 (SHP-1 inhibitor), Fludarabine (STAT1 inhibitor), LY294002 (PI3K inhibitor), and NSC781406 (PI3K inhibitor).
- Genetic Manipulation: siRNA knockdown of KLF6 and IRF1; overexpression of KLF6 and LILRB4; use of LILRB4-/- mice.
- Blocking: Neutralizing anti-LILRB4 antibodies.
Analytical Techniques: Bioinformatics screening (UCSC, JASPAR), Immunofluorescence (IF), Western blotting, Flow cytometry, Electrophoretic Mobility Shift Assay (EMSA), Chromatin Immunoprecipitation (ChIP)-qPCR, Co-immunoprecipitation (Co-IP), and re-analysis of single-cell RNA sequencing (scRNA-seq) data.

Key Contributions and Results

Identification of KLF6 as a Direct Transcriptional Regulator of LILRB4:
- Bioinformatics and experimental validation identified Krüppel-like factor 6 (KLF6) as a key transcription factor binding to the LILRB4 promoter region (-591 to -601 bp).
- T. gondii infection significantly downregulated KLF6 expression and its nuclear localization in both human and mouse dMφ.
- EMSA and ChIP-qPCR confirmed that KLF6 binds directly to the LILRB4 promoter and that T. gondii infection impairs this binding.
- Knockdown of KLF6 via siRNA mimicked the infection-induced reduction in LILRB4, while KLF6 overexpression restored LILRB4 levels.
The pSTAT3-KLF6 Axis Mediates LILRB4 Suppression:
- The study demonstrated that T. gondii infection inhibits the phosphorylation of STAT3 at tyrosine 705 (pSTAT3 Y705).
- Pharmacological inhibition of STAT3 (using Stattic) reduced KLF6 expression and LILRB4 levels, exacerbating APOs in mice (reduced fetal/placental weight, increased malformations).
- Co-immunoprecipitation revealed a physical interaction between pSTAT3 and KLF6. Crucially, STAT3 inhibition abolished the ability of KLF6 overexpression to rescue LILRB4 levels, establishing that STAT3 acts upstream of KLF6 to regulate LILRB4 transcription.
Downstream Dysregulation of IDO and iNOS via SHP-1:
- IDO Suppression: LILRB4 downregulation led to decreased Indoleamine 2,3-dioxygenase (IDO), a critical tolerance molecule. Mechanistically, LILRB4 loss reduced the phosphorylation of SHP-1 (pSHP-1). This triggered a cascade: reduced pSHP-1 $\rightarrow$ reduced pJAK2 $\rightarrow$ reduced pSTAT1 (Y701) $\rightarrow$ reduced IRF1 $\rightarrow$ decreased IDO transcription.
- iNOS Promotion: Conversely, LILRB4 downregulation increased inducible nitric oxide synthase (iNOS), a pro-inflammatory marker. This occurred via the pSHP-1 $\rightarrow$ pPI3K $\rightarrow$ c-Fos axis.
- Inhibitor studies confirmed that the JAK/STAT1 pathway was the dominant route for IDO regulation, while the PI3K/c-Fos axis drove iNOS expression.
Functional Consequences on Maternal-Fetal Tolerance:
- In vivo, LILRB4-deficient mice infected with T. gondii exhibited more severe APOs and greater suppression of IDO and activation of iNOS compared to wild-type infected controls.
- The study established that the loss of LILRB4 shifts dMφ from a tolerant phenotype (high IDO, low iNOS) to a dysfunctional, pro-inflammatory state.

Significance and Claims
The paper claims to elucidate a novel molecular framework linking T. gondii infection to pregnancy complications. Specifically, it identifies the pSTAT3(705)/KLF6/LILRB4 axis as the critical regulatory pathway through which the pathogen suppresses maternal-fetal tolerance. The study posits that T. gondii inhibits STAT3 phosphorylation, which in turn reduces KLF6 expression and its binding to the LILRB4 promoter. This loss of LILRB4 disrupts the recruitment of SHP-1, leading to a dual dysregulation: the suppression of the immunoregulatory IDO (via the JAK2/STAT1/IRF1 pathway) and the upregulation of the pro-inflammatory iNOS (via the PI3K/c-Fos pathway).

The authors conclude that LILRB4 serves as a central therapeutic hub, and restoring this axis or targeting the downstream signaling nodes could potentially mitigate infection-related adverse pregnancy outcomes. The study does not propose specific clinical trials or drug development plans but highlights LILRB4 and the identified signaling pathways as potential targets for future intervention strategies.