IL-35 produced by dendritic cells via TIM-3-STAT3 signaling contributes to the development of visceral leishmaniasis

This study identifies a TIM-3-STAT3-IL-35 signaling axis in dendritic cells as a critical driver of immunosuppression and disease progression in visceral leishmaniasis, demonstrating that inhibiting STAT3 restores protective immunity and reduces parasite burden.

The Gist

The Big Picture: A Broken Defense System

Imagine your body is a fortress, and the immune system is the army guarding it. When a parasite called Leishmania donovani (the cause of Visceral Leishmaniasis, or VL) invades, the army should wake up, sound the alarms, and attack the enemy.

However, in people with VL, the army doesn't just fail to attack; it actually goes to sleep and lets the enemy take over the fortress. This paper discovers why the army is falling asleep and finds a way to wake it up.

The Characters in the Story

1. The Invader (Leishmania): A sneaky parasite that hides inside your cells.
2. The Scouts (Dendritic Cells/DCs): These are the immune system's "scouts." Their job is to spot the enemy, sound the alarm, and tell the soldiers (T-cells) where to fight.
3. The Soldiers (T-cells): The heavy hitters that actually kill the parasites.
4. The "Sleeping Gas" (IL-35): A chemical signal that tells the army to stand down and stop fighting.
5. The Switch (TIM-3 and STAT3): The internal machinery inside the scouts that decides whether to sound the alarm or release the sleeping gas.

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The Discovery: How the Enemy Hacks the Scouts

1. The Scouts Get Hijacked
Usually, when a scout (DC) sees a parasite, it screams, "Alert! Attack!" But this paper found that when the Leishmania parasite infects the scout, it flips a switch inside the scout's brain. Instead of screaming "Attack," the scout starts pumping out a massive amount of "Sleeping Gas" (IL-35).

2. The Chain Reaction
This Sleeping Gas does two terrible things:

• It silences the other scouts: It stops them from waking up and getting ready to fight.
• It brainwashes the soldiers: It tells the T-cell soldiers, "Don't bother fighting; just relax." It even tricks the soldiers into making more Sleeping Gas themselves.

The Result: The entire immune system shuts down. The parasite multiplies unchecked, causing a severe, often fatal disease.

The "How-To" Manual: How the Parasite Does It

The researchers figured out the exact wiring diagram the parasite uses to hack the scouts:

1. The Doorbell (TIM-3): The parasite rings a specific doorbell on the scout called TIM-3.
2. The Light Switch (STAT3): Ringing that doorbell flips a light switch inside the cell called STAT3.
3. The Factory (IL-35): Once the switch is flipped, it turns on a factory inside the cell that manufactures the Sleeping Gas (IL-35).

The Analogy: Imagine a burglar (the parasite) walks into a security guard's booth (the DC). Instead of hitting the guard, the burglar presses a hidden button (TIM-3) that turns on a machine (STAT3) which starts spraying fog (IL-35) over the whole city, making everyone fall asleep.

The Solution: Cutting the Power

The researchers asked: What if we break the connection between the doorbell and the light switch?

They tested a drug called WP1066.

• What it is: A drug already approved by the FDA to treat a specific type of brain tumor in children.
• What it does here: It acts like a pair of pliers that cuts the wire between the doorbell (TIM-3) and the light switch (STAT3).

The Experiment:
When they gave this drug to mice infected with the parasite:

1. The scouts stopped making the Sleeping Gas.
2. The soldiers woke up and started fighting.
3. The parasite was cleared from the body, and the mice got better.

Why This Matters

• New Understanding: Before this, we knew the immune system was failing in VL, but we didn't know who was making the "Sleeping Gas" or how the parasite forced them to make it. This paper proves the Scouts (DCs) are the first ones to make the gas, and they do it using the TIM-3/STAT3 pathway.
• A New Cure: Since WP1066 is already an approved drug, it could be "repurposed" quickly to treat VL. Instead of inventing a new drug from scratch, doctors could potentially use this existing one to stop the parasite from hacking the immune system.

Summary in One Sentence

This paper reveals that a parasite tricks our immune scouts into producing a chemical that puts our entire defense force to sleep, but we can stop this by using an existing drug to cut the wire that controls the chemical, waking the immune system up to fight the infection.

Technical Summary

1. Problem Statement

Visceral leishmaniasis (VL), caused by Leishmania donovani (LD), is a fatal parasitic disease characterized by profound immunosuppression. While the roles of IL-10 and TGF-β in VL pathogenesis are established, the specific molecular and cellular mechanisms driving early immunosuppression remain poorly defined.

• Knowledge Gap: It was unclear whether Dendritic Cells (DCs) produce the immunosuppressive cytokine IL-35 (a heterodimer of IL-12p35 and EBI3) during LD infection. Previous studies on IL-35 in DCs were conflicting, and the role of the complete IL-35 heterodimer (as opposed to individual subunits) in VL was unknown.
• Therapeutic Need: There is an urgent need to identify novel therapeutic targets to restore host immunity and clear the parasite, particularly given the spread of VL to non-endemic regions.

2. Methodology

The study employed a comprehensive approach combining in vitro cell culture, in vivo mouse models, molecular biology, and pharmacological interventions.

• Cell Models: Bone marrow-derived dendritic cells (BMDCs) from BALB/c mice and human monocyte-derived DCs (HuMoDCs) were infected with LD promastigotes (LDPm) and amastigotes (LDAm).
• Molecular Characterization:
  • Flow Cytometry: To detect co-expression of IL-12p35 and EBI3 (defining IL-35+ cells) in DCs, T cells, and other splenocytes.
  • Confocal Microscopy & c-FRET: To confirm the physical interaction and colocalization of p35 and EBI3 subunits within the cell, proving heterodimer formation.
  • Co-immunoprecipitation (Co-IP): To biochemically verify the IL-35 complex.
  • ELISA: To quantify secreted IL-35 heterodimer in culture supernatants.
• Signaling Pathway Analysis:
  • Chromatin Immunoprecipitation (ChIP) & DNA Pull-down: To verify STAT3 binding to specific promoter regions of EBI3 and IL12A.
  • Reporter Assays: Luciferase assays in HEK293T cells to test promoter activation by STAT3.
  • Blocking/Inhibition: Use of anti-TIM-3 antibodies, STAT3 siRNA, and the STAT3 inhibitor WP1066.
• Functional Assays:
  • DC Maturation: Assessment of co-stimulatory molecules (CD40, CD80, CD86) and cytokine secretion (IL-12, TNF-α) in bystander DCs treated with LD-infected DC supernatants.
  • T Cell Proliferation: CFSE/eFluor 670 dilution assays to measure T cell proliferation in the presence of IL-35.
• In Vivo Models:
  • Adoptive Transfer: Transferring modified BMDCs (STAT3-silenced, IL-35 overexpressing, or anti-IL-35 antibody-transfected) into LD-infected mice to isolate the effect of DC-derived IL-35.
  • Pharmacological Treatment: Oral administration of WP1066 (an FDA-designated orphan drug) to LD-infected mice.
  • Readouts: Parasite burden (Leishman-Donovan Units), organ weight (hepatosplenomegaly), and T cell cytokine profiles (IFN-γ vs. IL-10).

3. Key Contributions

1. Identification of DCs as the Primary Source: The study establishes that DCs are the earliest and predominant cellular source of the IL-35 heterodimer during LD infection, preceding T cell production.
2. Novel Signaling Axis: It defines a previously unknown TIM-3 $\rightarrow$ STAT3 $\rightarrow$ IL-35 axis. The authors demonstrate that LD infection activates TIM-3 on DCs, which triggers STAT3 phosphorylation. Activated STAT3 then directly binds to the promoters of EBI3 and IL12A to drive IL-35 transcription.
3. Mechanism of Immunosuppression: The paper elucidates how DC-derived IL-35 creates a tolerogenic environment by:
   • Suppressing DC maturation via inhibition of the NF-κB pathway.
   • Inducing IL-35 expression in T cells (a feed-forward loop).
   • Inhibiting T cell proliferation and shifting the immune response toward a non-protective Type-2 (IL-10) phenotype.
4. Therapeutic Repurposing: The study identifies WP1066, a STAT3 inhibitor, as a potent therapeutic agent that reduces parasite burden by blocking the TIM-3-STAT3-IL-35 axis.

4. Key Results

• IL-35 Production: LD infection induced rapid IL-35 production in DCs (peaking at 48h in vitro). Confocal microscopy and c-FRET confirmed the formation of the functional IL-35 heterodimer.
• Signaling Mechanism:
  • STAT3 binds to specific sites in the EBI3 (-210, -386) and IL12A (-365) promoters upon LD infection.
  • Blocking TIM-3 or silencing STAT3 significantly reduced IL-35 production, confirming the pathway.
• Functional Impact:
  • Supernatants from LD-infected DCs suppressed LPS-induced maturation of bystander DCs and T cell proliferation. These effects were reversed by neutralizing anti-IL-35 antibodies.
  • IL-35 produced by DCs induced IL-35 expression in T cells, creating a self-amplifying immunosuppressive loop.
• In Vivo Efficacy:
  • Neutralization: Adoptive transfer of DCs transfected with anti-IL-35 antibodies reduced parasite load and hepatosplenomegaly while restoring IFN-γ+ T cells.
  • STAT3 Inhibition: Transfer of STAT3-silenced DCs or treatment with WP1066 significantly reduced parasite burden.
  • Rescue Experiment: Overexpression of IL-35 in STAT3-silenced or WP1066-treated DCs reversed the therapeutic benefits, confirming that the efficacy is specifically due to the reduction of IL-35.
  • Oral WP1066: Oral administration of WP1066 to infected mice significantly lowered parasite load and disease severity without significant toxicity.

5. Significance

• Pathogenesis Insight: This work redefines the immunopathology of VL by identifying DC-derived IL-35 as a critical driver of immunosuppression, distinct from the previously known roles of IL-10 and TGF-β.
• Mechanistic Clarity: It resolves the controversy regarding DC production of IL-35 and provides a clear molecular mechanism (TIM-3/STAT3) for its induction.
• Therapeutic Potential: The identification of the TIM-3-STAT3-IL-35 axis offers a new target for intervention. The successful use of WP1066 suggests a viable strategy for repurposing an existing FDA-approved drug (originally for pediatric brain tumors) to treat visceral leishmaniasis.
• Broader Implications: The findings suggest that targeting STAT3 or IL-35 could be a general strategy for controlling other infectious diseases where DCs drive immunosuppression.

In conclusion, the paper provides a robust mechanistic link between parasite recognition (TIM-3), host transcriptional regulation (STAT3), and cytokine-mediated immunosuppression (IL-35), offering a promising therapeutic avenue for a neglected tropical disease.