Hypothesis

Restoring FXR signaling specifically in hepatic Kupffer cells will disrupt the bile acid‑driven immune senescence cascade, reduce hepatic lymphocyte infiltration, and extend healthspan without requiring systemic FXR activation.

Rationale

• Aging increases primary/secondary bile acid ratio due to loss of microbial BSH activity [1].
• Hepatic FXR expression declines with age, removing an anti‑inflammatory brake on myeloid cells [2].
• FXR deficiency in Kupffer cells leads to aberrant TH1/TH17 responses and blocks hepatoprotection by FXR agonists [3].
• Primary bile acids (e.g., CDCA) up‑regulate CXCL16 on LSECs, recruiting lymphocytes that become senescent and secrete SASP factors [4].
• Senescent macrophages spread paracrine senescence via TGFβ, impairing regeneration [5].

Together, these data suggest that the hepatic immune compartment is a nodal point where bile acid dysregulation and FXR loss converge to produce inflammaging. Targeted FXR reactivation in Kupffer cells should re‑establish the brake on pro‑inflammatory T‑cell recruitment, lower CXCL16‑mediated lymphocyte trapping, and prevent the conversion of infiltrating immune cells into a senescent, SASP‑rich state.

Novel Mechanistic Insight

We propose that aged Kupffer cells acquire a bile acid‑sensor switch: declining FXR is compensated by up‑regulation of the G‑protein bile acid receptor TGR5, which drives cAMP‑PKA‑CREB signaling that sustains NF‑κB activity and CXCL16 production. Restoring FXR would not only repress NF‑κB directly but also antagonize TGR5 signaling via heterodimer‑dependent transcriptional repression, thereby collapsing the cytokine loop.

Predictions

1. Cellular level – Kupffer‑cell‑specific FXR agonist delivery will decrease p16^INK4a^ and SASP (IL‑6, TNF‑α) expression in hepatic immune cells.
2. Tissue level – Hepatic CXCL16 mRNA and protein will drop, lymphocyte (CD3^+, CD4^+, CD8^+) infiltration will fall by >50 %, and the secondary bile acid pool (DCA, LCA) will increase due to restored microbial BSH activity secondary to reduced inflammation.
3. Functional level – Liver regeneration after partial hepatectomy will improve to youthful levels, serum ALT/AST will normalize, and median lifespan will increase by ~15 % compared with vehicle‑treated aged mice.
4. Specificity – Systemic FXR agonist administration will produce modest improvements but will not replicate the full phenotype, confirming the Kupffer‑cell‑centric mechanism.

Experimental Design

• Animals: 20‑month‑old C57BL/6J mice (n=10 per group).
• Groups:
  1. Vehicle control (liposome PBS).
  2. Non‑targeted FXR agonist (gut‑restricted GW4064) administered orally.
  3. Kupffer‑cell‑targeted FXR agonist (mannose‑decorated lipid nanoparticles encapsulating GW4064) delivered intravenously.
  4. Kupffer‑cell‑specific FXR knockout + targeted agonist (to control for off‑target effects).
• Readouts (at 4 weeks):
  • Flow cytometry of hepatic immune subsets (CD45^+, F4/80^+, CX3CR1^+ for Kupffer cells; CD3^+, CD4^+, CD8^+ for lymphocytes).
  • qPCR/Western for FXR, TGR5, p16, IL‑6, TNF‑α, CXCL16 in sorted Kupffer cells.
  • Bile acid profiling (LC‑MS) of liver, serum, feces.
  • Histology (H&E, Sirius Red) for fibrosis and senescence (SA‑β‑gal).
  • Liver regeneration assay (2/3 PH, BrdU incorporation at 48 h).
  • Survival monitoring.

Potential Outcomes and Interpretation

• If targeted FXR activation reduces immune senescence markers, lymphocyte influx, and improves regeneration more than non‑targeted agonist, then the hypothesis is supported: hepatic immune FXR is a decisive node.
• If no difference is observed between targeted and non‑targeted groups, then the hypothesis is refuted, suggesting that other cell types (e.g., hepatocytes, LSECs) dominate the FXR‑mediated effect.
• If targeted agonist worsens inflammation, then FXR may have context‑dependent pro‑inflammatory roles in aged Kupffer cells, requiring reassessment.

Implications

A positive result would reposition hepatic immune cells—not merely as victims of aging—but as programmable drivers of bile‑acid‑mediated inflammaging. It would justify developing Kupffer‑cell‑specific FXR modulators as geroprotectors, potentially bypassing the metabolic side‑effects of systemic FXR activation (e.g., lipogenesis, HDL alteration).