Controlled Cathepsin Leakage as a Tunable Immune Recruitment Mechanism in Senescent Cells

Hypothesis

Senescent cells deliberately tune lysosomal membrane permeabilization to release cathepsins at sub‑lethal concentrations, converting lysosomal damage into a graded chemokine‑like signal that recruits and activates immune cells without triggering apoptosis.

Mechanistic Basis

Low‑level LMP permits limited cathepsin B and L efflux into the cytosol LMP enables graded cathepsin release. Cytosolic cathepsins cleave specific substrates that amplify danger signaling: they process pro‑IL‑1β and pro‑IL‑18 for NLRP3 inflammasome activation Cathepsins B and L support NLRP3 inflammasome assembly and, independently of the inflammasome, drive IL‑1β secretion through cathepsin‑sensitive pathways Particle-induced IL-1β secretion is cathepsin-sensitive. The decline of cystatin B (CSTB) removes endogenous inhibition, raising cathepsin activity just enough to sustain histone H3 tail cleavage and senescence‑associated chromatin changes CSTB deficiency sustains histone H3 tail cleavage while elevating cathepsin B in disomic contexts CSTB reduction elevates cathepsin B activity. In aging brain, cathepsin D, E and B rise, providing a reservoir that can be mobilized via LMP Cathepsin levels increase with age.

We propose that CSTB downregulation is not a passive collapse but a regulated rheostat that sets the cathepsin leak rate. This leak generates a diffusible danger cue that attracts monocytes/macrophages, promotes their polarization to a pro‑inflammatory phenotype, and stimulates phagocytic clearance of neighboring damaged cells. The senescent cell remains viable because the cathepsin concentration stays below the threshold that triggers mitochondrial outer membrane permeabilization or catastrophic lysosomal rupture.

Testable Predictions

1. CSTB restoration reduces cathepsin leak and immune recruitment. Overexpressing CSTB in senescent fibroblasts will lower cytosolic cathepsin B/L activity, diminish NLRP3 inflammasome signaling, and decrease chemotactic attraction of bone‑marrow‑derived macrophages in vitro.
2. Cathepsin leak concentration correlates with immune cell activation. Using a FRET‑based cathepsin activity sensor, we will quantify leak intensity across single senescent cells and show a positive linear relationship with the number of adhered macrophages and their IL‑1β production.
3. Transient LMP inhibition blocks the hostage‑negotiator function without inducing apoptosis. Treating senescent cells with a low dose of lysosomotropic agent that stabilizes lysosomal membranes (e.g., chloroquine at sub‑toxic concentrations) will suppress cathepsin release, reduce immune cell recruitment, yet leave senescence markers (p16^INK4a^, SA‑β‑gal) intact.
4. In vivo, CSTB haploinsufficiency amplifies senescent‑cell‑driven inflammation. Mice with heterozygous CSTB deletion will exhibit elevated cathepsin activity in tissue‑resident senescent cells, increased neutrophil and monocyte infiltration, and exacerbated age‑related tissue fibrosis compared with wild‑type controls.

Experimental Design

• In vitro: Induce senescence in human IMR‑90 fibroblasts via irradiation. Generate three groups: control, CSTB‑overexpressing (lentiviral), and CSTB‑knockdown (siRNA). Measure cytosolic cathepsin B/L activity with a Magic Red assay, NLRP3 inflammasome activation (ASC speck formation, caspase‑1 cleavage), and IL‑1β/IL‑18 secretion. Perform macrophage migration assays using conditioned media and quantify recruited cells by flow cytometry.
• Single‑cell resolution: Transduce cells with a cathepsin‑specific FRET probe (e.g., CatB‑Sensor). Correlate FRET ratio per cell with macrophage adhesion counts obtained via live‑cell imaging.
• Pharmacological LMP modulation: Treat senescent cultures with 10 µM chloroquine (lysosomal stabilizer) or 5 µM siramesine (LMP inducer) for 6 h. Assess cathepsin leak, immune recruitment, and viability (Annexin V/PI).
• In vivo: Use CSTB^+/− and wild‑type mice aged 18 months. Harvest liver and brain, isolate senescent cells via p16^INK4a^‑GFP sorting, measure cathepsin activity, and perform immunohistochemistry for F4/80^+ macrophages and collagen deposition.

Potential Implications

If validated, this hypothesis reframes senescent cells not as passive debris but as active immunomodulatory hubs that fine‑tune tissue surveillance via a cathepsin‑based signal. Therapeutic strategies would then aim to modulate the leak rate—bolstering CSTB to dampen excessive inflammation or enhancing controlled LMP to boost immune clearance—rather than indiscriminately removing senescent cells with senolytics, which could inadvertently abort a beneficial hostage‑negotiation signal.