Senescent stromal and follicular dendritic cells sustain germinal center architecture via SASP‑derived extracellular matrix and chemokine signals, and their removal impairs affinity maturation in aged mice

Hypothesis

In aged germinal centers (GCs), senescent stromal cells, follicular dendritic cells (FDCs), and related stromal populations act as beneficial chaperones that preserve GC structure and support B‑cell selection through senescence‑associated secretory phenotype (SASP) factors. Ablating these cells will deteriorate GC architecture, reduce somatic hypermutation, and lower high‑affinity antibody output, contrary to the expectation that senolysis universally improves immune function.

Mechanistic Rationale

Aged GCs exhibit defective antigen trapping by FDCs, weakened Tfh help, and impaired stromal adhesion molecule expression 2. Yet intrinsic B‑cell capacity for mutation and selection remains intact when placed in a young environment 1. This disconnect suggests that the microenvironment supplies compensatory signals. Senescent cells are known to deposit and remodel extracellular matrix (ECM) components such as fibronectin, laminin, and collagen IV, and to secrete chemokines like CXCL12 and growth factors including IGF‑1 and HGF 3. In regenerative contexts, these SASP factors create a permissive niche for progenitor proliferation and tissue patterning. We propose that in the aging GC, senescent stromal/FDCs similarly scaffold the light‑zone dark‑zone interface, retain antigen‑immune complexes on FDC networks via ECM‑mediated anchoring, and deliver CXCL12‑dependent retention signals that keep B cells within the selection zone long enough for affinity‑based competition. The SASP also includes tissue‑inhibitors of metalloproteinases (TIMPs) that balance MMP activity, preventing excessive matrix degradation that would disrupt follicle integrity.

Experimental Design

1. Model: Use p16‑3MR mice to inducibly eliminate senescent cells via ganciclovir administration after immunization with NP‑KLH in aged (18‑month) mice.
2. Groups: (a) Aged + vehicle (control senescent load), (b) Aged + senolytic (ganciclovir), (c) Young + vehicle (baseline), (d) Aged + senolytic + recombinant CXCL12 or IGF‑1 supplementation (rescue).
3. Readouts (day 7‑14 post‑immunization):
   • Flow cytometric quantification of GC B cells (GL7⁺FAS⁺) and Tfh cells.
   • Histological assessment of FDC network integrity (CD35 staining) and stromal cell PDPN⁺/VCAM-1⁺ patterns.
   • Antigen retention on FDCs measured by NP‑PE fluorescence intensity.
   • Somatic hypermutation frequency in IgG1 V‑region sequencing.
   • Affinity‑maturation assessed by ELISA using low‑ and high‑density NP.
   • Serum titers of high‑affinity anti‑NP IgG.
4. Controls: Verify senolytic efficacy by p16^Ink4a^ and SA‑β‑gal staining; confirm that young mice are unaffected by the regimen.

Predicted Outcomes

• If senescent stromal/FDCs are beneficial, senolysis will decrease GC B‑cell numbers, disrupt FDC networks, reduce antigen retention, lower mutation frequency, and diminish high‑affinity antibody titers compared with aged controls.
• Supplementation with CXCL12 or IGF‑1 should rescue these defects, restoring GC size and affinity maturation despite senescent cell loss.
• Conversely, if senescent cells are purely detrimental, senolysis will increase GC output and antibody affinity, matching or exceeding young‑mouse levels.

Potential Caveats and Alternatives

Senolytics may affect non‑stromal senescent immune cells (e.g., senescent Tfh or macrophages) that could independently influence GCs. To isolate stromal effects, future experiments could employ stromal‑specific Cre drivers (e.g., Ccl19‑Cre) to delete p16^Ink4a^ selectively in fibroblast reticular cells. Additionally, the SASP is heterogeneous; profiling aged GC senescent cells via single‑cell RNA‑seq would identify the exact factors responsible for the supportive niche.