Hypothesis

Aged B cells actively suppress bone marrow B lymphopoiesis not only through homeostatic competition but via B‑cell‑intrinsic insulin receptor (IR) signaling that remodels the stromal niche to lower IL‑7 production. Blocking IR specifically in aged B cells will restore IL‑7‑dependent lymphopoiesis, increase naïve B cell output, and rescue germinal center formation and vaccine‑induced affinity maturation in old mice.

Mechanistic Rationale

• Aged B cells show heightened IR expression and activity (see 6), leading to increased downstream AKT‑mTOR signaling.
• This signaling drives secretion of TGF‑β and IFN‑γ from B cells, which act on bone marrow mesenchymal stromal cells to suppress IL‑7 transcription (known IL‑7 regulators).
• Reduced IL‑7 diminishes pro‑B to pre‑B transition, contracting the naïve B cell pool and perpetuating the age‑associated repertoire contraction (1, 2).
• The resulting low naïve B cell input limits germinal center seeding, compounding AID deficiency (4, 5).
• Thus, IR signaling in aged B cells sits upstream of both the cellular competition defect and the molecular AID/E47 deficit.

Testable Predictions

1. B‑cell‑specific IR knockout in aged mice (using CD19‑Cre IR^fl/fl) will:

   • Increase bone marrow IL‑7 mRNA and protein levels (2).
   • Elevate frequencies of hematopoietic stem‑cell‑derived pro‑B and naïve B cells (1).
   • Restore AID induction after immunization to levels comparable to young adults (4).
   • Produce larger, more organized germinal centers and higher affinity IgG antibodies (7).

2. Pharmacologic IR inhibition (e.g., using the selective antagonist S961) administered to old mice for 2 weeks prior to vaccination will recapitulate the genetic rescue.

3. In humans, ex‑vivo culture of aged peripheral B cells with IR antagonist will reduce their secretion of TGF‑β/IFN‑γ and increase IL‑7 production by co‑cultured bone marrow stromal cells.

Falsifiability

If B‑cell‑restricted IR loss or blockade fails to raise IL‑7, naïve B cell numbers, AID induction, or germinal center quality in aged mice, the hypothesis is refuted. Likewise, if IR inhibition does not alter aged B cell cytokine output in vitro, the proposed mechanistic link is unsupported.