Hypothesis

Clonal mtDNA heteroplasmy in proliferating germinal center (GC) B-cells reduces AID activity by (1) elevating mitochondrial ROS that causes off-target nuclear DNA damage at Ig loci, and (2) depleting acetyl‑CoA needed for histone acetylation that opens chromatin for AID access. Consequently, high heteroplasmy loads act as a selective bottleneck that skews affinity maturation toward lower‑affinity clones.

Testable predictions

• Prediction 1: GC B-cells isolated from aged mice will show a higher proportion of mtDNA heteroplasmy (>5% variant allele frequency) compared with young mice, and this proportion will inversely correlate with the frequency of high‑affinity antibodies in serum.
• Prediction 2: Pharmacological reduction of mitochondrial ROS (e.g., with MitoQ) in aged mice will decrease nuclear γH2AX foci in GC B‑cells and restore SHM fidelity without altering proliferation rates.
• Prediction 3: Supplying acetate to rescue acetyl‑CoA levels in B‑cells with induced mtDNA mutations will increase histone H3K27ac at the IgH locus and improve AID‑dependent mutation frequency, even when ROS remains elevated.

Experimental approach

1. Measure heteroplasmy: Single‑cell mtDNA sequencing of GC B‑cells (GL7+ CD95+) from young (3 mo) and aged (24 mo) mice; calculate variant allele frequency for each cell.
2. Correlate with output: Sort GC B‑cells into low (<2%) and high (>8%) heteroplasmy bins; assay SHM depth and bias in IgV genes via Ig‑seq; quantify serum antibody affinity using ELISA with increasing antigen concentrations.
3. Intervene ROS: Treat aged mice with MitoQ (0.5 mg/L in drinking water) for 2 weeks; repeat heteroplasmy and SHM analysis; stain for γH2AX and 8‑oxo‑dG in GC sections.
4. Rescue acetyl‑CoA: Culture sorted GC B‑cells from mtDNA‑mutator mice (PolG^D257A) with 5 mM acetate; perform ChIP‑qPCR for H3K27ac at IgH enhancers and measure AID‑induced mutations using a GFP‑reporter switch assay.
5. Control for proliferation: Track Ki‑67 and BrdU incorporation in all conditions to ensure observed effects are not secondary to altered cell‑cycle kinetics.

Mechanistic reasoning

Rapid GC B‑cell division (8‑12 h) creates a metabolic bottleneck where mitochondria must supply ATP, biosynthetic precursors, and redox balance. mtDNA mutations impair Complex I/III activity, increasing electron leak and superoxide production. Elevated ROS can diffuse to the nucleus, causing base oxidation and double‑strand breaks that interfere with AID’s deamination activity or trigger error‑prone repair pathways. Simultaneously, reduced TCA cycle flux diminishes citrate export, limiting acetyl‑CoA for histone acetyltransferases; hypoacetylated chromatin restricts AID access to switch regions and variable domains. Together, these two hits lower the probability of productive SHM events, forcing the GC to select clones that have undergone fewer mutations—often lower affinity. This positions mtDNA heteroplasmy not as a passive bystander but as an active regulator of the affinity‑maturation checkpoint.

Falsifiability

If high mtDNA heteroplasmy fails to correlate with reduced SHM fidelity, or if ROS scavenging and acetyl‑CoA rescue do not restore mutation quality despite persistent heteroplasmy, the hypothesis would be refuted, indicating that mitochondrial genotype does not directly dictate GC B‑cell diversification.