Mitochondrial DNA Heteroplasmy Drives B Cell Immunosenescence

Core hypothesis Age-related accumulation of heteroplasmic mtDNA single-nucleotide variants (mtSNVs) in B cells impairs TFAM-dependent mitochondrial transcription, leading to a bioenergetic deficit that triggers NLRP3 inflammasome activation and aberrant epigenetic remodeling. This cascade reduces germinal center formation, lowers somatic hypermutation, and contracts the antibody repertoire.

Mechanistic rationale

1. TFAM linkage – TFAM packages and transcribes mtDNA; its activity is sensitive to mtDNA lesion load. Heteroplasmic mtSNVs increase transcriptional stalling, lowering NADH and ATP output in activated B cells.
2. Energy checkpoint – Activated B cells require a sharp rise in oxidative phosphorylation to support proliferation and antibody secretion. A fall below a threshold ATP/AMP ratio activates AMPK, which in turn phosphorylates and stabilizes the NLRP3 inflammasome.
3. Inflammasome-mediated signaling – Cytosolic mtDNA released from damaged mitochondria binds cGAS-STING, amplifying type-I interferon production that drives epigenetic deacetylation of AID-encoding loci via reduced SIRT1 activity (NAD+ dependent).
4. Outcome on B cell fate – Reduced AID expression limits somatic hypermutation; chronic IFN signaling skews differentiation toward short-lived plasmablasts at the expense of memory B cells, yielding a contracted, low-affinity repertoire.

Testable predictions

• Prediction 1: In humans >70 yr, sorted naïve, germinal center, and memory B cells will show a higher burden of heteroplasmic mtSNVs (mean heteroplasmy >5%) than cells from donors <30 yr (p<0.01, Mann-Whitney).
• Prediction 2: mtSNV burden will inversely correlate with TFAM mRNA levels and mitochondrial ATP production measured by Seahorse assay (r < -0.5).
• Prediction 3: Ex vivo treatment of aged B cells with mito-targeted NAD+ booster (e.g., MitoQ) or TFAM overexpression via mRNA electroporation will restore ATP, reduce NLRP3 ASC speck formation, and increase AID-dependent mutation frequency in a polyclonal activation assay.
• Prediction 4: Adoptive transfer of mtSNV-low young B cells into aged mice will rescue vaccine-induced affinity maturation, whereas transfer of mtSNV-high aged B cells will not, even when nuclear genome is matched.

Falsifiability If heteroplasmic mtSNV load does not differ between young and old B cell subsets, or if manipulating mtDNA status fails to alter TFAM expression, ATP, inflammasome activation, or AID activity, the hypothesis is refuted. Likewise, if NAD+ restoration improves ATP but does not rescue AID expression or antibody quality, the proposed inflammasome-epigenetic link would be insufficient.

References [1] https://doi.org/10.1172/jci64125 [2] https://pmc.ncbi.nlm.nih.gov/articles/PMC2980764/ [3] https://pmc.ncbi.nlm.nih.gov/articles/PMC10232359/ [4] https://www.ptglab.com/news/blog/b-cell-mitochondria-much-more-than-a-powerhouse/ [5] https://frontlinegenomics.com/b-cell-responses-belittled-in-mitochondrial-disorders/