MAIT Cell Dysfunction as the Immune‑System Engine of Inflammaging

Hypothesis: Age‑related loss of riboflavin‑producing gut microbes silences MAIT cell surveillance, turning these innate‑like lymphocytes into a source of senescence‑associated secretory phenotype (SASP) that drives epithelial leak, systemic inflammation, and further immune aging. Restoring MR1 ligands breaks this loop and should delay multi‑organ decline.

**Mechanistic rationale

• With advancing age, the Firmicutes/Bacteroidetes ratio rises and butyrate‑makers like Barnesiella fall (PMC7374892, PMC10967378).
• Concurrently, dietary‑derived riboflavin and bacterial synthesis of 5‑(2‑oxopropylideneamino)‑6‑D‑ribitylaminouracil (5‑OP‑RU), the potent MR1 ligand, drop sharply (inferred from loss of Lactobacillus and Enterococcus strains that produce riboflavin).
• MAIT cells, which depend on MR1‑presented 5‑OP‑RU for tonic signaling, receive weaker stimulation, shift toward an exhausted phenotype, up‑regulate PD‑1 and CD57, and begin secreting IL‑6, TNF‑α and ROS (PMC11464129, Frontiers in Aging 2022).
• SASP factors from MAIT cells impair goblet‑cell mucus production and tight‑junction proteins, worsening LPS translocation (PMC10967378, PMC11464129).
• The ensuing inflammaging further fuels myeloid‑biased hematopoiesis and adaptive‑immune senescence, completing a feed‑forward circuit where immune cells themselves become the primary agents of tissue damage (Frontiers in Immunology 2024, PMC11785404).

**Testable predictions

1. Ligand rescue – Oral administration of a stable MR1 agonist (e.g., 5‑OP‑RU analogue) or a riboflavin‑enriched probiotic cocktail will increase MAIT cell activation markers (CD69, CXCR6) and reduce PD‑1^hi CD57^+ frequencies in aged mice.
2. Barrier restoration – Ligand‑treated old animals will show higher mucin‑2 mRNA, lower FITC‑dextran serum leakage, and decreased circulating LPS compared with vehicle controls.
3. Senescence burden – Flow cytometry of splenic and lamina propria lymphocytes will reveal a decline in p16^INK4a^+ and γH2AX^+ MAIT cells after ligand treatment, alongside reduced SASP cytokine levels in plasma.
4. Functional outcome – Improved performance on rotarod and novel‑object‑recognition tasks, reflecting attenuated neuro‑inflammation, should correlate with the immunological readouts.

Falsifiability – If MAIT‑cell–targeted ligand supplementation fails to (a) elevate MAIT activation, (b) lower gut permeability, or (c) reduce systemic inflammatory markers in aged mammals, the hypothesis that MAIT dysfunction drives inflammaging would be refuted. Likewise, germ‑free mice colonized solely with riboflavin‑deficient bacteria should exhibit accelerated MAIT senescence despite normal butyrate levels, confirming the ligand‑specific axis.

Implications – Unlike broad SCFA approaches, targeting the MAIT‑MR1 axis offers a precise lever to reprogram immune surveillance without globally altering microbiota composition. Success would suggest that re‑educating a single lymphocyte subset can interrupt the immune‑system‑driven aging cycle, slowing senescence across gut, vasculature, and brain.