Hypothesis

The complement cascade in the aging retina does not merely reflect stochastic damage; it acts as a programmed signal that removes senescent retinal pigment epithelium (RPE) to reduce intra‑retinal competition for metabolites, thereby favoring the survival and reproductive success of younger kin. This aligns with the disposable soma view that aging is a selected trait, but adds a mechanistic layer: age‑dependent loss of heparan sulfate proteoglycans (HSPGs) in Bruch’s membrane lowers the threshold for Factor H‑mediated regulation, allowing a developmentally timed complement activation that mirrors the decline in systemic IGF‑1 after reproductive age.

Mechanistic reasoning

1. HSPG loss as a developmental timer – HSPG sulfation patterns are set during embryogenesis and gradually decay with age due to reduced expression of extensin‑like heparan sulfate synthases (EXTLs). This progressive loss diminishes Factor H anchoring, increasing local C3b conversion probability in a predictable, age‑correlated fashion (see [3]).
2. Complement‑driven RPE clearance – C3b opsonization tags RPE for phagocytosis by microglia and macrophages. The ensuing C3a/C5a signaling induces a senescence‑associated secretory phenotype (SASP) that includes IGF‑binding proteins, lowering systemic IGF‑1 and reinforcing the post‑reproductive endocrine shift ([4]).
3. Kin‑selection benefit – By eliminating RPE that would otherwise consume glucose and lipids, the retina frees resources for photoreceptor support in younger individuals. In a household‑style model, this increases the inclusive fitness of the individual's offspring.

Testable predictions

• Prediction 1: RPE‑specific overexpression of the complement inhibitor CRRY in mice will delay the age‑dependent rise in drusen‑associated C3 deposition and extend functional ERG responses beyond the normal lifespan, but will also lead to accumulation of p16^INK4a^‑positive senescent RPE and exacerbate photoreceptor loss under oxidative stress.
• Prediction 2: Accelerating complement activation via a heterozygous C3 gain‑of‑function allele will cause premature drusen formation and early‑onset geographic atrophy‑like lesions, yet offspring raised under limited food will show higher survival rates compared to wild‑type littermates.
• Prediction 3: Systemic administration of an IGF‑1 neutralizing antibody in young mice will phenocopy the age‑dependent HSPG loss pattern and increase retinal C3 deposition, linking endocrine status to complement timing.

Falsifiability

If complement inhibition does not alter the timing of RPE senescence or if clearing senescent RPE fails to improve kin survival under resource limitation, the hypothesis that complement‑mediated RPE clearance is a programmed kin‑selected mechanism would be refuted, supporting a purely stochastic damage model.