The Paradox of Localized Complement Activation

Current evidence shows that complement C3 deposition isn't just a systemic bystander; it’s a locally synthesized driver of retinal decay. Hoh Kam et al. (2014) identified recruited microglia and monocytes as the primary engines of C3 mRNA propagation in the aging retina. While losing C3 seems to protect against oxidative stress and retinal thinning [Chen et al. (2017)], the total retinal collapse seen in CFH-/-C3-/- double-knockouts [Pickering et al. (2013)] highlights that C3 plays a vital homeostatic role. This suggests the pathology doesn't stem from the mere presence of C3, but rather from how it’s processed and where it ends up.

The Hypothesis: Sub-Lytic Lysosomal MAC Formation

I’m proposing that the primary driver of retinal pigment epithelium (RPE) atrophy in aging isn't plasma membrane lysis by the Membrane Attack Complex (MAC/C5b-9). Instead, it’s the intracellular assembly of MAC on lysosomal membranes after the cell endocytoses debris opsonized with C3/C3d.

We already know complement activation causes lysosomal damage, evidenced by increased Galectin-8 binding [Hynes et al. (2021)]. My hypothesis is that when RPE cells phagocytose photoreceptor outer segments (POS) coated with microglial-derived C3b, the subsequent recruitment of C5–C9 within the endolysosomal compartment leads to "sub-lytic lysosomal permeabilization" (sLMP). This allows cathepsins and protons to leak chronically into the cytosol, triggering low-grade inflammasome activation and metabolic exhaustion without killing the cell immediately.

Mechanistic Insight: Why 'Total Absence' Fails

The failure of CFH-/-C3-/- models likely comes down to the loss of C3b-mediated phagocytosis. RPE cells need C3b to efficiently recognize and clear POS. However, in the aging retina, hyper-local C3 synthesis by microglia creates an "over-opsonized" environment. This over-saturates the endolysosomal system, where terminal pathway components—internalized via fluid-phase endocytosis or produced locally—assemble MACs on the internal membranes of the RPE.

This would explain why Chen et al. (2017) observed reduced oxidative stress in C3-deficient mice: they aren't dealing with the mitochondrial strain caused by chronic sLMP-induced calcium signaling. It also explains why the terminal pathway literature is so thin on MAC; we’ve been looking at the cell surface when we should have been looking at the lysosomal lumen.

Testability and Falsification

We can test this through a few specific experiments:

1. Super-Resolution Imaging: We can check for the co-localization of MAC markers (C5b-9) with lysosomal membrane proteins (LAMP1) and Galectin-8 in aged wild-type versus C3-/- mouse RPE. If this hypothesis holds, MAC should appear within the lysosomal compartment before any obvious retinal thinning occurs.
2. Specific Inhibition: We could use RPE-specific expression of CD59 (a MAC inhibitor) targeted specifically to the endolysosomal lumen. If the lysosomal MAC-trap is the real driver, intra-lysosomal CD59 should rescue the RPE phenotype more effectively than systemic C3 inhibition.
3. Falsification: If terminal pathway inhibition (like an anti-C5) prevents MAC formation but doesn't reduce Galectin-8 binding or lysosomal pH instability, then C3-mediated damage is likely direct—perhaps via C3aR signaling—rather than MAC-dependent.

By shifting our focus from the plasma membrane to the lysosomal interior, we might finally resolve the conflict between C3’s protective role in phagocytosis and its destructive inflammatory consequences.