What Happens in C3 Glomerulopathy (C3G)
C3 glomerulopathy is a rare kidney disease where your body's immune defense system (specifically the complement system) goes haywire and attacks your own kidney filters, causing them to become inflamed and damaged. 1
The Basic Problem
In plain terms, here's what's happening:
Your immune system has a component called "complement" that normally helps fight infections. One part of this system—the alternative pathway—acts like a security alarm that should turn on briefly and then shut off. In C3G, this alarm gets stuck in the "on" position and keeps attacking your kidney's filtering units (glomeruli) even though there's no infection to fight. 2
Why the Alarm Gets Stuck
The malfunction happens for two main reasons:
- Genetic mutations - You're born with faulty "off switches" in your complement system (occurs more in children) 2
- Autoantibodies - Your body makes antibodies that prevent the alarm from turning off (more common in adults) 2
Either way, a protein called C3 keeps depositing in your kidney filters like rust building up in pipes, causing progressive damage. 1
What You Experience
The disease causes:
- Protein leaking into urine (nephrotic syndrome) - your kidneys lose their ability to keep protein in your blood
- Blood and inflammation in urine (nephritic syndrome) - damaged filters allow blood cells through
- Progressive kidney function decline - about 60% of patients reach kidney failure within 10 years 3
- Low C3 levels in blood tests - the complement protein gets used up attacking your kidneys 4
Two Subtypes Under the Same Umbrella
C3G includes two patterns that look different under the microscope but share the same underlying problem of complement dysregulation 1:
- C3 glomerulonephritis (C3GN) - more common, with irregular deposits
- Dense deposit disease (DDD) - rarer, with characteristic "sausage-like" dense deposits
Both behave similarly and progress to kidney failure at comparable rates.
The Transplant Problem
If you need a kidney transplant, C3G has a high recurrence rate (up to 60%) because the underlying complement problem is in your blood, not just your kidneys - so it attacks the new kidney too. 5, 3 This makes transplant outcomes particularly challenging.
Current Treatment Landscape
The 2023 KDOQI guidelines recommend a stepwise approach for moderate-to-severe C3G 6:
- First-line: MMF (mycophenolate mofetil) plus corticosteroids
- Second-line: Eculizumab if first-line fails
- Clinical trials: Should be considered for all patients, not just treatment failures
However, traditional immunosuppression and terminal complement blockers (like eculizumab) have shown limited benefit because they don't target the root problem—the overactive alternative pathway. 5
Newer therapies targeting the alternative pathway directly (such as pegcetacoplan and iptacopan) are showing much more promise by actually shutting off the stuck alarm rather than just treating downstream inflammation. 7, 8 These represent a paradigm shift toward disease-specific treatment rather than non-specific immunosuppression.
The Bottom Line
C3G is fundamentally a disease where your complement system's alternative pathway won't turn off, continuously depositing C3 protein in your kidney filters and causing progressive damage. Without effective treatment targeting this specific pathway, most patients face declining kidney function and eventual kidney failure, with high risk of recurrence after transplantation.