Inflammation in acute poststreptococcal glomerulonephritis (APSGN) is initiated by specific proteins released by group A streptococcus bacteria that trigger immune complex formation and uncontrolled complement activation in the kidneys. The process begins days to weeks after a strep throat or skin infection, when bacterial proteins deposit in the glomeruli (the kidney’s tiny filtering units) and provoke an immune response that damages the kidney’s own tissue.
Two Streptococcal Proteins That Start the Process
Not every strep infection leads to kidney inflammation. Only certain strains produce what researchers call “nephritogenic antigens,” proteins specifically capable of triggering glomerular disease. Two have been identified as the primary culprits: streptococcal pyrogenic exotoxin B (SpeB) and nephritis-associated plasmin receptor (NAPlr). Earlier theories pointed to M proteins on the bacterial surface as the main drivers, but more recent work has shifted the focus to SpeB and NAPlr as the proteins most directly tied to kidney damage.
SpeB is a powerful enzyme that the bacteria secrete into surrounding tissue. It carries a positive electrical charge, which turns out to be critical. The glomerular basement membrane, the filtration barrier in the kidney, carries a strong negative charge. That charge difference allows SpeB to penetrate and lodge itself within the membrane, where it becomes a target for the immune system. Once embedded, SpeB also directly cleaves a complement protein called C3, kicking off one of the body’s most aggressive inflammatory cascades.
NAPlr sits on the bacterial cell surface and functions identically to an enzyme the bacteria use for energy metabolism (GAPDH). Its key property is that it binds tightly to plasminogen, a protein in your blood, and converts it into plasmin, an enzyme that breaks down clots and tissue. Both SpeB and NAPlr share this ability to activate plasmin, which contributes to local tissue damage in the glomerulus. A third protein called SIC, produced by certain M1 and M57 strep strains, has also been linked to kidney complications. SIC works differently: it blocks the complement system’s ability to kill bacteria, helping the infection persist long enough for nephritogenic antigens to accumulate.
How Immune Complexes Form in the Kidney
The inflammation that damages the glomerulus is driven by immune complexes: clumps of antibody bound to antigen. In APSGN, these complexes form through two routes. Some assemble in the bloodstream and then get trapped in the kidney’s filtration network. Others form directly at the site of injury, a process called in situ immune complex formation, where the antigen arrives in the glomerulus first and the antibody finds it there.
In situ formation is particularly important in APSGN because of the charge dynamics described above. Cationic (positively charged) antigens like SpeB are drawn through the negatively charged basement membrane, where they embed on the outer side. Antibodies circulating in the blood then bind to these planted antigens, forming immune complexes right on the membrane surface. Under electron microscopy, these deposits appear as characteristic “humps,” large, dome-shaped clumps sitting on the outer (subepithelial) surface of the basement membrane. Immunofluorescence staining reveals these humps contain IgG antibodies and C3 complement protein in a coarse, irregular “lumpy-bumpy” pattern along the capillary walls.
Complement Activation Drives the Damage
The complement system is a chain reaction of proteins in the blood that amplifies inflammation and destroys foreign material. In APSGN, the alternative complement pathway is the dominant route of activation, not the classical pathway. This distinction matters because it explains a hallmark lab finding: patients almost universally show low circulating C3 levels during the acute phase, while C1 and C4 (markers of the classical pathway) remain normal.
The alternative pathway runs continuously at a low level through a process called “tick over,” where C3 spontaneously breaks down and interacts with a protein called factor B. Factor B gets cleaved into fragments, and the active fragment (Bb) combines with C3b to form C3bBb, the alternative pathway’s key enzyme. This enzyme generates more C3b, which feeds back to create even more C3bBb in a self-reinforcing amplification loop. Under normal conditions, regulatory proteins keep this loop in check.
In APSGN, two things supercharge this loop. First, SpeB directly cleaves C3, flooding the system with complement fragments. Second, and perhaps more importantly, patients with APSGN develop transient autoantibodies against factor B. These autoantibodies stabilize the C3bBb enzyme, preventing it from breaking down naturally. The amplification loop runs unchecked, consuming massive amounts of C3 from the bloodstream (which is why serum C3 drops) and generating large quantities of inflammatory complement fragments in the glomerulus. These fragments recruit immune cells, damage capillary walls, and increase permeability, leading to the protein and blood that spill into the urine.
Inflammatory Cell Recruitment
Once complement activation and immune complex deposition are underway, the glomerulus becomes an active site of inflammation. Mesangial cells, the structural support cells within the glomerulus, respond to the injury by releasing signaling molecules called chemokines. Key among these are CCL2 (also known as MCP-1) and CX3CL1, which act as chemical beacons that draw macrophages and other immune cells out of the bloodstream and into the glomerular tissue. Podocytes, the specialized cells that wrap around the kidney’s capillaries, also release chemokines including CXCL1 and CCL2.
CCL2 plays a particularly central role. It recruits macrophages by binding to a receptor called CCR2 on their surface, pulling them into the injured tissue where they release further inflammatory signals and cause additional damage. CCL5 (also called RANTES) contributes to leukocyte migration as well. The result is a rapidly escalating inflammatory response: complement activation damages capillary walls, chemokines recruit immune cells, and those immune cells release more inflammatory mediators that perpetuate the cycle.
Timeline From Infection to Kidney Symptoms
The lag between strep infection and the onset of kidney inflammation reflects the time it takes for this immunologic cascade to build. After strep pharyngitis (throat infection), kidney symptoms typically appear about 10 days later. After streptococcal skin infections like impetigo, the latent period extends to about 3 weeks. This delay is what makes APSGN “post-streptococcal.” The original infection may have already resolved by the time the kidney disease appears, which is why the diagnosis often relies on antibody evidence of a recent strep infection rather than finding active bacteria.
The combination of anti-streptolysin O (ASO) and anti-DNase B antibody tests is the most reliable way to confirm prior strep exposure, with a combined sensitivity of 95.5% and specificity of 88.6%. A single antibody test alone catches only about 70-73% of cases.
Resolution of the Inflammatory Process
The autoantibodies against factor B that sustain complement activation in APSGN are transient, meaning the body stops producing them relatively quickly. This is why serum C3 levels typically return to normal within about 12 weeks of diagnosis. If C3 stays low beyond that window, it raises concern for a different underlying kidney disease rather than APSGN.
A long-term follow-up study tracking children for 9 years after diagnosis found that proteinuria (protein in the urine) resolved within 3 years and blood in the urine disappeared within 4 years. Children generally recover well, though the pace of full resolution can be slower than many families expect. The self-limiting nature of APSGN is directly tied to the transient nature of the immune dysregulation: once the nephritogenic antigens are cleared and the autoantibodies fade, the complement amplification loop loses its fuel, inflammation subsides, and the glomeruli heal.
Who Is Most Affected
APSGN remains a significant pediatric disease worldwide, with roughly 170,584 new cases among children and adolescents in 2021. The global incidence has been declining at about 1% per year, but the burden is unevenly distributed. Middle-income countries carry the highest rates, at nearly 9 per 100,000 children, roughly 1.7 times higher than in high-income countries. Brazil reports the highest national incidence at 32 per 100,000, followed by North Korea and Taiwan. China accounts for over 17% of global pediatric cases, followed by Brazil and India. In low-income regions, rates have actually been gradually increasing, highlighting persistent gaps in strep infection prevention and treatment.