Mednotes

L9 Renal Glomerulonephritis

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tagsrenal, lecture
aliasesL9 Renal Glomerulonephritis

23 min read

Overview

Glomerulonephritis (GN) is immune-mediated glomerular injury, and this lecture builds it in three parts: the basic immunopathogenesis of the main patterns of injury (proliferative, non-proliferative, and rapidly progressive), the specific immunology of acute post-streptococcal glomerulonephritis, and the clinical syndromes (nephritic and nephrotic) with their pathophysiology. The unifying idea is that where the antigen sits and where the immune deposit forms determine whether the injury has contact with the circulation, and that in turn determines whether inflammatory cells are recruited, which determines the clinical presentation.

Normal glomerular structure

Structures to know from the normal glomerulus: mesangial cell, capillary loop, endothelial cell, afferent arteriole, efferent arteriole, and the juxtaglomerular apparatus.

The capillary wall in cross-section, from lumen outwards:

  • Glomerular capillary lumen, with fenestrated endothelial cells (fenestrae) and an overlying glycocalyx.
  • Glomerular basement membrane (GBM).
  • Podocyte foot processes with filtration slits bridged by the slit diaphragm (40 nm).
  • Podocyte cell body, urinary space, parietal epithelial cell.

Key functional points about this barrier:

  • The GBM is not a solid barrier.
  • The barrier has charge- and size-selective properties.
  • Podocyte function is essential to it.
  • In nephritic forms of GN, normal capillary loops become occluded loops.
  • In nephrotic syndrome the foot processes are effaced (seen on SEM and TEM), compared with the normal foot processes, GBM, endothelium, Bowman’s space and slit diaphragm.

Initiating events and the antigens involved

Host immune responses to antigens (pathogen or self) produce specific immunoglobulins and sensitised T cells as the primary effector arm. This usually results in deposition of immunoglobulins or accumulation of activated T lymphocytes within the glomerulus.

Three antigen categories:

  1. Exogenous bacterial or viral product.
  2. Exogenous self: circulating DNA/ENA (SLE), altered IgA.
  3. Endogenous, a local component of the glomerulus: the non-collagenous domain of collagen A4, and components of the podocyte foot process.

Immune complex formation: binding of multiple IgM or IgG antibodies to a soluble antigen causes an insoluble complex to form, which is deposited at the surface of tissue. Immunoglobulin from B cells binds antigen to form an activated immune complex, and the site, the manner and the type of immunoglobulin all modify the immune response.

Humoral immunity contributes in three ways:

  • Antibody to normal glomerular constituents: Goodpasture antigen (collagen A4 in the basement membrane) and the podocyte antigen targeted by anti-PLA2R antibody.
  • Non-renal self antigens localised in glomeruli: SLE (DNA fragments) or cryoglobulins.
  • Exogenous antigens, free or as circulating immune complexes, localising according to charge and size, with nephrogenic potential depending on the type of protein recognition.

Mechanisms of immune glomerular injury

B cells (producing IgG) and T cells induce glomerular injury by several pathways:

  • Antibody can directly induce injury.
  • Antibody-activated complement, with the complement membrane complex directly inducing injury.
  • Complement chemotaxis recruiting PMNs.
  • Macrophage recruitment and activation via Fc fragments on deposited immunoglobulins.
  • Lymphokines produced by activated T cells (delayed-type hypersensitivity).

These principles apply to all immune-mediated disease, not just GN.

Complement

Three pathways converge on C3:

  • Classical: IgG1, IgG3, IgM with C1q, C2, C4, forming the classical-pathway C3 convertase C4bC2a.
  • Lectin: MBL and MASPs, triggered by microbial surfaces, agalactosyl IgG, IgG4, IgA and SpeB, also feeding into C4bC2a.
  • Alternative: spontaneous activation, damaged cells, LPS and IgA, with C3b plus properdin and factor B, via C3 tickover, forming the alternative-pathway C3 convertase C3bBb.

C4bC2a with C3b forms the C5 convertase (C4bC2aC3b / C3bBbC3b). C5 is then cleaved to give the two key mediators: C5a, a chemotactic factor, and C5b,C6,C7,C8,C9, the membrane attack complex. Regulators are the circulating complement regulatory proteins factor H (CFH), factor I (CFI) and membrane cofactor protein (MCP), and the cell-bound regulator CD59.

Innate and adaptive limbs together

Etiologic events generate PAMPs and DAMPs, which drive two arms:

  • Innate: complement and TLRs/NLRs. Complement generates C5b-9 and C5a; these plus TLR/NLR signalling activate resident glomerular cells and circulating inflammatory cells, which release proteases, chemokines, oxidants, growth factors, cytokines and eicosanoids, producing glomerular tissue injury.
  • Adaptive: antigen-presenting cell to CD4 T cells, which give TREGs (inhibitory), B cells (autoantibodies and antigen-antibody complexes), and TH1/TH2/TH17 subsets; all feed into circulating inflammatory cells and glomerular tissue injury.

Where deposits form, and why it matters

Three routes to deposit formation, each mapping to particular diseases:

  • A. Circulating immune complex trapping: circulating antigen-antibody complexes are trapped between the endothelial cell and the basement membrane. Post-infectious (streptococcal) GN, membranoproliferative GN (chronic infection), IgA nephropathy.
  • B. In situ immune deposit formation with exogenous or planted antigens: circulating antigen and antibody form deposits in situ at the capillary wall or mesangium. Lupus nephritis (non-renal self antigens) and exogenous bacterial antigens.
  • C. In situ immune deposit formation with endogenous antigens: antibody binds directly to fixed antigens on the capillary wall or podocyte. Anti-GBM GN (antigen collagen) and membranous GN (antigen PLA2R).

Consequences of immune deposit formation depend on:

  • Site of deposits: mesangial, subendothelial, subepithelial. These give the descriptive terms for GN.
  • Biological properties of the immunoglobulin: complement fixation, activation of serine proteases, ability to evoke a cellular inflammatory response via cytokines.
  • Mechanism of formation: in situ immune complexes are more nephritogenic.
  • Amount of immune deposit.

Patterns of glomerular injury

Endothelial, proliferative pattern (contact with the circulation).

  • Leucocyte accumulation, endothelial cell injury, endocapillary proliferation: post-infectious GN, lupus nephritis.
  • Associated immune complex deposition with mesangial proliferation: post-infectious GN, lupus nephritis, IgA nephropathy.
  • If severe, capillary wall destruction and crescent formation: anti-GBM GN, vasculitis, lupus nephritis class IV, membranoproliferative GN.

Epithelial, podocyte pattern (no contact with the circulation). Antibodies and complement inflict cytotoxic injury on the podocyte with no contact with the circulation, resulting in a non-exudative, non-proliferative capillary wall lesion. This is membranous GN.

Overall scheme of capillary wall damage: glomerular antibody deposits and sensitised T cells both activate complement, generating C5b-9 and C5a. C5b-9 acts on glomerular epithelial cells (GEC), glomerular endothelial cells (GEN) and mesangial cells (MC) in a non-inflammatory pathway causing capillary wall damage. C5a acts on PMNs and platelets, mesangial cells and macrophages in an inflammatory pathway, generating oxidants and proteases that cause capillary wall damage and proteinuria, and generating fibrin leading to crescents, with macrophages also contributing directly to crescents.

Case 1: acute post-streptococcal glomerulonephritis (APSGN)

An 18 year old presents with macroscopic haematuria, decreased urine output (oliguria), periorbital oedema and hypertension, on a background of skin infections (impetigo). That is an acute nephritic syndrome: haematuria, oliguria, oedema, hypertension, with acute kidney injury.

Pathogenesis

  • Triggered by recent infection with specific strains of group A beta-haemolytic streptococcus.
  • It is an immune complex disease that triggers complement activation and inflammation, by either in situ IC formation (streptococcal antigens deposited within the GBM with antibody binding) or glomerular trapping of circulating immune complexes.
  • Complement activation with C3 degradation, via the classical pathway.
  • Inflammation follows.

Sequence for the case: activated immune complexes and/or nephritogenic antigens circulate from the strep infection; they attract and activate complement, with injury as the complexes contact the endothelium; chemotaxis by C3a and C5a recruits neutrophils; proteases are activated. The result is a proliferative form of glomerulonephritis.

Deposit (“hump”) formation: a negatively charged nephritogenic antigen from repeated bacterial infection binds along the endothelial layer, antibody binds to form the immune complex, pro-collagenase and latent MMP are activated to collagenase and MMP which degrade GBM components, leading to effacement of podocyte foot processes and subepithelial “hump” deposits with protein loss into the urinary space.

Histology

  • Light microscopy: diffuse endocapillary proliferation with large amounts of PMNs, versus the normal glomerulus. Nephritic presentation.
  • Electron microscopy: subepithelial electron-dense deposits along the GBM plus an infiltrating neutrophil, versus the normal glomerulus (urinary space, epithelial cell, GBM, endothelium, lumen).

Group A streptococci (GAS)

  • Include harmless and pathogenic species. May cause pharyngitis, endocarditis, meningitis, pneumonia, impetigo, scarlet fever, necrotising fasciitis, sepsis, acute rheumatic fever and APSGN.
  • Beta-haemolytic strains are further characterised by Lancefield serotyping (20 groups, A to H and K to V).
  • Streptococcus pyogenes is group A beta-haemolytic and causes most infections.
  • Rheumatic fever and APSGN do not tend to occur in the same patient.

Virulence factors:

  • M protein: promotes adherence to epithelium and escape from phagocytosis in the absence of opsonising antibody. Encoded by the emm gene, which distinguishes more than 230 genetically different emm types (previously M serotypes). Antibodies to M protein are protective but type-specific with low cross-reactivity, which is the basis for GAS vaccine development.
  • Superantigens: some strains produce streptococcal pyrogenic exotoxin A1 (SPE-A1) or SPE-B, causing toxic shock. A superantigen bridges the T-cell receptor and MHC II outside the normal antigen groove, unlike conventional antigen presentation where the TCR binds antigen held in MHC II. SPE-B also cleaves complement C3b and degrades neutrophil extracellular traps.
  • Fibronectin binding proteins: promote invasion.
  • Capsule: some strains have hyaluronic acid capsules that inhibit phagocytosis.
  • Cytolysins: streptolysin O, hyaluronidase, DNAse; kill host cells.

Emm types:

  • Three major emm pattern groups are defined on sequence differences and associate with tropism (pharynx A to C plus E; skin D plus E).
  • All pattern groups can lead to APSGN.
  • A 2013 Auckland study of pharyngeal GAS samples (n = 282) showed three emm types predominate: 1, 89 and 12.
  • Emm 1 and 12 predominate in developed countries.

Epidemiology (New Zealand)

  • Leading cause of acute nephritis worldwide.
  • Risk greatest in children (especially 5 to 9 years) and older adults (over 60 years); males more than females.
  • NZ incidence per 100,000 children per year, total 9.7. By ethnicity: Asian 2.1, European/other 2.6, Maori 15.7, Pacific 45.5. By NZDep: quintile 1 to 2, 0.8; quintile 9 to 10, 45.3.

Clinical and laboratory features, Auckland 2007 to 2009 (n = 176)

Feature%Time to APSGN
S. pyogenes isolated from pharynx329.1 days
S. pyogenes isolated from skin4010.9 days
Gross haematuria87
Microscopic haematuria only13
Hypertension72
Oedema62
Low C3 complement93
Serum creatinine more than 2 SD above normal68

Important

Gross haematuria (87%) and low C3 (93%) are the standout features.

Outcomes

  • Most children make a complete recovery, and resolution begins within the first 2 weeks.
  • Prevalence of long-term complications in NZ is unknown. In an Australian Aboriginal population there was a 3 to 4 fold increase in risk of chronic kidney disease 5 years after APSGN; other studies found more than 90% have normal or modestly reduced renal function 5 to 18 years post APSGN.
  • Some patients present with extra-renal complications (acute encephalopathy or congestive heart failure), which may delay diagnosis.
  • Acute severe renal failure is uncommon in NZ (2%).
  • Post-streptococcal GN is a self-limiting GN presenting as acute nephritic syndrome: clearance of infection removes circulating immune complexes and the glomerulus repairs.

Case 2: membranous GN and the nephrotic presentation

A 60 year old male with progressive oedema and frothy urine. Haematology normal. Creatinine 80 umol/L, urea 6.0 mmol/L, albumin 20 g/L, protein 46 g/L (nephrotic). 24 hour urine protein excretion 5.5 g (nephrotic), protein/creatinine ratio 489 g/mmol (normal less than 23). Serum protein electrophoresis normal, autoantibody screen negative, normal renal ultrasound.

Histological features of membranous GN

  • No influx of inflammatory cells.
  • No expansion of the mesangium.
  • No occlusion of the capillary lumen.
  • Expansion of the basement membrane.
  • Immunofluorescence shows granular IgG deposition along the capillary loops; EM shows subepithelial deposits along the GBM; silver/PAS light microscopy shows the thickened basement membrane.

Mechanism

Three possible mechanisms of subepithelial deposit formation: deposition of small circulating immune complexes of low-avidity antibody with oligovalent antigen (primary); in situ immune complex formation involving a native podocyte antigen; and in situ formation involving a “planted” non-native antigen bound by circulating antibodies (secondary).

PLA2R and IgG4 co-localise along the glomerular capillary loops in membranous GN, shown by immunofluorescence with PLA2R and IgG4 staining merging.

Why membranous GN is non-inflammatory: the immune complex is not active until it is formed on the podocyte. Complement is activated at that site to form C5b-9 in the urinary space, so there is no contact with the circulation and C5a cannot recruit PMNs.

Staged podocyte injury:

  1. Healthy podocyte carrying THSD7A and PLA2R1 antigens on the GBM.
  2. Immune deposits form as PLA2R1-antibody (IgG4) and THSD7A-antibody (IgG4) bind, causing cytoskeleton disturbance, with proteinuria.
  3. Complement activation: C5b-9 forms, generating eicosanoids and O2 radicals.
  4. GBM thickening via the TGF-beta / MMP9 pathway affecting laminin and collagen IV, with proteinuria.

C5b-9 inserted into the podocyte membrane triggers ER stress, COX-2 activation, cytoskeletal changes, dissociation of CD2AP and nephrin from the slit diaphragm, and protease plus NADPH-oxidase/ROS generation degrading GBM components, resulting in proteinuria.

Progression to podocyte loss: C5b-9 drives actin/cytoskeletal changes, ROS, TGF-beta, angiotensin II and lack of VEGF, and DNA damage (p21 up, p57 down), leading to apoptosis and lack of proliferation, and separately to detachment via integrin loss and degradation. The net result is loss of podocytes.

From glomerular injury there are two paths: no podocyte depletion gives no glomerulosclerosis and no progression to ESKD; whereas podocyte loss (necrosis, apoptosis, detachment), glomerular enlargement and podocyte phenotype switch all converge on effective podocyte depletion, then glomerulosclerosis, then progression to ESKD.

Natural history of membranous GN

  • Untreated spontaneous remission in 20 to 30%.
  • 10 year renal survival 60 to 80%.
  • With persistent nephrotic syndrome, 40 to 50% develop kidney failure over a 10 year period.

Case 3: rapidly progressive (crescentic) GN

A 54 year old male with a 2 month prodromal illness of vague arthralgias and myalgias and low grade “flu-like” fever, then an acute 48 hour history of breathlessness and haemoptysis, oliguria and haematuria, anorexia and nausea.

Examination and investigations: hypertensive 180/100, JVP 7 cm, pulmonary and peripheral oedema. Urinalysis shows haematuria and proteinuria, with RBC casts and dysmorphic RBCs on microscopy. Creatinine 1500 umol/L, urea 48 mmol/L, pH 7.12, K+ 6.8 mmol/L, albumin 37 g/L (normal). This is acute nephritic, severe rapidly progressive GN.

Anti-GBM glomerulonephritis: crescentic GN on light microscopy (cellular crescent in Bowman’s space) with linear IgG staining along the GBM on immunofluorescence, as opposed to the granular staining of immune-complex GN. Crescents show proliferating macrophages and fibrin deposition in Bowman’s space.

Phases of the inflammatory process:

  1. Normal kidney: glomerular capillary loop and Bowman’s capsule with adhesion molecules, alongside peritubular capillary and interstitial cell architecture.
  2. Initiation phase: a primary insult (for example immune complex) triggers generation of proinflammatory mediators (cytokines, chemokines, lipid mediators, ROS) and activation of leucocytes, macrophages and intrinsic renal cells, with selectin-ligand interaction and leucocyte rolling.
  3. Amplification phase: amplification of the inflammatory signal by infiltrating cells and activation of an increasing number of intrinsic renal cells, with spillover of mediators into the peritubular capillaries and urinary space, and proteinuria.
  4. Progression phase: persistent release of mediators leads to interstitial infiltration, interstitial and mesangial proliferation, further damage of tubules, rupture of Bowman’s capsules and progressive fibrosis.

Clinical assessment and classification

Assessment of GN: history and examination, then urinalysis by dipstick, microscopy and proteinuria.

Classification according to clinical presentation:

  • Asymptomatic urinary abnormalities: subnephrotic-range proteinuria and/or microscopic haematuria, not accompanied by renal impairment, oedema or hypertension.
  • Nephritic syndrome: recent onset of haematuria and proteinuria, renal impairment, and salt and water retention causing hypertension.
  • Rapidly progressive glomerulonephritis: progression to renal failure over days to weeks, in most cases in the context of a nephritic presentation, typically with extensive glomerular crescent formation on renal biopsy.
  • Nephrotic syndrome: nephrotic-range proteinuria (more than 3.5 g per 1.73 m² in 24 h), hypoalbuminaemia, hyperlipidaemia and oedema, in many cases complicated by predisposition to venous thrombosis and bacterial infection.
  • Chronic glomerulonephritis: persistent proteinuria with or without haematuria and slowly progressive impairment of renal function.

Nephrotic syndrome: features and pathophysiology

Features: hypoalbuminaemia, oedema, proteinuria more than 3 g per 24 hr, hyperlipidaemia. Renal function is often normal. Histology shows thickened capillary walls.

Primary initiating event for sodium reabsorption: on the lumen side of the cortical collecting duct cell, plasminogen is converted to plasmin by urokinase-type plasminogen activator; plasmin cleaves the gamma-inhibitory domain that normally keeps ENaC channels closed, converting closed ENaC channels to open channels and driving Na+ transport from lumen to blood.

Oedema pathway: glomerular disease increases filtration of proteins, giving proteinuria, then hypoalbuminaemia, then decreased plasma oncotic pressure. Via a perceived low arterial perfusion pressure this contributes to sodium retention, which is also driven directly by glomerular disease through tubular Na+ uptake, SNS and RAAS. Sodium retention gives plasma volume expansion, then increased capillary hydrostatic pressure (with blood pressure low-normal), then water movement from the intravascular to the interstitial space, giving oedema. Remember Starling’s law.

Acute nephritic syndrome: features and pathophysiology

Features: haematuria, oliguria, acute kidney injury, hypertension. Histology shows proliferation that has occluded the capillary loops.

Pathophysiology: glomerular injury causes haematuria, proteinuria and a fall in GFR. The reduced GFR, together with RAAS, SNS and AVP activation, causes sodium retention, which gives hypertension and water retention, and hence oedema, oliguria, and a raised JVP with cardiomegaly.

Urinalysis in nephritic disease shows red-tinged urine, red blood cells, dysmorphic RBCs and RBC casts.

Take home points

  • Understand the basic immunology related to GN.
  • Understand the different forms of GN.
  • Understand how they represent different patterns of immune-mediated injury.
  • Link this to the clinical presentation.

Self-test

  1. List the three antigen categories that initiate glomerulonephritis, with an example of each.
  2. Describe how an insoluble immune complex forms and is deposited in tissue.
  3. List the pathways by which B cells and T cells induce glomerular injury.
  4. Name the two key complement mediators generated downstream of C5, and state what each does.
  5. Name the three complement pathways and the C3 convertase each generates.
  6. List the complement regulatory proteins named in the lecture and say which one is cell-bound.
  7. Distinguish the three routes of immune deposit formation, and name one disease associated with each.
  8. List the four factors that determine the consequences of immune deposit formation.
  9. Describe the endothelial proliferative pattern of injury and what happens when it is severe.
  10. Explain why membranous GN produces no influx of inflammatory cells despite complement activation.
  11. Distinguish the immunofluorescence pattern of anti-GBM GN from that of immune-complex GN.
  12. Describe the mechanism of complement activation in APSGN and which pathway is involved.
  13. Describe the steps of subepithelial “hump” formation in post-infectious GN.
  14. List the GAS virulence factors and give the function of each.
  15. What proportion of NZ children with APSGN in the Auckland 2007 to 2009 series had gross haematuria, and what proportion had a low C3?
  16. State the NZ incidence of APSGN per 100,000 children per year overall, and for Maori and Pacific children.
  17. Predict the outcome for a child with APSGN, and state one long-term risk reported in another population.
  18. Describe the four stages of podocyte injury in membranous GN.
  19. What happens intracellularly when C5b-9 inserts into the podocyte membrane?
  20. Explain how effective podocyte depletion leads to ESKD, and what happens if depletion does not occur.
  21. Give the natural history figures for untreated membranous GN.
  22. Define nephrotic syndrome using the clinical classification criteria.
  23. Define rapidly progressive glomerulonephritis.
  24. Describe the primary initiating event for sodium reabsorption in nephrotic syndrome.
  25. Explain the full pathway from glomerular disease to oedema in nephrotic syndrome.
  26. Explain the pathophysiology linking glomerular injury to hypertension and oliguria in acute nephritic syndrome.
  27. A 54 year old has a 2 month flu-like prodrome, then 48 hours of haemoptysis, breathlessness, oliguria and haematuria, with creatinine 1500 umol/L, K+ 6.8 mmol/L and RBC casts. What is the syndrome, the likely diagnosis, and the expected biopsy findings?
  28. Describe the four phases of the inflammatory process in crescentic GN.
  29. Distinguish the light-microscopic appearance of post-streptococcal GN from that of membranous GN.
  30. Integrative: explain how the anatomical site of immune deposit formation determines whether a patient presents with a nephritic or a nephrotic picture, using post-streptococcal GN and membranous GN as the two examples.

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