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Flashcards in 19 Pathology of Glomerular Disease Deck (29)
1

Glomerulus

  • Site of...
  • Capillary beds
  • Surrounded by...
  • Visceral epithelial cells
  • PT epithelial cells

  • Site of ultrafiltration of plasma resulting (following tubular modification) in the formation of urine
  • First of two capillary beds in the kidney that connects the AffAs & EffAs
    • Second capillary bed is the peritubular capillary plexus in the cortex & the vasa recta in the medulla
  • Surrounded by Bowman’s capsule
    • Covered by parietal epithelial cells
  • Visceral epithelial cells (podocytes) cover the glomerular capillary surfaces
    • Formed as the parietal epithelial cells
  • PT epithelial cells reflect over the vascular and tubular poles
  • Podocytes
    • Important part of the filtration barrier of the glomerulus
    • Cover the entire glomerular BM

2

Basement membrane overlying glomerular capillaries

  • 3 layers (from outer to inner)
  • Endothelial cell
  • Overlying mesangial areas, the BM has 2 layers
  • Deep to the BM is the...
  • Mesangial matrix contains mesangial cells that have 3 functions

  • 3 layers (from outer to inner)
    • Lamina rara externa
    • Lamina densa
    • Lamina rara interna
  • Endothelial cell
    • Internal to the BM
    • Unique flat cell with numerous holes or fenestrations that retard cells but allow plasma to freely enter the BM
  • Overlying mesangial areas, the BM has 2 layers
    • Lamina rara externa
    • Lamina densa)
  • Deep to the BM is the...
    • Mesangial matrix
    • Contiguous with the lamina rara interna
  • Mesangial matrix contains mesangial cells that have 3 functions
    • Tether BM to mesangium --> formation of glomerular segments with multiple peripheral capillaries & a central mesangial unit
    • Phagocytosis of cell debris & material that gets deposited in the mesangium & lamina rara interna
    • Production of cytokines that stimulate glomerular cellular proliferation in response to injury

3

Assessing glomeruli in a renal biopsy

  • Histologic sections
  • Glomerular mesangial areas
  • Glomerular capillaries
  • Glomerular capillary walls
  • Urinary space
  • H&E stain
  • Methenamine silver stains
  • PAS stains
  • Renal biopsy paraffin histologic sections

  • Histologic sections
    • 2D representation of a 3D structure
  • Glomerular mesangial areas
    • Relatively inapparent
    • Contain < 3 mesangial cells / mesangial area
      • Exception: hilum has a greater # of mesangial cells
  • Glomerular capillaries
    • Patent
  • Glomerular capillary walls
    • Glomerular BM + endotehlial cell + podocyte
    • Thin & expanded
  • Urinary space
    • Empty
  • H&E stain
    • Limited by its inability to distinguish cytoplasm of endothelial cells, podocytes, & mesangial cells from GBM & mesangial matrix
  • Methenamine silver stains
    • Stain type IV collagen in glomerular & tubular BMs, bowman's capsule, & extraglomerular blood vessels
  • PAS stains
    • Stain polysaccharides in glomerular & tubular BMs, bowman's capsule, & extraglomerular blood vessels
  • Renal biopsy paraffin histologic sections
    • Cut at 2-3 microns in thickness
    • Thicker sections --> cell overlapping --> appearnace of hypercellularity

4

Clinical classifications

  • Primary
  • Secondary
  • Onset
  • Single occurence
  • Chronic

  • Primary
    • Limited to the kidney
  • Systemic
    • Secondary glomerulonephropathies
  • Onset
    • Acute
    • Insidious (chronic)
    • Sublincial (detected as a lab abnormality only)
  • Single occurence
    • Resolves w/ no clinical or pathologic sequelae
      or
    • Organizes w/ a persistent & stable deficit
  • Chronic
    • Periods of alternating activity & inactivty (relapses & remissions)
    • Frequently --> progressive loss of glomeruli & progressive renal dysfunction

5

Glomerular syndromes

  • Acute nephritic syndrome
  • Rapidly progressive glomerulonephritis
  • Nephrotic syndrome
  • Chronic renal failure
  • Asymptomatic hematuria or proteinuria

  • Acute nephritic syndrome
    • Hematuria, azotemia/ARF, variable proteinuria, oliguria, edema, & hypertension
    • Reversible lesion that doesn't --> glomerular scarring
    • Renal biopsy: proliferative glomerular disorder w/o necrotizing lesions / crescents
  • Rapidly progressive glomerulonephritis
    • Often presents like an acute nephritic syndrome w/ proteinuria & ARF
    • Progressive w/o therapy
    • --> glomerular scarring w/ loss of functional glomeruli
    • In some pts, the disease is more slowly progressive
    • Renal biopsy: glomerular necrotizing lesions / crescents
  • Nephrotic syndrome
    • >3.5 gm proteinuria / day, hypoalbuminemia, hyperlipidemia, hyperlipiduria, & edema
    • Renal biopsy: non-proliferative glomerular disorder w/ consistent podocyte injury manifested by foot process fusion, in addition to other disease specific pathology
  • Chronic renal failure
    • Renal impairment (azotemia) progressing gradually to renal failure over a period of years
    • May be associated with...
      • All forms of progressive glomerular disease
      • Glomerular diseases which occur as a single episode --> significant loss of glomeruli w/ subsequent progressive hyperfiltration injury & glomerular loss occurring in the remaining glomeruli
  • Asymptomatic hematuria or proteinuria
    • Non-progressive subclinical hematuria or proteinuria
    • Detected on urine evaluation during a routine physical exam
    • Glomerular hematuria: sub-nephrotic proteinuria

6

Glomerulopathy:
Approach to morphologic classification

  • Establish...
  • Define...
  • Define...
  • Assess...

  • Establish glomerulus as primary target of injury
  • Define distribution of glomerular injury
    • Subcapsular vs. juxtamedullary vs. random
    • Diffuse vs. focal
    • Global vs. segmental
    • Peripheral (capillayr loop) vs. central (mesangial) vs. extraglomerular
  • Define light microscopic pattern of glomerular injury
  • Assess for involvement of other renal compartments

7

Morphologic (pathologic) classification:
Light microscopy

  • Normal subcapsular glomerulus including Bowman's space/capsule measures...
  • Juxtamedullary glomeruli may measure...
  • Compare the structure of the glomerulus to that of a tree
    • Trunk & branches -->
    • Leaves -->
  • More peripheral mesangium (in a 2-3 micron section)...
  • More centrally towards to the hilum...
  • Capillary loops
  • Capillary walls
  • Light microscopic (H&E) classification of glomerular disorders
  • Specific etiologic diagnosis
  • Renal biopsy
  • For those disorders which characteristically diffusely involve glomeruli...
  • For early stage focal glomerular disorders...
  • For non-random focal processes...

  • Normal subcapsular glomerulus including Bowman's space/capsule measures...
    • 250 microns in max diameter
  • Juxtamedullary glomeruli may measure...
    • Up to 300 microns
  • Compare the structure of the glomerulus to that of a tree
    • Trunk & branches --> mesangium
    • Leaves --> capillary loops
  • More peripheral mesangium (in a 2-3 micron section)...
    • Inapparent matrix
    • < 3 mesangial cells
  • More centrally towards to the hilum...
    • Both mesangial matrix & cellularity are increased
  • Capillary loops
    • Peripheral structures
    • Widely patent
  • Capillary walls
    • Combination of endothelial & epithelial cells & BM
    • Thin and uniform
  • Light microscopic (H&E) classification of glomerular disorders
    • Descriptive: based on the qualitative & quantitative morphologic alterations from the normal glomerulus
    • Iintended to define the distribution & pattern of glomerular injury
  • Specific etiologic diagnosis
    • Based on the integration of the clinical & serologic data + light (H&E & special histochemical stains), IF, & EM
  • Renal biopsy
    • Represents a sampling of a pathologic process occurring in the kidney 
    • May or may not be representative of this process
  • For those disorders which characteristically diffusely involve glomeruli...
    • Sampling which includes a single glomerulus may be sufficient to define the disease process
  • For early stage focal glomerular disorders...
    • Adequate sampling is critical for establishing the diagnosis
    • > 10 non-sclerotic glomeruli are arbitrarily required for an adequate biopsy only if the focality of the glomerular process is random
  • For non-random focal processes...
    • E.g. glomerular disorders which preferentially affect subcapsular or juxtamedullary glomeruli
    • Full thickness cortical (with medullary) sampling is optimal to evaluate glomeruli at all levels within the cortex
    • Best accomplished by examination of > 2 1.5 cm needle biopsy cores

8

Morphologic (pathologic) classification:
Light microscopy:
Distribution of injury

  • Diffuse vs. focal
  • Global vs. segmental
  • Mesangial vs. peripheral vs. extraglomerular

  • Diffuse vs. focal
    • Diffuse: > 50% of glomeruli affected
    • Focal: < 50% of glomeruli are affected
  • Global vs. segmental
    • Global: entire glomerulus
    • Segmental: subtotal of glomerular involvement by a lesoin affecting > 1 anatomic segments
  • Mesangial vs. peripheral vs. extraglomerular
    • Mesangial: mesangium
    • Peripheral: GBM + podocytes
    • Extraglomerular: bowman's space

9

Morphologic (pathologic) classification:
Light microscopy:
Patterns of injury:
Increased ECM

  • Sclerosis 
  • Types
  • On silver stain, 2 patterns of obsolescence are seen

  • Sclerosis (ex.)
    • Fibrillar collagen (scar)
    • Mucopolysaccharides
    • Non-collagen proteins
    • Immune complex deposits
  • Types
    • Mesangial
    • Peripheral (capillary loop) BM
    • Bowman's space/capsule
    • Obsolescence (entire glomerulus is eosinophilic and hypocellular on H&E stain)
  • On silver stain, 2 patterns of obsolescence are seen
    • Tuft is collapsed and fibrosis is present only in Bowman's space
      • Ischemic pattern
    • Fibrosis replaces part of or entire glomerulus and fills Bowman's space
      • Organization of a necrotizing inflammatory GN or crescent

10

Morphologic (pathologic) classification:
Light microscopy:
Patterns of injury:
Increased cellularity

  • Intraglomerular
    • Mesangioproliferative
    • Endocapillary proliferative = mesangiocapillary proliferative
    • Membranoproliferative
    • Exudative
  • Extraglomerular 
    • Pure epithelial
    • Crescent

  • Intraglomerular
    • Mesangioproliferative
      • Iincreased cellularity & matrix confined to mesangial areas
      • > 3 mesangial cells / mesangial area
    • Endocapillary proliferative = mesangiocapillary proliferative
      • Increased mesangial & capillary loop cellularity
      • Increased mesangial matrix
      • Capillary wall thickening
      • Capillary luminal occlusion secondary to endothelial swelling &/or BM thickening
    • Membranoproliferative
      • Special forms of mesangiocapillary proliferative GN
      • Distinctive features on silver stains
        • Double GBM contours
        • Mesangial cell ingrowth into contiguous subendothelial GBM (mesangial interposition)
    • Exudative
      • Mesangiocapillary proliferative GN
      • Increased numbers of glomerular intracapillary neutrophils
  • Extraglomerular (glomerulus peripheral to the BM)
    • Pure epithelial
      • Visceral &/or parietal)
      • Correlates with podocyte injury
    • Crescent
      • Results from glomerular necrotizing lesion (glomerular capillary vasculitis)
      • Ttransmural glomerular capillary wall breaks
      • Bleeding into Bowman’s space
      • Earliest crescents have blood and fibrin in Bowman’s space (fibrinous crescent)
      • Cytokines produced by incoming monocytes cause proliferation of parietal epithelial cells (cellular crescent)
      • Recruitment of fibroblasts --> fibrosis in the affected area of the glomerular tuft & adjacent Bowman’s space (fibrocellular --> fibrous crescent)

11

Morphologic (pathologic) classification:
Light microscopy:
Patterns of injury:
Other

  • Hyalinosis
    • Plasma protein insudation in mesangium, capillary BM, arterioles &/or Bowman's capsule
    • Due to endothelial or epithelial injury w/ serum protein leakage & entrapment
    • In diabetes, hyalinosis lesions are designated as
      • Fibrin caps (glomerular tuft)
      • Capsular drops (Bowman's capsule).
  • Necrosis or necrotizing lesion
    • Necrotizing glomerular capillaritis associated w/ inflammatory cells, karyorrhectic nuclear debris & fibrin
    • Organization --> segmental or global sclerosis / scar
  • Glomerular foam cells
    • Frequently seen in glomerular – associated proteinuric disorders
    • Correspond to resorption of protein & lipoprotein in macrophages within the glomerular tuft
  • Mesangiolysis
    • Disruption of mesangium
    • Because several capillary loops are tethered to one mesangial area, mesangiolysis may --> single large capillary loop (microaneurysm)

12

Morphologic (pathologic) classification:
Light microscopic description

  • LM classification/description (ex.s)
  • Etiological specificity
  • Specific diagnosis requiresintegration of...

  • LM classification/description = distribution pattern(s) + morphologic pattern(s) of injury (ex.s)
    • Focal segmental glomerulosclerosis with hyalinosis.
    • Focal segmental endocapillary proliferative and necrotizing glomerulonephritis with focal cellular crescents.
    • Diffuse global endocapillary proliferative and exudative glomerulonephritis.
    • Diffuse mesangioproliferative glomerulonephritis.
    • Etc.
  • Etiological specificity
    • A particular LM description may be shared by a variety of etiologically distinctive glomerular disorders
    • Conversely, a specific disorder (e.g. lupus) may display >1 glomerular morphologic pattern of injury
      • Depends upon factors such as the time course in the illness, physical properties of deposited materials (e.g. immune complex deposits), host-specific immune response to injury, etc.
  • Specific diagnosis requires integration of...
    • Clinical & serologic data
    • LM
    • IF
    • EM

13

Morphologic (pathologic) classification:
​Immunofluorescence microscopy

  • Used to assess...
  • Uses...
  • Panel of stains usually includes...
  • Procedure
  • A positive signal with a particular antibody indicates...

  • Used to assess...
    • Evidence of antibody-mediated immunologic injury
  • Uses...
    • A panel of commercially prepared fluorescein labeled antibodies which recognize specific antigens
  • Panel of stains usually includes...
    • Ig heavy chain classes (IgG, IgM, IgA) & light chains (kappa, lambda)
    • Complement components of the shared (C3), classical (C1q, C4) and alternative (properdin) pathways
    • Fibrinogen
    • Albumin
  • Procedure
    • One frozen section slide is prepared for each specific antibody used
    • The fluorescein labeled antibody is incubated with the frozen section tissue
  • A positive signal with a particular antibody indicates...
    • Presence & location of that antigen within the tissue

14

Morphologic (pathologic) classification:
​Immunofluorescence microscopy:
Distribution

  • Glomerular
  • Extragomerular

  • Glomerular (including glomerular tuft and Bowman's space/capsule)
    • Diffuse vs. focal
    • Global vs. segmental
    • Mesangial vs. peripheral (GBM + podocytes)
    • Bowman’s space
  • Extraglomerular components
    • Tubules (cytoplasm and tubular basement membrane)
    • Interstitium
    • Blood vessels

15

Morphologic (pathologic) classification:
​Immunofluorescence microscopy:
Patterns

  • Granular
  • Linear
  • Homogeneous, smudgy, or irregular
  • Intracellular droplet staining
  • Absence

  • Granular
    • Discrete immune complex deposits
  • Linear
    • Uniform distribution of targeted antigen
    • Weak BM linear staining for albumin and IgG is always present
      • Corresponds to small amounts of these proteins that get into the BM (background staining)
    • Specific pathologic staining only if albumin stain is less intense than another more intense stain
      • E.g. IgG in anti-GBM / Goodpasture disease
  • Homogeneous, smudgy, or irregular
    • Protein trapping / insudation within an area of sclerosis (usually IgM & C3).
    • Deposition of a non-immune complex protein (e.g. amyloid)
  • Intracellular droplet staining
    • Protein resorption droplets within glomerular epithelial cells or tubular cells
  • Absence of staining

16

Morphologic (pathologic) classification:
​Immunofluorescence microscopy:
Staining profile

  • Used to assess...
  • Uses...
  • Panel of stains usually includes...
  • Procedure
  • A positive signal with a particular antibody indicates...

  • Focal segmental glomerulosclerosis
    • Homogeneous
    • Segmental IgM and C3
  • IgA nephropathy
    • Granular mesangial IgA, C3
  • Membranous nephropathy
    • Granular peripheral staining for IgG and C3
  • Amyloid
    • Smudgy mesangial + GBM staining for deposited amyloid protein
  • Lupus
    • Granular mesangial and/or peripheral loop IgG, IgM, IgA, C3, C1q, C4 
    • "Full house"

17

Morphologic (pathologic) classification:
Electron microscopy

  • Transmission EM
  • Features assessed
    • Cellular alterations
    • ECM changes
    • Electron dense deposits

  • Transmission EM
    • Permits resolution of ultrastructural anatomy & morphologic alterations, which are below the resolution of LM or IF
    • Provides the highest sensitivity for diffuse glomerular disorders
    • However, b/c only 1-2 glomeruli are usually examined, focal processes may be missed.
  • Features assessed
    • Cellular alterations
      • Podocyte foot process effacement (fusion)
      • Intracellular inclusions (e.g. viruses)
    • ECM changes
      • BM (increase, remodeling (e.g. duplication, splitting, breaks, etc.), thinning, interruption)
      • Mesangium.
    • Electron dense deposits
      • Immune complexes, protein insudates, protein deposits w/ organized substructure (e.g. amyloid)
      • Location
        • Mesangial, subendothelial, subepithelial, intramembranous, Bowman's space/capsule
      • Substructural detail
        • Some immune complexes, e.g. those occurring in lupus or cryoglobulinemia, have distinctive substructures
      • Reaction of the glomerulus to the protein deposits
        • E.g. mesangial interposition with new subendothelial basement membrane formation in type 1 membranoproliferative GN
        • Subepithelial basement membrane encasement of deposits in membranous GN

18

Pathogenesis of glomerular disorders

  • Determinants of the clinical expression of glomerular disease
  • Glomerulopathies
  • Glomerular proteinuria
  • Glomerular hematuria
  • Glomerulopathies presenting with acutely impaired renal function
  • Unrecognized slowly progressive chronic renal disorders

  • Determinants of the clinical expression of glomerular disease
    • Relative quantity (proportion) of injured glomeruli
    • Type of injury
  • Glomerulopathies
    • Either focal or diffuse without impairment of overall glomerular filtration
    • May produce proteinuria or hematuria
    • Typically don't present with loss of renal function
  • Glomerular proteinuria
    • Results from injury to the podocyte or GBM
  • Glomerular hematuria
    • Results from GBM breaks with bleeding into Bowman’s space
  • Glomerulopathies presenting with acutely impaired renal function
    • Typically diffuse and proliferative
    • Associated with glomerular capillary narrowing & reduced glomerular perfusion / filtration
  • Unrecognized slowly progressive chronic renal disorders
    • May ultimately present with symptomatic renal failure simulating acute renal failure
    • Typically show diffuse chronic changes in all four renal compartments, with glomeruli showing diffuse (non-proliferative) segmental and global glomerulosclerosis

19

Pathogenesis of glomerular disorders:
Proteinuria

  • Proteinuria
  • Hematuria
  • Renal function
    • Normal
    • Azotemia

  • Proteinuria
    • Injury to GBM &/or podocytes w/o capillary wall break
  • Hematuria
    • Break in glomerular capillary wall
  • Renal function
    • Normal
      • Focal glomerulopathy +/- proliferation
      • Diffuse glomerulopathy w/o proliferation
    • Azotemia
      • Diffuse glomerulopathy w/ proliferation +/- necrosis/crescents (acute)
      • Diffuse glomerulosclerosis (chronic)

20

Pathogenesis of glomerular disorders:
Proteinuria

  • Glomeruli filter...
  • Blood cells, large molecules (proteins) and intermediate-sized negatively charged molecules (proteins)
  • Small molecules (water, electrolytes, glucose, small proteins, etc.) & positively charged intermediate sized molecules (proteins)
  • Damage to the podocyte filtration slit diaphragms (most important size barrier) or reduction in the negative charge in the GBM &/or podocyte & endothelial cell surfaces
  • If the tubules are unable to reabsorb the filtered proteins...
  • Albumin
  • Selective proteinuria
  • If the glomerular permeability defect is greater...
  • Low MW proteins irrespective of charge
  • Tubular proteinuria
  • Overflow proteinuria
  • The three types of proteinuria can be distinguished by...

  • Glomeruli filter...
    • A large volume of blood (blood cells and plasma) each day
  • Blood cells, large molecules (proteins) and intermediate-sized negatively charged molecules (proteins)
    • Prevented from being filtered b/c of size & charge barriers in the glomerular capillary wall
  • Small molecules (water, electrolytes, glucose, small proteins, etc.) & positively charged intermediate sized molecules (proteins)
    • Forced through the glomerular capillary walls by hydrostatic & oncotic pressure in the circulating blood within glomerular capillaries
  • Damage to the podocyte filtration slit diaphragms (most important size barrier) or reduction in the negative charge in the GBM &/or podocyte & endothelial cell surfaces
    • Allows progressively larger & less negatively charged molecules to be filtered
  • If the tubules are unable to reabsorb the filtered proteins...
    • Proteinuria develops
  • Albumin
    • Intermediate sized protein in high concentration in the blood
  • Selective proteinuria
    • Less severe glomerular capillary wall injury leads to selective proteinuria (albuminuria)
  • If the glomerular permeability defect is greater...
    • Larger proteins in high concentration in the blood (i.e. Igs) are also filtered
    • --> non-selective glomerular proteinuria (albumin + immunoglobulin)
  • Low MW proteins irrespective of charge
    • Normally filtered by the glomerulus
    • Don't appear in the urine because they're in small conc in the blood & filtrate, & are easily reabsorbed by proximal tubules
  • Tubular proteinuria
    • When there is tubular injury (w/o glomerular injury), filtered low MW proteins aren't reabsorbed & appear alone in the urine
  • Overflow proteinuria
    • Occurs when small proteins not normally present in the blood (e.g. monoclonal free light chain immunoglobulin or Bence-Jones protein occurring in patients with multiple myeloma) are overproduced
    • Proteins are freely filtered by a normal glomerulus
    • If the concentration of this protein in the filtrate exceeds the tubules’ capacity for reabsorption, the overflow protein appears in the urine (overflow proteinuria)
  • The three types of proteinuria can be distinguished by...
    • Urine protein electrophoresis

21

Pathogenesis of glomerular disorders:
Immune mediated glomerular injury

  • Clinical glomerular disease is the result of...
  • For immunologic diseases, the inciting process involves either...
  • Immunoglobulins produce glomerular injury through either...
  • Immune complex deposits form when...
  • In a naïve host...
  • Approximately 1-2 weeks after the introduction of the antigen...
  • As the antigen is cleared...
  • In immune complex associated glomerular disorders, this process may either...

  • Clinical glomerular disease is the result of...
    • An initiating pathologic process that may directly injure the glomerulus, or alternatively mediate injury indirectly via the host response
  • For immunologic diseases, the inciting process involves either...
    • Humoral (type II or III hypersensitivity) immune responses
    • Cell mediated (type IV hypersensitivity) immune responses
  • Immunoglobulins produce glomerular injury through either...
    • Direct antibody dependent cytotoxicity targeted against glomerular cells (type II hypersensitivity)
    • Formation of immune complexes forming or secondarily depositing in the mesangium and/or GBM (type III hypersensitivity reaction) (more common)
  • Immune complex deposits form when...
    • An antigen stimulates the immune system to produce an antigen-specific antibody, initially forming soluble single antigen - antibody complexes that are easily cleared by the reticuloendothelial system (RES; liver, spleen, lymph nodes, bone marrow, etc.)
  • In a naïve host...
    • There is a latency of several days before antigen-specific antibodies are produced, with the antigen initially present in higher concentration (antigen excess)
  • Approximately 1-2 weeks after the introduction of the antigen...
    • The antibody conc increases & becomes proportionate to the antigen concentration
    • Allows cross-linking of single antigen - antibody complexes
    • Allows the formation of insoluble larger immune complex deposits that deposit in vascular walls
    • When this occurs, the host develops an immune response and clinical features of an immune complex disorder (small vessel vasculitis and / or glomerulonephritis)
  • As the antigen is cleared...
    • The relative antibody conc increases (antibody excess)
    • The immune complex disorder resolves
  • In immune complex associated glomerular disorders, this process may either...
    • Occur s a single episode
    • May be recurrent

22

Pathogenesis of glomerular disorders:
Mechs of glomerular immune deposition

  • Most immunologic glomerular injury has been shown to occur...
  • Two distinctive forms of antibody-associated injury

  • Most immunologic glomerular injury has been shown to occur...
    • On a humoral basis
  • Two distinctive forms of antibody-associated injury
    • Injury mediated by circulating antibodies binding in-situ to intrinsic or planted glomerular antigens
    • Injury resulting from deposition of preformed soluble circulating antibody-antigen (Ab-Ag) immune complex deposits

23

Pathogenesis of glomerular disorders:
Glomerular immune complex localization

  • Immune complex deposits can form in either...
  • The location of intraglomerular immune complex deposits is determined by multiple physical factors of the immune complex deposit, including...
  • The intraglomerular location of immune complex deposits determines...

  • Immune complex deposits can form in either...
    • Glomerular locations
      • GBM (subepithelial, subendothelial)
      • Mesangial
    • Extraglomerular locations
      • Tubular BM
      • Interstitium
      • Extraglomerular blood vessels
  • The location of intraglomerular immune complex deposits is determined by multiple physical factors of the immune complex deposit, including...
    • Stability of the immune complex deposit in the circulation
      • Determined in part by antigen – antibody affinity
      • Preformed circulating immune complex deposit vs. dissociated antigen & antibody
    • Size and shape of the immune complex deposit
    • Charge of the immune complex deposit
  • The intraglomerular location of immune complex deposits determines...
    • The clinical presentation & pathologic appearance of the affected glomeruli

24

Pathogenesis of glomerular disorders:
In-situ immune deposition

  • In-situ immune deposition indicates that...
  • Two well-characterized models of GN arising on the basis of in-situ immune deposition
    • Anti-glomerular basement membrane (anti-GBM) disease
    • Heymann's nephritis
  • Subepithelial deposits are formed in-situ, either...

  • In-situ immune deposition indicates that...
    • The antigen & antibody arrive independently & initially form an immune complex at the site of deposition
    • As opposed to a preformed circulating antigen – antibody immune complex that deposits as an intact structure
  • Two well-characterized models of GN arising on the basis of in-situ immune deposition
    • Anti-glomerular basement membrane (anti-GBM) disease
      • Circulating antibodies which recognize an epitope of type IV collagen molecule within the lamina densa bind diffusely & uniformly to this portion of the GBM
        • Produce a linear fluorescence pattern w/o discrete electron dense deposits by EM
      • Inflammatory host response --> necrotizing / crescentic glomerulonephritis
    • Heymann's nephritis
      • An experimental model of membranous glomerulopathy characterized by a non-proliferative GN w/ diffuse GBM thickening due to numerous subepithelial deposits
      • Animals are immunized w/ a preparation of PT brush border
        • This preparation contains a 330 kD glycoprotein (GP330) which is also present on the basal surface of the podocyte foot processes
      • The resulting circulating GP330 antibody may traverse the GBM & bind to the podocyte foot process cell membrane, either...
        • Inducing a complement rxn at the cell surface --> podocyte foot process injury/lysis
        • Surface immune complex deposits that form may be shed into the lamina rara externa, altering GBM protein permeability by reducing negative charge in the GBM & inciting the formation of new GBM around the deposits
      • Lastly, antibodies may react with previously planted nonglomerular (circulating) antigens
        • Typically cationic molecules trapped in the anionic GBM (e.g. endostreptosin or hepatitis B surface antigen)
  • Subepithelial deposits are formed in-situ, either...
    • As described above
    • Less commonly by dissociation of preformed subendothelial deposits with reassociation in the subepithelial space

25

Pathogenesis of glomerular disorders:
Mesangial & subendothelial deposits

  • Circulating immune complex deposits are physiologically generated...
  • Some of these immune complexes...
  • While most...
  • Most free & adsorbed immune complexes are removed by the...
  • Mesangial/subendothelial immune complex deposits usually form by...
  • The mesangium...
  • When the capacity of the mesangium is overwhelmed...
  • Immune complex deposit localization is determined by...
  • Preformed immune complex deposits must be...
  • Positively-charged, small-intermediate size immune complexes are preferentially deposited in the...
  • Later, these immune complex deposits may...
  • Deposits localizing to the mesangium are typically...
  • Negatively charged immune complex deposits do not deposit in the...

  • Circulating immune complex deposits are physiologically generated...
    • In small numbers, e.g. during transient bacteremias, etc
  • Some of these immune complexes...
    • Remain solubilized within the plasma
  • While most...
    • Become adsorbed to the surfaces of RBCs via the CR1 complement receptor
  • Most free & adsorbed immune complexes are removed by the...
    • Reticuloendothelial system (RES)
  • Mesangial/subendothelial immune complex deposits usually form by...
    • Deposition of preformed circulating immune complex deposits which have escaped the RES
  • The mesangium...
    • Serves as the RES of the kidney
    • Normally removes small numbers of circulating physiologically generated immune complex deposits
  • When the capacity of the mesangium is overwhelmed...
    • Immune complex deposit expand the mesangium and may diffuse into the continuous paramesangial / subendothelial space
  • Immune complex deposit localization is determined by...
    • The size & charge of the immune complex deposit
  • Preformed immune complex deposits must be...
    • Initially small enough to traverse the endothelial cell fenestrations
  • Positively-charged, small-intermediate size immune complexes are preferentially deposited in the...
    • GBM lamina rara interna (subendothelial space) which is rich in negatively charged heparan sulfate proteoglycans
  • Later, these immune complex deposits may...
    • Enlarge by coalescence
  • Deposits localizing to the mesangium are typically...
    • Less positively charged (more often IgM or IgA containing)
  • Negatively charged immune complex deposits do not deposit in the...
    • Mesangium or GBM

26

Pathogenesis of glomerular disorders:
Mesangial & subendothelial deposits

  • Complement activation occurs most intensely with...
  • Complement activation may directly injure...via...
  • Neutrophil degranuation at endothelial surfaces results in...
  • A primary T-cell response may result in...
  • Both humoral & cell mediated responses ultimately lead to the generation of...
  • Damage to the podocyte filtration slit diaphragms, detachment of the epithelial cells from the GBM, and/or alteration of the size and charge barriers in the GBM results in...
  • Transmural breaks in the capillary wall results in...
  • Mesangial cells and inflammatory cells may secrete additional cytokines resulting in...

  • Complement activation occurs most intensely with...
    • IgG-containing immune complex deposits
  • Complement activation may directly injure glomerular cells & the GBM via...
    • Neutrophil chemotaxis (C5a) & activation
      • Immune complexes have antibody Fc components & C3b which bind to receptors on neutrophils & stimulate neutrophil degranulation in glomerular capillaries
    • The lytic portion of the complement cascade (C5-9)
  • Neutrophil degranuation at endothelial surfaces results in...
    • Vascular wall injury (necrotizing capillaritis)
  • A primary T-cell response may result in...
    • Cytokine generation w/ stimulation of macrophages & mesangial cells
  • Both humoral & cell mediated responses ultimately lead to the generation of...
    • Oxidants, cytokines, proteases, growth factors, complement activation, etc., mediating glomerular cell and/or GBM injury
  • Damage to the podocyte filtration slit diaphragms, detachment of the epithelial cells from the GBM, and/or alteration of the size and charge barriers in the GBM results in...
    • Proteinuria
    • The quantity and selectivity of the proteinuria are related to the magnitude of the permeability barrier alteration
  • Transmural breaks in the capillary wall results in...
    • Leakage of a small amt of blood (blood cells and plasma) manifested as hematuria w/ RBC casts
    • B/c the blood loss through these lesions is small, patients don't become anemic and don't have significant proteinuria
  • Mesangial cells and inflammatory cells may secrete additional cytokines resulting in...
    • Glomerular cellular proliferation, & later fibroblast proliferation

27

Pathogenesis of glomerular disorders:
Glomerular proliferation, immune complex removal, & inflammatory response

  • Cellular proliferation correlates w/...
  • The site of proliferation is dependent on...
  • While the magnitude of the cellular response is in part determined by...
  • Cellular proliferation is mediated through...
  • The quantity of immune complex deposits is determined by...
  • Immune complex deposits form a latticework structure when there is...
  • Immune complexes are spontaneously reabsorbed when...
  • Empirically, older immune complex tend to contain proportionately greater amounts of...
  • Within the mesangium, immune complexes are further removed via...
  • Within the subendothelial space, immune complex deposits may be...
  • Subepithelial deposits are removed principally via...& are isolated by...

  • Cellular proliferation correlates w/...
    • The presence of immune complex deposits in the mesangial-subendothelial space, but not in the subepithelial space
  • The site of proliferation is dependent on...
    • The distribution of the immune complex deposits
  • While the magnitude of the cellular response is in part determined by...
    • The quantity of immune complex deposits
  • Cellular proliferation is mediated through...
    • The production of various cytokines
  • The quantity of immune complex deposits is determined by...
    • The relative rates of their formation vs. removal
  • Immune complex deposits form a latticework structure when there is...
    • An optimal ratio of Ab:Ag (zone of equivalence), permitting Ab-Ag crosslinking
  • Immune complexes are spontaneously reabsorbed when...
    • Ab or Ag production ceases or when the relative concentrations of Ab and Ag change
    • --> either Ag or Ab excess & dissolution of the immune complex
  • Empirically, older immune complex tend to contain proportionately greater amounts of...
    • Complement relative to Ig
    • Suggests that immune complexes may be removed in part by complement-mediated solubilization (steric disruption of immune complex lattice)
  • Within the mesangium, immune complexes are further removed via...
    • Mesangial cell phagocytosis & transmigration of immune complexes back into the glomerular capillaries
  • Within the subendothelial space, immune complex deposits may be...
    • Isolated & eliminated by the ingrowth of mesangium into the continuous lamina rara interna w/ the production of a new subendothelial layer of GBM
    • --> capillary wall mesangial interposition
      • Seen as tram tracking by silver stain
      • Occurs in type I membranoproliferative GNs
  • Subepithelial deposits are removed principally via...& are isolated by...
    • Removed principally via solubilization
    • Isolated by the glomerulus by forming new GBM around each individual deposit
      • Spikes --> domes --> train tracks

28

Pathogenesis of glomerular disorders:
Glomerular proliferation, immune complex removal, & inflammatory response

  • The extent to which complement is activated in part determines the...
  • Some immune complexes, e.g. IgG-strep Ag occurring in post-streptococcal GN...
  • Whereas other deposits, e.g. IgA-Ag deposits occurring in IgA nephropathy...
  • As a consequence, glomerular lesions in IgA nephropathy characteristically show...
  • Significant complement activation may be reflected by...
  • Pts with hypocomplementemia almost always have...
  • Subendothelial complement-activating immune complex deposits produce some (but not all)...
  • Like complement-activating subepithelial deposits, these immune complex deposits are capable of...
  • However, b/c subendothelial ICs are physically accessible to circulating intracapillary neutrophils which contain Fc and C3 receptors on their surfaces, these immune complex deposits can...

  • The extent to which complement is activated in part determines the...
    • Magnitude of the inflammatory response
  • Some immune complexes, e.g. IgG-strep Ag occurring in post-streptococcal GN...
    • Readily activate complement with avid chemotaxis
  • Whereas other deposits, e.g. IgA-Ag deposits occurring in IgA nephropathy...
    • Typically don't intensely activate complement
  • As a consequence, glomerular lesions in IgA nephropathy characteristically show...
    • Increased mesangial cellularity w/o an inflammatory response
  • Significant complement activation may be reflected by...
    • Serum hypocomplementemia
    • In particular if the hepatic synthetic capacity is overwhelmed by the rate of complement consumption
  • Pts with hypocomplementemia almost always have...
    • Proliferative glomerulonephritis
    • However, not all proliferative GNs are hypocomplementemic
  • Subendothelial complement-activating immune complex deposits produce some (but not all)...
    • Necrotizing lesions
  • Like complement-activating subepithelial deposits, these immune complex deposits are capable of...
    • Inflammatory cell chemotaxis
  • However, b/c subendothelial ICs are physically accessible to circulating intracapillary neutrophils which contain Fc and C3 receptors on their surfaces, these immune complex deposits can...
    • Directly activate the neutrophils within the circulation
    • --> degranulation & the production of a necrotizing capillaritis (glomerulonephritis)

29

Immune complexes:
Removal

  • Spoontaneous solubilization
  • Actie elimination
    • Mesangium
    • Subendothelial
    • Subepithelial

  • Spoontaneous solubilization
    • Ab or A excess
    • Complement mediated solubilization
  • Actie elimination
    • Mesangium
      • mesangial cell phagocytosis
      • Transmigration of ICs into vasculature
    • Subendothelial
      • Capillary loop mesangialization / mesangial interposition
    • Subepithelial
      • GBM surrounds individual ICs w/ incorporation of ICs into GBM