Chapter 7 Diseases of the Glomerulus
Introduction
A. Glomerular diseases affect primarily the glomeruli. However, destruction of the glomerulus renders the remainder of the nephron nonfunctional and progressive destruction of glomeruli can lead to decreased glomerular filtration rate, azotemia, and renal failure.
B. Glomerular disease is an important cause of chronic renal failure in humans and has been increasingly recognized in veterinary medicine.
C. The two important glomerular diseases of domestic animals are glomerulonephritis (GN) and glomerular amyloidosis.
E. The term nephrotic syndrome traditionally has been used to describe patients with proteinuria, hypoalbuminemia, hypercholesterolemia, and edema or ascites. Human patients excreting more than 3.5 grams of protein per 1.73 m2 body surface area per day in their urine are said to have nephrotic range proteinuria. Patients with several different glomerular diseases can present as having nephrotic syndrome.
Normal Glomerular Anatomy and Function
A. The glomerulus is a unique vascular structure consisting of a capillary bed between two arterioles (Figure 7-1).
B. An ultrafiltrate of plasma is forced through the glomeruli as blood is pumped through the kidney by the heart.
C. The filtration barrier of the glomerulus consists of three layers (from the vascular space to the urinary space) (Figure 7-2):
1. Endothelium.
a. The fenestrated capillary endothelium is 100 to 500 times more permeable to water and crystalloids than are systemic capillaries.
2. Glomerular basement membrane.
a. A trilaminar structure consisting of a central dense region (lamina densa) and outer less dense regions (lamina rara interna, lamina rara externa).
c. Proteoglycans are large, highly negatively charged molecules consisting of a protein backbone with polysaccharide (glycosaminoglycans) side chains. The proteoglycans are responsible for the charge selectivity of the basement membrane.
D. The glomerular filter functions as a size and charge selective barrier.
1. Size selectivity: The glomerular filter excludes particles of <35 Å in radius (serum albumin has a molecular weight of 70,000 daltons and molecular radius of 36 Å).
E. Other components of the glomerulus.
1. Mesangial cells provide structural support for the glomerular capillary loops and also possess contractile and phagocytic properties (Figure 7-3).
a. Reside in the glomerular interstitium in areas where the podocytes do not completely surround the capillary endothelium.
d. Contain contractile elements that can alter the amount of glomerular surface area available for filtration.
2. Parietal epithelial cells line the urinary side of the glomerular capsule (Bowman’s capsule) and are continuous with the visceral epithelial cells at the vascular pole of the glomerulus and with the proximal tubule at the urinary pole.
3. The juxtaglomerular apparatus, at the vascular pole, consists of specialized smooth muscle cells of the afferent and efferent arterioles containing electron dense renin granules, and the macula densa, a specialized segment of the distal tubule. The juxtaglomerular apparatus mediates tubuloglomerular feedback.
FIGURE 7-1 Schematic representation of normal glomerular morphology at the light microscopic level.
(Drawn by Tim Vojt.)
Pathogenesis of Glomerular Disease
Immune-Mediated Glomerulonephritis
1. Immune-complex GN is a disease of glomeruli caused by deposition of immunoglobulin or complement in the glomerular capillary wall.
2. Immune complexes deposit in the glomerular filter in two ways (Figure 7-4):
a. Soluble circulating immune complexes are trapped in the glomerulus in conditions of antigen-antibody equivalence or slight antigen excess (the classic example is serum sickness). In antibody excess, immune complexes tend to be large and insoluble and are rapidly removed from the circulation by phagocytic cells. In large antigen excess, immune complexes do not readily bind complement and are less likely to produce immune injury.
3. Immune complexes may be deposited in a subepithelial, subendothelial, or intramembranous position. Complexes also may deposit in the mesangium.
4. Factors affecting the location of deposition include size of the complexes (dependent on antigen-to-antibody ratio), charge of the complexes, removal of complexes by phagocytosis, and damage to the basement membrane itself.
b. Subendothelial complexes are large or highly negatively charged, and, therefore, do not easily pass through the basement membrane.
5. The location of deposition determines the histologic changes observed and the severity of glomerular dysfunction.
a. Subepithelial complexes are associated with basement membrane thickening and minimal inflammatory cell infiltration. Proteinuria may be severe.
b. Subendothelial complexes are associated with recruitment of inflammatory cells and basement membrane thickening, but proteinuria may only be moderate in severity.
c. Intramembranous deposition is most commonly associated with antiglomerular basement membrane disease.
6. Immune complexes can be detected in the glomeruli by staining histologic sections with fluorescein-labeled antibody against immunoglobulins or complement of the species being studied. This technique requires renal biopsy specimens to be frozen in liquid nitrogen or stored for less than 7 days in special preservative solutions (e.g., Michel solution). More recently, immunohistochemistry using peroxidase-antiperoxidase methods can be applied to specimens preserved routinely in 10% buffered formalin.
a. Glomerular deposition of preformed immune complexes usually results in a lumpy bumpy or granular discontinuous immunofluorescence pattern with mesangial and subendothelial location of the immune complexes (Figure 7-5).
b. In situ formation of immune complexes can occur within glomeruli when circulating antibodies react with endogenous glomerular antigens or “planted” nonglomerular antigens in the glomerular capillary wall. In this instance, a smooth linear continuous pattern of immunofluorescence usually results.
Mechanisms of Immune Injury
1. Deposition of immune complexes in the mesangium may or may not be associated with pathologic lesions or glomerular dysfunction.
2. Immune complex deposition in glomeruli may reduce the amount of fixed negative charge and enhance filtration of negatively charged circulating macromolecules (e.g., albumin).
3. Complement activation results in membrane damage and proteinuria. Soluble complement components also recruit inflammatory cells.
4. Platelet activation and aggregation may occur due to endothelial damage or antigen antibody interaction and exacerbate glomerular damage by release of a variety of mediators. These mediators cause activation and proliferation of mesangial cells and endothelial cells, vasospasm and increased coagulation.
5. Mesangial cells contribute to glomerular inflammation by release of eicosanoids, cytokines, and growth factors and by increased matrix production.
6. Inflammatory cells also contribute to glomerular injury.
a. Neutrophils and macrophages localize in the glomeruli in response to soluble mediators including complement components, platelet activating factor, platelet-derived growth factor, and eicosanoids.
b. Activated neutrophils release reactive oxygen species and proteinases, leading to further damage.
7. Many bioactive mediators may be involved in these interactions. Such mediators are produced by resident glomerular cells (mesangial cells, endothelial cells) or by recruited blood cells (neutrophils, platelets) in response to deposition of immune complexes in the glomerular basement membrane. Important mediators include:
a. Eicosanoids (prostaglandins, thromboxanes, and leukotrienes) are products of arachidonic acid metabolism. Thromboxanes, and leukotrienes cause vasoconstriction and mesangial cell contraction, both of which may result in decreased glomerular filtration rate (GFR). These mediators also attract and activate neutrophils.
b. Cytokines and growth factors are produced by inflammatory cells and resident glomerular cells. These include tumor necrosis factor, interleukin-1, interleukin-6, platelet- derived growth factor, transforming growth factor, and epidermal growth factor, and contribute to mesangial cell activation (cellular proliferation, matrix production) and incite inflammation.
8. Several infectious and inflammatory diseases have been associated with glomerular deposition or in situ formation of immune complexes in dogs and cats (Box 7-1). In most cases, however, the antigen source or underlying disease process is not identified and the glomerular disease is referred to as idiopathic.
BOX 7-1 CAUSES OF IMMUNE-MEDIATED GLOMERULONEPHRITIS IN DOGS AND CATS
I. Pyometra: Commonly associated with proliferative or membranoproliferative GN. Continuous high levels of bacterial antigens are thought to be responsible for immune complex formation and deposition, although these antigens have not been demonstrated in the kidney. The glomerular changes usually disappear once ovariohysterectomy is performed.
II. Dirofilaria immitis infection: Also commonly associated with GN and proteinuria. One theory is that immune complexes form in response to shed microfilarial antigens. Another is that the mechanical presence or products of the microfilaria damage the glomerular vessels.
III. Canine adenovirus-1 (infectious canine hepatitis): Associated with mild proliferative GN and interstitial nephritis. Dense deposits suggestive of immune complexes are seen in the glomerular basement membrane, and tubular epithelial antigens are shed in the urine. Intranuclear inclusion bodies may be present in endothelial cells.
VII. Other disease associations
D. Proteinuria has been observed in dogs with hyperadrenocorticism, and normal dogs treated with glucocorticoids develop proteinuria and glomerular lesions (e.g., hypercellularity, thickened glomerular basement membranes, foot process fusion, glomerular adhesions) but no deposition of immune complexes.
IX. Familial glomerular disease
B. Membranoproliferative GN associated with hereditary deficiency of complement component III in Brittany spaniels
I. Feline leukemia virus infection: Frequently associated with membranous GN with or without signs of renal dysfunction. These cats usually do not die from renal failure.
Nonimmune Glomerular Injury
1. Most idiopathic GN in animals is thought to be immune-mediated, but glomerular damage also may occur by nonimmune mechanisms.
2. Whereas the primary target for immune-mediated glomerular disease is the filtration barrier, the primary target for nonimmune injury appears to be the endothelial cell.
3. Damaged endothelial cells release factors (e.g., endothelin) that have vasoactive, proliferative and proinflammatory effects. Additionally, endothelial damage stimulates coagulation.
Progression of Glomerular Disease
1. Continued deposition of complexes and release of inflammatory mediators eventually lead to glomerulosclerosis.
3. Glomerular disease may induce tubulointerstitial disease and progress to end-stage kidney disease and chronic renal failure.
a. Obstruction of glomerular capillaries may result in ischemia of the tubules and tubulointerstitial disease.
c. Protein in the ultrafiltrate is taken up and degraded by proximal tubular cells. Overload of the lysosomal systems of these cells may lead to hypoxia and cell death. Increased protein uptake also may lead to increased transcription of inflammatory mediators.
