Renal Pathophysiology Iain MacLeod, Ph.D imacleod@hsph.harvard.edu

1. Renal Pathophysiology Iain MacLeod, Ph.D imacleod@hsph.harvard.edu Iain MacLeod 23 November 2009

2. Renal Anatomy • 800,000 to 1 million nephrons / kidney • initial filtering component – renal corpuscle • tubule extending from renal corpuscle to collecting ducts • Renal corpuscle forms filtrate of blood free of cells and protein • Filtrate leaves and enters tubule – as it flows through the tubule, substances are exchanged • Fluid remaining at the end of each nephron combines in collecting ducts – leaves as urine • Renal corpuscle: • interconnected capillary loops –glomerulus • supplied with blood from afferent arteriole • glomerulus protrudes into Bowman’s capsule • as blood flows through glomerulus, 1/5 of plasma filters into Bowman’s capsule • remaining blood leaves through efferent arteriole

3. Renal Corpuscle A – Renal corpuscle
B – Proximal tubule
C – Distal convoluted tubule
D – Juxtaglomerular apparatus
1. Basement membrane (Basal lamina)
2. Bowman's capsule – parietal layer
3. Bowman's capsule – visceral layer
3a. Pedicels (podocytes)
3b. Podocyte 4. Bowman's space (urinary space)
5a. Mesangium – Intraglomerular cell
5b. Mesangium – Extraglomerular cell
6. Granular cells (Juxtaglomerular cells)
7. Macula densa
8. Myocytes (smooth muscle)
9. Afferent arteriole
10. Glomerulus Capillaries
11. Efferent arteriole

4. Renal Corpuscle • The endothelial cells of the glomerulus • Glomerular basement membrane • Podocytes

5. Renal Corpuscle

6. Membranous Glomerulonephritis • Progressive disease thought to occur as a result of autoimmunity – 85% of cases areidiopathic • Disease can also occur secondary to other autoimmune diseases (eg. systemic lupus erythematosus) or through inflammation in response to bacterial / viral infection. • Symptoms: • Edema • Foamy appearance of urine • Frequent urination • High blood pressure • Membranous glomerulonephritis is the most common cause of nephrotic syndrome in Caucasians (20-35%).

7. Membranous Glomerulonephritis - Pathology Characterized by subepithelial immune deposits - presence of immunoglobulins, complement, and membrane attack complex (MAC) (C5b-9) Immune complexes serve as a trigger for the complement cascade and formation of the MAC – but what triggers Ab formation ?

8. Membranous Glomerulonephritis - Pathology Characterized by subepithelial immune deposits - presence of immunoglobulins, complement, and membrane attack complex (MAC) (C5b-9) Immune complexes serve as a trigger for the complement cascade and formation of the MAC – but what triggers Ab formation ? Serum samples from 26 of 37 patients (70%) with idiopathicbut not secondary membranous nephropathy specifically identifieda 185-kD glycoprotein in nonreduced glomerular extract. Reactive serum specimensrecognized recombinant PLA2R and bound the 185-kD glomerularprotein. PLA2R was expressed in podocytesin normal human glomeruli and colocalized with IgG4 in immunedeposits in glomeruli of patients with membranous nephropathy.IgG eluted from such deposits in patients with idiopathic membranousnephropathy, but not in those with lupus membranous or IgA nephropathy,recognized PLA2R.(N Engl J Med. 2009 Jul 2;361(1):11-21)

9. Membrane Attack Complex (MAC)

10. Membrane Attack Complex • What happens ? • Lysis of glomerular epithelial cells • Stimulates release of proteases and reactive O2 species that induce further cell damage • Capillary walls become more porous, increasing the amount of fluid that can enter Bowman’s capsule • Inhibitor of nephrin is released, decreasing the selectivity of filtration through podocytes.

11. Membranous Glomerulonephritis – Clinical Features Any thoughts on clinical / laboratory features?

12. Membranous Glomerulonephritis – Clinical Features Any thoughts on clinical / laboratory features? Proteinuria is generally in nephrotic range and can be massive (>10 g/24h) (corresponding hypoproteinemia) Hypoalbuminemia Hypercholesterolemia Microhematuria is common and macrohematuria is unusual No hypocomplementemia (at least in idiopathic forms) and in many cases increased levels of MAC (C5b-C9)

13. Membranous Glomerulonephritis – Clinical Features • Any thoughts on clinical / laboratory features? • Proteinuria is generally in nephrotic range and can be massive (>10 g/24h) • (corresponding hypoproteinemia) • Hypoalbuminemia • Hypercholesterolemia • Microhematuria is common and macrohematuria is unusual • No hypocomplementemia (at least in idiopathic forms) and in many cases increased levels of MAC (C5b-C9) • The reason for most of these changes should be obvious – why do patients present with hypercholesterolemia ? • triglycerides are normally cleared by the liver and other organs not the kidney • mechanism is unknown but enzymes involved in fatty acid metabolism are not as active • one route could be that an enzyme inhibitor that is normally cleared by the kidney can accumulate

14. Membranous Glomerulonephritis – Outcomes Prognosis ~25% of patients have partial or complete remission (with 1/3 of those having recurrence) 50% will lack any proteinuria or impairment of renal function A small percentage will have rapid loss of renal function, possibly leading to death. Varying outcomes with no obvious differences in pathology make treatment options difficult.

15. Membranous Glomerulonephritis – Outcomes • Prognosis • ~25% of patients have partial or complete remission (with 1/3 of those having recurrence) • 50% will lack any proteinuria or impairment of renal function • A small percentage will have rapid loss of renal function, possibly leading to death. • Varying outcomes with no obvious differences in pathology make treatment options difficult. • Treatment • Corticosteroids and other immunosuppresants – limited use with variable data on the effectiveness • Side effects of immunosuppresants may be more dangerous than the underlying disease • Calcineurin inhibitors (CNIs) induceremission of proteinuria in most nephrotic patient • 60% of patients becometreatment dependent and are at risk of chronic nephrotoxicity

16. Membranous Glomerulonephritis – Outcomes • Is Rituximab the answer? • Humanized monoclonal antibody designed to target the cellular protein, CD20 • CD20 is found on the surface of all mature B cells – involved in the progression of B cells in to plasma cells, which produce high quantities of antibodies (but actual role is unknown) • Rituixmab causes B cells to undergo programmed cell death, allowing new, less-reactive B cells to develop in the bone marrow. • Destroys memory B cells, those responsible for recognising previous exposure to an antigen.

17. Membranous Glomerulonephritis – Outcomes Is Rituximab the answer? Design: Thirteen patientswith membranous glomerulonephritis , normal renal function, and proven dependence on calcineurin inhibitors ,despite previous treatment with other immunosuppressant drugs,received a single trial of four weekly doses of rituximab (375mg/m2).

18. Membranous Glomerulonephritis – Outcomes Is Rituximab the answer? Design: Thirteen patientswith membranous glomerulonephritis , normal renal function, and proven dependence on calcineurin inhibitors ,despite previous treatment with other immunosuppressant drugs,received a single trial of four weekly doses of rituximab (375mg/m2). Outcome measures: the percentage of patients withCNI withdrawal and no evidence of relapse and the percentageof patients with complete or partial remission 30 mo after CNIwithdrawal.

19. Membranous Glomerulonephritis – Outcomes Is Rituximab the answer? Clin J Am Soc Nephrol. 2009 Jun;4(6):1083-8. Epub 2009 May 28 Design: Thirteen patientswith membranous glomerulonephritis , normal renal function, and proven dependence on calcineurin inhibitors ,despite previous treatment with other immunosuppressant drugs,received a single trial of four weekly doses of rituximab (375mg/m2). Outcome measures: the percentage of patients withCNI withdrawal and no evidence of relapse and the percentageof patients with complete or partial remission 30 mo after CNIwithdrawal. Results: After rituximab, proteinuria decreased significantly(2.5 ± 0,76 basal versus 0.85 ± 0.17 at 6 mo; p = .0003) CNIs and other immunosuppressant drugs could bewithdrawn in all patients with no evidence of relapse. AfterCNI withdrawal, glomerular filtration rate increased significantly (90.3 ± 15basal to 106.4 ± 20 at 3 mo with a mean increase of 15.3%[range 0–20]). Three patients suffered a relapse of nephroticproteinuria 19, 23, and 28 mo after rituximab treatment; allwere successfully treated with a second course of rituximab. At 30 mo, all patients were in remission.