Membranoproliferative glomerulonephritis

Membranoproliferative glomerulonephritis (MPGN) is a pattern of glomerular injury in which immune-complex deposition, mesangial expansion, endothelial damage, and remodeling of the glomerular capillary walls combine to impair renal filtration. Within the book’s account of Hepatitis C, it is not merely an incidental biopsy label: it is the most frequent renal lesion associated with HCV-related mixed cryoglobulinemia, an important explanation for proteinuria and declining kidney function, and a recurring manifestation of systemic viral disease in both nontransplanted and transplanted patients.

Definition and Clinical Setting

HCV-infected patients, including kidney and liver transplant recipients, have an increased risk of glomerular lesions leading to Chronic kidney disease. These lesions may occur with or without substantial liver disease, but patients with HCV-associated glomerulonephritis are consistently HCV RNA positive in serum. Epidemiologic evidence places MPGN at the center of this association: the prevalence of HCV seropositivity in case series of patients with MPGN was reported to be approximately ten times that observed among HCV-negative patients. In a large United States comparison involving 18,002 people with HCV infection and 25,137 controls whose baseline estimated glomerular filtration rate exceeded 60 ml/min/1.73 m2, HCV infection was associated with a higher risk of developing CKD stages 3–5. Another comparison found CKD in 9.6% of anti-HCV-positive patients versus 5.1% of HCV-negative patients, with a shorter time to CKD and End-stage renal disease among the HCV-positive group; a higher baseline viral load independently predicted CKD.

The lesion is especially prominent in Mixed cryoglobulinemia, a systemic vasculitis for which HCV is the major cause. Mixed cryoglobulinemia can produce arthralgias, arthritis, Raynaud phenomena, purpura, peripheral neuropathy, hypocomplementemia, and kidney disease. Its renal manifestations range from nephrotic or non-nephrotic proteinuria and hematuria to reduced estimated glomerular filtration rate. Acute nephritic syndrome is the presentation in about 25% of affected patients, while nephrotic syndrome occurs in about 20%. Purpura, arthralgias, or neuropathy may precede the renal disease by several years, so the kidney lesion can appear late in a broader systemic process rather than as an isolated renal disorder.

Pathology and Mechanism

HCV is mainly associated with type II mixed cryoglobulinemia, and type I MPGN, also called Cryoglobulinemic glomerulonephritis, is the most frequent kidney lesion in that setting. The pathological picture typically combines immune-complex deposition with the structural changes of type I MPGN. On light microscopy, glomeruli may be markedly hypercellular because mononuclear and polymorphonuclear leukocytes have massively infiltrated the capillary loops. The tuft becomes more lobulated, while increased mesangial matrix and mesangial cells, swollen capillary endothelium, splitting of the capillary basement membrane, intracapillary thrombi, and eosinophilic material representing precipitated immune complexes or cryoglobulins may all be present. Small- and medium-sized renal-artery vasculitis can accompany the glomerular injury.

Immunofluorescence usually demonstrates IgM, IgG, and C3 deposited in the mesangium and along capillary walls. Electron microscopy commonly identifies subendothelial immune complexes and may reveal fibrillar or immunotactoid structures suggestive of cryoglobulin deposits. These findings give the lesion a visible architecture of circulating immune material becoming trapped at the filtration barrier: the clinical proteinuria, hematuria, complement consumption, and renal dysfunction correspond to a pathology in which deposits, inflammatory cells, and altered capillary walls obstruct and injure the glomerular microcirculation.

A related but distinct form is non-cryoglobulinemic MPGN. Some patients with HCV infection develop MPGN despite having no detectable cryoglobulins in serum. Their clinical presentation, pathological findings, and laboratory data are otherwise often indistinguishable from idiopathic type I MPGN, except for the presence of HCV antibodies and HCV RNA in serum. Electron microscopy shows subendothelial and mesangial immune complexes without a distinctive substructure. The distinction therefore depends less on a wholly different morphology than on the demonstrable viral context and the absence of circulating cryoglobulins.

Diagnosis and Disease Assessment

The relationship between MPGN and HCV depends on distinguishing exposure from active infection. Diagnosis of HCV infection begins with anti-HCV antibodies detected by third-generation enzyme-linked immunosorbent assay. Antibody positivity alone cannot distinguish resolved infection from chronic infection; active disease is established by detecting HCV RNA in serum with a sensitive molecular assay, with a threshold of approximately 15 IU/ml. Chronic HCV infection should therefore be demonstrated by both anti-HCV and HCV RNA positivity. This distinction is crucial when MPGN is being attributed to HCV, because the book’s evidence repeatedly links HCV-associated renal disease to ongoing viral RNA positivity rather than to an antibody record alone.

Evaluation must address both hepatic and extrahepatic injury. Liver fibrosis can be assessed by biopsy or by noninvasive biochemical and morphological methods, including FibroScan measurement of liver stiffness. Such tests can avoid approximately half of liver biopsies and permit yearly fibrosis follow-up in untreated patients. The kidney assessment, by contrast, depends on the clinical pattern of proteinuria, hematuria, renal dysfunction, complement abnormalities, cryoglobulins, and renal biopsy. In kidney-transplant candidates, screening is particularly important because up to 25% of patients found to have active HCV infection have advanced fibrosis on liver biopsy despite apparently well-compensated liver disease. In low-prevalence regions, initial enzyme immunoassay screening is followed by nucleic acid testing when anti-HCV is positive; in high-prevalence settings, nucleic acid testing should be first-line because a negative antibody test cannot reliably exclude infection.

Clinical Course and Outcomes

The course of cryoglobulinemic MPGN is variable. In most patients, renal disease progresses slowly and does not reach kidney failure despite persistent proteinuria and some degree of renal impairment. Even patients with massive thrombi and vasculitis may experience spontaneous reversal. The risk of progressing to End-stage renal disease is higher with older age, recurrent purpura, an elevated cryocrit, hypocomplementemia, and high serum creatinine at diagnosis. Thus, the biopsy can look dramatically active without making an inevitable prediction of renal failure, while apparently systemic clues such as recurrent purpura and complement consumption help identify a more dangerous trajectory.

The prognosis becomes much worse in HIV/HCV coinfection. In one series of 12 HIV- and HCV-coinfected intravenous drug users, patients presented with proteinuria, microhematuria, and elevated serum creatinine; most had edema, hypertension, and hypocomplementemia, while 42% had cryoglobulinemia. Renal biopsy showed MPGN in five patients, mesangial proliferative glomerulonephritis in five, membranous nephropathy in one, and collapsing glomerulopathy with immune-complex deposits in one. HCV RNA was found in renal tissue and/or serum in nine of the eleven patients tested. During follow-up, three patients died and five progressed to end-stage renal failure, with a mean time to dialysis of 8.4 months after biopsy.

A second series of 14 coinfected patients, 13 of whom were African-American and all of whom were intravenous drug users, showed an equally severe pattern. Ten had nephrotic syndrome with renal insufficiency, two had chronic renal insufficiency, one had acute renal failure, and one presented with uremia. Six had AIDS at presentation, six had hypocomplementemia, and four had cryoglobulinemia. MPGN was found in eleven biopsies, including type III patterns with immune complexes on both sides of the glomerular basement membrane. Three patients received alpha-interferon without clear benefit, and the majority progressed rapidly to chronic dialysis or death; the median time to dialysis or death was 5.8 months. This outcome contrasted sharply with the ten-year survival free of death or chronic renal failure reported in a historical cohort of HCV-infected patients with renal disease but without HIV infection.

Transplantation and Therapeutic Significance

MPGN also appears after renal transplantation, where HCV-associated immune-complex disease can damage an otherwise functioning allograft. HCV-infected renal and liver allograft recipients are prone to immune-complex-mediated glomerulonephritis, as are nontransplanted patients. De novo MPGN was first reported in HCV-infected renal-allograft recipients without demonstrable cryoglobulinemia, although indirect evidence suggested circulating immune complexes. Later observations identified type II mixed cryoglobulinemia, including an IgM-kappa monoclonal component, in patients with de novo MPGN after transplantation.

Cryoglobulin levels in HCV-infected allograft recipients are often very low, usually below 5%, which can make detection and characterization difficult. HCV RNA is commonly much more concentrated in the cryocrit than in serum, supporting a pathogenic role for HCV in the cryoglobulinemia and suggesting activation of the classical complement pathway. Clinically, allograft-associated MPGN may cause hypertension, microhematuria, proteinuria, renal dysfunction, and sometimes nephrotic syndrome. Biopsy can show chronic damage in multiple compartments, focal or diffuse mesangial proliferation, double contours, and parietal IgM and C3 deposits.

The diagnosis may be difficult because MPGN can resemble Transplant glomerulopathy. Electron microscopy helps separate them: MPGN is characterized by subendothelial electron-dense deposits, whereas transplant glomerulopathy shows radiolucent areas in the glomerular basement membrane. Both processes can coexist, an overlap that supports the simultaneous contribution of humoral immunity and HCV-associated autoimmunity to the poor outcome of HCV-positive renal allografts. The broader transplant literature identifies HCV infection as an important risk factor for renal allograft loss and associates it with glomerulitis, transplant glomerulopathy, anti-HLA antibodies, and antibody-mediated rejection, although the relative contributions of viral injury and heightened immune risk may vary.

Treatment is consequently directed at both the virus and the renal or systemic consequences of immune-complex disease. The book describes treatment of HCV infection as appropriate at any stage, with therapy guided by viral kinetics and by hepatic or extrahepatic impact. In the period discussed, standard therapy relied principally on pegylated interferon and ribavirin, while genotype 1 disease could be treated with triple therapy adding the direct-acting antiviral protease inhibitors telaprevir or boceprevir. Sustained virologic response represents virologic recovery and, in the absence of liver comorbidities, remodeling of fibrosis. The emergence of direct-acting antiviral drugs therefore mattered to MPGN not only as an advance in viral control but as a potential way to remove the initiating infection from an immune-complex renal disease whose consequences may persist after the liver appears clinically stable.