In passive or active Heymann nephritis (HN) in the rat, the immune complexes that form in the glomerular subepithelial space result in complement activation and the urinary (U) excretion of S protein-membrane attack complex (SC5b-9, MAC). Because of the similarities between HN in rats and membranous nephropathy (MN) in humans, it has been suggested that measurement of SC5b-9 in urine (UMAC) could be useful in assessing the immunologic activity of MN in patients. The present study was undertaken in normal individuals and in patients with nephrotic syndrome to determine: 1) the conditions of urine collection and preservation needed for accurate measurement of UMAC for clinical purposes; and 2) whether UMAC levels are a sensitive and/or specific test for MN. In studies conducted on urine specimens from patients with increased UMAC levels, we found that UMAC in freshly voided urine was stable for at least three hours at 37°C. with or without the addition of the enzyme inhibitors that were used to stabilize UMAC levels in the studies of HN in the rat. Urine pH, leukocytes and erythrocytes, over the ranges usually encountered, did not influence UMAC levels. However, freezing urine at -70°C artifactually raised UMAC levels (1500 ± 550 to 1800 ± 580 SE ng/ml, P < 0.001 by paired t-test). Normal urine contained low UMAC levels: 80 ± 3 ng/mg urinary creatinine (UCr). By contrast, patients with glomerulopathies tended to have elevated UMAC levels: 18 of 38 patients had levels that ranged from 200 to 20,000 ng/mg UCr. UMAC levels did not distinguish patients with MN from those with other causes of glomerulopathy. Indeed, the highest UMAC levels were seen in patients with nephrotic syndrome due to focal glomerulosclerosis or diabetic glomerulosclerosis. In such patients the ratio: urine protein (UPr)/UCr was correlated with the logarithm of the ratio UMAC/UCr (r = 0.85, P < 0.001). Biopsy specimens of normal kidney showed no staining for SC5b-9. Patients with diabetic glomerulosclerosis or focal glomerulosclerosis and elevated UMAC levels showed heavy deposits of SC5b-9 in tubular epithelium but little or no SC5b-9 in glomeruli, suggesting that in these patients UMAC resulted from SC5b-9 formed on tubular epithelium. Patients with MN and elevated UMAC levels had SC5b-9 deposits in both glomeruli and tubular epithelium, suggesting that in these patients UMAC resulted from SC5b-9 formed both in glomeruli and on tubular epithelium. Patients with idiopathic MN who received a complete course of immunosuppressive treatment had lower UMAC levels than those who did not: 1.5 ± 1.0 (N = 7) versus 1120 ± 380 (N = 8) ng/mg UCr, P < 0.02 by unpaired t-test, even though substantial proteinuria (UPr/UCr = 4.9 ± 2.1 mg/mg) was still present in the treated patients. We conclude that: 1) UMAC is stable over a wide range of conditions of collection and storage; and 2) there are two mechanisms for UMAC formation: (a) glomerular immune complex formation, as demonstrated in previous studies of HN, and (b) heavy proteinuria (generally UPr/UCr> 4.0 mg/mg), that apparently causes complement activation on renal tubular epithelial cells, as demonstrated in this study. The fact that heavy proteinuria itself results in increased UMAC excretion compromises, but may not invalidate the use of UMAC measurement to monitor the immunologic activity of MN. Longitudinal studies will be needed to test that hypothesis.
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