Background: Sera from some patients with focal segmental glomerulosclerosis (FSGS) increase glomerular albumin permeability (P(alb)) in vitro. The hypothesis that a component of normal serum can protect the glomerular permeability barrier was tested using sera from FSGS patients, normal individuals, and several mammalian and avian species.
Methods: In most experiments, isolated rat glomeruli were incubated in medium containing FSGS serum known to increase P(alb) in vitro, normal serum, or both active FSGS and normal serum. In other experiments, fractions of normal serum and serum from other vertebrate species were incubated with active FSGS serum. P(alb) was calculated from glomerular capillary expansion in response to an oncotic gradient. To enrich the blocking activity, normal pooled human plasma was subjected to various biochemical manipulations.
Results: Normal human serum prevented the increase in P(alb) (active FSGS sera, 0.77 +/- 0.12; active FSGS sera:normal serum, 1:1 mix, 0.06 +/- 0.30, P < 0.001). Protection diminished as the concentration of normal serum was decreased. Specific fractions of human serum, including human albumin and immunoglobulin fractions, were not protective. Blocking activity was present in 80% ammonium sulfate precipitate and certain fractions from size-exclusion chromatography of normal pooled human plasma. Normal serum from each of the vertebrate species tested also prevented the increase in P(alb). Preincubation with normal serum was protective during subsequent incubation with FSGS serum, but normal serum was not protective after preincubation with FSGS serum.
Conclusions: We conclude that a factor or factors in normal serum block the permeability effect of active FSGS sera. This phenomenon may account for variability in proteinuria among patients with FSGS and may explain inconsistent proteinuria following injection of FSGS sera into experimental animals. Characterization of the protective substance(s) and the mechanism by which the increase in permeability is blocked may provide insight into the pathogenesis of FSGS.