The mesangium in anti-Thy-1 nephritis. Influx of macrophages, mesangial cell hypercellularity, and macromolecular accumulation.

Mesangial pathology is a hallmark of focal and segmental glomerulosclerosis (FGS). In an immunologically mediated mesangial cell (MC) injury model, we analyzed the relationship between mesangial hypercellularity, increased macromolecular uptake by the mesangium, and long-term pathologic sequelae. A single injection of monoclonal anti-Thy-1 (AT) antibody induces MC apoptosis, extensive mesangiolysis, proteinuria, MC proliferation, and hypercellularity. Immunohistologic analysis indicated influx of ED 1-positive macrophages after 24 hours, which gradually subsiding thereafter. At day 12, hypercellularity was due to smooth musclelike MCs, and macrophagelike MCs were absent. Injection of iron dextran in nephritic rats indicated that mesangial uptake of iron correlated with mesangial hypercellularity, but was independent of proteinuria. Long-term studies showed no difference after 19 weeks in FGS between nephritic and control rats. In conclusion, although mesangiolysis is accompanied by influx of macrophages, a phase of smooth musclelike MC proliferation and increased macromolecular uptake, these pathologic events do not result in chronic mesangial pathology and FGS.