FTY720 is a novel immune modulator whose primary action is blood lymphocyte depletion through interaction with sphingosine-1-phosphate (S1P) receptors. The present study analyses the effect of FTY720 on both the early mesangial cell injury and the subsequent matrix expansion phase in experimental mesangioproliferative glomerulonephritis. Disease was induced by injection of OX-7 anti-thy1 antibody into male Wistar rats. In both protocols, FTY720 administration (0.3 mg/kg body weight) resulted in a selective and very marked reduction in blood lymphocyte count. In the injury experiment, the S1P receptor modulator was given starting 5 days before and continued until 1 day after antibody injection, but did not significantly affect the degree of anti-thy1-induced mesangial cell lysis and glomerular inducible NO production. In the matrix expansion experiment, FTY720 treatment was started 1 day after antibody injection and continued until day 7. In this protocol, the S1P modulator reduced proteinuria, histological matrix expansion and glomerular protein expression of TGF-1, fibronectin and PAI-1. Glomerular collagen III staining intensity was decreased. FTY720 reduced markedly glomerular lymphocyte number per cross section and to a lesser degree macrophage infiltration. In conclusion, FTY720 significantly limits TGF- overexpression and matrix protein expression following induction of acute anti-thy glomerulonephritis, involving reductions in blood and glomerular lymphocyte numbers. The results suggest that lymphocytes actively contribute to matrix expansion in experimental mesangioproliferative glomerulonephritis. Our study expands on findings on FTY720's beneficial effects on tubulointerstitial and functional disease progression previously reported in anti-thy1-induced chronic glomerulosclerosis.