We demonstrated that an indirect pathway of alloantigen presentation via liver sinusoidal endothelial cells (LSEC) is involved in alloreactive T-cell tolerance induced by portal venous injection (PI) of donor cells. Thirty million C57BL/6 (B6) splenocytes that were either untreated or treated with 30-Gy irradiation were injected via the portal vein into Balb/c mice. Host LSEC expressing major histocompatibility complex class II actively endocytosed the allogeneic naive splenocytes as well as irradiated splenocytes after PI. Using a transendothelial migration assay, it was demonstrated that host-type Balb/c CD4(+) T cells that transmigrated across LSEC that had captured irradiated B6 splenocytes were rendered tolerant to subsequent alloantigen presentation by host professional antigen-presenting cells. Consistently, PI of irradiated donor-type splenocytes led to remarkable prolongation of the survival of subsequently transplanted heart allografts. These results indicate that indirect antigen presentation by LSEC significantly contributes to alloreactive T-cell tolerance induced by PI of irradiated donor splenocytes.