CD4 and graft versus host disease: To investigate whether the addition of WT Treg would rescue mice from the severe GVHD caused by Lag-3−/−Tcon, 5×105 highly purified CD4+CD25+Foxp3+ Treg from C57BL/6 mice were co-transferred on day 0 with 5×106 TCD-BM, followed by 1×106 Lag-3−/− or WT Tcon on day 2.