Tregs interact with cerebral endothelial cells and platelets to promote ischemic neurodegeneration after stroke. (A left) Purified CD4⁺CD25⁺ (750 000) Tregs or CD4⁺CD25⁻ non-Tregs were transferred into Rag1^−/− mice 24 hours before 60 minutes of tMCAO in the presence of anti–LFA-1 blocking Abs or control Abs. Infarct volumes were determined on day 1. Blocking of LFA-1 reversed the stroke-enhancing effect of CD4⁺CD25⁺ Tregs but not of CD4⁺CD25⁻ non-Tregs in Rag1^−/− mice. Right, circulating CD4⁺CD25⁺ Tregs express higher amounts of LFA-1 than CD4⁺CD25⁻ non-Tregs on day 1 after 60 minutes of tMCAO. MFI indicates mean fluorescence intensity. (B) In vitro binding capacity to ICAM-1 (left) but not VCAM-1 (right) is more pronounced in CD4⁺CD25⁺ Tregs than in CD4⁺CD25⁻ non-Tregs both under basal conditions and after stimulation with CXCL12. (C) Purified CD4⁺CD25⁺ (750 000) Tregs or CD4⁺CD25⁻ non-Tregs were transferred into Rag1^−/− mice 24 hours before 60 minutes of tMCAO. Rag1^−/− mice were treated with antiplatelet serum to deplete circulating platelets or controls serum and infarct volumes were determined on day 1. Depletion of platelets reversed the stroke-enhancing effect of CD4⁺CD25⁺ Tregs and also of CD4⁺CD25⁻ non-Tregs in Rag1^−/− mice. *P < .05 and ***P < .0001 between the indicated groups; ns indicates not significant.