Several mechanisms have been proposed for the dual effect of Tregs in regard to GvHD and the GvL effect, e.g., inhibition of JAK1/JAK2 signaling, expression of NKG2D by CD8+ cells, IL-21 signaling, or the differential expression of granzyme B by CD4+CD25+ and CD8+ cells37–40. Here, IL21 is linked to graft versus host disease.