On day 28 after transplant, intracellular FoxP3 staining used to quantify all CD4+ Treg revealed a significant reduction in Treg frequency within the CD4 T cell compartment of the SP and to an even greater extent in the BM of mice receiving GVHD-inducing grafts as compared to syngeneic graft recipients (Figure 2B). Here, CD4 is linked to graft versus host disease.