IL21 and chronic graft versus host disease: Skin-infiltration by ICOS+ follicular helper (Tfh)-like CD4+ T cells and ICOS+ Th1 cells may also be important in driving skin fibrosis in murine chronic GVHD, the latter process requiring the generation of the Tfh-related cytokine, IL-21 (40); of note, antibody-mediated depletion of ICOS-expressing T cells or IL-21 blockade was able to block collagen deposition.