CD4 and cystic fibrosis: Although vitamin D is known to module immune responses by up-regulating the expression of PD-1 in Crohn’s disease [34] and both PD-L1 and PD-L2 in cell-based models [60], vitamin D supplementation has also reduced the expression of PD-1 in CD4+ and CD8+ T cells in cystic fibrosis [35].