PDCD1 and cystic fibrosis: However, the modulation of PD-1 remains unclear since vitamin D has been demonstrated to increase the PD-L1 expression (and, therefore, PD-1/PD-L1 interaction) and decrease the expression of co-stimulatory receptors, such as CD80 and CD86 in healthy individuals [33] and enhance PD-1 expression in Crohn’s disease [34], while this inhibitory expression has been reduced in both helper and cytotoxic T cells with vitamin D treatment in cystic fibrosis [35].