IntroductionInterferon-␥ (IFN-␥), produced by T and natural killer (NK) cells, is considered the principal effector cytokine of cell-mediated immunity and exerts its effects on target cells through a highaffinity receptor complex linked to a specific Janus kinase (Jak)/ signal transducer and activator of transcription (STAT) signaling cascade. 1,2 The IFN-␥ receptor (IFN-␥R) complex consists of 2 chains: an IFN-␥R1 binding chain and an IFN-␥R2 signaling chain. 1 The intracellular portions of the 2 chains provide the Jak1 and Jak2 docking sites. Upon phosphorylation, Jak1 and Jak2 activate STAT1: Following phosphorylation and dimerization, STAT1 is translocated into the nucleus where it activates transcription of numerous sets of IFN-␥-inducible genes. 2 The IFN-␥/STAT1 pathway plays an essential role in controlling the expansion of normal and neoplastic cell types of different origin. Activation of this pathway switches on many proapoptotic and antiproliferative genes such as interferon regulatory factor 1 (IRF1), p21 waf/cip , Fas and FasL, and activates caspases. [3][4][5][6][7][8][9][10] However, the signals transduced by IFN-␥ do not always induce apoptosis or block proliferation, and lymphoid cells become resistant to the antiproliferative effects of IFN-␥. Resting, malignant, or normal T cells that develop toward the T helper 1 (Th1) pathway become resistant to the antiproliferative effects of the IFN-␥/STAT1 pathway 9,11,12 or rather, IFN-␥ favors their proliferation and differentiation. [13][14][15] The refractoriness of T cells to the IFN-␥/STAT1 pathway has been attributed mainly to down-regulation of the IFN-␥R chains, especially IFN-␥R2, which protects these cells from the antiproliferative/proapoptotic effects of [16][17][18][19] Both IFN-␥-dependent and -independent mechanisms have been reported to downregulate IFN-␥R2 expression in T lymphocytes. During murine Th cell differentiation, IFN-␥ itself induces IFN-␥ resistance by down-regulating IFN-␥R2, 16 whereas in human T lymphocytes, IFN-␥R2 internalization occurs mostly in clathrin-coated pits independently from IFN-␥ 17 and is selectively induced by insulinlike growth factor 1 (IGF1). 20 Since the IFN-␥/STAT1 pathway is usually down-regulated in T lymphocytes, information on the mechanisms that maintain low IFN-␥R2 expression in these cells might prove useful for devising therapeutic approaches centered on selectively reinstating the IFN-␥/STAT1 apoptotic signaling pathway in autoreactive or neoplastic T cells.Besides T-cell receptor (TCR) engagement, 9,12 up-regulation of surface IFN-␥R2 in T cells may also be induced by serum deprivation, 5 exposure to nitric oxide (NO), 21 Among the plethora of factors present in serum, iron has profound effects on numerous critical cell functions, such as electron and oxygen transport, mitochondrial energy metabolism, and detoxification, thus requiring tight homeostatic regulation. 24 Iron binds to cytoplasmic iron regulatory protein 1 (IRP1) and IRP2 which in turn regulate expression of proteins such as fe...
