Hepcidin: A Clue for a New Role for Lymphocytes.

Recent evidence suggests the involvement of lymphocytes in the severity of iron overload disorders, with increased severity of iron overload in Hereditary Hemochromatosis patients and in animal models with lymphocyte number deficiencies. However, no mechanism(s) has been suggested to explain these observations. The aim of this study was to analyze hepcidin expression in lymphocytes.

Expression of hepcidin was analyzed by Real-time PCR in human and mouse Peripheral Blood Lymphocytes (PBLs) and in selected resting lymphocyte populations, in response to holotransferrin and ferric citrate. The effect of hepcidin in the expression of the iron exporter Ferroportin was analyzed by FACS and confocal immunofluorescence. Cellular iron traffic was analyzed by measurement of ⁵⁵Fe and ¹²⁵I-TF, cell proliferation assessed by BrdU incorporation and silencing of gene expression in lymphocytes performed with siRNAs.

Hepcidin is expressed in PBLs and is up-regulated in response to holotransferrin and ferric citrate. The response to holotransferrin was observed in CD8⁺ and not in CD4⁺ lymphocytes, a result confirmed by the failure of lymphocytes from β₂-microglobulin-KO mice to respond to holotransferrin, in comparison with Bl6/J controls. Hepcidin up-regulation induced by holotransferrin decreases ferroportin expression, inducing its co-localization with the proteasome marker LMP2. Tumor Necrosis Factor-á (TNF-á) expression in PBLs increases with holotransferrin treatments. siRNA-mediated silencing of TNF-á in PBLs abrogates hepcidin up-regulation by holotransferrin and incubation of PBLs with recombinant TNF-á increases hepcidin expression, suggesting the involvement of this cytokine in the basal and holotransferrin-induced hepcidin expression in these cells. The role of hepcidin in a situation of high iron demand - lymphocyte activation and proliferation - was assessed. Hepcidin expression increases with T-lymphocyte activation and siRNA-mediated silencing of hepcidin in activated T lymphocytes causes a decrease in intracellular iron levels, by increasing ferroportin-mediated iron export. The low intracellular iron levels were associated with impaired T lymphocyte proliferation.

The ability of PBLs to increase hepcidin expression in response to ferric citrate distinguishes lymphocytes from hepatocytes and places peripheral blood lymphocyte numbers as a first line of response to increases in transferrin saturation and presence of NTBI, characteristic of iron overload disorders. The findings that hepcidin modulates ferroportin expression, intracellular iron levels and cell proliferation in lymphocytes demonstrate the importance of this protein for lymphocyte iron homeostasis. The control of the intracellular iron levels of lymphocytes confers to hepcidin a pivotal role in the postulated ability of circulating lymphocytes to function as “biological iron chelators”.

With the demonstration, for the first time, of hepcidin synthesis by peripheral blood lymphocytes, of its regulation by elemental iron and of its involvement in T cell proliferation, the present results put forward a molecular mechanism for the described modifier role of lymphocytes in protection from iron toxicity.

No relevant conflicts of interest to declare.