Studies on iron and immunity: Effect of iron on lymphocyte proliferation stimulated by polyclonal B cell activators and mechanism of iron action

Iron deposition in synovial tissue from patients with rheumatoid arthritis (RA) was demonstrated coincidently with infiltration of immune cells. In order to investigate if ferric iron interacts directly with B-cells, the effect of ferric citrate on lymphocyte proliferation stimulated by mitogens was examined. Human peripheral blood mononuclear cells (PBM) were separated from venous blood by Conray-Ficoll gradients. PBM were cocultured with or without different polyclonal B-cell activators: Pokeweed mitogen (PWM), Staphylococcal phage lysate (SPL) and Staphylococcus aureus Cowan I (SAC). Ferric citrate significantly inhibited PWM, SPL and SAC responses as well as Con A responses. Furthermore, PBM were separated into PBM depleted of monocytes, E-RFC (T-cell enriched) and non E-RFC (B-cell enriched), and each cell fraction was cultured with mitogens and ferric citrate. Mitogen responses of fractionated cells were inhibited by co-addition of ferric citrate in the same way as PBM. These results indicated that iron may inhibit SAC responses of human B-cells by interacting directly with B-cells. Iron also inhibited specific antigen (PPD) stimulated lymphocyte proliferation in vitro. Iron was shown to act in the early phase of mitogen responses. Purified IL-1 or IL-2 was added to the PBM culture system with mitogen and ferric citrate. Neither IL-1 nor IL-2 reversed the inhibitory activity of iron on lymphocyte proliferation. Con A or ferric citrate itself was not toxic to PBM after 5 days of culture. However, low cell viability of PBM was noted when the cells were cultured together with Con A and ferric citrate for 5 days. The decrease in cell viability was blocked by co-addition of catalase or gold in PBM culture system, indicating that iron might release oxygen radicals from activated macrophages