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.
Polyclonal B cell activator