Immunological Inhibition Arising From Misplaced Iron: Implications for Thalassemia and Sickle Cell Disease

I. Mustafa, D. Wang and M.D. Scott

Canadian Blood Services and the Centre for Blood Research and the Department of Pathology and Laboratory Medicine at the University of British Columbia, Vancouver, BC, Canada

Background: The thalassemias and Sickle Cell Disease (SCD) arise from mutations to the globin subunits of adult hemoglobin (HbA) resulting in destabilized hemoglobin and, potentially, a life-threatening anemia due in part to iron-driven redox reactions. While transfusions corrects the anemia, secondary iron overload can occur. Thus, both the primary and secondary pathology of thalassemia and SCD arise from “misplaced” iron. Removal of oxidatively damaged RBC in vivo occurs primarily via erythrophagocytosis by the mononuclear phagocytic system (MPS). This clearance mechanism may result in negative immunoregulatory effects such as the observed increased risk bacterial infections in these patients.

Methods: To determine the functional consequences of iron on the MPS, the effects of ferric iron (Fe³⁺; ferric ammonium citrate, FAC), heme, purified HbA and oxidized RBC on antigen presentation/proliferation by PBMC and cultured dendritic like (DC) cells was examined. Antigens examined included tetanus toxoid (TT Ag), formalin-fixed Streptococcus mutans (SM Ag) and RhD peptide. PBMC proliferation was determined by ³H-thymidine incorporation or via flow cytometry using carboxyfluorescein diacetate, succinimidyl ester (CFSE) stained cells. To determine if iron-driven immunomodulation could be reversed, an iron shuttle chelation system using Desferal (DFO; shuttle chelator) and S-DFO (a high molecular weight DFO-starch conjugate) was examined.

Results: Importantly, all forms of iron, including oxidized RBC, significantly inhibited antigen presentation and PBMC proliferation. For example, 100 µM hemin resulted in a >98% reduction in proliferation in response to the TT or SM Ag. Similarly, phagocytosis of oxidized RBC virtually abolished the ability of antigen presenting cells within the PBMC to present antigen and abolished the response to the TT and SM antigens. DC cells were similarly affected by FAC (200 µM) exposure (7 days) with a ~78% reduction PBMC response to an immunodominant RhD peptide. Iron chelators could partially overcome the effects of the bioreactive iron. Of interest, prolonged treatment with S-DFO (unlike DFO) did not adversely affect purified hemoglobin.

Conclusions: As shown, iron has significant immunodepressive effects on immune function (antigen presentation and lymphocyte proliferation). Iron chelation can effectively bind and remove free and complexed iron /heme preventing both redox-driven damage and immuosuppression. These data suggest that a two component iron shuttle chelation system may effectively slow/prevent iron-driven damage within cells and may also protect immune competency.

Note: This was published Transfusion AABB Annual Meeting, New Orleans, LA USA 24-27 Oct 2009.