Iron Overload Diminishes the Effectiveness of the Innate Immune Response in Thalassemia Major: a Possible Mechanism for Increased Infection Risk.

Infection is the second most common cause of death in thalassemia. The innate immune system provides a first line of defense against infection and specificity depends on pattern recognition receptors (PRRs) specific to microbial pathogens. One class of PRR called the toll-like receptors (TLRs) are important for transducing the signal for bacterial Lipopolysaccharide (LPS), resulting not only in cytokine production, but also in the control of extracellular iron levels through production of neutrophil gelatinase associated Lipocalin (NGAL). However, the exact role that NGAL plays and the expression level of PRRs are unknown in thalassemia. Thus, the goal in these studies is to investigate the relationship of iron overload to the innate immune cell expression of PRRs and NGAL in thalassemia.

Fifteen transfusion dependent thalassemia patients (11 – 29 yrs old) participating in the combination trial of deferasirox (an oral iron chelator) and deferoxamine were enrolled (Novartis sponsored CICL670AUS24T). Fasting blood samples were obtained i) at baseline after a 72 hr washout of chelator, and ii) at 6 and 12 months on study. Five healthy controls (13 - 18 yrs old) were also enrolled. Fresh monocytes were isolated using antibody-linked magnetic microbeads (Miltenyi Biotec Inc). Highly enriched populations of CD14⁺ monocytes were verified by flow cytometry. The expression of TLR4, also examined by flow cytometry is reported as the mean fluorescent intensity (MFI). In patients with thalassemia, liver iron concentration (LIC) was analyzed by biomagnetic susceptibility (“SQUID”, Ferritometer®). The plasma levels of NGAL were analyzed by ELISA.

At baseline the expression of monocyte TLR4 (mean 18.8 ± 3.5 MFI) was reduced 30% compared to the healthy controls (mean 26.9 ± 7.6 MFI, p<0.05). The expression of TLR4 over the follow-up period of 52 weeks in patients receiving intensive combination chelator therapy significantly increased 27% / year (7 MFI / year, p=0.005). Interestingly the expression of monocyte TLR4 was negatively correlated with LIC (r=-0.6, p=0.04). Finally, thalassemia patients at baseline have significantly higher levels of NGAL (80 ± 20 ng/ml) compared to controls (42 ± 15 ng/ml, p=0.01).

These preliminary studies support the hypothesis that iron burden has a negative impact on the innate immune response in thalassemia as demonstrated by the decreased expression of TLR4. After intensive chelation, the levels of TLR4 increased, indicating that decreased iron overload with chelation may improve innate immune responsiveness. Finally, the iron transport protein NGAL is significantly elevated in thalassemia possibly acting to prevent essential iron uptake by pathogenic bacteria.

Harmatz:Novartis: Research Funding; Apotex : Membership on an entity's Board of Directors or advisory committees; Ferrokin: Membership on an entity's Board of Directors or advisory committees. Vichinsky:Novartis: Consultancy, Research Funding.