Regulatory T cells are important to suppress unwanted immune responses. Infusion of Tregs can be used for therapeutic purposes, such as to suppress inflammatory diseases and transplantation reactions. We have found an unexpected role for iron metabolism in these cells. Iron is a critical element for all cells. It is incorporated in a number of cellular proteins, among which mitochondrial enzymes, and is necessary for the enzymatic functions of those proteins. Soluble iron is, however, toxic when it is not bound to proteins, due to its ability to promote the production of free oxygen radicals. Iron that is not incorporated in proteins can be stored in a non-toxic form inside cells in cages made by FTH proteins, from which it can be released when necessary. We have found that Tregs express elevated levels of FTH compared to other types of T cells, suggesting a special function for iron metabolism in these cells. In this project, we examined this function. We found that FTH is unexpectedly necessary to preserve Treg cellular identity. When FTH is removed from Tregs, these cells change their DNA expression programs and become more like the T cells they are supposed to suppress. Correspondingly, these Tregs no longer function well, allowing the generation of stronger immune responses. Such stronger responses can for instance exacerbate autoimmune disease in a model for multiple sclerosis and kill mice infected with a malaria parasite. Plasma iron concentrations typically drop promptly during infections (presumably to curb growth by micro-organisms). Our results, therefore, suggest that Tregs may carry an iron reserve to allow them to control unfettered immune responses during infections. Indeed, we have found that Tregs possess a greater ability than other T cells to grow when the iron concentration is low in medium.

Mechanistically, our results show that FTH probably regulates Treg-specific gene expression through making iron available to mitochondrial enzymes, necessary to generate metabolites that control gene expression programs. In particular, Tregs that lack FTH produce lower amounts of alpha-keto-glutarate, a metabolite that is required for the function of certain enzymes that modify the DNA. These so-called Tet enzymes (which themselves also depend on iron as a cofactor) remove chemical groups from DNA and this is necessary to activate certain genes, including the gene that codes for the FoxP3 protein, the central protein controlling Treg-specific gene expression. Cellular metabolic programs also control signal transduction pathways that regulate expression of FoxP3. We have used a special technique (Mass Spectrometry) to study the activity of thousands of signal transduction events in parallel and have discovered multiple Treg-specific events. How these events control Treg specific gene expression remains to be tested.

Studying the mechanisms underlying stable expression of Treg-specific genes is important for the therapeutic use of these cells: as shown in our results, loss of this specific gene expression program interferes with Treg function and can even make them act as aggressive cells that cause pathology. By understanding the molecular mechanisms involved we hope to find critical factors that can be manipulated to prevent destabilization of Treg-specific gene expression and function.