For some years now a small group of scientists have been pioneering a revolutionary idea - that the vertebrate immune system could have a role in the regulation of iron in the body. And now, a work soon to be published in the journal Immunology, shows that human lymphocytes (white blood cells) actually produce hepcidin, the most important protein in the regulation of iron levels in the body. What was also unexpected was the fact that hepcidin affected lymphocyte multiplication - which occurs for example during an infection - showing that the two systems seem to be much more interlinked than even previously imagined.The discovery has far reaching implications as Jorge Pinto, the first author of the study, resumes: " from new players in both iron metabolism and immunology to a new understanding of how cell division in itself is controlled (both normal cell division and cancer) to even help us to better understand the evolution of the immune system". The work comes from a team of Porto University, Portugal, led by Maria de Sousa, the scientist that first proposed the idea of a link between the immune system and iron homeostasi.
Thirty years ago Maria de Sousa, then at the beginning of her career, noticed that lymphocytes were attracted to places with surplus of iron. This, together with
1- the fact that the vertebrate immune system (IS) was incredibly more complex that those of its ancestors (and evolution rarely increases complexity, which is energetically costly, unless something is gained)
2- the IS unique capacity to reach everywhere in the body
led her to a revolutionary new idea - could this new complexity be evolutionary sound, because it allowed the IS to perform some important new function, maybe protecting the body against iron toxicity?
In fact iron, although an essential element for most life forms, can also be toxic to these same organisms when free (not attached to proteins). This means that in this form it needs to be "watched" and regulated around the clock. In vertebrates, this is done through hepcidin, a liver protein that "moves" iron between cells and plasma according to the body needs (or potential dangers). The problem is that the hepcidin liver cells have limited mobility so a complementary far reaching iron control system was needed. Lymphocytes, with their unique capacity to move throughout the body were the perfect candidates and since 1978, de Sousa and her group have been chasing this idea.
Much of their work has been done on hemochromatosis - a disease where there are problems in the absorption of iron through the digestive track leading to too much iron in the organism and to its toxic accumulation in the organs.
From this work we know now that hemochromatosis patients also have a defective IS, and more, that their iron overload levels correlate with their lymphocyte deficiency - the less lymphocytes they have the more severe the disease. Work in animal models with iron overload problems or instead, with lymphocyte deficiencies have again found links between excess of iron in the body and deficient IS further supporting de Sousa's "immuno-iron idea".
And meanwhile, human lymphocytes were shown to produce several proteins crucial for the regulation of iron levels - ferritin, which acts as the body storage of iron (so holding to it when there is too much in the body or releasing it when there is deficiency) and ferroportin, which is the cells' iron "exit door" (again releasing or retaining iron as necessary) . The fact that lymphocytes had both proteins gave them the potential to be a "mobile" and easily "mobilizable" iron-storage compartment, characteristics perfect for an important role in iron homeostasis.
Nevertheless, the exact mechanism how this could happen remained elusive
But hepcidin, the central piece of iron regulation, is known to be also an important player in the immune response what has raised the possibility that it could be in it the clue to this problem. In fact, during infection hepcidin shuts down the "door" through which iron leaves the cell (ferroportin) reducing iron availability in the plasma and thus helping to control infection - as bacteria need iron to divide. And now several studies have shown that hepcidin is produced by a variety of cells involved in the immune response. Finally, last year, a study suggested, for the first time, that lymphocytes were also capable of producing the protein putting the possibility that hepcidin could actually be "the missing link" of de Sousa's theory.
To clarify this hypothesis Jorge Pinto, Maria de Sousa and colleagues at the Institute for Molecular and Cell Biology (IBMC) of Porto University looked at hepcidin production in human lymphocytes in situations of toxic iron concentrations or immune activation, as de Sousa's theory proposed that lymphocytes could play a role in both situations. They found that hepcidin not only was produced by all classes of lymphocytes, but also that its production increased both in the presence of high quantities of iron, and when lymphocytes were activated, backing de Sousa's proposals.
Pinto explains: "We show, for the first time, that lymphocytes can "feel" the toxic levels of iron in circulation and respond by increasing their own capacity to retain it within, restoring "normality". The same mechanism is seen being used in situations of (iron) demand, such as when the cells are activated by the occurrence of an infection and need to divide."
They also found something else totally unexpected - that hepcidin was involved in this second mechanism, suggesting an even closer dependence between the two systems than de Sousa had thought.
To Hal Drakesmith, a researcher at the University of Oxford working on the possibility of manipulating iron transport as a way to combat infections such as HIV, malaria and Hepatitis C these results raise particularly interesting questions as he explains "This seems to suggest that control of iron metabolism may be an integral component of lymphocyte immunity. Withholding iron from pathogens is an accepted part of our defence against infection, but a role for lymphocytes in controlling iron transport has not been proposed before.
"Crucially - says Pinto - we still believe that the main regulator of systemic iron levels is the liver but not only are lymphocytes (and not liver cells) able to sense toxic forms of iron, but they are also able to travel and be activated in specific places where the pathogens accumulate helping to control infection. "
These results are a major step to understand the link between the IS and iron and, if confirmed in live organisms -all this work was done on human cells in the laboratory - can be the beginning of a totally different view of what the immune system is and how to approach immunologic problems.
As Hal Drakesmith says "the paper describes several new findings which are highly likely to be of interest and importance to the iron and immunity fields of research" A simple example is the anaemia that usually accompanies chronic inflammatory diseases and that so far can not be clearly explained. Pinto and Sousa's results suggest that lymphocyte chronic activation, so characteristic of these diseases, by leading to hepcidin production could be part of the phenomenon as iron is an integral part of red blood cells.
Pinto, de Sousa and colleagues now plan to go back to those diseases of iron overload associated to immune abnormalities and see if hepcidin proves to be, in fact, the connection between them. Other possibility is the construction of mice without the hepcidin gene in the bone marrow - where lymphocytes develop - to analyse the changes that this could bring to both iron homeostasis and the immune response.
Whatever happens this is a strikingly interesting story with decades of persistence and believe behind it and which, I am sure, still has much to tell us.