In a recent post I wrote about some ideas of Kendall Smith and his role in discovering the cytokine IL-2. On 8th December I heard him give a talk in which he presented various ideas about IL-2, its receptor and Tregs. I discussed some aspects of IL-2 in the last post. When mutant mice are engineered which cannot produce IL-2 they show a strange combination of symptoms which combine immunodeficiency (a reduced capability of the immune system to fight pathogens) and autoimmune disease (an inappropriate reaction of the immune system to host tissues). This probably has to do with the fact that IL-2 is important for the production of both effector T cells and Tregs, which act in opposite directions. Similar phenomena are seen in the disease of humans called IPEX (immune dysregulation, polyendocrinopathy, enteropathy, X-linked syndrome). It is often attributed to a lack of the transcription factor Foxp3 which is of central importance for the function of Tregs. The gene for Foxp3 is on the X chromosome and this explains the way IPEX is inherited and the term X-linked in its name. However, as pointed out by Smith in his talk, one third of patients diagnosed with IPEX have no mutation in the Foxp3 gene. In this context he referred to a paper of Caudy et al. (J. Allergy Clin. Immunol. 119, 482). What is shown in this paper is that there is a different possible cause of IPEX-like symptoms, namely mutations in the gene for CD25, a surface molecule associated to Tregs.
The paper concerns a patient (an eight year old boy) who had suffered a horrific combination of diseases. It was found that he had mutations in both copies of the CD25 gene. The mutation in one copy came from the mother and was a frame shift due to an insertion. In other words, there is a extra base in the DNA which makes the part of the gene after it look like nonsense when it is being transcribed. The mutation in the other copy came from the father and consisted of one base being exchanged. This happed to cause a stop codon so that reading stopped at that point. The combination of these circumstances meant that the boy could not produce CD25 and this was the presumed cause of his disease. His Foxp3 gene was normal. On the other hand other IPEX patients can produce CD25. Thus there appear to be two diseases with related symptoms. The gene coding for CD25 is on chromosome 10, not the X chromosome. This is why two mutations are necessary to produce CD25 deficiency.
What is the connection to IL-2? The IL-2 receptor, which was also discovered by Kendall Smith and his collaborators, consists of three chains called , and . The second and third are always present on the surface of T cells but the first is only present in variable amounts. In fact the chain of the IL-2 receptor is nothing other than CD25. The and chains together allow for some IL-2 signalling but strong signalling in response to normal concentrations of IL-2 is only possible with the help of the chain. In this case it is not only the case that the receptor signals when IL-2 is bound to it. Binding also causes the receptor to be taken into the interior of the cell and destroyed. This process is an important part of the dynamics associated to IL-2. The chain of the IL-2 receptor also forms part of the receptor for many other cytokines, for instance IL-4. The gene for this receptor is on the X chromosome. When it cannot be produced due to a mutation this leads to a disease called X-linked severe combined immunodeficiency (SCID). In this case the immune system does not function since so much of its signalling system has been disrupted. This is also known as the ‘bubble boy disease’ since children affected by it have to live in a sterile environment.