T cells carrying the surface molecule CD4 are responsible for directing the behaviour of other immune cells. The Th1 and Th2 cells within this class activate the immune response. Inhibitory influences on the immune response come from the regulatory T cells, also known as Treg cells. These carry CD4. It has been recognized that a molecule called Foxp3 is a master regulator of their activity. Foxp3 is a transcription factor, not a surface molecule. Thus it is not so helpful for identifying these cells experimentally. Yesterday I heard a talk by Shohei Hori from Yokohama on research he and others have done recently on Treg cells. He started by talking about cell differentiation and discussing the issue of whether this always goes in one direction. He recalled that this picture is too simple and that it is known that cells can be induced by artificial means to go backwards in the usual sequence of differentiation or ‘sideways’ onto another branch of the tree of differentiation. A question which arises in this context is whether Treg cells can turn into Th1 cells. This is of medical importance for the following reason. Treg cells might be used for the therapy of autoimmune diseases. If, however, Treg cells given to a patient could turn into Th1 cells this could aggravate the disease. It has been reported that in experimental settings this kind of transformation can take place.
The experimental findings appear contradictory, at least superficially, and a central theme of the talk was how to resolve this conflict. A key idea proposed is that Treg cells are a heterogeneous population. In this view the majority of Treg cells are stable and cannot turn into Th1 cells. At the same time there is a small population called transient Foxp3+ cells which can undergo this kind of transformation. What are the differences between these types? Apparently there is a difference on an epigenetic level with a certain DNA sequence being methylated or not. The ordinary Foxp3+ cells show a high level of expression of the surface molecule CD25 while the transient Foxp3+ cells do not. There remains the question of how the experimental finding of transformation of many Foxp3+ T cells can be reconciled with the relative rarity of the transient Foxp3+ cells. A mechanism for doing this is that under suitable conditions the transient Foxp3+ T cells proliferate in the periphery while the other Foxp3+ cells do not. Once they have proliferated they can transform in relatively large numbers. An open question is how these two types of cell diverge during their development . It may be that this happens before they express Foxp3.