The numbers of Foxp3+ Treg cells in the MLN and spleen during H. polygyrus and S. mansoni infections were greatly reduced in ICOS−/− mice, showing that the expansion of Foxp3+ Treg cells in these settings depends upon ICOS co-stimulation; similar to previously described requirements for ICOS in Foxp3+ Treg-cell function in models of tolerance and autoimmunity 18,19,21. Here, FOXP3 is linked to Autoimmunity.