Alongside the expected upregulation of ICOS by CD4+Foxp3− Teff cells during both infections (Fig. 1A–C), CD4+Foxp3+ Treg cells showed increased expression of ICOS over the first 4 weeks of H. polygyrus infection (Fig. 1D) and during the acute egg phase (weeks 6–8) of S. mansoni infection (Fig. 1E). Here, ICOS is linked to infection.