FOXP3 and infection: Helios is a marker of tTregs,36 which, while inducible in other T cell populations,45, 46 is known to synergize with Foxp3 in silencing the IL-2 locus.47 Our data demonstrate that Helios+Foxp3+ tTregs are the predominant population that increases in proportion and proliferation in response to infection in the more resistant BALB/c mouse strain, although, as previously demonstrated, pTreg induction can also occur.12 A key question was therefore to identify whether Treg expansion following H. polygyrus infection contributed to the parasite immune response and immune pathology in BALB/c mice.