T cells in helminth infection: the regulators and the regulated.

Helminth parasites survive through a combination of parasite longevity, repeated re-infection and selective immune suppression to prevent protective Th2 responses. To counteract helminth-induced immunosuppression, and to induce long-term immunological memory, understanding of the multiple regulatory pathways within the T cell compartment is needed. Extrinsic inhibition by regulatory T cells is a key element of Th2 suppression. In addition, Th2 cells in chronic regulatory environments become functionally impaired, indicating cell-intrinsic regulation, which compromises protective Th2 memory. We discuss these pathways and consider the potential for reversing unresponsiveness through stimulatory signals or replacement by new responder populations. Future vaccine or therapeutic strategies should aim to minimize extrinsic regulatory effects and simultaneously negate Th2 anergy to drive effector responses into a long-term functionally competent state.