Keeping score of antigraft T cells

Transplanted organs function well in their new home only if they are left alone by the host's immune system. Now, Codarri et al. (page 1533) have potentially found a way to monitor the health of the new replacements—they identify the T cells that harass grafts in transplant patients.

Unlike grafts that are rejected immediately, those that fail gradually are thought to be rejected in part by CD4⁺ T cells. But activated CD4⁺ T cells are difficult to distinguish from the regulatory T (T reg) cells that suppress rejection, as both express CD4 and CD25. Only the T reg cells also express the transcription factor FOXP3, but their sparse numbers are difficult to follow in vivo. FOXP3⁺ cells express much less IL-7Rα than do other CD4⁺ T cells....