Allogeneic environment required for Treg proliferation and localization. Adoptive transfer of FVB luc⁺ Tregs with or without cotransfer of Tcons at a 1:1 cell ratio (A) or Tcons (B) into lethally-irradiated allogeneic Balb/c or syngeneic FVB hosts. Representative bioluminescent images that indicate changes in trafficking or signal intensity up to 4 weeks following transplantation are shown. Treg proliferation and localization were not observed or detected in syngeneic hosts. No significant differences in homing or signal intensity were noted for Treg proliferation in the presence or absence of Tcons. Delayed and moderate expansion of Tcons in lymph nodes was detected in syngeneic hosts. (C) Rag2^−/−γC^−/− unirradiated hosts received allogeneic luc⁺ Tregs or Tcons at transplantation. Images of a single animal are shown, which is representative of at least 3 animals per group per experiment. All experiments were performed at least twice. Color bars represent signal intensity scale over whole body: 3000 to 30 000.