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, 2000). However, a more recent study questioned the requirement for Treg activation: Szymczak-Workman et al. (2009) showed that TCR-transgenic Tregs were able to suppress Tcons with different antigen specificity in the absence of the Treg-cognate antigen. Differences in the type of APCs used in the assays, the transgenic system, or "pre-activation" by the cell purification procedure might provide