To examine CTLA-4 expression in vivo, a single dose of Surek was injected into mice along with irradiated B78-D14 cells. In this setting, a significant upregulation of CTLA-4 was observed after 4 days (Figure 2A). The frequency of FoxP3+ CD4+ regulatory T cells (Tregs) was also markedly increased at this time point (Figure 2B). As shown by Ki67 staining, this was likely due to enhanced Treg proliferation (Figure 2C). Interestingly, CTLA-4 upregulation on the CD4+ T cells was mainly restricted to the FoxP3+ population (Figure 2D). Since CTLA-4 mediates suppressive functions of Tregs, the data supported the concept of blocking this molecule during trAb therapy.