PDCD1 and systemic sclerosis: In this study, the level of CTLA-4 was lower in the CD4+CD25+FoxP3+ cells of SSc patients compared with the cells of healthy controls (p = 0.034), but not the levels of CD69 (p = 0.104), PD-1 (p = 0.268), and GITR (p = 0.296; Figure 1B).