PDCD1 and autoimmune disease: They found that the expression of sPD-1/sPD-L1 was upregulated in patients with autoimmune diseases compared with healthy subjects, but downregulated after treatment, suggesting that sPD-1 may resist the inhibitory effect of membrane-bound PD-1 on T cells, thereby indirectly enhancing immune responses (Wu et al., 2009; Shi et al., 2013; Liu et al., 2015; Aarslev et al., 2017; Yan et al., 2019).