PDCD1 and Arthritis: Our findings revealed that compared to the mice receiving CD4+PD-1−T cells adoptive transfer, CIA mice subjected to adoptive transfer of CD4+PD-1+T cells increased the severity of arthritis, indicated by a higher arthritis score, substantial enhancement of synovial cell infiltration, pronounced cartilage damage, and enhanced expression of IL-1β and IL-6 in the synovium (Fig. 2G).