PDCD1 and rheumatoid arthritis: Reduced expression of VSTM1 and increased levels of its soluble form in autoimmune disease are not unique to VSTM1; other inhibitory receptors also exhibit similar changes, such as decreased expression of programmed cell death protein 1 (PD‐1) and increased soluble forms of leukocyte‐associated immunoglobulin‐like receptor 1 (LAIR‐1) in SLE or RA [42, 52].