Co-stimulatory receptor-ligand pairs in the synovium. Effector T-cell populations frequently lose the expression of CD27 and CD28 and gain the expression of regulatory receptors that bind to ligands in the synovium. In general, these receptors as well as their ligands are constitutively expressed independent of cell activation. CD47 is expressed on all T cells irrespective of their differentiation status and receives a co-stimulatory signal from a trimolecular complex formation including CD36 on synoviocytes or endothelial cells and thrombospondin produced in the synovial tissue. KIR, killer immunoglobulin-like receptor; MHC, major histocompatibility complex; TCR, T-cell receptor.