PRL and osteoarthritis: Expression of the prolactin receptor (PRLR) is higher in synovial tissue from RA and PsA patients than in synovial tissue from osteoarthritis (OA) patients, and prolactin (PRL) cooperates with other pro-inflammatory stimuli such as CD40L and TNF to activate macrophages by increasing the expression of pro-inflammatory cytokines including IL-6, IL-8 and IL-12β (127).