Constitutive expression of ligand for natural killer cell NKp44 receptor (NKp44L) by normal human articular chondrocytes

• Isolated human chondrocytes express ligand for NK cell NKp44 stimulatory receptor.
• NKp44 ligand is constitutively expressed in normal articular cartilage.
• NKp44 ligand may account for chondrocyte destruction by activated NK cells.

Normal chondrocytes display susceptibility to lysis by natural killer (NK) cells and this phenomenon may play a role in some inflammatory cartilage disorders. The mechanisms of chondrocyte recognition and killing by NK cells remain unclear. Using flow cytometry and immunohistochemical staining we found that normal human articular chondrocytes constitutively express a ligand for NKp44, one of stimulatory NK cell receptors involved in recognition and killing of target cells. Expression of NKp44 ligand by normal articular chondrocytes is not involved in their killing by unstimulated NK cells; however, it is responsible for anti-chondrocyte cytotoxicity mediated by long-term activated NK cells. Thus, expression of NKp44 ligand may play a role in chondrocyte destruction in course of chronic inflammatory cartilage disorders.