A Role of NKR-P1A (CD161) and Lectin-like Transcript 1 in Natural Cytotoxicity against Human Articular Chondrocytes.

Normal cartilage cells are susceptible to lysis by NK cells. This phenomenon may play a role in immune cartilage destruction; however, the mechanisms of chondrocyte recognition by NK cells remain poorly understood. Therefore, the aim of this study was to reveal a possible role of NKR-P1A/lectin-like transcript 1 (LLT1) interaction in NK cell-mediated cytotoxicity against normal human articular chondrocytes. Chondrocytes were isolated from articular cartilage obtained during talonavicular joint surgery. PBMC or polyclonal NK cells isolated from normal donors served as effector cells. Cell-mediated cytotoxicity against chondrocytes was evaluated by means of 18-h Cr-release assay. Specific mRNA expression was evaluated by classical and quantitative RT-PCR, and proteins were detected by Western blot analysis. We found that lysis of articular chondrocytes by PBMC or polyclonal NK cells was potentiated by stimulation with IL-2. Stimulation of effector cells with IL-2 downregulated mRNA expression of inhibitory NKR-P1A NK cell receptor, and blocking of NKR-P1A with specific mAbs resulted in increased chondrocyte killing. Chondrocytes constitutively expressed LLT1, a ligand of NKR-P1A. LLT1 expression by chondrocytes could be upregulated by IL-1α and TNF. Chondrocyte treatment with IL-1α resulted in their increased resistance to killing by natural cytotoxic cells. This could be reversed by blocking of NKR-P1A. These results show that susceptibility of normal articular chondrocytes to lysis by NK cells is modulated by NKR-P1A/LLT1 interactions. Thus, NKR-P1A/LLT1 interaction might provide some novel target for therapeutic interventions in the course of pathological cartilage injury.