Cytotoxic T lymphocytes recognize and lyse chondrocytes under inflammatory, but not non-inflammatory conditions.

The human major histocompatibility complex (MHC) class I allele HLA-B27 is strongly associated with seronegative spondyloarthropathies including ankylosing spondylitis and reactive arthritis. Although of unknown aetiology, one hypothesis suggests that a cytotoxic T cell (CTL) response against a self-antigen at sites of inflammation, such as entheses or joints may be involved. The chondrocyte is one of the major specialized cell types found both in articular cartilage and cartilaginous entheses and therefore is a possible source of such an antigen. CTL recognition of these cells is a potential mechanism for inflammation and cartilage damage, both through direct lysis of chondrocytes and the secretion of pro-inflammatory cytokines such as tumour necrosis factor and interferon-gamma (IFN-gamma). We test the feasibility of this hypothesis by examining the ability of chondrocytes to present antigen to CTL in vitro. Chondrocytes isolated from the ribcages of mice did not constitutively express detectable levels of MHC class I by fluorescence-activated cell sorting analysis. In addition, they were resistant to lysis by alloreactive and influenza A virus nucleoprotein (NP)-specific CTL. However, treatment of chondrocytes with IFN-gamma up-regulated MHC class I expression and rendered the cells susceptible to lysis by CTL. Similarly, IFN-gamma-treated chondrocytes infected with influenza A virus were recognized by NP-specific CTL, though with variable efficiency. Thus, we suggest that under certain circumstances CTL-mediated lysis of chondrocytes is potentially a potent mechanism for cartilage damage in vivo, but that low levels of MHC class I on healthy chondrocytes protects from immune recognition in health.