From immunogenetical studies we learnt that Behçet's disease (BD) is associated with HLA-B*51, and to a lesser extent with HLA-B*2702. There may even be a role of MICA genes in the pathogenesis of BD. Various of these MHC class I molecules can be recognized by NK receptors on NK and NK-T-cells independently from peptides. These NK cell receptors may be activating (DS) or inhibitory (DL). Interestingly, increased percentages of CD4⁺CD16⁺ and CD4⁺CD56⁺ T-cell subsets have already been described in BD patients.

In patients with rheumatoid arthritis and ankylosing spondylitis, unusual proinflammatory and cytotoxic CD4⁺ T cells marked by the lack of the costimulatory molecule CD28 express stimulatory NK cell receptors on their surface. In rheumatoid arthritis, MHC class I recognizing NK receptors are even considered as disease risk genes. In CD4⁺CD28^- T cells from patients with ankylosing spondylitis we recently showed functional NK cell features and an enrichment of these cells in the CD4⁺CD25⁺ T-cell compartment by costimulation with HLA-B27 transfected cells.

We hypothesize that CD4⁺CD28^- T-cells as markers of a chronic inflammatory process are also elevated in BD patients, express MHC class I recognizing NK receptors, and thus recognize HLA-B*51 via NK receptors. This mechanism could explain the chronicity of BD as an MHC class I associated disease.