NKG2D and natural cytotoxicity receptors are involved in natural killer cell interaction with self-antigen presenting cells and stromal cells.

It is thought that human natural killer (NK) lymphocytes should not damage self-tissues due to the inhibiting signal initiated by the engagement of one or another inhibitory receptor superfamily (IRS) members with self-human histocompatibility antigen (HLA)-I. During viral infection, the low expression of self-HLA-I on infected-cells leads to a reduction of the inhibiting signal and thus NK cells kill self-cells (missing self-hypothesis). Here, we have analyzed human NK cell interaction with self-cells as antigen-presenting cells (APC) or stromal cells isolated from bone marrow or skin. Despite the expression of high levels of HLA-I, APC and stromal cells are killed by interleukin (IL)-2-activated NK cells upon lymphocyte function antigen (LFA)1-(intracellular adhesion molecule) (ICAM)1 interaction. The natural cytotoxicity receptors NKp30 and NKp46 are responsible for the delivery of lethal hit to APC, whereas NKG2D-activating receptor, the ligand of the major histocompatibility complex (MHC)-related molecule MICA, and the UL16-binding protein are involved in stromal cell killing. These events are dependent on the activation of phosphoinositol 3-kinase and consequent release of perforins and granzymes. Both bone marrow stromal cells and skin fibroblasts inhibit T cell proliferation to alloantigen or triggering through CD3/T cell receptor complex. Importantly, NK cells can revert this veto effect. Altogether, these findings support the notion that NK cells can recognize self-cells possibly affecting both APC function and interaction between lymphocytes and microenvironment leading to autoreactivity.