Cutting edge: Class II transactivator-independent endothelial cell MHC class II gene activation induced by lymphocyte adhesion

NK cells induce MHC class H molecules on the surface of allogeneic endothelial cells in an adhesion-dependent, IFN-γ-independent manner. Here, we demonstrate that NK cells induce HLA-DR on the surface of a mutant cell line that is defective in IFN-γ-induced MHC class II expression. RNA analysis in these cells and in a cell line that is defective in class II transactivator (CIITA) demonstrates that NK cell-induced HLA-DRα mRNA expression is also CIITA-independent. The Janus kinase-1-deficient cell line U4A expresses HLA-DRα mRNA in response to NK cell activation, and HLA-DRα promoter constructs transfected into these cells are induced by NK cells but not IFN-γ. These data indicate that the IFN-γ-independent component of the target cell HLA-DR expression induced by lymphocyte adhesion uses a signaling pathway that is distinct from the IFN-γ-dependent mechanism and also suggest that CIITA is not required.