NK lymphocytes adhere avidly to allogeneic endothelial cells (ECs) and induce their membrane expression of MHC class II Ags in vitro. Endothelial class II expression augments EC-driven CD4+ T cell proliferation in vitro, and may amplify T cell recruitment and clonal expansion in vivo. Using an ex vivo lymphocyte-skin organ coculture model, NK cells could be found lining and inducing class II HLA on microvessel endothelium. Using neutralizing anti-IFN-gamma and anti-IFN-gamma receptor Abs, a spectrum of IFN-gamma dependence was observed for NK-mediated EC HLA-DR induction at the membrane and transcriptional levels, from negligible to moderate. Trans-well experiments displayed that direct NK-EC contact is required, and Ab inhibition studies indicated that the beta 2 integrin-ICAM-1 pathway(s) is critical in the generation of these responses. The use of HLA-DR alpha promoter constructs in transient transfection assays demonstrated that the highly conserved X and S transcription boxes are required in both IFN-gamma- and NK-mediated gene activation. As expected, because of the receptor species specificity, human IFN-gamma did not induce HLA-DR alpha promoter constructs transfected in Chinese hamster ovary cells, whereas NK cells did. Taken together, these results indicate that human allogeneic NK lymphocytes induce EC class II HLA gene activation and membrane expression in an adhesion-dependent, IFN-gamma-independent fashion and suggest that, in concert with any IFN-gamma-dependent component, this induction could represent an efficient mode of endothelial activation and immune amplification in vivo.