Membrane-associated lymphotoxin on natural killer cells activates endothelial cells via an NF-kappaB-dependent pathway. 
BACKGROUND: Inhibition of complement in small animal models of xenotransplantation has demonstrated graft infiltration with natural killer (NK) cells and monocytes associated with endothelial cell (EC) activation. We have previously demonstrated that human NK cells activate porcine EC in vitro, which results in adhesion molecule expression and cytokine secretion. In this study, we used the NK cell line NK92 to define the molecular and cellular basis of NK cell-mediated EC activation. METHODS: EC were transfected with either reporter constructs containing the luciferase gene driven either by E-selectin or interleukin (IL)-8 promoters or a synthetic NF-kappaB-dependent promoter. In addition, a dominant-negative mutant tumor necrosis factor receptor I (TNFRI) expression vector was co-transfected in inhibition studies. Forty-eight hours after transfection, EC were stimulated with NK cells or NK cell membrane extracts for 7 hr and activation was measured by a luciferase assay. RESULTS: Co-culture of NK cells with transfected EC enhanced E-selectin, IL-8, and NF-kappaB-dependent promoter activity. NK cell membrane extracts retained the capacity to activate EC and induced nuclear translocation of NF-kappaB (p50 and p65). Western blotting of NK cell and membrane extracts detected the presence of Lymphotoxin-alpha (LTalpha) but not tumor necrosis factor-alpha. Furthermore, LTalpha was secreted in NK:EC co-cultures. Co-transfection with dominant-negative mutant TNFRI inhibited EC activation by NK cell membrane extracts and by NK cells by 80% and 47%, respectively. The same pattern of inhibition was observed using anti-human LT sera. CONCLUSIONS: Human NK cell membrane-bound LT signals across species via TNFRI, leading to NF-kappaB nuclear translocation and transcription of E-selectin and IL-8, which results in EC activation. The discrepancy in the degree of inhibition by membrane extracts and NK cells with mutant TNFRI suggests that additional pathways are utilized by NK cells to activate EC.