Gangliosides shed by tumor cells exert potent inhibitory effects on cellular immune responses. Here we have studied ganglioside inhibition of APC function. When human monocytes were preincubated in 50 micro M highly purified ganglioside G(D1a), pulsed with tetanus toxoid (TT), and washed, the expected Ag-induced proliferative response of autologous normal T cells added to these monocytes was inhibited by 81%. Strikingly, there was also almost complete (92%) and selective inhibition of the up-regulation of the monocyte costimulatory molecule CD80, while I-CAM-1, LFA-3, HLA-DR, and CD86 expression were unaffected. Purified LPS-stimulated monocytes that had been preincubated in G(D1a) likewise showed inhibition of CD80 up-regulation (59%) as well as down-regulation of CD40 (54%) and impaired release of IL-12 and TNF-alpha (reduced by 59 and 51%). G(D1a)-preincubated human dendritic cells (DC) were also affected. They had reduced constitutive expression of CD40 (33%) and CD80 (61%), but not CD86, and marked inhibition of release of IL-6 (72%), IL-12 (70%), and TNF-alpha (46%). Even when pulsed with TT, these ganglioside-preincubated DC remained deficient in costimulatory molecule expression and cytokine secretion and were unable to induce a normal T cell proliferative response to TT. Finally, significant inhibition of nuclear localization of NF-kappaB proteins in activated DC suggests that disruption of NF-kappaB activation may be one mechanism contributing to ganglioside interference with APC expression of costimulatory molecules and cytokine secretion, which, in turn, may diminish antitumor immune responses.