To better understand immune modulation by endotoxin and facilitate the development of novel vaccine adjuvants, the structural requirements of Neisseria meningitidis lipopoly(oligo)saccharide (LOS) for activation of human monocyte derived dendritic cell (MDDC) was determined. Highly purified LOS from wild type and genetically-defined mutants of N. meningitidis serogroup B were used. Unglycosylated or penta-acylated meningococcal KDO(2)-lipid A failed to induce human MDDC maturation and activation. However, both wild type meningococcal LOS and KDO(2)-lipid A, significantly up-regulated CD80, CD83 and CD86 and released significantly higher amounts of IL-12p70, IL-6, IL-10, TNFalpha, MCP-1, IP-10 and RANTES. Further, DCs stimulated with wild type or KDO(2)-lipid A but not meningococcal lipid A or penta-acylated KDO(2)-lipid A stimulated naïve allogeneic CD4+ T cells to secrete enhanced levels of IFN-gamma, relative to T cells primed with immature DCs. In contrast to Escherichia coli LPS, IL-5 production was enhanced or maintained in CD4+ T-cells stimulated with MDDC exposed to wild-type meningococcal LOS and KDO(2)-lipid A. These data suggest that KDO linked to a fully acylated meningococcal lipid A is required for meningococcal endotoxin's immunostimulatory activity of human MDDC via TLR4/MD-2 and that different endotoxin structures influence Th responses mediated by MDDC.