Abstract: The present invention relates to neisserial LPS having a tetra-acylated lipid A moiety, wherein the tetra-acylated lipid A moiety is modified as compared to the lipid A moiety of a wild-type neisserial LPS in that it lacks one of the secondary acyl chains and lacks a primary acyl chain on the 3-position of the glucosamine at the reducing end of the lipid A moiety. The invention further relates to neisserial bacteria that have been genetically modified to reduce expression of either one of the endogenous lpxL1 or lpxL2 genes and to introduce expression of a heterologous pagL gene. The neisserial LPS of the invention has TLR4 agonist properties and is therefore useful in compositions for inducing or stimulating immune responses, such as vaccines, as well as in other forms of immunotherapy.