The present invention relates to neisserial LPS having a hexa-acylated lipid A moiety, wherein the hexa-acylated lipid A moiety is modified as compared to the lipid A moiety of a wild-type neisserial LPS in that it comprises a palmitoleoyl instead of a lauroyl secondary acyl chain on the glucosamine at the non-reducing end of the lipid A moiety. The invention further relates to mixtures of the hexa-acylated LPS with the corresponding penta-acylated LPS, lacking a secondary acyl chain on the glucosamine at the non-reducing end of the lipid A moiety. The invention also relates to neisserial bacteria that have been genetically modified to reduce expression of the endogenous lpxL1 gene and to introduce expression of a heterologous thermosensitive lpxP gene for producing the hexa- and penta-acylated LPS. By selecting the time and/or temperature at which the bacterium is grown, it is feasible to increase or decrease the amount of hexa-acylated lipid A structure relative to the corresponding penta-acylated structure and thereby modulate the TLR4 agonist activity of the neisserial LPS of the invention, to the exact level of activity required for a particular immunotherapeutic approach.