The subject of the invention is a method for adjuvanting LPS of a Gram-negative bacterium, according to which LPS or LPS liposomes (LPS formulated in liposomes) is (are) mixed with the lipidated human-transferrin receptor subunit B (TbpB protein) of Neisseria meningitidis or a lipidated fragment thereof; or (ii) LPS and the lipidated TbpB of N. meningitidis or a lipidated fragment thereof are formulated together in liposomes; or (iii) LPS is conjugated with the lipidated TbpB of N. meningitidis or a lipidated fragment thereof; in order to obtain a preparation which does not contain OMVs and which is capable of inducing, after administration to a mammal, an anti-LPS immune response which is improved by comparison with the anti-LPS immune response observed after administration of the corresponding preparation in which the lipidated TbpB of N. meningitidis or a lipidated fragment thereof is omitted; as well as vaccine compositions thereof. The LPS may, for example, be the LOS of a non-enteric Gram-negative bacterium such as N. meningitidis.