The invention relates to a method for admixing the LPS of a gram-negative bacteria, according to which: (i) LPS or LPS liposomes (LPS formulated in liposomes) is/are mixed with the lipidated sub-unit B of the receptor of human transferrine (lipidated TbpB) of Neisseria meningitidis or a lipid fragment thereof or (ii) LPS and the lipidated TbpB of N. meningitidis or a lipid fragment thereof are formulated together in liposomes or (iii) LPS is conjugated with the TbpB of N. meningitidis or a lipid fragment thereof in order to obtain a preparation that does not contain any OMVs and can induce, following administration to a mammal, an improved anti-LPS immune response compared to the anti-LPS immune response observed following administration of the corresponding preparation in which the lipidated TbpB of N. meningitidis or a lipid fragment thereof is omitted. The invention also relates to the vaccine compositions obtained by said method. The LPS can be, for example, the LOS of nonenteric gram-negative bacteria such as N. meningitidis.
