Meningococcal outer membrane vesicle composition-dependent activation of the innate immune response

Meningococcal outer membrane vesicles (OMVs) have been extensively investigated and successfully implemented as vaccines. They contain pathogen associated molecular patterns including lipopolysaccharide (LPS), capable of triggering innate immunity. However, Neisseria meningitidis contains an extremely potent hexa-acylated LPS, leading to adverse effects when its OMVs are applied as vaccines. To create safe OMV vaccines detergent treatment is generally used to reduce LPS content. While effective, this method also leads to loss of protective antigens such as lipoproteins. Alternatively, genetic modification of LPS can reduce its toxicity. In the present study, we have compared standard OMV isolation methods using detergent or EDTA with genetic modifications of LPS to yield a penta-acylated lipid A (lpxL1 and pagL), on the in vitro induction of innate immune responses. The use of detergent decreased both TLR4 and TLR2 activation by OMVs, while the LPS modifications only reduced TLR4 activation. Mutational removal of PorB or fHbp, two proteins known to trigger TLR2 signaling, had no effect indicating that multiple TLR2 ligands are removed by detergent treatment. Detergent treated OMV and lpxL1 OMV showed similar reduction of cytokine profiles in the human monocytic cell line MM6 and human DCs. OMVs with the alternative penta-acylated LPS structure obtained after PagL-mediated deacylation showed reduced induction of pro-inflammatory cytokines IL-6 and IL-1β but not of IP-10, a typical TRIF dependent chemokine. Taken together, these data show that lipid A modification can be used to obtain OMVs with reduced activation of innate immunity, similar to what is found after detergent treatment.