Understanding and exploiting lipoprotein translocation in Gram negative pathogens for improved vaccine antigen delivery platforms

Lipoproteins of pathogenic Gram-negative bacteria are involved in different biological processes due to their highly immunogenic nature; and have proven to be good vaccine antigens. The lipoprotein translocation machinery of model organisms such as Escherichia coli is well characterized but unlikely E. coli, N. meningitidis displays many lipoproteins on its surface and recently an additional translocation component, Surface lipoprotein assembly modulators, involved in the surface exposure of specific N. meningitidis lipoproteins, has been identified. Several aspects were investigated: the sorting signals for Neisserial lipoproteins localization with a particular focus on the outer membrane crossing and surface localizations and the sorting differences of N. meningitidis with respect to the E. coli model were pointed out. The role of Slam1 for lipoproteins surface exposure was better characterized. Our results described a new role for Slam1 in outer membrane translocation of NHBA (Neisserial-Heparin-Binding-Antigen) and Mip (Macrophage-infectivity-potentiator). fHbp as Slam1 substrate was confirmed but also a new role for Slam1 in fHbp expression stabilization was elucidated. Surface expression of N. meningitidis lipoproteins in the heterologous E. coli background was attempted, by taking advantage of Slam1 component which we demonstrate to be necessary to actively translocate NHBA and fHbp on bacterial surface. Finally, the immunogenicity of OMVs with different lipoproteins surface display modulated by Slam1 expression was evaluated in homologous or heterologous systems. Slam1 is able to drive bactericidal activity in N. meningitidis OMVs through modulating lipoprotein surface localization and furthermore other Slam-dependent bactericidal surface lipoproteins, different from fHbp and NHBA, might be present in N. meningitidis. We demonstrated the importance of the co-expression of surface lipoproteins with Slam1 for the correct display of antigens and we showed that, depending on the presence of Slam1, heterologous E. coli OMVs, enriched with N. meningitidis SLPs, elicit different antibody responses.