The adhesin NadA favors cell adhesion/invasion by hypervirulent Neisseria 
meningitidis B (MenB). Its recombinant form NadA(Δ351-405,) devoid of the outer 
membrane domain, is an immunogenic candidate for an anti-MenB vaccine able to 
stimulate monocytes, macrophages and dendritic cells. In this study we 
investigated the molecular mechanism of NadA(Δ351-405) cellular effects in 
monocytes. We show that NadA(Δ351-405) (against which we obtained polyclonal 
antibodies in rabbits), binds to hsp90, but not to other extracellular 
homologous heat shock proteins grp94 and hsp70, in vitro and on the surface of 
monocytes, in a temperature dependent way. Pre-incubation of monocytes with the 
MenB soluble adhesin interfered with the binding of anti-hsp90 and anti-hsp70 
antibodies to hsp90 and hsp70 at 37°C, a condition in which specific 
cell-binding occurs, but not at 0°C, a condition in which specific cell-binding 
is very diminished. Conversely, pre-incubation of monocytes with anti-hsp90 and 
anti-hsp70 antibodies did not affected NadA(Δ351-405) cell binding in any 
temperature condition, indicating that it associates to another receptor on 
their plasma membrane and then laterally diffuses to encounter hsp90. 
Consistently, polymixin B interfered with NadA(Δ351-405) /hsp90 association, 
abrogated the decrease of anti-hsp90 antibodies binding to the cell surface due 
to NadA(Δ351-405) and inhibited adhesin-induced cytokine/chemokine secretion 
without affecting monocyte-adhesin binding. Co-stimulation of monocytes with 
anti-hsp90 antibodies and NadA(Δ351-405) determined a stronger but polymixin B 
insensitive cell activation. This indicated that the formation of a recombinant 
NadA/hsp90/hsp70 complex, although essential for full monocyte stimulation, can 
be replaced by anti-hsp90 antibody/hsp90 binding. Finally, the activation of 
monocytes by NadA(Δ351-405) alone or in the presence of anti-hsp90 antibodies 
were both inhibited by neutralizing anti-TLR4 antibodies, but not by anti-TLR2 
antibodies. We propose that hsp90-dependent recruitment into an hsp90/hsp70/TLR4 
transducing signal complex is necessary for the immune-stimulating activity of 
NadA(Δ351-405) anti-MenB vaccine candidate.