Structural Investigation of Antigens Using Electron Microscopy

In this research the phenomenon of monoclonal antibodies (mAbs) cooperative bactericidal activity and the structure of the N.meningitidis antigen NadA var.3 are investigated using Transmission Electron Microscopy (TEM). The mAbs cooperativity is a mechanism that occurs when mAbs that individually show low or no bactericidal activity become bactericidal when coupled together. Revealing the structural bases of cooperative bactericidal activity is fundamental for a thorough understanding of antibody-based mechanisms of protection. Meningococcal factor H binding protein (fHbp), surface-exposed lipoprotein presents in the vaccine against serogroup B Neisseria meningitidis, Bexsero®. A comparison of the structure of the murine cooperative complex with the human cooperative one has been performed revealing a higher level of flexibility and instability of the first complex. Moreover we have been able to prove a simultaneous binding of factor H (fH) to the immune cooperative complex. The 3D structures of the cooperative complexes demonstrated that the angle formed between fHbp-antibodies is orientated only depending on the epitope location on the antigen. Non-cooperative human complexes show a structural dissimilarities compared with the cooperative couples mainly for the absence of the complex formation due to the partial epitopes overlapping, as proved by Hydrogen/Deuterium Exchange Mass Spectrometry (HDX_MS). The NadA var. 3, the variant included in Bexsero® vaccine, structure was determined by the Cryo-Electron Microscopy (Cryo-EM) technique combined with Single Particle (SP) reconstruction method. The 3D reconstruction of NadA var.3, generated from Cryo-TEM data, shows an elongated and thin stalk decorated by a globular compact head characterized by a three-fold symmetry. Moreover the 3D EM map shows three evident points of interruption in the density of the stalk region presumably correlated with the three interruptions present in the coiled-coil periodicity of NadA var.3 the sequence. This result indicates a possible alternative mechanism of the antigen flexibility.