Hamers-Casterman et al (1989) have demonstrated that, unexpectedly in camelidae, about 50 % of IgG were antibodies devoid of light chain. These antibodies interact with the antigen by the virtue of only one single variable domain, referred to as VHH. Despite the absence of light chain, these single chain antibodies exhibit a broad antigen-binding repertoire by enlarging their hypervariable regions.
Phage display technology enables the selection from repertoires of VHH displayed on the surface of filamentous bacteriophage. Llamas or Alpacas are immunized with antigens provided by the users; once the serum shows an adequate immune response, blood lymphocytes are purified and the VHH genes population is isolated from lymphocytes then converted to phage-display format using PCR. Subsequent transformations usually yield libraries of 107 to 109 clones, each clone corresponding to a specific VHH. The library is panned against the antigen to select specific VHH. These VHH are then expressed in Escherichia coli and their characteristics are analyzed (purity, affinity, specificity).
VHHs are complimentary of mAbs in the sense that we demonstrated they recognize non-conventional epitopes, they cross the Blood-Brain Barrier and they bind intracellular antigens. VHHs are useful either for brain imaging, viral neutralization and/or for cristallization studies.
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