title: Nanobodies as tools for in vivo imaging of specific immune cell types.
abstract: Nanobodies are single-domain antigen-binding fragments derived from heavy-chain antibodies that are devoid of light chains and occur naturally in Camelidae. We have shown before that their small size and high affinity and specificity for their target antigen make Nanobodies ideal probes for in vivo tumor imaging. In the present study, we have evaluated the use of Nanobodies as a generic method for imaging the in vivo biodistribution of specific immune cell types, using myeloid cells as an example.
  
  
  METHODS
  The cellular specificity of Nanobodies raised against murine bone marrow-derived dendritic cells was verified using flow cytometry on a range of myeloid and nonmyeloid cell types. The Nanobodies were then labeled with (99m)Tc and their biodistribution was analyzed using SPECT. The biodistribution was also assessed by measuring radioactivity in various organs and tissues. To verify whether the observed biodistribution was due to specific targeting through the antigen-binding loops, rather than retention in organs because of effects of the framework regions, we genetically grafted the antigen-binding loops of the Nanobodies onto the framework region of a Nanobody scaffold that by itself showed low background retention in the periphery. The cellular specificity and biodistribution of these grafted Nanobodies were determined as before.
  
  
  RESULTS
  Nb-DC2.1, which recognizes a wide range of myeloid cells, targets most strongly to the liver, spleen, and lungs. Nb-DC1.8, which recognizes immature bone marrow-derived dendritic cells in vitro, gives a much smaller signal in the liver and spleen than does Nb-DC2.1 but mainly targets to the lungs and gives a pronounced signal in the skin. Grafting of the antigen-binding loops of Nb-DC1.8 or Nb-DC2.1 to the scaffold of Nb-BCII10 alters the observed biodistribution of the Nanobodies to resemble that of the Nanobody from which the antigen-binding loops have been derived.
  
  
  CONCLUSIONS
  The observed in vivo biodistribution of the Nanobodies reflects the main in vivo locations of the cells recognized by the Nanobodies and is determined by the antigen-binding loops of the Nanobodies. Thus, Nanobodies represent elegant targeting probes for imaging the in vivo biodistribution of specific immune cell types.
authors.affiliation: Laboratory of Cellular and Molecular Immunology, Vrije Universiteit Brussel, Brussels, Belgium.
authors.forename: Kurt
authors.initials: K
authors.lastname: De Groeve
authors.forename: Nick
authors.initials: N
authors.lastname: Deschacht
authors.forename: Celine
authors.initials: C
authors.lastname: De Koninck
authors.forename: Vicky
authors.initials: V
authors.lastname: Caveliers
authors.forename: Tony
authors.initials: T
authors.lastname: Lahoutte
authors.forename: Nick
authors.initials: N
authors.lastname: Devoogdt
authors.forename: Serge
authors.initials: S
authors.lastname: Muyldermans
authors.forename: Patrick
authors.initials: P
authors.lastname: De Baetselier
authors.forename: Geert
authors.initials: G
authors.lastname: Raes
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D001853:Bone Marrow / Q000000981:diagnostic imaging; D003713:Dendritic Cells / Q000276:immunology* / Q000648:ultrastructure*; D004926:Escherichia coli / Q000378:metabolism; D005260:Female; D005434:Flow Cytometry; D005456:Fluorescent Dyes; D005464:Fluorobenzenes; D051379:Mice; D008810:Mice, Inbred C57BL; D008969:Molecular Sequence Data; D022423:Myeloid Cells / Q000276:immunology* / Q000648:ultrastructure*; D053758:Nanoparticles; D014018:Tissue Distribution; D014055:Tomography, Emission-Computed; D015899:Tomography, Emission-Computed, Single-Photon
chemical_list: C513915:AlexaFluor 488 carboxylic acid tetrafluorophenyl ester; D005456:Fluorescent Dyes; D005464:Fluorobenzenes