Affinity is an important determinant of the anti-trypanosome activity of nanobodies

Caljon, Guy; Stijlemans, Benoit; Saerens, Dirk; Van Den Abbeele, Jan; Muyldermans, Serge; Magez, Stefan; De Baetselier, Patrick

doi:10.1371/JOURNAL.PNTD.0001902

Title

Affinity is an important determinant of the anti-trypanosome activity of nanobodies

Author

Caljon, Guy

Stijlemans, Benoit

Saerens, Dirk

Van Den Abbeele, Jan

Muyldermans, Serge

Magez, Stefan

De Baetselier, Patrick

Abstract

Background: The discovery of Nanobodies (Nbs) with a direct toxic activity against African trypanosomes is a recent advancement towards a new strategy against these extracellular parasites. The anti-trypanosomal activity relies on perturbing the highly active recycling of the Variant-specific Surface Glycoprotein (VSG) that occurs in the parasite's flagellar pocket. Methodology/Principal Findings: Here we expand the existing panel of Nbs with anti-Trypanosoma brucei potential and identify four categories based on their epitope specificity. We modified the binding properties of previously identified Nanobodies Nb_An05 and Nb_An33 by site-directed mutagenesis in the paratope and found this to strongly affect trypanotoxicity despite retention of antigen-targeting properties. Affinity measurements for all identified anti-trypanosomal Nbs reveal a strong correlation between trypanotoxicity and affinity (KD), suggesting that it is a crucial determinant for this activity. Half maximal effective (50%) affinity of 57 nM was calculated from the non-linear dose-response curves. In line with these observations, Nb humanizing mutations only preserved the trypanotoxic activity if the KD remained unaffected. Conclusions/Significance: This study reveals that the binding properties of Nanobodies need to be compatible with achieving an occupancy of >95% saturation of the parasite surface VSG in order to exert an anti-trypanosomal activity. As such, Nb-based approaches directed against the VSG target would require binding to an accessible, conserved epitope with high affinity.

Language

English

Source (journal)

PLoS neglected tropical diseases

Publication

2012

ISSN

1935-2727

1935-2735

DOI

10.1371/JOURNAL.PNTD.0001902

Volume/pages