Immunogenicity and humanization of single-domain antibodies

Single-domain antibodies (sdAbs), the autonomous variable domains of camelid and shark heavy-chain antibodies, have many desirable properties as components of biologic drugs. However, their sequences may increase the risk of immunogenicity and antidrug antibody (ADA) development in humans, and thus, sdAbs are routinely humanized during development. Here, we review and summarize the available evidence regarding the factors governing immunogenicity of sdAbs and our current state of knowledge of strategies to mitigate immunogenicity risks by humanization. While several sdAb properties, including high homology of camelid V_H Hs with human IGHV3 gene products, favor low immunogenicity in humans, epitopes absent in the human repertoire including the exposed V_H :V_L interface may be intrinsically immunogenic. While most clinical trials have demonstrated minimal sdAb immunogenicity, two notable exceptions (the tetrameric DR5-specific V_H H TAS266 and the TNFR1-specific V_H GSK1995057) illustrate that special caution must be taken in identifying preexisting ADAs against highly potent sdAbs. Nonhuman sequence alone does not adequately explain sdAb immunogenicity, as some camelid V_H Hs are nonimmunogenic while some fully human V_H s elicit ADAs. The presence of preexisting ADAs directed against the exposed C-termini of some sdAbs in a significant proportion of individuals awaits a molecular explanation. Whether sdAb humanization reduces or promotes immunogenicity remains unclear: reduction of nonhuman sequence content at the expense of introducing low-level aggregation in humanized variants may be counterproductive. Further work will establish thresholds for V_H H and V_NAR humanization to maximize human sequence content while avoiding loss of binding affinity and/or immunogenicity resulting from aggregation or decreased stability.