Circumvention of common labeling artifacts using secondary nanobodies

The most common procedure to reveal the location of specific (sub)cellular elements in biological samples is via immunostaining followed by optical imaging. This is typically performed with target-specific primary antibodies (1.Abs), which are revealed by fluorophore-conjugated secondary antibodies (2.Abs). However, at high resolution this methodology can induce a series of artifacts due to the large size of antibodies, their bivalency, and their polyclonality. Here we use STED and DNA-PAINT super-resolution microscopy or light sheet microscopy on cleared tissue to show how monovalent secondary reagents based on camelid single-domain antibodies (nanobodies; 2.Nbs) attenuate these artifacts. We demonstrate that monovalent 2.Nbs have four additional advantages: 1) they increase localization accuracy with respect to 2.Abs; 2) they allow direct pre-mixing with 1.Abs before staining, reducing experimental time, and enabling the use of multiple 1.Abs from the same species; 3) they penetrate thick tissues efficiently; and 4) they avoid the artificial clustering seen with 2.Abs both in live and in poorly fixed samples. Altogether, this suggests that 2.Nbs are a valuable alternative to 2.Abs, especially when super-resolution imaging or staining of thick tissue samples are involved.