A magnetic bead-based assay (Brown et al., 2012 (link)) was used to measure antibody expression and biodistribution. Beads were conjugated to anti-human antigen-binding fragment (Fab) to capture mAbs of interest. Briefly, anti-human IgG F(ab’)2 fragment (109-006-097; Jackson Immune Research) was conjugated to fluorescent microspheres (MagPlex-C Microspheres, Luminex Corp.) at a ratio of 6.5 μg protein/100 μl microspheres. Samples were diluted as indicated in mAb expression and biodistribution figures and then incubated with microspheres (500–750 beads/well) overnight at 4°C and washed in PBS with 1% BSA, 0.05% Tween-20, and 0.1% sodium azide. Anti-human IgG PE (2040-09; Southern Biotech) was incubated at 0.65 μg/ml for 45 min in PBS-TBN. The microspheres were washed and resuspended in 90 μl of sheath fluid (Luminex) and read using a Bio-plex array reader (FlexMap 3D, OR MAGPIX). The median fluorescence intensity of the PE signal was determined for each sample at indicated dilutions. For biodistribution assessment, the signal is reported as the fold increase in PE signal in treated pups relative to untreated controls.