Dietary Iron Deficiency in Adult Mice Increases Brain Uptake of High-Affinity, Anti-Transferrin Receptor Antibody RI7217.

Brain capillary endothelial cells (BCECs) express transferrin receptor 1 (TfR1) to ensure sufficient iron transport into the brain. Our main objective was to examine adult mice subjected to dietary iron deficiency (ID) for possible changes in the content of TfR1 in BCECs and the influence thereof on the uptake and possible transport across the blood-brain barrier (BBB) of high-affinity, rat anti-mouse transferrin receptor IgG2a antibody (clone RI7217) targeting the TfR1. We subjected adult, female mice to dietary ID for 8 weeks. Iron and copper were measured using inductively coupled plasma mass spectrometry (ICP-MS) in various tissues, including total brain, and fractions of brain tissue separated to contain a capillary enriched fraction ("capillary fraction") and a post-capillary, non-endothelial-containing brain parenchymal fraction ("brain fraction"). Possible effects of ID on the cerebral angioarchitecture were estimated using 3D confocal microscopy of optically cleared brain samples labeled using intravenous injection of wheat germ agglutinin with subsequent machine learning-based segmentation and vascular tracing. TfR1 was quantified using ELISA. RI7217 antibodies were conjugated with 1.4 nm nanogold and brain uptake quantified using ICP-MS. ID significantly reduced the iron content in the capillary fraction, liver, spleen, kidney, heart, and skeletal muscles. ID increased the copper content in the brain. Analysis of cerebral cortical angioarchitecture revealed no changes following dietary ID, except for a minor increase in tortuosity of small-caliber vessels. Following ID, the concentration of TfR1 protein remained unchanged in total brain, and the isolated capillaries and brain fraction. In contrast, the uptake of nanogold-conjugated RI7217 was increased in total brain, the brain fraction, liver, spleen, and isolated retinae. The targeting to TfR1 in ID hence suggested increased brain uptake of RI7217. Hypothetically, elevated transport of RI7217 could occur due to increased trafficking of TfR1-containing vesicles through BCECs in ID.