Key Points
- The use of HuScan®’s phage display library, consisting of over 731,000+ unique peptide sequences and representing ~48,000 proteins, enabled comprehensive profiling of patient autoantibodies across an entire human proteome, facilitating the identification of autoantibodies relevant to neuroinflammatory disease.
- Insights from this screen suggested the possibility of autoimmune-mediated B12 deficiency restricted to the CNS and informed a successful targeted treatment strategy, demonstrating the potential of HuScan in guiding personalized medical interventions.
HuScan® Identifies Autoantibody Targeting the Transcobalamin Receptor
In a recent study investigating the role of autoimmunity in suspected neuroinflammatory disease, cerebral spinal fluid (CSF) from a single patient was profiled using the HuScan phage display library to screen for autoantibodies. This comprehensive phage library (731,000+ peptide sequences representing ~48,000 proteins) enables epitope-level autoantibody profiling across an entire human proteome. Through Phage ImmunoPrecipitation Sequencing (PhIP-Seq), researchers were able to identify an autoantibody targeting the transcobalamin receptor (CD320), a cell surface protein that is enriched at the blood-brain barrier and is responsible for cellular uptake of biologically active vitamin B12. Notably, while the patient’s serum levels of vitamin B12 were within normal range, levels in the CSF were almost 6-fold lower than healthy controls. Whole genome CRISPRi screening revealed an alternative pathway for uptake of B12 via the LDLR receptor, which enables hematopoietic, but not brain endothelial uptake, suggesting the possibility of B12 deficiency restricted to the CNS. Upon initiation of B12 supplementation, levels of vitamin B12 in the patient’s CSF were increased 3-fold, and after 9 months of treatment the patient reported subjective improvement in neurological symptoms.
FIGURE: Autoantibodies in the blood may interfere with vitamin B12 transport across the blood-brain barrier by targeting CD320 on endothelial cells, possibly through internalization and depletion of CD320 from the cell surface. This impairment affects the journey of holotranscobalamin (holoTC, the biologically active form of vitamin B12) as it crosses multiple cellular layers, including endothelial cells (red), pericytes (orange), and astrocytes (purple). Consequently, various cell types in the brain, particularly those with high metabolic demands like neurons and oligodendrocytes, could be affected. This potentially leads to neurological problems depending on factors such as antibody levels, duration of exposure, and individual susceptibility.
Autoimmune-mediated Vitamin B12 Deficiency Found
This study underscores the power of proteome-wide profiling of patient autoimmune response, as it revealed an autoimmune-mediated vitamin B12 deficiency that could not have been detected through routine blood work. Retrospective analysis of PhIP-seq datasets for patients with unexplained neurological symptoms revealed seven additional patients with anti-CD320 seropositivity. Additionally, patients diagnosed with neuropsychiatric systemic lupus erythematosus (NPSLE), but not non-neurologic SLE, showed significant enrichment for CD320 autoantibodies, suggesting that anti-CD320 may contribute to neurologic deficits in certain patient subsets. Interestingly, approximately 6% of healthy controls were also seropositive. This finding could represent a non-pathogenic fingerprint from previous immune interactions, but could also indicate subclinical disease or disease predisposition. As the precise role of autoimmunity in neurologic conditions continues to be revealed, HuScan® can serve as a powerful tool for advancing our understanding of autoimmune pathogenesis.
GRAPHS: Left: Serum (red) and CSF (blue) concentrations of vitamin B12 in three healthy controls compared to the symptomatic patient. Right: Serum and CSF concentrations of the patient before and after high-dose oral B12 supplementation.
REFERENCE
Pluvinage JV, Ngo T, Fouassier C, et al. "Transcobalamin receptor antibodies in autoimmune vitamin b12 central deficiency". Sci Transl Med. 2024; 16(753) doi:10.1126/scitranslmed.adl3758