Anti-CD320 Autoantibodies Unveil a Novel Mechanism of CNS Vitamin B12 Deficiency in Idiopathic Myelopathy

Highlight

This study identifies anti-CD320 autoantibodies as a novel biomarker in idiopathic myelopathy (IM), linked to central nervous system (CNS) vitamin B12 deficiency. The presence of these autoantibodies corresponds with distinct clinical features, a unique MRI pattern, and metabolic evidence of impaired vitamin B12 transport. Vitamin B12 supplementation with or without immunosuppression resulted in clinical improvement in most treated patients, suggesting potential therapeutic avenues.

Study Background

Myelopathies, or disorders involving the spinal cord, present a significant clinical challenge due to their disabling nature and diverse etiologies. Despite advances in diagnostics, 12% to 18% of cases remain idiopathic, limiting targeted interventions. Vitamin B12 deficiency is a known reversible cause of myelopathy, but classic peripheral deficiency markers often fail to capture CNS-specific transport abnormalities. Transcobalamin receptor CD320 facilitates vitamin B12 cellular uptake in the CNS, yet immune-mediated disruption of this pathway has remained unexplored as a cause of idiopathic myelopathy.

Study Design

This retrospective, multi-center, case-control study spanning May 2014 to October 2025 examined biofluids from 148 patients with idiopathic myelopathy (IM) drawn from four tertiary care centers. Control groups consisted of patients with other neurological diseases (ONDs; n=32) and autoimmune myelitis with known etiology (n=30). Utilizing proteome-wide phage display, researchers discovered autoantibodies against the CD320 receptor. Subsequent immunoassays validated anti-CD320 presence, and cerebrospinal fluid (CSF) analyses quantified bioactive vitamin B12 and metabolites to assess CNS vitamin B12 status. Clinical data, CSF profiles, and spinal MRI scans were analyzed to characterize the cohort with positive anti-CD320 antibodies.

Key Findings

Anti-CD320 autoantibodies were detected in 18 of 32 discovery cohort patients (56%) with idiopathic myelopathy, representing a substantial subset. Notably, these patients had significantly reduced CSF bioactive vitamin B12 levels compared to neurological controls (mean 15.1 vs 22.9 pmol/L, P=0.03), indicative of autoimmune central vitamin B12 deficiency despite normal systemic vitamin B12 status. Clinical correlations revealed that anti-CD320-positive patients more frequently exhibited a subacute disease course (56% vs 7%, P=0.008), a predominantly normal CSF profile without significant pleocytosis or elevated protein (83% vs 50%, P=0.04), and a distinctive dorsolateral spinal cord lesion pattern on MRI (61% vs 7%, P=0.003).

Two independent validation cohorts confirmed these findings with anti-CD320 positivity detected in 45% (41/91) and 48% (12/25) of IM patients, respectively. While anti-CD320 antibodies were less commonly found in patients with known autoimmune myelitis, their occasional presence suggests a possible overlap or comorbid autoimmune process. Treatment with vitamin B12 supplementation, alone or combined with immunosuppression, was administered to five patients with anti-CD320 positivity; four demonstrated clinical improvement, underscoring a potential treatment responsiveness in this subgroup.

Expert Commentary

The identification of anti-CD320 autoantibodies establishes a mechanistic link between autoimmunity and CNS-restricted vitamin B12 deficiency in idiopathic myelopathy. This expands the conceptual understanding beyond systemic vitamin B12 deficiency by highlighting a targeted autoimmune disruption of vitamin B12 transport into CNS cells. Clinically, these findings emphasize the importance of incorporating autoimmune serology and CSF metabolic assessment alongside conventional workup when evaluating unexplained myelopathy. Moreover, the MRI dorsolateral lesion pattern may serve as a radiographic hallmark aiding diagnosis.

Limitations include the retrospective design and relatively small treatment cohort, which necessitate prospective longitudinal studies to clarify the natural history, confirm causality, and optimize therapeutic strategies targeting both autoimmunity and metabolic correction. The possibility of anti-CD320 antibodies overlapping with other autoimmune myelopathies also warrants further investigation.

Conclusion

Autoimmune-mediated CNS vitamin B12 deficiency, identified via anti-CD320 autoantibodies, represents a novel and potentially treatable cause of idiopathic myelopathy. Screening for these antibodies followed by metabolic confirmation presents a promising diagnostic paradigm. Recognizing this entity may lead to targeted supplementation and immunomodulatory therapies, improving outcomes for patients with previously unexplained spinal cord dysfunction.

Funding and Trial Registration

The study was conducted by a consortium of institutions including UCSF Neurohospitalist Division and others. Detailed funding sources and clinical trial registration information were not specified in the publication.

References

1. Pluvinage JV, Acero-Garces D, Greco G, et al. Anti-CD320 Autoantibodies and Central Nervous System Vitamin B12 Deficiency in Idiopathic Myelopathy. JAMA Neurology. 2026 Aug 17. PMID: 42606839.

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4. Lindenbaum J, Healton EB, Savage DG, et al. Neuropsychiatric disorders caused by cobalamin deficiency in the absence of anemia or macrocytosis. N Engl J Med. 1988;318(26):1720-1728.