Autoimmune Encephalitis Biomarkers

Introduction and Clinical Utility

Autoimmune encephalitis (AE) is a critical, treatable cause of acute encephalopathy, frequently overlapping with acute encephalitis syndrome (AES).
Biomarkers are essential to confirm definite AE, differentiate from viral or metabolic encephalopathies (like PANS/PANDAS), guide targeted immunotherapy, and track therapeutic response.

Routine cytobiochemistry typically reveals mild-to-moderate lymphocytic pleocytosis (<100 cells/µL) and mildly elevated protein (<150 mg/dL) with normal glucose.
Intrathecal immunoglobulin synthesis is demonstrated by CSF-restricted oligoclonal bands (OCBs) in 50–60% of cases.
An elevated IgG Index (>0.7) strongly implies independent central nervous system antibody production.

Autoantibodies remain the diagnostic gold standard and are categorized based on the cellular location of their target antigens.

Category	Clinical Characteristics	Key Biomarkers
Cell-Surface & Synaptic Antibodies	Directly pathogenic by causing receptor internalization. Excellent response to early immunotherapy. Rarely paraneoplastic in pediatric cohorts.	- Anti-NMDAR: Most common pediatric marker; highly sensitive in CSF.- Anti-LGI1: Highly sensitive in serum.- Anti-CASPR2: Associated with Morvan syndrome.- Anti-MOG: Identifies MOG antibody-associated disease (MOGAD).
Intracellular & Onconeuronal Antibodies	Markers of T-cell-mediated cytotoxic response. Strong association with occult malignancies (paraneoplastic). Poor response to traditional immunotherapy.	- Anti-Hu (ANNA-1).- Anti-Ma2.- Anti-CRMP5.- Anti-Yo.

Seronegative or probable antibody-negative AE accounts for 30–40% of cases, necessitating the use of supportive paraclinical biomarkers.

Biomarker	Diagnostic and Prognostic Utility
CSF Neopterin	Frequently elevated in anti-NMDAR encephalitis; critically differentiates AE from PANS (where levels remain normal).
Neurofilament Light Chain (NfL)	Structural axonal protein that leaks into CSF/blood; sensitive biomarker for active neurodegeneration and long-term disease severity.
Total Tau Protein	Markedly elevated during acute, destructive phases of cortical inflammation.
CSF Osteopontin & Cytokines	Elevated IL-6, IL-10, and TNF-α serve as diagnostic markers to differentiate autoimmune from viral encephalitis.
Serum Syncytin-1	Levels correlate actively with blood-brain barrier disruption.

Paired Testing Protocol: Diagnostic validation mandates synchronous paired testing of both serum and CSF via cell-based assays (CBA) to minimize false negatives (e.g., missing NMDAR on isolated serum testing) and false positives.
International Consensus (Graus Criteria): Pediatric adaptations require paraclinical evidence of neuroinflammation (CSF pleocytosis, OCBs, elevated neopterin) for a "probable antibody-negative AE" diagnosis.
Indian Implementation: The Indian Academy of Pediatrics (IAP) and ICMR Standard Treatment Workflows mandate early CSF antibody panels alongside viral PCRs to accurately differentiate autoimmune AES from infectious AES, avoiding unnecessary empirical antimicrobial therapy.