A 62-year-old woman with small-cell lung carcinoma develops limbic encephalitis with anti-Hu antibodies. The anti-Hu (ANNA-1) target antigen is a nuclear RNA-binding protein. Which mechanism best explains why antibody-positive patients do not respond well to immunotherapy and have poor prognosis?

• A The primary injury is cytotoxic T-cell mediated, not antibody mediated
• B Anti-Hu antibodies block the BBB-transport system preventing immunoglobulin entry
• C Anti-Hu antibodies promote tumor growth by activating HuD on tumor cells
• D Intracellular target location renders antibodies non-pathogenic and T-cells cause irreversible neuronal apoptosis ✓

Explanation

Anti-Hu targets an intracellular nuclear protein (HuD), so the antibodies themselves are not directly pathogenic — they serve as biomarkers. The actual neuronal injury is mediated by cytotoxic CD8+ T-cells that recognize the same antigen on neurons; this results in irreversible apoptosis, explaining why immunotherapy is largely ineffective. This contrasts with antibodies against cell-surface targets (e.g., anti-NMDAR) where antibody-mediated therapies can be effective.

Reference: Robbins & Cotran Pathologic Basis of Disease, 10th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.