Polysorbate 20-Mediated Transcytosis as the Initiator of Engerix-B Demyelination: Challenging the Molecular Mimicry Paradigm

Abstract Background An epidemiologically robust signal linking Engerix-B (GlaxoSmithKline) hepatitis B vaccination to multiple sclerosis (MS) and a spectrum of demyelinating disorders has been documented in France and independently replicated in the United Kingdom. The signal is vaccine-specific: it is absent for other recombinant hepatitis B vaccines administered in the same populations and sharing the same antigenic component (hepatitis B surface antigen, HBsAg) and aluminium-based adjuvant. The dominant mechanistic hypothesis — molecular mimicry between HBsAg epitopes and myelin components — fails to explain this antigen-matched selectivity and has not achieved experimental confirmation as an initiating event. Objective To identify the pharmacologically active differential component of Engerix-B responsible for the observed demyelinating signal, to propose a mechanistic framework sufficient to account for the full phenotypic spectrum of documented neurological adverse events, and to advance a paradigm-level critique of molecular mimicry as the initiating mechanism in organ-specific autoimmunity. Methods Intraclass natural experiment comparing Engerix-B, GenHevac B, and HB-VAX DNA across shared and differential compositional elements. Systematic analysis of regulatory documents (Summary of Product Characteristics, EMA Article 30 referral, Australian Product Information). Integration with the two-level excipient-mediated autoimmunity framework developed in prior publications by the same authors. Results / Argument Polysorbate 20 (PS20), confirmed in all presentations of Engerix-B by the EMA Article 30 harmonised SmPC (2000) and by the Australian Product Information (GSK, April 2026), is the sole compositional variable distinguishing Engerix-B from antigen-matched comparators. Thiomersal, co-formulated with PS20 in the historical formulation, is mechanistically excluded because ethylmercury — whether free or albumin-bound — does not form polyoxyethylated corona structures and is not a substrate for the ApoE/LDLR transcytosis pathway. PS20, sharing the polyoxyethylene -(OCH2CH2 )n- backbone with PS80, is hypothesised to form an ApoE corona on antigen-containing particulates, mediating LDLR/LRP1-dependent transcytosis across the blood-brain barrier and delivering HBsAg to immunologically privileged compartments. Local innate immune activation in these compartments — not molecular mimicry — is proposed as the initiating mechanism of demyelination. Molecular mimicry, if it occurs, functions as a secondary amplifier in HLA-susceptible individuals. Conclusions The heterogeneous phenotypic spectrum of Engerix-B-associated demyelinating disorders maps predictably onto distinct anatomical destinations of PS20-mediated transcytosis, providing a unified mechanistic account. This framework challenges molecular mimicry as the primary initiating event not only for Engerix-B-associated demyelination but, by structural extension, for the broader class of organ-specific autoimmune conditions in which an excipient-antigen vector achieves access to an immunologically privileged compartment.