Keystone Epitope Theory: Implications for the Investigation of Multiple Sclerosis

Multiple sclerosis (MS) has long presented a paradox: a strong and reproducible association with Epstein–Barr virus (EBV) exposure despite EBV’s near-universal prevalence. Large prospective cohorts show that EBV seroconversion precedes MS onset and is associated with a marked increase in risk 1, while earlier work links risk to prior infectious mononucleosis and to elevated anti-EBV nuclear antigen 1 (EBNA1) antibody responses 2,3. Yet EBV is clearly not sufficient, implying additional “gates” governed by host genetics, developmental timing, tissue context, and the quality of antiviral control. Keystone Epitope Theory (KET) proposes that persistent, human-adapted pathogens such as EBV imprint high-gain immune priorities toward constrained peptide-HLA geometries (“keystone epitopes”), creating plausible vulnerabilities when modified self or foreign antigens accidentally mimic those geometries in the right HLA and niche 4-10. Applied to MS, KET reframes EBV association as a conditional recall problem and separates at least two related but nonidentical endpoints: liability to develop MS and quantitative control of the EBV reservoir. In this formulation, HLA-DRB1*15:01 may act mainly through qualitative class II presentation, altered-self presentation, and/or cross-reactive CD4 programs, whereas HLA-A*02:01 is more consistent with protective class I antiviral surveillance. Other class I backgrounds, including HLA-A*03:01 and HLA-B*07, may define relatively higher-risk or impaired-control contexts, although direct CNS cross-reactivity of the relevant class I clonotypes remains unproven. Blood EBV DNA, antibody features, exhaustion, and CNS-residency phenotypes measured after onset may be state-dependent consequences or modifiers and should not automatically be interpreted as the causal predisposition. The review summarizes key evidence for EBV-focused immunity in MS, including host–virus genetic convergence 11, and integrates recent findings supporting several nonexclusive routes from EBV to CNS autoimmunity: direct EBNA1–CNS mimicry, EBV-induced remodeling of HLA-DR15 self-peptide presentation, survival signals that sustain antigen-capturing B cells within CNS niches, and alterations in antiviral cellular and humoral responses 12-17,18l,19. It then proposes a lesion-anchored, receptor-first workflow that defines the cognate peptide–HLA surface before testing structural mimicry, altered self-presentation, niche-dependent amplification, and post-translationally modified targets, treating the CNS niche as the gate where specificity, antigen pressure, and inhibitory tone converge. The endpoint-separated model also has broader implications for autoimmunity by distinguishing class II-dependent immune focusing and humoral organization from class I-dependent antigen-source control or tissue execution, while treating tissue access and post-onset immune state as separate endpoints. The model also treats focal blood-brain barrier (BBB) activation as a distinct local access gate: gadolinium enhancement can precede conventional lesion evolution, but the temporal sequence linking barrier opening to entry of latently EBV-infected memory B cells remains unproven.