Autoimmune diseases affect 5-8% of the population, yet the upstream events that start them remain unidentified. Genetic association studies and epidemiology implicate hundreds of risk variants and environmental exposures, but these explain risk statistically, not mechanistically: autoreactive lymphocytes circulate in every healthy person, autoantibodies appear years before symptoms in many diseases, and propagation mechanisms such as neoepitope generation and determinant spreading are partially mapped. The unresolved question is what actually converts physiological self-reactivity into a sustained, tissue-destructive autoimmune response — the identity and sequence of the initiating events before clinical onset, and the mechanism by which central and peripheral tolerance, including regulatory T cells, fails to contain the nascent response. The question is posed as a general one across autoimmune diseases; partial disease-specific answers exist (for example Epstein-Barr virus as a causal trigger of multiple sclerosis), but no causally validated account of initiation and regulatory failure exists for any major human autoimmune disease.

Background

The immune system normally distinguishes host tissue from pathogens through a set of mechanisms called self-tolerance. Central tolerance deletes most self-reactive lymphocytes during their maturation in the thymus and bone marrow, while peripheral tolerance (anergy, deletion, and suppression by regulatory T cells, abbreviated Tregs — a specialized T-cell population that restrains immune responses) controls self-reactive cells that escape. Autoimmune diseases arise when this control fails.

Three lines of evidence frame the open question. First, genome-wide association studies (GWAS — screens associating genetic variants with disease across populations) have linked hundreds of mostly non-coding risk variants to autoimmune diseases, each of small effect and together explaining only part of heritability; concordance between identical twins is incomplete, implicating environmental triggers. Second, the healthy immune system is not free of self-reactivity: studies of early human B-cell precursors (Wardemann et al. 2003) showed that a substantial fraction produces autoantibodies — antibody molecules binding host components — implying that autoreactivity alone is not disease, and that conversion to disease is a separate, unexplained event. Third, the asymptomatic 'preclinical' phase is well documented: autoantibodies precede systemic lupus erythematosus by years (Arbuckle et al. 2003), antibodies to citrullinated proteins and rheumatoid factor precede rheumatoid arthritis (Rantapää-Dahlqvist et al. 2003), and presymptomatic type 1 diabetes is now formally staged by autoantibody number and glucose tolerance (Insel et al. 2015). These studies prove a long window between the first self-directed response and clinical disease but do not reveal what starts it.

Two propagation mechanisms are comparatively well understood. Post-translational modifications (chemical alterations of proteins after synthesis, such as oxidation, citrullination, or glycation) can create neoepitopes — new structures on self-proteins that the immune system has never been tolerized to and that can be recognized by T cells; this 'autoantigenesis' has been reviewed by Doyle and Mamula (2012) and is an active program in type 1 diabetes, where oxidatively modified insulin neoepitopes are recognized by autoantibodies and autoreactive T cells (Yang et al. 2022; Vinci et al. 2026). Determinant spreading (also called epitope spreading), documented experimentally since Lehmann et al. (1992) and reviewed by Vanderlugt and Miller (2002), is the diversification of the immune response from the initially targeted epitope — the part of a protein recognized by immune receptors — to additional epitopes of the same or other proteins, driven by tissue release of new antigens; it propagates destruction but presupposes an initial breach whose cause it does not explain.

For triggers, the strongest causal evidence in any human autoimmune disease is the longitudinal serological demonstration that Epstein-Barr virus (EBV) infection dramatically increases risk of multiple sclerosis (Bjornevik et al. 2022), but even there the mechanism linking infection to brain-directed autoimmunity is incomplete, and recent reviews conclude that microbial causation of autoimmune disease in general 'remains to be demonstrated' (Zhang et al. 2026). Framework reviews of human autoimmunity (La Cava 2015; Rosenblum, Remedios and Abbas 2015) organize pathogenesis into initiation and propagation stages and identify defective elimination or control of self-reactive lymphocytes as the fundamental lesion — while leaving the identity of the initiating events and the mode of regulatory failure as the central unanswered questions.

Problem Statement

Identify, with causal validation, the events that initiate the loss of self-tolerance in human autoimmune disease before clinical onset: the upstream processes that trigger the first sustained self-directed adaptive immune response, and the mechanism by which central and peripheral tolerance — including regulatory T cell function — fails to arrest it. The question is posed at the level of human autoimmune disease in general; per-disease answers are admissible components.

References

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