Molecular Mimicry Concept
Molecular mimicry occurs when microbial antigens structurally resemble host antigens, such that immune responses targeting the pathogen cross-react with self-antigen. T cells or B cells primed by the microbe recognize and attack host tissues. This mechanism has been proposed for many autoimmune diseases, though proving causation remains challenging: the pathogen may be long gone, making it difficult to demonstrate temporal relationship.
Campylobacter jejuni and Guillain-Barré Syndrome
Campylobacter jejuni, a diarrheal pathogen, expresses lipooligosaccharides (LOS) that structurally mimic gangliosides (GM1, GQ1b), key nervous system glycolipids. Following C. jejuni infection, some individuals develop antibodies to these LOS structures. Due to mimicry, antibodies cross-react with gangliosides on motor nerves, causing demyelination and paralysis (Guillain-Barré syndrome, GBS). This is the clearest example of mimicry-induced autoimmunity: >30% of GBS cases are preceded by C. jejuni infection, and C. jejuni antibodies cross-react with gangliosides in GBS sera.
Streptococcus and Rheumatic Fever
Group A Streptococcus (GAS), a skin and throat pathogen, expresses M protein structurally similar to cardiac myosin, tropomyosin, and other cardiac proteins. Following GAS infection, some individuals develop antibodies to M protein. Due to mimicry, anti-M antibodies cross-react with cardiac tissue, causing inflammation, valve damage, and rheumatic fever (RF). RF remains a leading cause of acquired heart disease globally. The temporal relationship (post-streptococcal infection) is clear, supporting mimicry as a causal mechanism.
Klebsiella and Ankylosing Spondylitis (HLA-B27 Hypothesis)
Ankylosing spondylitis (AS), a severe spinal arthritis, is strongly associated with HLA-B27 (>90% of patients). Klebsiella pneumoniae LPS shares epitopes with HLA-B27. The hypothesis: HLA-B27+ individuals infected with Klebsiella develop antibodies to Klebsiella LPS that cross-react with HLA-B27, triggering autoimmunity. This mechanism is supported but unproven; not all HLA-B27+ individuals develop AS, suggesting additional factors (genetic, environmental) are required.
Bystander Activation and Epitope Spreading
Molecular mimicry isn't the only link between microbes and autoimmunity. Bystander activation occurs when pathogenic inflammation broadly activates T cells near sites of microbial infection, regardless of specificity. Epitope spreading—where immune responses broaden to additional epitopes on the same or nearby proteins—can extend autoimmunity beyond the initial mimicry target. These mechanisms explain why some autoimmune diseases involve multiple autoantigens, not just one mimicked epitope.
Microbiome-Autoimmunity Emerging Research
Recent studies link dysbiosis to autoimmunity: loss of Faecalibacterium and other butyrate producers correlates with IBD, ankylosing spondylitis, and other autoimmune conditions. Dysbiosis may increase intestinal permeability, allowing luminal bacteria (or their antigens) to translocate and trigger immune activation. Additionally, dysbiosis-reduced Tregs (due to low butyrate) may inadequately suppress autoreactive T cells. Microbiome-targeting interventions (prebiotics, probiotics, antibiotics) are being explored but evidence for clinical benefit remains limited.