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Spreading of antibody reactivity to non‐thyroid antigens during experimental immunization with human thyroglobulin

Intermolecular spreading of antibody reactivity has been implicated in the evolution of autoimmune disease. In this study, spreading of antibody reactivity to non‐thyroid autoantigens after experimental immunization with thyroglobulin (Tg) was investigated. For this purpose, two rabbits were injected with human Tg six times (stages 1–6) every 3 weeks. Animals were also bled before priming. Antisera were tested by enzyme‐linked immunosorbent assay (ELISA) for reactivity to several non‐thyroid antigens: bovine serum albumin (BSA), native DNA (nDNA), human myosin, human globular (G) and filamentous (F) actin and porcine tubulin. Tg‐immunized animals developed the following serological reactivity pattern: (a) high reactivity to myosin from stage 2 onward, (b) significant reactivity to F‐actin, remaining high up to stage 6, (c) reactivity to BSA with a peak at stage 3, (d) a small increase of reactivity to G‐actin at stage 3 and (e) no increase of reactivity to nDNA and tubulin. The study of affinity‐purified anti‐Tg antibodies and the use of competitive assays revealed that reactivity to F‐actin was not due to cross‐reaction with Tg. On the contrary, reactivity to myosin during the first stages of immunization was due to cross‐reaction with Tg, while at stage 6 it became myosin‐specific. Reactivity to BSA at stage 3 was also due to cross‐reaction with Tg. We conclude that at least part of the induced anti‐Tg antibodies may result from the expansion of B cell clones producing polyreactive natural autoantibodies, and polyreactivity of anti‐Tg antibodies during the first stages of Tg‐immunization may be responsible for the intermolecular spreading of antibody response.