The role of microchimerism in autoimmune diseases
Autoimmunity has been associated with maternalfetal microchimerism both in animal models and in human observational studies. The concept has been studied in various model systems, organ involvements and epidemiological studies. The sensitivity to detect such microchimers (fetal cells in maternal tissue: fetalmaternal microchimerism, maternalfetal: maternal cells in a childs compartment) varies with the methods applied and the reproducibility of detection limits for Y-chromosomal cells in maternal thyroid tissue obtained at surgery is also variable. There is transplacental exchange of cells between mother and fetus during any pregnancy, whereas long-term persistence in either organism is not infrequent but less common. We have shown that mothers that have given birth to sons have Y-chromosome positive cells within their thyroids more frequently if they are affected by either Hashimotos thyroiditis or Graves disease than if they have non-autoimmune thyroid disesase such as adenomas. Furthermore this finding is associated with the susceptibility alleles HLA DQA1*0501-DQB1*0201 or DQB1*0201. This has served as a model to test a hypothesis that organ specific autoimmunity might in fact represent alloimmunity by a graft-versus-host disease mechanism through fetal cells acting against maternal antigens. Thyroid autoimmunity is exacerbated in the year following parturition as Graves disease or can be underrecognised as postpartum thyroiditis. Postpartum thyroiditis occurs in Denmark with 1.5 per 100 000 person-years, where in a study of randomly selected individuals no association could be observed between thyroid antibody titres and the history for childbirth. However a recent study on twins with opposite sex finds a significantly higher prevalence of thyroid antibodies in female twins from malefemale twin pairs than in females from femalefemale twin pairs with similar findings in males (Brix TH 2009). Thereby microchimerism may contribute to the pathogenesis of thyroid and other autoimmune disease.