By: Jenna Pappalardo
Autoimmune disorders include a variable set of diseases in which a person’s immune system abnormally targets its own normal body tissue rather than an invading pathogen or other threat. Development of these diseases is a result of genetic and environmental factors synergizing to induce immune responses that would normally be prevented by a series of checkpoints in the immune system. These diseases disproportionately target the sexes, with estimates suggesting 78% of those affected by autoimmune diseases are women, but why this divide exists is unknown. When considering the biological differences between males and females, one obvious answer would be hormones—but how would they affect these processes? A recent study provides a new clue into how hormones could affect a barrier to autoimmunity that, when functioning normally, results in the death of self-reactive cells.
To understand this new link to hormones, I’ll pause to explain the very cool process that should normally happen in the immune system to prevent autoimmunity. T cells are part of the adaptive immune system that mounts specific and robust responses against pathogens. These cells work in various ways, including facilitating immune responses and directly killing infected cells, which makes them great defenders against invaders, but has severe consequences when aimed at their host’s tissues. Adaptive cells gain specificity for what they recognize through essentially random genetic recombination and mutations, so some cells develop to recognize a normal host protein and have the potential to attack it if they aren’t eliminated. T cells develop in a small organ by the heart called the thymus (hence their name), where they undergo stringent selection to delete any T cells that are targeted against normal tissue. But wait—I thought when I first learned of this process—how can their exposure only to the thymus ensure that T cells reactive to the brain or eye or pancreas are also killed before they’re released into the body? The elegant solution to this is the autoimmune regulator or AIRE. Similar to how differential expression of genes allows the same genome to give rise to hundreds of kinds of cells by different expression of proteins, AIRE allows for cells in the thymus to express proteins from other tissues throughout the body. AIRE induces the expression of proteins from all over the body (proteins that are normally expressed in the brain, kidney, liver, etc.) in the thymic stromal cells. T cells are then tested to see if they react against these proteins that represent other tissues and are killed if they recognize them too strongly. This process eliminates T cells that will be activated, and subsequently induce an active immune response, in response to a person’s own protein (ie autoimmune triggering T cells).
The authors of the previously mentioned study decided to investigate how AIRE expression differs in males and females and found that females have lower expression of AIRE, as well as the representative proteins (or tissue-specific antigens, TSAs) that it regulates. Recapitulating lowered estrogen levels in castrated male mice suggested that hormonal differences may be responsible. For more direct assessment of how hormones affected AIRE expression, the authors introduced either estrogen or DHT (basically activated testosterone) to human thymic epithelial cells in culture. Adding estrogen caused a decrease both in AIRE expression and an AIRE-dependent TSA, while testosterone actually caused a slight increase in both. If cells were given both estrogen and DHT, the effect of estrogen won out with an overall reduction in AIRE and its TSA. There are TSAs that do not depend on AIRE, which were measured in this study and not affected by estrogen. These results were mirrored in experiments where human thymus fragments were transplanted into mice. When estrogen was administered, there was lower AIRE and AIRE-dependent TSA expression compared to mice not receiving estrogen. The impact of estrogen was further confirmed by removing the estrogen receptor from cells in the thymus to prevent estrogen from acting on them, which restored AIRE and AIRE-dependent TSA expression to levels comparable to males.
The authors hypothesized that estrogen might be affecting AIRE expression by altering the areas of DNA that are available for transcription. DNA methylation can hide regions of DNA from transcription, which prevents those regions from being translated into the protein they encode. They found that adding estrogen increased the number of methylated sites in cultured human thymic epithelial cells, while DHT did not significantly change the level of methylation. This suggests that estrogen may affect how much AIRE is expressed by causing an increase in DNA methylation, turning off the gene that encodes it. These results were tied into susceptibility to autoimmunity using experimental autoimmune thyroiditis (EAT), which is an autoimmunity model in mice. In this model, an adaptive immune response is inappropriately launched against thyroglobulin, a protein expressed in the thyroid. As female mice are more prone to develop EAT, the next step was assessing if lower AIRE expression contributes to EAT susceptibility. Sure enough, there was more pronounced autoimmunity in males when AIRE expression was lowered to mimic levels in females by preventing the protein from being synthesized.
There are still many open questions about the interaction of genetics and environment in autoimmune susceptibility, but this study provides new insight into how sex contributes to that balance by proposing an estrogen-mediated process that could allow for the escape of more autoreactive T cells. These factors comprise such a complex interplay that at a recent dinner with a visiting speaker and prominent immunologist, he suggested that whoever untangles the mechanisms accounting for the sex disparity in autoimmunity deserves the Nobel Prize (and joked that he isn’t smart enough to tackle that challenge). Sex doesn’t just affect susceptibility to autoimmunity, but also its severity and immunological characteristics, making it a potential avenue for developing new preventatives and therapeutics.