Gut microbiome changes may shape genetic risk in autoimmune encephalitis

Background

Autoimmune limbic encephalitis (LE) is a rare but serious brain condition in which the body’s immune system mistakenly attacks parts of the brain. This can cause seizures, memory loss, confusion, and changes in behaviour. The most common form is linked to antibodies against a brain protein called LGI1 and is an important cause of autoimmune‑related epilepsy. 

Research has shown that most people with LGI1‑antibody encephalitis share a specific inherited change in an immune system gene (called HLA‑DRB107:01*). However, this same genetic change is also found in many healthy people. This means that genes alone cannot explain why some people develop the disease while others do not. 

Scientists have therefore suggested that another factor may be involved — something in the environment that works alongside genetic risk. One possible contributor is the gut microbiome: the trillions of bacteria and other microbes that live in our digestive system and play a key role in regulating the immune system and inflammation, including in the brain. 

Research

In this study led by researchers at FutureNeuro and RCSI, the gut microbiome was examined in 42 people with LGI1‑antibody encephalitis and compared with 27 healthy relatives or close contacts. This approach helped account for shared genes, diet, and living environment. Using advanced genomic sequencing techniques, the team analysed both the types of gut microbes present and what they are capable of doing. 

The researchers found that the condition was not caused by a single harmful bacterium or infection. Instead, people with LGI1‑antibody encephalitis showed small but consistent changes in the overall balance of gut microbes, similar to patterns seen in other inflammatory brain conditions. These changes were more noticeable in individuals who carried more copies of the HLA‑DRB107:01* risk gene. 

The study also showed that gut microbes in patients were less likely to produce short‑chain fatty acids — natural substances that help keep the immune system balanced and reduce inflammation. Together, these findings suggest that changes in gut bacteria might support an over‑active immune response in people who are already genetically vulnerable, increasing the risk of brain inflammation and seizures. 

Potential Impact

This is the largest gut microbiome study carried out so far in LGI1‑antibody encephalitis. While it does not show that gut bacteria cause the condition, it provides important evidence that genetic risk and environmental factors may work together to influence who develops this rare condition. 

By highlighting a possible role for the gut–brain connection in autoimmune encephalitis and epilepsy, this research opens the door to new ways of understanding disease risk. In the future, larger studies could explore whether gut‑based markers help identify people at higher risk, or whether microbiome‑informed approaches could support existing immune treatments. More broadly, the findings may also help advance understanding of epilepsy and other inflammatory brain disorders.