From barrier breach to brain invasio: Modulators of T cell migration in Multiple Sclerosis

Abstract

Multiple Sclerosis (MS) is the most common neurological disorder in young adults, and every 5 minutes someone in the world is diagnosed with MS. The symptoms of MS present in unpredictable episodes impairing visual, sensory, physical, emotional and cognitive functions, which severely impact the lives of patients in the prime of their lives. The disease is caused by immune cells infiltrating into the brain and spinal cord attacking the insulating layer, called myelin, around the nerves. This causes disturbed signalling between neurons, leading to the array of symptoms. The immune system consists of many different cell types, of which the CD4+ T cell is believed to play a critical role in MS development, by opening and migrating through brain barriers. In healthy brains, T cells are not able to cross these barriers, but in people with MS these barriers are permeable, allowing T cells to enter and cause inflammation. In the first part of this thesis, I focussed on intercellular communication via small messengers called extracellular vesicles (EV). I investigated whether EV derived from blood brain barrier endothelial cells (BBB-EC) impacted BBB integrity, and if the size of these vesicles mattered. When the BBB was inflamed, much like in a brain of an MS patient, BBB-EC released more EV and their content mimicked the inflamed BBB. Large EV showed the most overlap with inflamed BBB cells. Thereby, EV were able to negatively regulate the integrity of the BBB. Next, EV were administered to mice induced with a MS-like disease, and we expected worsening of the disease. However, the group that received small EV actually got less sick and had less leakage of the BBB, indicating a protective effect. More research with EV derived from other origins is necessary. Nevertheless, this study indicates that BBBEV have a disease modifying effect in the context of MS. In the second part of this thesis, I investigated inflammasome activation within T cells, and determined whether it is crucial for BBB-transmigration, or if it is the result of migration. I first analysed inflammasome components in immune cells of MS patients and HD, and found no differences in directly isolated cells. However, I found that stimulation of CD46 induced inflammasome activation in both donor populations, and I established that in CD4+ T cells, the amount of inflammasome activation was higher in MS patients compared to HD. In the MS-like disease in mice, I found that inflammasome activation in CD4+ T cells increased in the central nervous system over time but not in peripheral lymphoid organs. I therefore looked further into the effect of inflammasome activation on the migratory capacities of T cells. Herein, I found indications that inflammasome activation is the result of BBBtransmigration rather than just contact with endothelial cells, and that this only happens in CD4+ T cells and not in other immune cells present in the blood. Finally, I looked into a specific molecule found on the endothelial cell layer that could be the trigger for inflammasome activation. In future research, I aim to validate this and show that blocking this interaction limits the pathogenicity of infiltrating T cells.