Co-receptor interaction changes Treg fitness in Juvenile Idiopathic Arthritis

Specific white blood cells called regulatory T cells (Tregs), characterised by FOXP3 expression, are crucial for maintaining immune tolerance and preventing autoimmunity.

But in Juvenile idiopathic arthritis (JIA), a rare disease but the most common rheumatic autoimmune condition with childhood onset, Tregs fail to stop overactive effector cells from attacking the child’s joints, leading to pain, inflammation and ultimately joint destruction and disability.

Although modern therapies have improved outcomes, they often come with severe side effects and are not always effective with 30-50% of children experience inflammatory flares without any obvious trigger, even while on medication. We believe that Tregs in inflamed joints are not working appropriately.

Tregs communicate with their environment through co-receptors, their signalling then determines the action of the Treg.

Interestingly, co-receptors of the same family, such as TIGIT and CD226, share the same ligands but have opposite downstream effects when engaged individually.

We found that in contrast to healthy blood Tregs, which predominantly are TIGIT single positive, Tregs in synovial fluid (SF) from inflamed joints of children with JIA express high levels of both co-receptors TIGIT and CD226 on the same cells, and that the microenvironment is rich in their ligands.

We determined that TIGIT and CD226 can directly interact, and in cell line models TIGIT-CD226 co-expression changes their signalling and cell behaviour in response to ligand stimulation.

We believe that in the inflamed environment of JIA Treg signalling, activation, proliferation and functions are changed by TIGIT and CD226 co-expression and this keeps JIA inflammation going.

Firstly, we will look at single cell gene expression data, which genes are on and which are off, from synovial tissue biopsies and compare the expression of TIGIT, CD226 and their ligands in tissue to synovial fluid derived cells.

To determine what pathways TIGIT and CD226 use when alone or when co-expressed we will analyse gene expression and epigenetic modification data, marks that decide whether a gene is on or off, from engineered Tregs (TIGIT+CD226-, or TIGIT+CD226+, TIGIT-CD226+, TIGIT-CD226-) with and without ligand stimulation.

Additionally, we will test Tregs from the inflamed joint of JIA on their signalling, activation, proliferation and functionality in response to activation and TIGIT/CD226 ligand stimulation compared to control blood Tregs and Tregs engineered to have distinct TIGIT and CD226 expression patterns.

Finally, to be able to see TIGIT and CD226 interacting with each other on SF Tregs we will tag native TIGIT/CD226 genes with fluorescent proteins using gene editing (CRISPR-Cas9 homology directed repair) and visualise TIGIT-CD226 interaction in response to their ligand in JIA SF Tregs with high resolution microscopy.

Taken together we will learn more about the fundamental mechanisms behind TIGIT and CD226 as individual co-receptors as well as when interacting with each other in the inflamed joint of JIA.

This will tell us how TIGIT and CD226 change SF Treg functions, and determine whether breaking CD226-TIGIT interaction could be a novel treatment target to restore the immunoregulatory balance in JIA and potentially other autoimmune conditions.