Interestingly, not all individuals carrying variants develop symptoms, indicating an incomplete disease penetrance of approximately 70 %. Moreover, the clinical presentation and organ involvement vary significantly among those affected. These findings suggest the existence of modifiers that either trigger disease onset or influence its severity. While several potential modifiers (such as HLA or past infections) have been investigated as potential disease modifiers in the context of CTLA-4 insufficiency, none have been conclusively identified thus far. Proposed modifiers include a gain-of-function mutation in JAK3 and a loss-of-function mutation in CLEC7A. However, these rare variants have only been observed in individual patients.

In our pursuit to identify modifiers for CTLA-4 insufficiency, we aim to test the hypothesis that affected CTLA4 mutation carriers exhibit abnormal molecular signatures. These signatures can potentially be discerned through a combination of epigenome, transcriptome, and/or proteomic profiling. Furthermore, after excluding a second somatic hit in CTLA4 itself and germline mutations in other genes within the TCR signaling pathway, we are expanding our search for genetic modifiers of disease penetrance by investigating somatic hits in the whole exome and exploring differences in the genetic makeup between affected and unaffected mutation carriers. Additionally, we are examining whether specific TCR sequences are over-represented in affected patients and whether these sequences can be linked to a specific set of viral or autoantigens.

Finally, we are investigating the potential role of the microbiome as a modifier of disease. The microbiome is known to play a critical part in shaping immune responses and maintaining tolerance to self-antigens. Our group has studied the composition of the gut microbiota in patients with common variable immunodeficiency (CVID), a disease that many CTLA4 mutation carriers suffer from. Additionally, autoimmune gut disease is commonly observed in CVID and is one of the most common organ involvements in CTLA-4 insufficiency. We have found that CVID patients have an altered gut microbiota composition characterized by decreased alpha diversity and the expansion of the class Gammaproteobacteria. Based on these findings, we aim to explore whether the gut microbiota acts as a disease modifier in CTLA-4 insufficiency.