GARP derived TGFβ promotes acetylation-mediated Foxp3 protein stabilization and Treg functionality

Diseases of immune dysregulation are frequently caused by single gene mutations in central pathways of immune tolerance. They are characterised by heterogeneous clinical manifestations with autoimmune symptoms based on defects in mechanisms regulating self-tolerance. Identification of the genetic cause of such diseases has critical implications for the treatment of patients. To identify underlying molecular etiologies, patients suffering from immune dysregulation were subjected to whole-exome sequencing. Two patients bearing previously undescribed mutations of LRRC32 encoding glycoprotein A repetitions predominant (GARP) were identified. GARP has been recently described to be specifically expressed on regulatory T cells (Tregs) and is important for their suppressive capacity. GARP is involved in TGFβ1 activation by binding latent TGFβ1 in the cytoplasm and translocating it to the cell surface. The importance of fully functional Tregs is well described for maintaining immunological self-tolerance and homeostasis of the immune system. Tregs control destructive immune responses against pathogens and limit reactions towards self antigens. However, the function of this TGFβ1 source and the underlying mechanisms are not yet completely understood. My PhD thesis presents data that reinforces the importance of GARP as a regulator of Treg function and stability. It describes two patients with LRRC32 mutations, characterises the function of GARP in cellular and molecular detail and demonstrates the importance of functional Tregs for physiological immune homeostasis in men and mice. The study identifies a novel link between GARP dependent TGFβ signalling in Tregs and expression of the Treg specific histone deacetylase (HDAC)9 that promotes Foxp3 deacetylation, which contributes to an instable dysfunctional Treg phenotype. The research shows that Tregs from patients with LRRC32 mutations have only minimal GARP expression on the cell surface and reduced TGFβ signalling. Tregs from these patients further show a strongly diminished Treg suppressor function and significant reduction in Treg numbers and frequency. GARP functions are characterised in a novel molecular detail using a model of conditional Garp-deficiency in mice. Here the study confirms increased susceptibility to inflammatory diseases once GARP expression is decreased on Tregs. Consistent with the effects observed in patients, Garp-deficiency in mice leads to absence of latent TGFβ on the cell surface of Tregs, reduced TGFβ signalling and diminished suppressor function. Further, Treg from Garp-deficient mice exhibit an unstable phenotype due to diminished Foxp3 protein acetylation and stability. In sum, the PhD thesis reinforces the understanding of immunological mechanisms of immune dysregulation and expands the knowledge of immunological function of GARP as an important regulator of Treg stability