Altered ligand-interaction contributes to functional insufficiency due to CTLA-4 gene mutation

Abstract

CTLA-4 is one of the key inhibitory immune receptors and plays a critical role in maintaining immune tolerance. Both loss of CTLA-4 gene in mice and mutations in humans result in systemic autoimmune diseases with a similar phenotype. However, the precise mechanisms of CTLA-4 have not yet been entirely clarified. Structural and functional analysis of the reported autoimmune cases revealed that pathogenic CTLA-4 mutations occur frequently in ligand binding and dimerization regions. Through genetic mutation screening study, we have discovered a new CTLA-4 point mutation in a patient with recurrent autoimmune inflammation. We used steered molecular dynamics (SMD) simulation, single-molecule biophysical approaches, as well as functional assays to compare the function of this CTLA-4 mutant to the WT molecule. We demonstrated that, mechanical force induces conformational changes in CTLA-4 to enhance pre-existing contacts and activates new interactions at the CTLA-4-CD80 and CD86 binding interface. Although the mutant CTLA-4 can function to induce downstream signals, its ability in trans-endocytosis of CD80 and CD86 is impaired due to altered ligand interaction. The conclusions from this study will help us elucidate the immunoregulatory mechanisms of CTLA-4 and provide new strategies for more effective targeting of CTLA-4 in autoimmune diseases and tumors.