Immune dysregulation from a novel CTLA-4 haploinsufficiency variant.

Cytotoxic T lymphocyte antigen 4 (CTLA-4) is a key immune checkpoint receptor that regulates T cell activation through ligand competition and transendocytosis. Heterozygous loss-of-function variants in CTLA4 result in CTLA-4 haploinsufficiency with autoimmune infiltration (CHAI), characterized by immune dysregulation and autoimmunity. We report a multigenerational family carrying a novel heterozygous CTLA4 variant, c.654T>A (p.Tyr218*), which truncates the cytoplasmic tail. Affected individuals presented with recurrent infections and autoimmune manifestations. Patient T cells showed reduced CTLA-4 expression at baseline and after stimulation, suggesting impaired stability. Jurkat cells expressing CTLA-4 Y218* exhibited enhanced degradation, partially rescued by lysosomal inhibition, and reduced transendocytosis of CD80. Together, these findings suggest that the CTLA-4 p.Tyr218* variant compromises protein stability and ligand uptake, contributing to CTLA-4 haploinsufficiency and immune dysregulation. This work broadens the spectrum of CTLA4 variants and underscores the importance of the C-terminal cytoplasmic domain in CTLA-4 function and immune regulation.