We identified germline heterozygous mutations in CTLA4 in members of four families with severe immune dysregulation. Human CTLA4 haploinsufficiency caused dysregulation of FoxP3+ regulatory T (Treg) cells and lymphocytic infiltration of target organs, mimicking Ctla4 homozygous mice. Patients also exhibited a B cell phenotype, with progressive loss of B cells and accumulation of autoreactive CD21^lo B cells. This study demonstrates a critical quantitative role for CTLA-4 in human immune homeostasis.

We identified germline heterozygous mutations in CTLA4 in members of four families with severe immune dysregulation. Human CTLA4 haploinsufficiency caused dysregulation of FoxP3+ regulatory T (Treg) cells and lymphocytic infiltration of target organs, mimicking Ctla4 homozygous mice. Patients also exhibited a B cell phenotype, with progressive loss of B cells and accumulation of autoreactive CD21^lo B cells. This study demonstrates a critical quantitative role for CTLA-4 in human immune homeostasis.