Loss of human ICO SL results in combined immunodeficiency

Abstract

Primary immunodeficiencies represent naturally occurring experimental models to decipher human immunobiology. We report a patient with combined immunodeficiency, marked by recurrent respiratory tract and DNA-based viral infections, hypogammaglobulinemia, and panlymphopenia. He also developed moderate neutropenia but without prototypical pyogenic infections. Using whole-exome sequencing, we identified a homozygous mutation in the inducible T cell costimulator ligand gene (ICO SLG; c.657C>G; p.N219K). Whereas WT ICO SL is expressed at the cell surface, the ICO SLN219K mutation abrogates surface localization: Mutant protein is retained in the endoplasmic reticulum/Golgi apparatus, which is predicted to result from deleterious conformational and biochemical changes. ICO SLN219K diminished B cell costimulation of T cells, providing a compelling basis for the observed defect in antibody and memory B cell generation. Interestingly, ICO SLN219K also impaired migration of lymphocytes and neutrophils across endothelial cells, which normally express ICO SL. These defects likely contributed to the altered adaptive immunity and neutropenia observed in the patient, respectively. Our study identifies human ICO SLG deficiency as a novel cause of a combined immunodeficiency.