Disrupted Ca homeostasis and immunodeficiency in patients with functional IP receptor subtype 3 defects.

Calcium signaling is essential for lymphocyte activation, with genetic disruptions of store-operated calcium (Ca) entry resulting in severe immunodeficiency. The inositol 1,4,5-trisphosphate receptor (IPR), a homo- or heterotetramer of the IPR1-3 isoforms, amplifies lymphocyte signaling by releasing Ca from endoplasmic reticulum stores following antigen stimulation. Although knockout of all IPR isoforms in mice causes immunodeficiency, the seeming redundancy of the isoforms is thought to explain the absence of variants in human immunodeficiency. In this study, we identified compound heterozygous variants of ITPR3 (a gene encoding IPR subtype 3) in two unrelated Caucasian patients presenting with immunodeficiency. To determine whether ITPR3 variants act in a nonredundant manner and disrupt human immune responses, we characterized the Ca signaling capacity, the lymphocyte response, and the clinical phenotype of these patients. We observed disrupted Ca signaling in patient-derived fibroblasts and immune cells, with abnormal proliferation and activation responses following T-cell receptor stimulation. Reconstitution of IPR3 in IPR knockout cell lines led to the identification of variants as functional hypomorphs that showed reduced ability to discriminate between homeostatic and induced states, validating a genotype-phenotype link. These results demonstrate a functional link between defective endoplasmic reticulum Ca channels and immunodeficiency and identify IPRs as diagnostic targets for patients with specific inborn errors of immunity. These results also extend the known cause of Ca-associated immunodeficiency from store-operated entry to impaired Ca mobilization from the endoplasmic reticulum, revealing a broad sensitivity of lymphocytes to genetic defects in Ca signaling.