Development of functional assays for CD25/Interleukin-2 axis in primary immunodeficiencies

A substantial proportion of patients with clinical primary immunodeficiencies remain without a definitive molecular diagnosis, and the functional consequences of identified variants are not always predictable from genotyping alone. Functional assays that directly assess the capacity of immune cells to mount appropriate responses to defined stimuli therefore represent an essential complement to genetic testing, particularly in the context of the incomplete genotype-phenotype correlations that characterize many inborn errors of immunity. Here we report a functional assay for detection of membrane CD25 protein levels that shows differential expression between healthy donors and patients with primary immunodeficiency. The assay we developed can detect significant upregulation of CD25 in both CD4 + and CD8 + T cells in either PBMC or total blood cultures in samples from healthy individuals, whereas all samples form primary immunodeficiency patients failed to do so. In parallel, we detected higher soluble CD25 levels in the sera of patients compared to healthy donors. Interleukin-2 synthesis was not impaired in all in vitro approaches we tested. Therefore, we suggest that detection of in vitro membrane CD25 levels can be potentially used as functional assay for supporting primary immunodeficiency diagnosis.