Signal transducer and activator of transcription 3 (STAT3) mutations underlying autosomal dominant hyper-IgE syndrome impair human CD8⁺ T-cell memory formation and function

Background The capacity of CD8⁺ T cells to control infections and mediate antitumor immunity requires the development and survival of effector and memory cells. IL-21 has emerged as a potent inducer of CD8⁺ T-cell effector function and memory development in mouse models of infectious disease. However, the role of IL-21 and associated signaling pathways in protective CD8⁺ T-cell immunity in human subjects is unknown. Objective We sought to determine which signaling pathways mediate the effects of IL-21 on human CD8⁺ T cells and whether defects in these pathways contribute to disease pathogenesis in patients with primary immunodeficiencies caused by mutations in components of the IL-21 signaling cascade. Methods Human primary immunodeficiencies resulting from monogenic mutations provide a unique opportunity to assess the requirement for particular molecules in regulating human lymphocyte function. Lymphocytes from patients with loss-of-function mutations in signal transducer and activator of transcription 1 (STAT1), STAT3, or IL-21 receptor (IL21R) were used to assess the respective roles of these genes in human CD8⁺ T-cell differentiation in vivo and in vitro. Results Mutations in STAT3 and IL21R, but not STAT1, led to a decrease in multiple memory CD8⁺ T-cell subsets in vivo, indicating that STAT3 signaling, possibly downstream of IL-21R, regulates the memory cell pool. Furthermore, STAT3 was important for inducing the lytic machinery in IL-21-stimulated naive CD8⁺ T cells. However, this defect was overcome by T-cell receptor engagement. Conclusion The IL-21R/STAT3 pathway is required for many aspects of human CD8⁺ T-cell behavior but in some cases can be compensated by other signals. This helps explain the relatively mild susceptibility to viral disease observed in STAT3- and IL-21R-deficient subjects.