Functional Characterization of an <i>IL2RG</i> Variant, a Case Report of X‐Linked T‐ B + NK + SCID

ABSTRACT Objectives Pathogenic variants in IL2RG , encoding the common γ chain (γ c /CD132), usually lead to T − B + NK‐ X‐SCID but can, occasionally, generate a T − B + NK+ phenotype. We wanted to delineate potential mechanisms for this discrepancy. Methods The immunological work‐up of our patient comprised: whole genome sequencing and subsequent bioinformatics, flow‐cytometry (lymphocyte surface receptor expression, STAT5 phosphorylation, NK cell proliferation and degranulation), lymphocyte stimulation (IL‐2, IL‐4 and IL‐15) as well as restriction enzyme digestion and fragment size separation (evaluation of relative WT/variant expression). Findings Our patient, hemizygous for a maternally derived, c.677 G > A IL2RG missense variant displayed a T − B + NK+ phenotype, no dysmorphic features and no thymus. The γ c was surface expressed. In contrast to B cells from cord‐blood and adults (including maternal B cells), only pre‐gene therapy (patient) B cells did not decrease IL‐4Rα surface expression upon IL‐4 stimulation, consistent with compromised IL‐4R (and γ c ) function. After gene therapy, patient B cells decreased IL‐4Rα upon Il‐4 stimulation. Pre‐gene therapy NK cells displayed normal, K562 cell, induced degranulation and, in response to IL‐2 and IL‐15 and exhibited normal initial pSTAT5 kinetics but clearly attenuated activation and proliferation day six. By restriction enzyme digestion and fragment size separation, selected T and B cells from the healthy mother exhibited skewed expression (92% and 84%, respectively) of the WT IL2RG allele. Conclusion The selective WT IL2RG expression in maternal B cells was consistent with the compromised IL‐4R signaling (and compromised IL‐21R signaling) in her offspring (the patient), as both IL‐4 and IL‐21 are critical for normal human B cell germinal center reactions but not for peripheral B cell homeostasis. The c.677 G > A IL2RG variant permitted normal NK cell degranulation and initial STAT5 phosphorylation but was incapable of sustaining normal NK cell activation and proliferation in vitro. As IL‐2 and IL‐15 induced in‐vitro NK cell proliferation is primarily mediated through the low affinity βγ c (CD122‐CD132) complex, our data indicate the importance of high affinity IL‐15Rα and IL‐2 Rα mediated signaling in‐vivo for sustaining NK cell numbers in X‐linked SCID. Consequently, we further hypothesize that in cases of X‐linked SCID, where even initial IL‐15 and IL‐2 STAT5 phosphorylation is compromised, in‐vivo trans‐presentation of IL‐15 (and IL‐2), via the high affinity IL‐15Rα and IL‐2Rα receptor subunits, will not be able to sustain normal peripheral NK cell numbers”.

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