Functional Characterization of an IL2RG Variant, a Case Report of X-Linked T- B + NK + SCID

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.

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).

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.

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".