Characterization of expanded γδ T cells from patient with atypical X-linked severe combined immunodeficiency reveals preserved function and IL2RG-mediated signaling

Abstract

Abnormally high γδ T cell numbers are common among atypical severe combined immunodeficiency (SCID) patients but detailed immunophenotyping and functional characterization of these expanded γδ T cells are limited. We have previously reported atypical SCID phenotype caused by hypomorphic IL2RG c.172C > T;p.(Pro58Ser) variant causing failure in IL2RG (CD132) plasma membrane targeting leading to impaired IL-2 responses in CD4 and CD8 T cells. Our aim was to investigate the phenotype and function of the index patient's abnormally large peripheral γδ T cell population.We found that in contrast to his αβ T cells, the patient's γδ T cells showed normal IL2RG cell surface expression and normal or enhanced IL2RG-mediated signaling. Vδ2+ population was proportionally increased with a preponderance of memory phenotypes and high overall tendency towards perforin expression. The patient's γδ T cells showed enhanced cytotoxicity towards A549 cancer cells. TCRvγ repertoire was versatile but sequencing of IL2RG revealed a novel c.534C>A; p.(Phe178Leu) somatic missense variant restricted to γδ T cells. Over time this variant became predominant in patient's γδ T cells and was enriched during in vitro expansion of patient's γδ T cells, indicating that the clones with this somatic variant may have growth or survival advantage. In silico analysis with DynaMut2 tool indicated that the overall effect of Phe178Leu substitution for IL2RG was predicted to be stabilizing and IL2RG-Pro58Ser/Phe178Leu to be more stable than IL2RG-Pro58Ser variant alone. Furthermore, IL2RG-Pro58Ser/Phe178Leu showed significantly increased cell surface expression when compared to IL2RG-Pro58Ser variant in stable HEK293 cell lines, suggesting that somatic IL2RG p.(Phe178Leu) variant may at least partially rescue the pathogenic effect of germline IL2RG p.(Pro58Ser) variant.In conclusion, our results indicate that expansion of γδ T cells associated with atypical SCID needs further studying and cannot exclusively be deemed as a homeostatic response to low numbers of conventional T cells. (Tuovinen et al., Journal of Clinical Immunology 2022)