Characterization of Expanded Gamma Delta T Cells from Atypical X-SCID Patient Reveals Preserved Function and IL2RG-Mediated Signaling

Abnormally high gamma delta T cell numbers among individuals with atypical SCID have been reported but detailed immunopheno typing and functional characterization of these expanded gamma delta T cells are limited. We have previously reported atypical SCID phenotype caused by hypomorphic IL2RG (NM_000206.3) c.172C > T;p.(Pro58Ser) variant. Here, we have further investigated the index patient's abnormally large gamma delta T cell population in terms of function and phenotype by studying IL2RG cell surface expression, STAT tyrosine phosphorylation and blast formation in response to interleukin stimulation, immunophenotyping, TCRv gamma sequencing, and target cell killing. In contrast to his alpha beta T cells, the patient's gamma delta T cells showed normal IL2RG cell surface expression and normal or enhanced IL2RG-mediated signaling. V delta 2 + population was proportionally increased with a preponderance of memory phenotypes and high overall tendency towards perforin expression. The patient's gamma delta T cells showed enhanced cytotoxicity towards A549 cancer cells. His TCRv gamma repertoire was versatile but sequencing of IL2RG revealed a novel c.534C > A; p.(Phe178Leu) somatic missense variant restricted to gamma delta T cells. Over time this variant became predominant in gamma delta T cells, though initially present only in part of them. IL2RG-Pro58Ser/Phe178Leu variant showed higher cell surface expression compared to IL2RG-Pro58Ser variant in stable HEK293 cell lines, suggesting that somatic p.(Phe178Leu) variant may at least partially rescue the pathogenic effect of germline p.(Pro58Ser) variant. In conclusion, our report indicates that expansion of gamma delta 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.Peer reviewe