A reversion of an IL2RG mutation in combined immunodeficiency providing competitive advantage to the majority of CD8+ T cells

Mutations in the common gamma chain (γ_c, CD132, encoded by the IL2RG gene) can lead to B⁺T⁻NK⁻ X-linked severe combined immunodeficiency, as a consequence of unresponsiveness to γc-cytokines such as interleukins-2, -7 and -15. Hypomorphic mutations in CD132 may cause combined immunodeficiencies with a variety of clinical presentations. We analyzed peripheral blood mononuclear cells of a 6-year-old boy with normal lymphocyte counts, who suffered from recurrent pneumonia and disseminated mollusca contagiosa. Since proliferative responses of T cells and NK cells to γc -cytokines were severely impaired, we performed IL2RG gene analysis, showing a heterozygous mutation in the presence of a single X-chromosome. Interestingly, an IL2RG reversion to normal predominated in both naïve and antigen-primed CD8⁺ T cells and increased over time. Only the revertant CD8⁺ T cells showed normal expression of CD132 and the various CD8⁺ T cell populations had a different T-cell receptor repertoire. Finally, a fraction of γδ⁺ T cells and differentiated CD4⁺CD27⁻ effector-memory T cells carried the reversion, whereas NK or B cells were repeatedly negative. In conclusion, in a patient with a novel IL2RG mutation, gene-reverted CD8⁺ T cells accumulated over time. Our data indicate that selective outgrowth of particular T-cell subsets may occur following reversion at the level of committed T progenitor cells.