The potential of a single progenitor cell to establish and maintain long-term protective T-cell immunity in humans is unknown. For genetic disorders disabling T-cell immunity, somatic reversion was shown to support limited T-cell development attenuating the clinical phenotype. However, the cases reported so far deteriorated over time leaving unanswered the important question of long-term activity of revertant precursors and the robustness of the resulting T-cell system.

We applied TCRβ-CDR3 sequencing and mass cytometry on serial samples of a now 18 year-old SCIDX1 patient with somatic reversion to analyse the longitudinal diversification and stability of a T-cell system emerging from somatic gene rescue.

We detected close to 10⁵ individual CDR3β sequences in the patient. Blood samples of equal size contained about 10-fold fewer unique CDR3β sequences compared to healthy donors, indicating a surprisingly broad repertoire. Despite dramatic expansions and contractions of individual clonotypes representing up to 30% of the repertoire, stable diversity indices revealed that these transient clonal distortions did not cause long-term repertoire imbalance. Phenotypically, the T-cell system did not show evidence for progressive exhaustion. Combined with immunoglobulin substitution, the limited T-cell system in this patient supported an unremarkable clinical course over 18 years.

Genetic correction in the appropriate cell type, in our patient most likely in a T-cell biased self-renewing hematopoietic progenitor, can yield a diverse T-cell system that provides long-term repertoire stability, does not show evidence for progressive exhaustion and is capable of providing protective and regulated T-cell immunity for at least two decades.

DFG EH 145/9-1, DFG SCHW 432/4-1 and the German Research Foundation under Germany's Excellence Strategy–EXC-2189–Project ID: 390939984.