Thymus-dependent T-cell regeneration is a major pathway for immune reconstitution after stem cell transplantation in children. Therefore, we prospectively assessed T-cell dynamics and thymic function in 164 pediatric patients between 1 and 124 months after transplantation by measuring T-cell receptor recombination excision circles and spontaneous expression of Ki67 in peripheral T-cell subsets. We analyzed the effect of recipient age, conditioning regimen, type of donor and graft, stem cell dose, and graft-versus-host disease on the onset and the plateau of thymic output. A high rate of spontaneous proliferation in early-reconstituting naive and memory T cells inversely correlated with total T-cell numbers. Accordingly, T-cell receptor recombination excision circle content was diminished in early-appearing naive T cells. A multivariate analysis revealed that the onset of thymic recovery was inversely correlated only with recipient age ( P < .0002), whereas the plateau of thymic output was higher in patients receiving increased stem cell numbers ( P < .0022). Donor type, stem cell source, and conditioning regimen influenced none of the analyzed parameters. In conclusion, lymphopenia-driven proliferation is important for T-cell homeostasis in children early after stem cell transplantation, but it might result in underestimation of thymic function. Onset and plateau of thymic activity are independently regulated by different transplant-related factors.

Our data indicate that immunomodulatory molecules such as PD-L1 rather than Tregs play pivotal roles in the tissue-specific regulation of alloresponses. Further studies are needed to refine the significance of the PD-L1 pathway in GVHD and its versatility for therapeutic intervention.

T‐cell re‐constitution after allogeneic stem cell transplantation (alloSCT) is often dampened by the slow differentiation of human peripheral blood CD34+ (huCD34+) hematopoietic stem cells (HSCs) into mature T cells. This process may be accelerated by the co‐transfer of in vitro‐pre‐differentiated committed T/NK‐lymphoid progenitors (CTLPs). Here, we analysed the developmental potential of huCD34+ HSCs compared with CTLPs from a third‐party donor in a murine NOD‐scid IL2Rγnull model of humanised chimeric haematopoiesis. CTLPs (CD34+lin−CD45RA+CD7+) could be generated in vitro within 10 days upon co‐culture of huCD34+ or cord blood CD34+ (CB‐CD34) HSCs on murine OP9/N‐DLL‐1 stroma cells but not in a novel 3‐D cell‐culture matrix with DLL‐1low human stroma cells. In both in vitro systems, huCD34+ and CB‐CD34+ HSCs did not give rise to mature T cells. Upon transfer into 6‐wk‐old immune‐deficient mice, CTLPs alone did not engraft. However, transplantation of CTLPs together with huCD34+ HSCs resulted in rapid T‐cell engraftment in spleen, bone marrow and thymus at day 28. Strikingly, at this early time point mature T cells originated exclusively from CTLPs, whereas descendants of huCD34+ HSCs still expressed a T‐cell‐precursor phenotype (CD7+CD5+CD1a+/−). This strategy to enhance early T‐cell re‐constitution with ex vivo‐pre‐differentiated T‐lymphoid progenitors could bridge the gap until full T‐cell recovery in severely immunocompromised patients after allogeneic stem cell transplantation.