Thymus-independent expansion of T lymphocytes in children after allogeneic bone marrow transplantation

The contribution of the thymus-dependent pathway and thymus-independent pathways for T cell regeneration after BMT in children is still unclear. We analyzed the kinetics of T cell regenerative pathways after allogeneic BMT. The number of CD4⁺CD45RA⁺ T cells, a thymus-dependent population, was very low until 3 months after BMT. The numbers of CD28^- T cells and CD8⁺ T cells expressing CD8α/α homodimer (CD8α/α⁺ T cells), a thymus-independent population, increased shortly after BMT, beyond the levels of healthy children in some patients. The numbers of Vγ9⁺Vδ2⁺ and Vα24⁺ T cells, which represent populations of extrathymic development, were less than 200/μl during the 6 months after BMT. There was a significant inverse correlation between the percentages of CD4⁺CD45RA⁺ and CD28^- T-cells at 1 month, and a positive correlation between the percentages of CD28^- and CD8α/α⁺ T cells at 2 and 3 months after BMT. The mean age at BMT was higher in patients with a high level of CD8α/α⁺ T cells than in those without an increase in these cells, suggesting the influence of thymic function on the regenerative pathways. These results suggest that the thymus-independent pathway is the dominant source of T cells even in children shortly after allogeneic BMT.