Abstract • 164

Background: After stem cell transplantation (SCT) the immune system is known to be compromised. Phenotypic abnormalities in the T cell and B cell compartments, impaired mitogen and antigen responses are common and persisting for a long time. It is believed that the repopulation of the immune system from a limited pool of donor lymphocytes is achieved by division of peripheral, mature T cells. Several reports show that extrathymical maturation of T cells is possible and results in an expansion of usually minor T cell subpopulations like double positive T cells (CD3+ CD4+ CD8+) or double negative (CD3+ CD4- CD8-) T cells.

Methods: We analyzed peripheral blood of 18 children after stem cell transplantation (median 200 days after SCT, range 42-2727 days). The percentage and absolute counts of double negative T cells and double positive T cells were analyzed using triple marker flow cytometry.

Results: Of all CD3+ T cells, 43,5 % were CD8 single positive (SP), 48% were CD4 SP. 2,9% were CD4 and CD8 double positive (DP). CD4 and CD8 double negative (DN) T cells were found to be slightly expanded. When patients within the first year after SCT were compared with patients later after SCT there was a significant increase of DN T cells in the latter group. Patients in their first year (mean 4,7 months) after SCT had 3,8% DN T cells, whereas patients later (mean 52 months after SCT) showed 8,3% DN T cells (p< 0,001). CD4 and CD8 DP T cells were not different between the two groups (3,1% vs 2,6%).

Conclusion: We propose that this increase of DN T cells in the peripheral blood after SCT is the reflection of ongoing peripheral (extrathymically) expansion and that this extrathymical maturation is at least in part the basis of the observed long lasting functional abnormalities after stem cell transplantation.