Requirement of residual thymus to restore normal T-cell subsets after human allogeneic bone marrow transplantation.

BACKGROUND: To determine the effect of residual thymic activity in reconstituting the T-cell system after T cell-depleting therapy, we monitored T-cell subsets of a unique thymectomized cancer patient in comparison to thymus-bearing patients after allogeneic bone marrow transplantation (BMT). METHODS: T cells and T-cell subsets previously shown in murine studies to be regulated by the thymus were analyzed by FACS from 6 to >48 months after BMT. The investigation of thymus-bearing patients included 32 examinations of 9 children and 14 adults. None of the investigated cases had severe graft-versus-host disease or severe infections when examined. RESULTS: In the thymectomized host, T-cell regeneration occurred by donor cell expansion and was characterized by two prominent features: (i) a persistent failure to regenerate naive (CD45RA+) T-helper cells (14%, median), consistent with the recently developed concept of a thymus-dependency; and (ii) persistently elevated proportions of CD3+CD4-CD8- cells (double-negative cells, median 29%), which were identified in T cell receptor (TCR)gamma delta+ (22%, median of CD3+ cells, 88% double negatives) but also TCRalpha beta+ T-cell populations (78%, median of CD3+ cells, 17% double negatives). In thymus-bearing patients, 10 of 12 and 6 of 14 examinations of children and adults, respectively, performed later than 12 months after BMT showed the proportion of CD4+CD45RA+ cells appropriate for age (>52% and >28% in children and adults, respectively). Elevated double-negative cells (>10%) were found in only three patients, but none had elevated double-negative cells with a TCRalpha beta+ phenotype. CONCLUSION: Residual thymic activity might, in addition to its well-established role for regenerating naive T-helper (CD4+CD45RA+) cells, control the expansion of double-negative cells. A normal T-cell subset regeneration in a proportion of thymus-bearing adult hosts indicates the potential of an effective residual thymic activity even beyond childhood.

Effective T cell regeneration following high-dose chemotherapy rescued with CD34+ cell enriched peripheral blood progenitor cells in children.

The ex vivo enrichment for the CD34+ cell fraction of PBPC, while it retains the capacity to restore haematopoiesis and potentially reduces a contamination by tumour cells, implements a depletion of T cells. To test whether such a setting adversely affects T cell reconstitution, we monitored T cells in four paediatric patients after CD34+ selected PBPC transplantation. The dose of CD34+ cells, which were enriched to 74%, median, was 7.1 x 10(6)/kg, median, that of T cells was 0.071 x 10(6)/kg, median. The patients were homogenous with respect to features with a potential to effect T cell reconstitution (low median age, (35 years); stage IV malignant tumours in first CR, uncomplicated post-treatment course). The results of sequential FACS analyses showed that by 9 months after treatment all four patients had recovered (1) a normal T cell count (CD3+ cells 1434/microl, median); (2) a normal CD4+ cell count (816/microl, median), while CD8+ cells were recovered (>330/microl) already by 3 months; (3) a normal CD4/CD8 ratio (1.8, median), as a result of an augmented growth of CD4+ cells between 3 and 6 months (increase of CD4+ cells 4.9-fold, median, CD8+ cells 1.1-fold, median). Expansion of cells with a CD45RA+ phenotype (thymus-derived T cells) predominated; from 3 to 6 months the increase of CD4+/CD45RA+ T cells was 130-fold, that of CD4+/CD45RO+ cells was 1.7-fold; CD8+/CD45RA+ cells increased 9-fold, CD8+/CD45RO+ cells increased 2.1-fold, indicating effective thymopoiesis. The findings demonstrate that in paediatric patients the setting of HD-CTX rescued with autologous CD34+ selected PBPC per se is not predictive of an impaired T cell recovery. High thymic activity may be a key factor for the rapid restoration of T cells.