Reconstitution of T-Cell Immunity in the Early Period after Allogeneic Hematopoieticstem Cell Transplantation.

Allogeneic hematopoietic stem cell transplantation (AHSCT) is often the only available treatment for leukemia. The transplantation procedure/conditioning regimen generally leads to a prolonged state of immunodeficiency characterized by persisting low levels of naïve T cells. Successful AHSCT requires reconstitution of normal T-cell immunity. Thymic function is necessary for de novo generation of T cells after transplantation.

In order to assess the reconstitution of T-cell immunity after AHSCT, we quantitatively analyzed thymic function through quantification of T cell receptor excision circles (TRECs) frequencies, in conjunction with immunophenotype analyses in a cohort of patients sampled at various times (at pre-HSCT, week 2, week 4, week 6, week 8, week 12, week 16 and week 20) after AHSCT.

We evaluated circulating T-cell phenotypes (CD45RA and CD45RO) and TRECs levels in 91 blood samples from 26 patients (median age, 32.0±12.7 years; range, 7–56 years) who had undergone AHSCT. All patients had received myeloablative conditioning regimens and were full donor chimeras in remission. Transplanted cells were obtained from the bone marrow or the peripheral blood of an HLA genotypically identical sibling. No specific procedure was carried out to enrich or deplete for a specific cell population. Normal controls were obtained from 16 age-matched healthy volunteers (median age, 30.8±7.6 years; range, 17 to 48 years). TRECs levels in DNA of peripheral blood mononuclear cells(PBMNCs)was detected by real-time PCR analysis. Early after AHSCT (within 5 weeks), the CD45RO+ T cells that expand were predominated and they were not thymus derived. But after 6 weeks post-AHSCT, CD45RA+ T cells predominated over CD45RO+ T cells in PBMNCs. TRECs levels were low or undetectable in the first 6 weeks after AHSCT. The mean value of TRECs levels lowered from 0.971±1.462 copies per 1000 PBMNCs on week 2 to 0.918±1.055 copies per 1000 PBMNCs at week 4, and near baseline at week 6 (0.107±0.108 copies per 1000 PBMNCs) after transplant. These results confirmed previous observations that TRECs levels in PBMNCs were restored in the short-term post-HSCT via peripheral expansion of graft-derived mature T cells.

TRECs levels raised after 8 weeks post-AHSCT. The mean value of TRECs levels at week 20 after AHSCT (1.247±1.100 copies per 1000 PBMNCs) was similar to the TRECs levels at pre-HSCT (1.119±1.549 copies per 1000 PBMNCs), however, it still lowered than the normal controls (3.011±0.838 copies per 1000 PBMNCs). Some cases after AHSCT had almost normal thymic output function recovery, but the others still had poor T cells immune reconstitution. In this study, 3 AHSCT recipients had early relapse, and their TRECs levels in PBMNCs were returning to the baseline or undetectable. It suggested that TRECs could be a relevant prognostic factor for patients who receive AHSCT.

In conclusion, although T-cell phenotypes had recovered by 6 weeks after AHSCT, TREC levels remained low for 8 weeks after AHSCT. T-cell neogenesis became evident by 16 weeks, and normal levels of thymic function began to restore at 20 weeks after AHSCT.