Theoretically, the T-lymphocyte population after HDIT and SCT can be regenerated with T lymphocytes from four sources: from expansion of T cells that are reinfused along with the stem cells; from transfused or residual stem cells through a process of thymic education, comparable to what happens in early childhood; from stem cells through an extrathymic pathway; and from residual memory T cells that survive the conditioning regimen.
Obviously, in case of HDIT alone, reconstituting T lymphocytes can only be derived from either endogenous stem cells or mature T lymphocytes that have survived the treatment. Age (which is associated with diminished thymopoiesis) and the extent of T-cell depletion (by in vivo or ex vivo manipulation of the graft or by [myelo]lymphoablation) are the major determinants of the relative contributions of these sources to the recovery of T-lymphocyte population after HDIT and SCT [5**]. This is reflected mainly in the CD4+ lymphocyte compartment, as assessed by immunophenotyping of circulating T cells. Naïve CD4+ T lymphocytes bear the CD45RA isoform, whereas memory CD4+ T lymphocytes express the CD45RO isoform. The level of naïve (CD45RA+) CD4+ cells drops rapidly after HDIT (whether followed by SCT or not) and recovers slowly, whereas levels of memory (CD45RO+) CD4+ and CD8+ cells recover more rapidly, and sometimes even 'overshoot' [6*,7,8*]. As a result, a prolonged T-cell subset imbalance with inverted CD4:CD8 ratios is found that can last for longer than a year, depending on the protocol.
These differential effects are explained by differences in maturation pathways for T-cell subsets. In principle, reconstitution of the naïve T-cell compartment is achieved by thymus-dependent and thymus-independent pathways. The development of naïve CD4+ T lymphocytes from stem cells has been shown to be mainly thymus dependent. The observation of an inverse correlation between the size of the thymus and the level of the CD4+cells in the peripheral blood after HDIT (without SCT) supports the notion that CD4+ development depends on residual thymus function [9*]. Additional evidence was recently obtained by T-cell receptor (TCR) recombination excision circle analysis [10*], which showed that thymic output of immature naïve CD4+ T lymphocytes decreases rapidly early in life, and stabilizes at a low level after adolescence and until old age. In contrast, levels of circulating memory CD4+ T lymphocytes and of CD8+ T lymphocytes are not severely affected by HDIT and SCT. This is because thymus-independent maturation pathways dictate the regeneration of CD8+ T lymphocytes comprising both extrathymic lymphopoiesis from haematopoietic precursors and peripheral expansion of mature CD8+T cells, especially the CD8+CD28- subset [6*].
Cotransfused or residual memory CD4+ T cells also expand during the first months after transplantation [11,12*]. The peripheral expansion of mature T lymphocytes observed after HDIT (with or without SCT) has been attributed in part to the encounters with viruses, and is reflected by the elevated expression of activation markers CD25, CD38 and most notably human leucocyte antigen-DR [13,14]. The wave of activated memory CD4+T lymphocytes subsequently subsides due to increased susceptibility to apoptosis [15*]. In the autologous transplant setting it is difficult to determine the precise origin (cotransfused or residual) of this early expanded T-cell pool. Data from studies in recipients of T-cell-depleted allogeneic bone marrow transplant (BMT) [11,12*,16**] indicate that the early post-transplant T-cell compartment has a mixed origin, with T cells derived from both transferred donor T cells and surviving host T cells. In contrast, in patients who underwent unmanipulated BMT, only few recipient T-cell clones were detected, and most were of donor origin. It is likely that similar competing mechanisms apply in the autologous setting. In keeping with this is the observation that recovery of total numbers of circulating CD4+ T lymphocytes after HDIT and SCT is related to the number of T lymphocytes present in the graft. T-cell depletion or CD34+ stem-cell enrichment of a graft leads to delayed CD4 lymphocyte recovery, and CD4 lymphocyte recovery occurs more rapidly after autologous peripheral blood SCT than after autologous BMT because higher numbers of lymphocytes in the former graft [13,16**,17*].
Whatever the origin of these early memory T cells, the initial expansion of a limited number of mature T-cell clones results in a skewed oligoclonal repertoire, whereas subsequent diversification of TCR repertoires after T-cell-depleted SCT is dependent on the capacity of the thymus to generate naïve thymic emigrants [18*,19]. Molecular analysis of TCR repertoires by CDR3 size spectratyping has revealed differences in T-cell populations isolated before and after transplantation with CD34+-enriched autologous peripheral blood stem cells, possibly in response to antigenic stimuli or to a stochastic process [20**].
Analogous to repopulating T lymphocytes, post-transplant B lymphocytes are derived from either cotransfused or residual B cells, or from transplanted or residual stem cells, depending on the treatment regimen employed. The documented transfer of humoral immunity in allogeneic transplantation suggests that differentiated antigen-selected B cells in the graft can be a significant contributor to post-transplant B cells, whereas residual B cells have also been demonstrated to contribute [5**]. Finally, immunophenotyping data lend support to a role of stem cells as a source of post-transplant circulating B cells. After an initial drop in circulating CD19+ and CD20+ mature B cells, B cells emerge with an undifferentiated phenotype. In the case of CD34+-selected SCT, an over-representation of CD5+CD19+ B cells can be detected during the early post-transplant period, but the significance of this is not well understood [20**]. Although the regeneration of B cells after transplantation has been interpreted as a recapitulation of foetal ontogeny, this has been disputed on the basis of structural differences in B-cell receptor repertoires .