Immune Reconstitution after Allogeneic Stem Cell Transplantation: a Pilot Flow-Cytometry Study for T-Cell Sub-Populations

Immune reconstitution after allogeneic hematopoietic stem cell transplantation (allo-SCT) is a complex process. From January 2014 we enrolled 6 patients with haematological disorders who were to undergo myeloablative allo-SCT (medium age: 42 years). All patients received apheretically collected stem cells, two from sibling HLA-identical donor, two from MUD full matched, two from haploidentical familiar donor. Peripheral blood samples were collected and analyzed one day before the beginning of the conditioning regimen and at days +30, +100, +180, +360 after stem cell infusion. Circulating T-cell populations are identified by 4-colour flow cytometry on the base of the co-expression of CDs according to the most recent knowledge. We studied absolute population counts and their percentage, and described the medium trend of reconstitution of different sub-populations. At time 0, CD45RA+ T lymphocytes were predominant; lymphocytes and T-reg were well represented in all patients. After allo-SCT, at day +30 memory T-cells seem to be the first expanding population. At the same time it is possible to observe a first increase of total circulating T cells (Fiure 1 A) after an extremely low count period. At day +100, Treg cells are measurable in most patients for the first time. Their mean number increases at day +180, and again at day +360. From day +180, mean T naïve cells percentage tends to rise. Between day +180 and day +360 almost every patient shows increased naïve T cells levels, while memory T-cells reach their minimum (at day +180). The expression of CD57, considered as a marker of senescence, is high in circulating T cells after 30 days and even higher after 100 days, this percentage keeps rising at days +180. Then, it’s possible to observe a plateau until +360. Some of the cases we studied seem to confirm the hypothesis of Treg cells inhibiting NK cells. The cases in which Tregs rise is the steepest seem to be the ones where NK cells levels drop more abruptly (Figure 1 B). The heterogeneous clinical history and the low number of patients do not allow to correlate a specific subtype of lymphocytes to a particular clinical feature (GvHD, outcome, CMV reactivation). In conclusion, this pilot study confirms some of the latest data on the timing of recovery after allo-SCT. A functional study of lymphocyte population, and an indirect measurement of thymic activity, linked to clinical features, could clarify the in vivo role of different lymphocyte populations.