In the last few years, allogeneic hematopoietic stem cell transplantation (HSCT) has improved clinical outcomes, with less transplant-related mortality (TRM) and better overall survival (OS).1 Despite the advance in the knowledge of immune reconstitution in HSCT, there are still gaps in this field, such as the mechanisms of immunotolerance, graft-versus-host-disease (GvHD) or graft-versus-leukemia (GvL) effect.2

The umbilical cord blood (UCB) transplant was performed for the first time in 1988.3 The UCB transplant is associated with delayed engraftment, increased risk of infections, greater engraftment failure and worse TRM, compared to bone marrow (BM) and peripheral blood (PB) sources.4 However, the UCB transplant usually causes less acute GvHD (aGvHD) and chronic GvHD (cGvHD) with similar rates of leukemia-free survival.5 This could be explained by distinctive patterns of immune reconstitution according to the graft source,6 regardless of the conditioning regimen.7

Dendritic cells (DCs) are antigen-presenting cells, making the connection between innate and adaptive immunity,8 as well as coordinating the immune system through activation and stimulation of T and B cells, the tolerance by removal of self-reactive T cells 9 and the linkage with regulatory T cells.10 So, the DCs play a key role in the immune system control. In the PB, the DCs are present in two subsets: myeloid (or conventional) DCs (mDCs) and plasmacytoid DCs (pDCs). The pDCs (CD123+CD11c) produce interferon (IFN) I and are implicated in the viral immune response, immune tolerance and memory. At the same time, mDCs (CD123-CD11c+) are responsible for a proinflammatory effect. Each DC subset is flexible in vivo and its responses vary according to factors, such as its activation state, the nature of the stimulus received and the inflammatory microenvironment.11 In the immune recovery after the HSCT, the DCs play a relevant role in the aGvHD, cGvHD, GvL and host response to infections,12 while a poor DC reconstitution is associated with an increased risk of relapse and poor survival.13,14

The NKT cells (NKTs) are T cells that express NK receptors, including NK 1.1 (CD161c) and semi-invariant CD1d-restricted αβ T-cell receptors (TCRs). There are two types of NKTs: the type I NKT (or iNKT), that expresses the Vα24-Jα18, and the type II NKT that expresses TCRs other than the Vα24-Jα18.15

Natural killer cells (NKs) are components of the innate immune system, eliminating tumor and infected cells, promoting cellular lysis in the absence of the HLA class I receptor. The NK cells are categorized as the NK 56dim16bright (90% of NKs in PB), with a predominant cytotoxic effect through the release of granzyme B and perforin, and the NK 56bright16−, related to the interferon gamma (INF-γ) and tumor necrosis factor α (TNF-α) secretion.16 The NKs are the first cells to recover after the HSCT and strongly contribute to the GvL effect, possibly due to the killer-cell immunoglobulin-like receptor (KIR) mismatch.17 It has been shown that low counts of the NK 56bright16− after the HSCT are associated with a lower survival rate 18 and that KIR mismatches improve survival.19

To better understand the mechanisms involved in different outcomes in the HSCT using UCB or PB, we compared the composition of immune-related cells in these two HSC sources (monocytes, B lymphocytes, T cells, NK cells and DCs and their subsets) in samples of UCB and PB.

In the present study, we compared concentrations (cells per liter) and frequencies (%) of WBCs and subsets in the UCB and PB from healthy individuals and observed higher counts and frequencies of the NK 56bright16− NK cells and pDCs and lower counts and frequencies of non-classic monocytes in the UCB.

To the best of our knowledge, this is the first study that compared subsets of the NK (NK 56dim16bright and NK 56bright16−) between the UCB and PB. The NK 56bright16− is a more immature NK cell, presenting a more humoral immune response profile, secreting the INF-γ, TNF-α, IL-10, IL-13 and granulocyte-macrophage colony-stimulating factor, rather than the direct cytotoxic effect of the NK 56dim16bright. They usually do not present KIR receptors and present higher expressions of the heterodimer CD94/NKG2A (major inhibitor receptor), which confers a lower alloreactivity than their more mature NK 56dim16bright counterpart.20 Therefore, there are higher counts and higher frequencies of NK 56bright16− NK cells and plasmacytoid dendritic cells in the UCB than in the PB.

Of interest, the monocyte maturation was clearly more evident in the PB than in the UCB samples. The PB showed high proportions of the late phases of monocyte differentiation, such as intermediate and non-classic monocytes, which was also observed by some previous studies.21 These populations represent a shift towards higher antigen-presenting activity and they are considered as monocyte-derived DCs 22 and are also being related to autoimmune diseases, such as rheumatoid arthritis and Crohn's disease.23

We found higher counts of pDCs in the UCB than in the PB, corroborating the findings of Prabhu SB et al. 21; the pDC produces the IFN type I, which is related to antiviral response, immune tolerance and anti-tumor immunity through secretion of the type-I IFN and TNF-α.24 Moreover, our group previously demonstrated that grafts with a high pDC content lead to a lower aGVHD and lower mortality risk after the HSCT.18

The GvHD is a reaction caused in part by donor alloreactive T CD8+ and T cell depletion (in vivo or ex vivo), which is a common strategy to mitigate this complication.25,26 Here we demonstrated a higher concentration of the T CD8+ in the PB, compared to the UCB, as has been shown in previous studies.27–29

All these findings in the PB: more mature monocytes, a higher concentration of T CD8+ and lower counts of pDC and NK 56bright16−, compared to the UCB, might explain the higher incidence of the cGvHD when the PB is used as a stem cell source for the HSCT.30 Conversely, the more tolerogenic profile observed in the UCB, compared to the PB, could explain the lower risk of the cGvHD with similar relapse rates as evidence of the sustained GvL effect.5,31,32

Our study has limitations, such as small sample size and the use of PB samples without the stimulation of granulocyte colony-stimulating factors, as commonly used for the HSCT, which could change the cells counts and frequencies.

In conclusion, we observed higher counts of NK 56bright16− cells and pDCs and lower counts of non-classic monocytes in the UCB, compared to the PB from healthy individuals. Even though the PB analyzed in this study was not obtained through apheresis, our findings could suggest that the UCB cell profile is more tolerogenic than the PB and this finding might explain the lower incidence and severity of the cGVHD in UCB recipients, compared to other stem cell sources, although maintaining the GVL effect. Future research is needed for a better understanding of immune reconstitution in the HSCT and to explain the differences regarding immune reconstitution and the GvHD risk, the GvL effect and other clinical outcomes in the HSCT among different stem cell sources.

M.C.R.B., V.C.M, M.V.G. and C.A.R. designed the research, performed research, analyzed data and wrote the paper; V.C.M, M.V.G. and C.A.R. performed statistical analysis; A. M. J. and C. A. A provided cord blood samples, M.C.R.B., M.Y. and M.V.G. performed flow cytometry analysis, critical review and revised the manuscript. All authors drafted and approved the manuscript and agreed with its submission.

The authors have nothing to disclose.