Recovery Of Myeloid Derived Suppressor Cell Subsets Following Allogeneic Hematopoietic Stem/Progenitor Cell Transplantation

Myeloid derived suppressor cells (MDSCs) are a heterogeneous population of immature myeloid cells that expand during many inflammatory conditions and malignancies. MDSCs may play an important role following allogeneic hematopoietic stem/progenitor cell transplant (HSCT). MDSCs suppress T-cell, B-cell and dendritic cell responses by a number of mechanisms, including promoting regulatory T cell expansion and producing soluble mediators such as Arginase 1 (Arg-1) and iNOS. MDSCs are divided into two subsets: monocytic (M-MDSCs) and granulocytic (G-MDSCs). MDSC morphology and function differ in various tissues under different inflammatory conditions. In a murine asthma model, M-MDSCs inhibit airway inflammation, but the other subset of MDSCs exacerbated airway inflammation. In a sepsis model, MDSCs exaggerated inflammation in the early stage, but suppressed inflammation in the later stage of sepsis. As the early post-transplant period is characterized by the rapid expansion of immature myeloid cells, we postulated this time period may also be a time when MDSCs might play a major role in modulating immune recovery post-transplant, and aid in the development of immune regulatory networks potentially important in the pathophysiology of graft-versus-host disease (GVHD).

In nine patients undergoing allogeneic HSCT, peripheral blood was drawn on the day prior to the start of conditioning, days +4-5, +7-9, +14-16, +21-23, +27-29 and +80-100 post HSCT. White blood cells were quantified, red cell depleted using HetaSep (Stem Cell Technologies), then stained with fluorescence-labelled antibodies against CD45, CD15, CD14, HLA-DR, CD33 and CD66b and analyzed by flow cytometry for MDSC subsets. The soluble mediators iNOS and Arg-1were evaluated by intracellular staining for iNOS and Arg-1 and analyzed by flow cytometry.

Four of the nine patients developed acute GVHD (II-IV) and/or extensive chronic GVHD. Early recovery of CD33⁺CD14⁺HLA-DR^-/low M-MDSCs and CD33⁺CD15⁺CD66b⁺ G-MDSCs was seen post-transplant. Compared to healthy donors, the percentage of M- and G-MDSCs was increased by 3 weeks post-transplant. Interestingly, the patients who went on to develop GVHD had lower percentage and number of M-MDSCs, but inversely had higher numbers of G-MDSCs by day +27-29 and day +80-100 post-transplant (Fig. 1). When compared with healthy donors, the expression of Arg-1 in G-MDSCs, a measure of activation of MDSCs, was increased in patients pre- and post-HSCT, especially at day +80-100; while there was no difference seen iNOS expression in G-MDSCs (Fig 2). The expression of Arg-1 and iNOS in M-MDSCs was increased pre-transplant but fell by day +80-100 post HSCT (Fig 2).

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Taken together, our pilot data indicates that both M- and G- MDSCs recover early post HSCT and may contribute to the pathophysiology of GVHD. Patients with lower numbers of M-MDSCs and higher numbers of G-MDSCs at earlier time points post-transplant might be at greater risk for developing GVHD.