Myeloid-derived suppressor cells (MDSCs) are regulatory cell populations that have the ability to suppress effector T cell responses and promote the development of regulatory T cells (Tregs). They are a heterogeneous population of immature myeloid progenitors that include monocytic and granulocytic subsets. We postulated that given the rapid expansion of myeloid cells post-transplant, these members of the innate immune system may be important contributors to the early immune environment post-transplant. To evaluate the kinetics of recovery and function of MDSCs after allogeneic hematopoietic stem cell transplant (HSCT), 26 patients undergoing allogeneic HSCT were studied at 6 time points in the first 3 months after HSCT. Both MDSC subsets recovered between 2 and 4 weeks, well before the recovery of T and B lymphocytes. MDSC subset recovery positively correlated with T, B, and/or double-negative T cell numbers after HSCT. MDSCs isolated from patients post-transplant were functional in that they suppressed third-party CD4(+) T cell proliferation and Th1 differentiation and promoted Treg development. In conclusion, functional MDSC are present early after HSCT and likely contribute to the regulatory cell population post-transplant.

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Background: Autologous myoblasts have been tested in the treatment of muscle-related diseases. However, the standardization of manufacturing myoblasts is still not established. Here we report a flask and animal-free medium-based method of manufacturing clinical-grade myoblast together with establishing releasing criteria for myoblast products under Good Manufacturing Practice (GMP). Methods: Quadriceps muscle biopsy samples were donated from three patients with myogenic ptosis. After biopsy samples were digested through enzymatic dissociation, the cells were grown in T175 flasks (passage 0) and hyperflasks (passage 1) in the animal-free SkGM TM -2 skeletal muscle cell growth medium containing 5% human platelet lysate for 15-17 days. The harvested cells were released based on cell morphology, cell dose, viability, sterility, endotoxin, mycoplasma and immunophenotype. Myotube differentiation was also evaluated. Results: 400 to 500 million myoblast cells were manufactured within 15 to 17 days by the end of passage 1, which met predetermined releasing criteria. The manufactured myoblast cells could differentiate and fuse into myotubes in vitro, with the possible trend that the Electronic supplementary material The online version of this article (

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). 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. Disclosures: No relevant conflicts of interest to declare.