IL-3 Specifies Early Hematopoietic Development from Human iPSCs and Synergizes with M-CSF and G-CSF on Myeloid Differentiation

Hematopoietic in-vitro-differentiation of pluripotent stem cells (PSCs) such as embryonic stem cells (ESCs) or induced pluripotent stem cells (iPSCs) holds great promise for disease modeling, drug testing, as well as cell- and gene-therapy strategies. Although hematopoietic differentiation of PSC has been shown to be feasible, the majority of current protocols apply a large number of different cytokines to direct differentiation. In this line, priming the differentiation process by a multitude of cytokines may alter the endogenous hematopoietic differentiation program of PSCs, thus hampering the usefulness of such protocols to gain insight into physiologic human hematopoietic development. To overcome this problem we have investigated the hematopoietic differentiation potential of human PSC, based on minimal cytokine application. Given the emerging role of IL3 as a critical factor in adult hematopoiesis and the pivotal role of M-CSF and G-CSF for terminal myeloid differentiation, we here employed IL3 in combination with either M-CSF or G-CSF on hematopoietic development.

To prove our concept, human CD34⁺ cell-derived iPSC clones were subjected to an embryoid body (EB)-based myeloid differentiation protocol employing cytokines from day 5 onwards and yielding so-called “myeloid cell forming complexes” (MCFCs) within 7-10 days. Analysis of MCFC within 10 days of differentiation revealed expression of MIXL1, KDR1, GATA2, and RUNX1, as well as an early CD34⁺/CD45^- population undergoing transition to a CD34⁺/CD45⁺ and thereafter CD34^-/CD45⁺ phenotype. The hypothesis of a primitive hematopoietic cell arising from a population with dual (hematopoietic and vascular epithelial) potential was supported by co-staining of these populations with VE-cadherin (CD144). Here primarily the CD34⁺/CD45⁺/CD144^- cells were capable of colony formation in vitro.

Differentiation of PSC for more than 15-days resulted in the continuous shedding of hematopoietic cells from MCFCs and further differentiation along the IL3/M-CSF let to the generation of >99% pure monocytes/ macrophages (iPSC-MΦ), while IL3/G-CSF promoted granulopoiesis (iPSC-gra, purity >95%). Of note, hardly any CD34⁺ cells were detected among MCFC-shedded cells for the IL3/M-CSF as well as the IL3/G-CSF combination. In contrast, differentiation in IL3 only resulted in 10% MCFC-derived CD34⁺ cells, an observation further confirmed by a 10-times increased clonogenicity for cells shedded from MCFC exposed to IL3 only when compared to IL3/G-CSF or IL3/M-CSF cultures. Furthermore, cells cultured in IL3 maintained the capacity of subsequent M-CSF-driven terminal differentiation, whereas no suspension cells were observed following differentiation of PSC with G-CSF alone.

Most strikingly, IL3/M-CSF or IL3/G-CSF cultures generated iPSC-MΦ or iPSC-gra from day 14-15 onwards over a period of 3-5 months at a quantity of 0.4-2.0 x 10⁶ cells/week (cumulative 0.8-4.0 x 10⁷ cells) per 3.5 cm well. For IL3/M-CSF cultures detailed characterization of mature myeloid cells demonstrated a typical MΦ-morphology of iPSC-MΦ by cytospins and a surface-marker profile of CD45, CD11b, CD14, CD163, and CD68. In addition, iPSC-MΦ had the ability to phagocytose latex-coated beads similar to peripheral blood (PB)-MΦ polarized to M2 and upon LPS stimulation secreted MCP1, IL6, IL8, and IL10, whereas IFNy, IL1b, IL4, IL5, and IL12 were absent. iPSC-gra showed surface expression of CD45, CD11b, CD16, CD15, CD66b and a differential count containing pro-myelocyte (3%), myelocyte (5%), meta-myelocyte (30%), bands (22%), eosinophils (2%), basophils (1%), and segmented-neutrophils (37%) . Moreover, iPSC-gra were able to migrate towards an IL8 or fMLP gradient, formed neutrophil extracellular traps, and up-regulated NADPH activity and ROS production upon PMA stimulation to a similar degree as PB granulocytes.

In summary, we here present an in vitro differentiation protocols for human iPSC requiring minimal cytokine stimulation, which appears highly suited to model human hematopoietic development or generate cells for gene and cell-replacement strategies. We further provide evidence that IL3 constitutes a key cytokine driving the early hematopoietic specification of human PSC, whereas M-CSF and G-CSF function primarily as downstream “supporter” cytokines regulating the terminal differentiation towards macrophages and granulocytes, respectively.