A Novel, Three Dimensional (3D) Culture System for the Efficient, Single-Step Erythropoietic Differentiation of Mouse Embryonic Stem Cells (mESCs)
Authors: Fauzi Iliana , Panoskaltsis Nicki , Mantalaris Athanasios 
Published: October 2019

Abstract
Abstract 2044
Current established protocols for the culture and differentiation of embryonic stem cells (ESCs) utilise two-dimensional (2D) tissue culture flasks/dishes. These culture methods are cumbersome and inefficient involving three stages: a) maintenance/expansion of undifferentiated ESCs, b) spontaneous differentiation through formation of embryoid bodies (EBs), and c) dissociation of EBs and replating leading to the terminal differentiation to the desired lineages. One of the major challenges in the use of ESCs for the production of red blood cells is controlling their differentiation pathway(s). Optimal culture conditions and requirements as well as precise differentiation mechanisms and cellular interactions within EBs are still not well characterised, resulting in sub-optimal control of homogenous differentiation especially due to the formation of all three germ layers. Furthermore, cavitation within EBs results in loss of available cell numbers, which reduces the yield and quality of the cellular product outcomes. To date, the most efficient protocols for the generation of oxygen-transporting, enucleated red blood cells from ESCs require co-culture with feeder cells and a multi-step process that lasts for approximately one month rendering such protocols difficult to scale-up.
We have developed an integrated, single-step bioprocess that: a) uses conditioned medium (CM) derived from HepG2, a human hepatocarcinoma cell line, that stimulates mesoderm formation, b) facilitates 3D culture through encapsulation of undifferentiated mESCs in hydrogels, c) bypasses EB formation, and d) involves culture in a rotating wall vessel bioreactor that does not require passaging of the cells and is scalable and automatable.
Previously, we have shown that in traditional 2D culture systems use of HepG2-CM facilitated early differentiation of mESCs into hematopoietic cells, as shown by expression of C-Myb, C-kit, Gata-2, SCL, and beta-globin genes, in comparison with that of control cultures. A significantly higher number (p≤0.001) of hematopoietic colonies was also achieved in conditioned medium-treated murine embryonic stem cells (CM)-mESC, at day 7 and 14, with a two-fold enhancement of all myeloid-erythroid progenitor colonies. Nucleated eythrocytes and macrophages were identified in the CM-mESC group as early as day 7 of culture. However, attempting to bypass the EB formation step in the 2D culture system did not produce any hematopoietic cells even by the conditioned medium-treated embryonic stem cells. Here, we now demonstrate, that single-step 3D cultures of encapsulated mESCs can produce hematopoietic cells bypassing EB formation. Specifically, undifferentiated mESCs were encapsulated (20,000 cells per hydrogel bead) and placed inside the rotating wall vessel bioreactor. The experimental group was exposed to conditioned medium supplemented with LIF for 3 days to stimulate mesoderm formation bypassing EB formation and terminal hematopoietic differentiation was accomplished by simply changing the culture medium and replacing it with 3U/ml hEPO and 40ng/ml mSCF. A significant increase in the number (p≤ 0.05) of hematopoietic colonies was observed from CM-mESCs at day 14, with a total five-fold expansion as well as enhancement of erythroid progenitors, BFU-E and CFU-E formation. A higher expression of hematopoietic genes, C-Myb, C-kit, Gata-2, SCL, as well as erythroid genes, EKLF and beta-major globin was also reported at 3 weeks of culture in low concentrations of cytokines in the bioreactor. Immunophenotypic analysis of the highly viable cells collected from the CM-mESCs group confirmed the positive expression of the proerythroblast markers (TER-119 and CD71). In conclusion, we have devised a scalable, automatable, single-step process for the derivation of mature erythrocytes from mESCs which may be used for further study of erythroid development and applications in the human system.

Disclosures:
No relevant conflicts of interest to declare.