The role of microenvironment in the in vitro directed hematopoietic pathway of murine embryonic stem cell differentiation (review)

Monocytes and macrophages are the targets for many animal lentiviruses, including the equine infectious anemia virus (I.P. Savchenkova et al., 2017). The complexity of the pathogenesis and insufficient knowledge of retroviral infections necessitate the search for an adequate cell model for their in vitro study. In this regard, obtaining macrophages via directed differentiation of embryonic stem cells (ESCs) in vitro, including those genetically transformed with equine gene, is of interest for veterinary medicine (I.P. Savchenkova et al., 2016). Mouse ESCs isolated from preimplantation embryos (M.J. Evans et al., 1981; G.R. Martin, 1981) have unique properties compared to other cell types (T.C. Doetschman et al., 1985; I.P. Savchenkova et al., 1996; A.M. Wobus et al., 2003), namely an unlimited capacity to proliferate and form all types of cells of the embryo and adult organism in vitro. They can be a valuable source for in vitro production of all types of mammalian tissues and organs for experimental research, including for the study and modeling of early hematopoiesis in in vitro culture. The review discusses issues related to the in vitro hematopoietic differentiation of ESCs (A.L. Olsen et al., 2006; I. Orlovskaya et al., 2008; J.A. Briggs et al., 2017). For this, various methodological approaches are used, which have advantages and disadvantages. Effects of cytokines, hematopoietic growth factors, and feeder layers, e.g. a monolayer of stromal cells, on differentiation in vitro of ESCs are under consideration. The attention extremely focuses on indirect method of differentiation by creating embryonic bodies (EBs) in vitro and simulating a microenvironment for differentiation. The microenvironment is shown to activate the hematopoietic cytodifferentiation pathways in mouse ESCs. It has been demonstrated that the conditions of culture and differentiation in vitro closest to those enabling hematopoiesis development in vivo, increases the efficiency of hematopoietic differentiation of ESCs. It is necessary to continue the search for a panel of factors that selectively direct the development of ESCs in the mesoderm and prevent their differentiation into ectoderm and endoderm. Obtaining new data will improve existing and develop new methods for creating specialized homogeneous populations of blood cells and the immune system in vitro with desired properties. Methods are currently being developed that make it possible to obtain macrophages in culture from ESCs (A. Subramanian et al., 2009; L. Zhuang et al., 2012; M. Pittet et al., 2014). Data are presented, including the author's own findings, on the role of the microenvironment in the differentiation of ESCs in macrophages in vitro. An indirect method of ESC differentiation through the creation of ETs in vitro and imitation of the microenvironment (addition of recombinant cytokines, the interleukin 3 and granulocyte macrophage colony-stimulating factor) can be considered as a more promising way to obtain macrophages in in vitro culture. An understanding of the regulatory mechanisms that drive the innate immune system may contribute to more effective research on lentiviruses with tropism for these cells. Production of monocytes and macrophages from ESCs in a culture of homogeneous cell population opens up new opportunities for studying the dependence of replication lentiviruses on the degree of cell differentiation.