Review articleDesigner macrophages: Pitfalls and opportunities for modelling macrophage phenotypes from pluripotent stem cells
Introduction
Macrophages (Mф) are specialised phagocytes that are resident in every tissue. Phagocytes are a primitive form of host defence, and one of the most evolutionary conserved components of the immune system (Reviewed by Tauber, 2003). In addition to host defence, mфs have also been implicated in roles as diverse as organogenesis and tissue homeostasis. Innate immune cells, including mфs, have memory-like features often referred to as ‘training’, resulting in an enhanced or hypo-responsiveness to repeated pathogenic challenge. These different functions and roles that mфs play serve to emphasise the importance of developing appropriate in vitro model systems to investigate different aspects of their biology. There has been an increasing number of protocols describing mф derivation processes from stem cells (Fig. 1; Table 1; Supplementary Table 2). These protocols utilise various culture strategies and factors capable of influencing mф maturation, phenotype, function and responsiveness (Fig. 2). This review will first provide an overview of the use of pluripotent stem cells (PSCs) in modelling mф biology and explore aspects of the in vitro environment, including culture conditions, supplemented factors and exposure history, which may impact on the function or polarisation of hPSC-derived mфs.
Section snippets
Opportunity: human pluripotent stem cells for disease modelling in vitro
The ability to derive mфs from human pluripotent stem cells (hPSCs) provides new opportunities to develop models relevant to human genetics. This has resulted in a progressive accumulation of studies that have described human mф functions in both health and disease. For example, human induced pluripotent stem cell (iPSC)-derived mфs have been utilised to investigate Blau Syndrome, a hereditary systemic granulomatosis (Takada et al., 2018). In this study, CRISPR-Cas9 genome editing was used to
Opportunity: macrophage tissue-resident specialisation and ontogeny
The phenotype of hPSC-mфs are largely based on the expression of common blood or myeloid cell surface markers, such as CD14, CD16 and CD68. Assessment of mф function commonly includes assessment of phagocytosis or inflammatory cytokine production (see Supplementary Table 1). However, these general phenotypic assays don’t reflect the diverse functions and complex roles that mфs play in tissue homeostasis, injury or infection. Mфs are found in different tissues with specialised functions that are
Pitfall: the impact of cell culture on macrophage function
As we increasingly become aware of the factors that can contribute to mф heterogeneity and specialisation, it seems obvious to consider the impact of the in vitro environment on derived mфs. For example, exposure of iPSC-derived mфs to feeder cells, serum and embryoid-bodies (EBs) may influence the adoption of a ‘tissue’ mф-like phenotype (or activation-state). EBs, by definition, give rise to cells from the three germ layers (endoderm, ectoderm and mesoderm), so may predispose mфs to resemble
Opportunity: the influence of growth factors and cytokines on macrophage phenotype and function
Since the earliest days of clonal agar expansion of mouse bone marrow progenitors, in vitro models have been used to describe mф differentiation and activation. Typically, macrophage colony stimulating factor (mCSF; CSF-1) and granulocyte macrophage colony stimulating factor (GM-CSF) have been used for clonal expansion of myeloid cells from blood or bone marrow progenitors (Metcalf and Nicola, 1983). The derivation of mфs from hPSCs has evolved over the years, particularly with sequential
Pitfall: the impact of exposure history on cell responsiveness
There is renewed interest in the impact of exposure history on the subsequent responsiveness of a mature mф to pathogenic challenge, although the consequences of early instruction by pathogen, or cytokines, or microenvironment during mф development remains to be fully explored. This is encapsulated in the concept of innate immune ‘training’, a critical mechanism for optimising and resolving inflammatory responses. An aspect of innate immune training has been the ability of mфs to become
Concluding remarks
The importance of mфs in health and disease has led to an increasing demand for in vitro models relevant to human studies. The development of methodologies utilising human stem cells for mф derivation has brought us closer to establishing such models. Upon recapitulation of the essential consequences of ontogeny or tissue niche, we will be closer to developing more specialised tissue-resident mф populations. However, formal evaluation of the impact of the derivation history on the function and
Funding acknowledgements
N.R. is funded by the Centre for Stem Cell Systems and the CSIRO Synthetic Biology Future Science Platform, and C.A.W. is funded by the Australian Research Council (FT150100330) and Stem Cells Australia (SR110001002).
Declarations of interest
None.
Author contributions
N.R.: conception, analysis, figure design and writing
M.R.: conception, figure development and editing
A.L.L: conception and editing
C.A.W: conception and editing
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