Classic and new mediators for in vitro modelling of human macrophages

Abstract

Macrophages are key immune cells in the activation and regulation of immune responses. These cells are present in all tissues under homeostatic conditions and in many disease settings. Macrophages can exhibit a wide range of phenotypes depending on local and systemic cues that drive the differentiation and activation process. Macrophage heterogeneity is also defined by their ontogeny. Tissue macrophages can either derive from circulating blood monocytes or are seeded as tissue-resident macrophages during embryonic development. In humans, the study of in vivo-generated macrophages is often difficult with laborious and cell-changing isolation procedures. Therefore, translatable, reproducible, and robust in vitro models for human macrophages in health and disease are necessary. Most of the methods for studying monocytederived macrophages are based on the use of limited factors to differentiate the monocytes into macrophages. Current knowledge shows that the in vivo situation is more complex, and a wide range of molecules in the tissue microenvironment promote and impact on monocyte to macrophage differentiation as well as activation. In this review, macrophage heterogeneity is discussed and the human in vitro models that can be applied for research, especially for monocytederived macrophages. We also focus on new molecules (IL-34, platelet factor 4, etc.) used to generate macrophages expressing different phenotypes. K E Y W O R D S macrophages, monocytes, Immunotherapy, innate cell mediated immunity, in vitro model 1 INTRODUCTION Macrophages are immune cells with heterogeneous phenotypes and complex functions in tissue homeostasis and innate and acquired immunity. These cells belong to the mononuclear phagocyte system (MPS). 1-3 In the original MPS model, macrophages present in the tissues were all thought to be derived from monocytes. 4 In the 2000s this concept started to change when lineage-tracing studies showed that populations of macrophages with different origins were found in Abbreviations: BHA, Butylated hidroxyanisole; CaOx, Calcium oxalate; CD200, cluster of differentiation molecule 200; CRISPR/Cas9, clustered regularly interspaced short palindromic repeats/CRISPR associated protein 9; CX 3 CL 1 , Fractalkine; CX 3 CR 1 , fractalkine receptor; HIF-1 /HIF-1 , Hypoxia-inducible factor 1 / ; HSC, hematopoietic stem cell; iPSC, induced pluripotent stem cells; iPSDM, iPSC-derived macrophages; IRF4/5, IFN regulatory factor 4/5; LIPA, Lysosomal acid lipase; LL-37, Cathelicidin antimicrobial peptides LL-37; MDM, monocyte-derived macrophage; MPS, mononuclear phagocyte system; PF4, Platelet factor 4; TREM2, Triggering receptor expressed on myeloid cells 2. This is an open access article under the terms of the Creative Commons Attribution-NonCommercial-NoDerivs License, which permits use and distribution in any medium, provided the original work is properly cited, the use is non-commercial and no modifications or adaptations are made.

Macrophages derived from pluripotent stem cells: prospective applications and research gaps

“…Another advantage of iMphs over MDMs is the fact that the generation of iMphs recapitulates embryonic hematopoiesis and results in the formation of macrophages that reliably model TRMs [ 34 , 41 , 44 ]. Moreover, the differentiation of iMphs in tissue-specific conditions allows generating “tissue-specific” macrophages, e.g., cells recapitulating Kuppfer cells or microglia [ 20 , 152 , 153 ]. In contrast to iMphs, MDMs primarily model macrophages that repopulate tissues in inflammatory conditions [ 20 ].…”

“…Moreover, the differentiation of iMphs in tissue-specific conditions allows generating “tissue-specific” macrophages, e.g., cells recapitulating Kuppfer cells or microglia [ 20 , 152 , 153 ]. In contrast to iMphs, MDMs primarily model macrophages that repopulate tissues in inflammatory conditions [ 20 ].…”

“…For their part, MDMs have their own benefits. First, the generation of MDMs is significantly cheaper compared to that of iMphs, as it requires only one exogenous factor (most often, M-CSF) and can be accomplished in 7 days [ 20 ]. This stands in contrast to the use of at least two or even up to nine exogenous factors for iMph generation and a much longer period required to generate iMphs (especially if considering the time frame needed for the generation of iPSCs; reviewed in [ 21 ]).…”

“…Until recently, macrophages derived from peripheral blood monocytes (monocyte-derived macrophages, MDMs) were the main source of human macrophages [ 20 ]. Lately, approaches to generating macrophages (iMphs) from pluripotent stem cells, either induced or embryonic, were developed and were shown to have several advantages over the MDM model (discussed below).…”

IFN-γ Drives Human Monocyte Differentiation into Highly Proinflammatory Macrophages That Resemble a Phenotype Relevant to Psoriasis

“…Thus far, most human macrophage models use in vitrodifferentiated macrophages due to the difficulties associated with isolating human macrophages ex vivo. Although differentiation protocols are usually restricted to M-CSF or GM-CSF with only limited use of alternative differentiation factors, other factors can yield functional macrophages, too (31). Considering the importance of IFN-g as a potent regulator of human disease, we hypothesized that IFN-g would drive monocyte differentiation into proinflammatory macrophages.…”