The Elegance of a Macrophage

Cellular & Molecular Immunology (2018) 15, 196–198 & 2018 CSI and USTC All rights reserved 2042-0226/18 $32.00 www.nature.com/cmi RESEARCH HIGHLIGHT The elegance of a macrophage Maria De Santis1, Massimo Locati1,2 and Carlo Selmi1,3 Cellular & Molecular Immunology (2018) 15, 196–198; doi:10.1038/cmi.2017.64; published online 31 July 2017 In the latest issue of Nature Immunology, significantly contributed to the evolu- suchasbeesmodulatethesamegenome Piccolo et al.1 elegantly explored the tion of eukaryotic organisms.2 In other by nutritional input to have two pheno- effect of coexisting interferon-γ (IFN-γ) scenarios, the epigenome is what dis- typically distinct females, queens and and interleukin 4 (IL-4) antagonistic tinguishes humans from a nematode workers, by simply altering the epigen- signals on macrophage transcriptional such as Caenorhabditis elegans, in ome with royal jelly, which globally and epigenetic profiles and, interestingly, which the genome encodes nearly the inhibits DNA methylation.4 identified a plastic cross-talk as opposed same number of genes as the human Macrophages are crucial guardians of to mutually exclusive programs between genome but results in 300 neurons tissue homeostasis and host defense. M1 and M2 polarized macrophages. instead of 100 billion.3 Social insects Although they are the only innate Their results support the fascinating hypothesis that transcriptional and epigenetic reprogramming overcame genet- ics to drive the evolution of eukaryotic organisms. A rapid reshaping of chromatin acetylation redirected the expression of hundreds of genes under the control of a few transcription factors in response to two coexisting but opposing signals that indicated inflammation and its resolution simultaneously. We are currently understanding increasingly more about epigenetics in different areas of biology. Epige- netic changes initially allowed adapta- tion against external perturbations; they then became the basis of a complex organization that led to a flexible, coordinated and context-dependent expression of a multitude of genes that 1Rheumatology and Clinical Immunology, Huma- nitas Research Hospital, Rozzano, Italy; 2Labora- tory of Leukocyte Biology, Humanitas Research Hospital, Rozzano, Italy and 3BIOMETRA Depart- ment, University of Milan, Milan, Italy Correspondence: Dr C Selmi, MD, PhD, Division of Rheumatology and Clinical Immunology, Uni- versity of Milan, Humanitas Research Hospital, via A. Manzoni 56, Rozzano, Milan 20089, Italy. E-mail: [email protected] or carlo.sel- [email protected] Received: 9 June 2017; Accepted: 12 June Figure 1 The original illustration of a macrophage provided by Elie Metchnikoff who was 2017 awarded the Nobel prize in 1908 ‘in recognition of his work on immunity’.7 The elegance of a macrophage MDeSantiset al 197 immune cell type established during transcription factors, such as JUNB and the human response to complex inputs embryonic development and maintained C/EBPβ, for activation and were vulner- and for the transition from a pro- in adulthood by longevity and local self- able to the inhibitory effects of IL-4. On inflammatory to an anti-inflammatory renewal, they possess an extraordinary the other hand, only 317 IL-4-related program during the resolution of inflam- epigenetic plasticity that allows them to acetylated regions were resistant to cross- mation. A fascinating and well-studied acquire different functional properties to inhibition by INF-γ, and the majority example of complex macrophage func- best cope with a variety of external contained the canonical E-box tion is represented by granulomas: a stimuli and coordinate an immune recognized by Myc, while the larger macrophage-driven structure typical of response in all primary pathological set- IFN-γ-sensitive group showed the different diseases such as mycobacterial tings, ranging from inflammatory condi- enrichment of the STAT6-binding motif. infection, ANCA-associated vasculitis, tions to infections, or from tumors to Phosphorylation of both STAT1 and Crohn’s disease and sarcoidosis, all of degenerative disorders.5 Macrophage STAT6 occurred normally when IFN-γ which are typically associated with gran- plasticity has been explained by the and IL-4 antagonistic signals coexisted, ulomas. M2 plus Th2 are predominant in definition of the M1 and M2 programs, but IFN-γ caused a diminished associa- polyangitis with granulomatosis,11 but in driven by interferon-γ (IFN-γ) and inter- tion of STAT6 with chromatin. The Crohn’s disease and sarcoidosis, a clear leukin 4 (IL-4), respectively, which antagonistic effect of IL-4 on IFN-γ- M1 plus Th1 profile characterize the indeed represent two extremes of a broad induced histone acetylation was not due granuloma.12,13 Conversely, different spectrum of activation states.6 In life, to a reduction in the association of macrophage subsets can be identified macrophages are exposed to multiple STAT1 with its genomic targets, resulting under other conditions such as mycobac- microenvironmental signals that do not in an overall prevalence of IFN-γ signal- terial infection. However, rather than a induce irreversible differentiation pro- ing over the IL-4 program. mixed macrophage phenotype as grams, indicating that a better under- TheseresultssuggestthattheM1 reported in vitro when antagonistic standing of the inter-conversion between program is dominant over the M2 pro- stimuli coexist, distinct differently polar- macrophage states when microenviron- gram, and M1 inhibits M2 without ized macrophage subpopulations com- mental conditions change is a key step to completely blocking it. Interestingly, the pete for arginine in succession within a taking full advantage of macrophage so-called ‘alternative macrophage activa- mycobacterial granuloma during the dif- pleiotropism. tion’ is more ancient than the cytotoxic ferent stages of the disease. In fact, M1 Via a high-throughput RNA-based activity of the M1 macrophages, which macrophagesareprevalentintheinter- sequencing approach, Piccolo and col- was first discovered by Elie Metchnikoff mediate region of a granuloma and in the leagues extensively investigated gene in 1882 and is illustrated in Figure 1. early stage of the disease, while M2 expression programs in mouse bone Macrophage specialization precedes the macrophages are predominant in the marrow-derived macrophages after con- development of T and B cells and is peripheral area of a granuloma and in ditioning them with IFN-γ and IL-4, based on two ancient mechanisms called thechronicstageofthedisease.14,15 which were individually able to induce ‘the arginine fork in the road’: ornithine Nevertheless, we should also note that the phosphorylation of the key transcrip- and nitric oxide originate from the same the in vivo observation does not fully tion factors STAT1 and STAT6. The amino acid via different enzymatic reflect the cell transcriptome and that the results demonstrate that only a fraction reactions.8 The former is involved in M1/M2 discrimination is based only on a of the IFN-γ- and IL-4-induced genes, extracellular matrix formation for orga- limited number of markers. Interestingly, such as the canonical markers of M1 and nogenesis and subsequent wound heal- while the role of M1 is not essential to M2 polarization-inducible nitric oxide ing, while the latter is involved with the granuloma formation during mycobac- synthase (NOS2) and arginase 1, respec- primitive function as an endogenous terial infection, as shown by the ability of tively, were largely inhibited by the mediator.9 Both processes were under NOS2 knockout mice to form necrotic antagonistic stimulus, while many the respective positive and negative con- granulomas,16 M2-like macrophages are others were still inducible after trol of TGF-β.10 Only in vertebrates did thoughttohaveaprominentrole.How- co-stimulation. The main finding is the the M1 macrophages acquire the ability ever, IL-4 alone is not able to induce a identification of genomic regions with to produce a large amount of nitric oxide typical granuloma with multinucleate variable sensitivity to the co-stimulation in response to microbial infection with giant cells,17 while IL-10 depletion results by mapping genome-wide histone acet- the synergistic action of LPS and in a mature fibrotic granuloma.18 This ylation through chromatin immunopre- IFN-γ, and M2 macrophages were evidence suggests the existence of an cipitation coupled with next-generation involved in defense against parasites intricate landscape during infection and sequencing. Indeed, the authors found and allergy under the control of IL-4 immune diseases that involves a complex that the 773 IL-4-resistant elements acti- and IL-13.9 balance of different cytokines. Although vated by IFN-γ were enriched exclusively The power of the IFN-γ-stimulated Piccolo’s results warrant translation to a for consensus motifs recognized by response is now critical for survival from human setting and to specificdisease STAT1 and IRF1, while the 736 IFN-γ- infection, but the plasticity of macro- contexts, the data have crucial implica- inducible genes required auxiliary phages is probably essential to modulate tions for our understanding of Cellular & Molecular Immunology The elegance of a macrophage MDeSantiset al 198 mechanisms of macrophage polarization 2 Maleszka R, Mason PH, Barron AB. Epige- 13 Lissner D, Schumann M, Batra A, Kredel LI, and potentially for the development of nomics and the concept of degeneracy in Kuhl AA, Erben U et al. Monocyte and M1 biological systems. Brief Funct Genom

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