Gwendalyn J. Randolph
Author’s address Gwendalyn J. Randolph1 1 Department of Pathology and Immunology, Washington University School of Medicine, St. Louis, MO, USA. Correspondence to: Gwendalyn J. Randolph Department of Pathology and Immunology Washington University School of Medicine St. Louis, MO 63110, USA Tel: +1 314 286-2345 e-mail: [email protected] Acknowledgement The author has no conflicts of interest.
This article introduces a series of reviews covering Monocytes and Macrophages appearing in Volume 262 of Immunological Reviews.
Immunological Reviews 2014 Vol. 262: 5–8 Printed in Singapore. All rights reserved
© 2014 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd
Immunological Reviews 0105-2896
A macrophage revolution—and beyond
State-of-the-art technologies have fueled an explosion of excitement about macrophages in the last few years. Gene expression profiling and mouse lines that allow for lineageselective gene ablation or temporal induction of lineage tracers comprise leading technological advances that have radically reshaped our views about the macrophage life cycle. The previous paradigm, with roots extending back more than 50 years, held that resident tissue macrophages throughout the body originated from circulating precursors in the blood, namely monocytes, and that monocytes continuously replaced them as needed throughout life. The new paradigm is that as resident macrophage populations appear within developing organs embryonically, those macrophages expand as the organ does and repopulate to maintain the macrophage pool at a steady level thereafter. In this paradigm, monocytes are relegated to the role of macrophage precursors particularly during inflammation. At the same time, large-scale gene expression profiling reveals a remarkable diversity of macrophages between different organs and evidence is mounting that discrete transcription factors, expressed and activated only in single organ’s macrophages in some cases, accounts for this diversity. Given the extent to which these advances have affected the field, it is appropriate that a number of reviews in this present volume directly address macrophage origin in various organs and the regulation of the specialized macrophages found in them (1–6). Gordon et al. (3) give readers a comprehensive background on how the phenotypic evaluation of many different distinct macrophage populations in vivo and in vitro evolved over time as tools and reagents were developed. Differences in interpretation that are especially present at early stages in the development of a new paradigm come through in the reviews. For example, Guilliams et al. (1) focus on macrophages in mucosae: the dermis, lung, and intestines. Interestingly, the dermis and the intestines are two organs where the life cycle of macrophages clearly does not fit the stated paradigm of embryonic origins
© 2014 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd Immunological Reviews 262/2014
5
Randolph A macrophage revolution
for resident macrophages. Monocytes appear to be the major precursor for intestinal macrophages and for a substantial proportion of skin macrophages. Guilliams et al. (1) discuss these populations in detail and argue that contact with microbes, especially relevant in skin and intestine, may be a driving force behind the rapid turnover of macrophages in these organs that requires monocyte input for replenishing. However, Sieweke et al. (4) argue this concept is flawed, because monocyte input into resident macrophages occurs in central organs like the heart not in contact with a microbial interface and that sites like the epidermis, where contact with microbes is more substantial than the dermis, do not promote continuous recruitment of monocytes to replace epidermal Langerhans cells. These discussions reveal a healthy, active debate about the current data from different mouse organs that make reading all the reviews on partially overlapping topics worthwhile. The articles by Hume (5) and Gordon (3) both discuss how the field must cope with and develop new analytical approaches to best take advantage of the large volume of data generated by genomic, proteomic, and lipidomic datasets on macrophages in multiple species. Hume (5) delves into the complex transcriptional architecture that underlies macrophage gene expression, and then an article by Glass and Gosselin (7) beautifully reveals how the field of epigenomics facilitates an understanding of the complex transcriptional regulation that contributes to macrophage identity and diversity. The article discusses in a very accessible way how lineage-determining transcription factors (LDTFs), such as PU.1 and CEBP/b in macrophages, serve to make DNA-binding sites accessible to signal-dependent transcription factors (SDTFs) that in turn explains why activation of the same SDTFs in different cell types leads to engagement of distinct transcriptional targets. Non-coding RNAs called enhancer RNAs are often produced at the site of LDTF binding to promote accessibility of an enhancer. For instance, Rev-Erb suppresses the production of eRNA from enhancers of the Cx3cr1 gene, thereby downmodulating transcriptional activity for producing Cx3cr1 mRNA, because production of the eRNA serves as a physical mechanism to maintain accessibility of the gene to RNA polymerase II (7). Self-replenishment of resident macrophages in homeostatic organs relies on proliferation, and the regulation of homeostatic proliferation is extensively discussed by Sieweke et al. (4), including evidence that macrophages proliferate in their fully differentiated state rather than new macrophages arising from a less differentiated macrophage progenitor res-
6
ident in tissues. Interestingly, Glass and Gosselin (7) point out how rates of proliferation affect the accumulation of transcription factors and thus proliferation rate itself can serve as an independent effector producing transcriptional diversity (7). An article by Allen et al. (8) expounds on the regulation of macrophage proliferation in the context of infection with helminths, wherein local macrophage proliferation can substantially contribute to the expansion of macrophage numbers associated with type 2 inflammation. This article also beautifully introduces one major form of activation, alternative activation, canonically mediated by interleukin-4 (IL-4). The article makes a strong case for alternative activation being associated with tissue repair and discusses the biological roles of key mRNA transcripts typically associated with alternative activation, like arginase 1 involved in collagen synthesis and hence tissue repair and fibrosis. This discussion sets the stage for other articles that consider macrophages in inflammatory and infectious disease states, including obesity (9) and atherosclerosis (10), two other articles that importantly include discussion of alternatively activated macrophages, as well as classically activated macrophages, and more. Hasty et al. (9) provide a balanced discussion of the literature on macrophages in obesity including a discussion of data from different groups that do not always reach similar conclusions. These two articles also bring us back to the consideration of monocyte-derived macrophages in the context of inflammation. Dutta and Nahrendorf (11) round off this discussion with an article that focuses on how monocyte availability in the circulation is regulated and how that in turn affects the course of inflammatory diseases, at least in part by governing the number of monocytes that are available to give rise to inflammatory macrophages. The newly emergent paradigm on the origin of resident tissue macrophages has raised much speculation on the possibility and potential implications that resident macrophages from embryonic origins and those derived from adult monocytes may have distinct biological roles in a given organ. Sieweke et al. (4) propose that the major difference might not be in distinct end states of differentiation but rather differences in propensity to display subtly different phenotypes that are accessible to both types of macrophages but in different proportions. This concept is in line with the notion that changes in the enhancer landscape would likely give rise to quantitative rather than qualitative differences (7). Finally, in the consideration of macrophages in disease states, tuberculosis has long been a classic example of a disease, where macrophages are cen© 2014 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd Immunological Reviews 262/2014
Randolph A macrophage revolution
trally involved. Ernst et al. (12) discuss the role of macrophages in tuberculosis extensively, even as they point out the diversity of myeloid cells that work in concert to affect the course of disease, including granulocytes and dendritic cells. Critical to the biology of macrophages in health or disease, no matter the organ where they are found, are fundamental functional properties that truly define macrophages. Focus on such functional properties and the frontier of research in these important areas sometimes gets lost in the excitement over the developing new concepts on macrophage origin. In that light, this review series contains two articles that respectively focus on the key functions of phagocytosis and migration in macrophages, providing
state-of-the-art insight into the cell biology of macrophages. Indeed, when one delves into these two articles, it becomes apparent that the classical disciplines of cell signaling and cell biology need not be forgotten in the race to understand the transcriptional control of macrophage identity, diversity, and activation. In their article on phagocytosis—arguably the core macrophage function—by Grinstein and Freeman (13), signal integration from a variety of phagocytic receptors is discussed and the point is made that signal output is influenced by multiple parameters, including cell shape and degree of adhesion. Maridonneau-Parini (14) provides a compelling article on signaling and coordination of macrophage motility. She discusses the evidence that macrophages adapt very different modes of migration, ameboid or
Fig. 1. Model for diversity in macrophages promoted by diversity in the microenvironment. The left of the figure depicts resting resident macrophages (top and lower left) in distinct microenvironments taking on distinct shapes. Packing of chromatin is schematized on the right, with lineage-determining transcription factors creating sites of chromosomal accessibility for the binding of signal-dependent transcription factors. These factors are likely distinct in different environments. Mobilization of macrophages in response to signals that include phagocytic cargo or pathogens for engulfment lead the macrophage to change its shape and overall interaction with the environment. Cytoskeletal rearrangements serve as part of the critical signaling events that occur in response to phagocytosis and migration; these new signals generate yet additional signaldependent transcription factors that work together with lineage-determining transcription factors to effect gene expression. Further reading on this concept is found throughout this volume, including imagery modeled after the figures in reviews by Gordon (3), Glass (7), Grinstein (13), and Maridonneau-Parini (14) and their colleagues. © 2014 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd Immunological Reviews 262/2014
7
Randolph A macrophage revolution
mesenchymal, depending upon the porosity and composition of the extracellular matrix. She makes a case that macrophages are uniquely adaptable in selecting between different modes of migration, compared with other cell types that exhibit particular default modes (14). Gordon et al. (3) show a figure in which the shape of brain macrophages, microglia, is strikingly different in different regions of the central nervous system, as they also point out that we know relatively little about stromal cell-macrophage interactions that likely affects the assumption of the distinct shapes observed. The upshot of this discussion is that macrophage diversity in different organs, and corresponding diversity in macrophage activities, is as much a
problem of cell biology as it is genomics and epigenomics, because the extracellular interactions between macrophages and their environment no doubt pivotally influence, if not dictate, macrophage phenotypes (Fig. 1). Like the macrophage able to integrate numerous inputs and adapt accordingly, future advances in macrophage biology will be richly found among those scientists able to integrate research on macrophages coming from diverse disciplines. This volume, containing a series of reviews that collectively covers many different aspects of macrophage biology, provides a wonderful starting point for integrative reading on macrophages across disciplines.
References 1. Scott CL, Henri S, Guilliams M. Mononuclear phagocytes of the intestine, the skin, and the lung. Immunol Rev 2014;262:9–24. 2. Haldar M, Murphy KM. Origin, development, and homeostasis of tissue-resident macrophages. Immunol Rev 2014;262:25–35. 3. Gordon S, Pl€ uddemann A, Martinez Estrada F. Macrophage heterogeneity in tissues: phenotypic diversity and functions. Immunol Rev 2014;262:36–55. 4. Gentek R, Molawi K, Sieweke MH. Tissue macrophage identity and self-renewal. Immunol Rev 2014;262:56–73. 5. Hume DA, Freeman TC. Transcriptomic analysis of mononuclear phagocyte
8
6.
7. 8.
9.
differentiation and activation. Immunol Rev 2014;262:74–84. Gautier EL, Yvan-Charvet L. Understanding macrophage diversity at the ontogenic and transcriptomics levels. Immunol Rev 2014;262:85–95. Gosselin D, Glass CK. Epigenomics of macrophages. Immunol Rev 2014;262:96–112. R€ uckerl D, Allen JE. Macrophage proliferation, provenance, and plasticity in macroparasite infection. Immunol Rev 2014;262:113–133. Hill AA, Bolus WR, Hasty AH. A decade of progress in adipose tissue macrophage biology. Immunol Rev 2014;262:134–152.
10. Colin S, Chinetti-Gbagiudi G, Staels B. Macrophage phenotypes in atherosclerosis. Immunol Rev 2014;262:153–166. 11. Dutta P, Nahrendorf M. Regulation and consequences of monocytosis. Immunol Rev 2014;262:167–178. 12. Srivastava S, Ernst JD, Desvignes L. Beyond macrophages: the diversity of mononuclear cells in tuberculosis. Immunol Rev 2014;262:179–192. 13. Freeman SA, Grinstein S. Phagocytosis: receptors, signal integration, and the cytoskeleton. Immunol Rev 2014;262:193–215. 14. Maridonneau-Parini I. Control of macrophage 3D migration: a therapeutic challenge to limit tissue infiltration. Immunol Rev 2014;262:216–231.
© 2014 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd Immunological Reviews 262/2014
Author’s address Gwendalyn J. Randolph1 1 Department of Pathology and Immunology, Washington University School of Medicine, St. Louis, MO, USA. Correspondence to: Gwendalyn J. Randolph Department of Pathology and Immunology Washington University School of Medicine St. Louis, MO 63110, USA Tel: +1 314 286-2345 e-mail: [email protected] Acknowledgement The author has no conflicts of interest.
This article introduces a series of reviews covering Monocytes and Macrophages appearing in Volume 262 of Immunological Reviews.
Immunological Reviews 2014 Vol. 262: 5–8 Printed in Singapore. All rights reserved
© 2014 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd
Immunological Reviews 0105-2896
A macrophage revolution—and beyond
State-of-the-art technologies have fueled an explosion of excitement about macrophages in the last few years. Gene expression profiling and mouse lines that allow for lineageselective gene ablation or temporal induction of lineage tracers comprise leading technological advances that have radically reshaped our views about the macrophage life cycle. The previous paradigm, with roots extending back more than 50 years, held that resident tissue macrophages throughout the body originated from circulating precursors in the blood, namely monocytes, and that monocytes continuously replaced them as needed throughout life. The new paradigm is that as resident macrophage populations appear within developing organs embryonically, those macrophages expand as the organ does and repopulate to maintain the macrophage pool at a steady level thereafter. In this paradigm, monocytes are relegated to the role of macrophage precursors particularly during inflammation. At the same time, large-scale gene expression profiling reveals a remarkable diversity of macrophages between different organs and evidence is mounting that discrete transcription factors, expressed and activated only in single organ’s macrophages in some cases, accounts for this diversity. Given the extent to which these advances have affected the field, it is appropriate that a number of reviews in this present volume directly address macrophage origin in various organs and the regulation of the specialized macrophages found in them (1–6). Gordon et al. (3) give readers a comprehensive background on how the phenotypic evaluation of many different distinct macrophage populations in vivo and in vitro evolved over time as tools and reagents were developed. Differences in interpretation that are especially present at early stages in the development of a new paradigm come through in the reviews. For example, Guilliams et al. (1) focus on macrophages in mucosae: the dermis, lung, and intestines. Interestingly, the dermis and the intestines are two organs where the life cycle of macrophages clearly does not fit the stated paradigm of embryonic origins
© 2014 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd Immunological Reviews 262/2014
5
Randolph A macrophage revolution
for resident macrophages. Monocytes appear to be the major precursor for intestinal macrophages and for a substantial proportion of skin macrophages. Guilliams et al. (1) discuss these populations in detail and argue that contact with microbes, especially relevant in skin and intestine, may be a driving force behind the rapid turnover of macrophages in these organs that requires monocyte input for replenishing. However, Sieweke et al. (4) argue this concept is flawed, because monocyte input into resident macrophages occurs in central organs like the heart not in contact with a microbial interface and that sites like the epidermis, where contact with microbes is more substantial than the dermis, do not promote continuous recruitment of monocytes to replace epidermal Langerhans cells. These discussions reveal a healthy, active debate about the current data from different mouse organs that make reading all the reviews on partially overlapping topics worthwhile. The articles by Hume (5) and Gordon (3) both discuss how the field must cope with and develop new analytical approaches to best take advantage of the large volume of data generated by genomic, proteomic, and lipidomic datasets on macrophages in multiple species. Hume (5) delves into the complex transcriptional architecture that underlies macrophage gene expression, and then an article by Glass and Gosselin (7) beautifully reveals how the field of epigenomics facilitates an understanding of the complex transcriptional regulation that contributes to macrophage identity and diversity. The article discusses in a very accessible way how lineage-determining transcription factors (LDTFs), such as PU.1 and CEBP/b in macrophages, serve to make DNA-binding sites accessible to signal-dependent transcription factors (SDTFs) that in turn explains why activation of the same SDTFs in different cell types leads to engagement of distinct transcriptional targets. Non-coding RNAs called enhancer RNAs are often produced at the site of LDTF binding to promote accessibility of an enhancer. For instance, Rev-Erb suppresses the production of eRNA from enhancers of the Cx3cr1 gene, thereby downmodulating transcriptional activity for producing Cx3cr1 mRNA, because production of the eRNA serves as a physical mechanism to maintain accessibility of the gene to RNA polymerase II (7). Self-replenishment of resident macrophages in homeostatic organs relies on proliferation, and the regulation of homeostatic proliferation is extensively discussed by Sieweke et al. (4), including evidence that macrophages proliferate in their fully differentiated state rather than new macrophages arising from a less differentiated macrophage progenitor res-
6
ident in tissues. Interestingly, Glass and Gosselin (7) point out how rates of proliferation affect the accumulation of transcription factors and thus proliferation rate itself can serve as an independent effector producing transcriptional diversity (7). An article by Allen et al. (8) expounds on the regulation of macrophage proliferation in the context of infection with helminths, wherein local macrophage proliferation can substantially contribute to the expansion of macrophage numbers associated with type 2 inflammation. This article also beautifully introduces one major form of activation, alternative activation, canonically mediated by interleukin-4 (IL-4). The article makes a strong case for alternative activation being associated with tissue repair and discusses the biological roles of key mRNA transcripts typically associated with alternative activation, like arginase 1 involved in collagen synthesis and hence tissue repair and fibrosis. This discussion sets the stage for other articles that consider macrophages in inflammatory and infectious disease states, including obesity (9) and atherosclerosis (10), two other articles that importantly include discussion of alternatively activated macrophages, as well as classically activated macrophages, and more. Hasty et al. (9) provide a balanced discussion of the literature on macrophages in obesity including a discussion of data from different groups that do not always reach similar conclusions. These two articles also bring us back to the consideration of monocyte-derived macrophages in the context of inflammation. Dutta and Nahrendorf (11) round off this discussion with an article that focuses on how monocyte availability in the circulation is regulated and how that in turn affects the course of inflammatory diseases, at least in part by governing the number of monocytes that are available to give rise to inflammatory macrophages. The newly emergent paradigm on the origin of resident tissue macrophages has raised much speculation on the possibility and potential implications that resident macrophages from embryonic origins and those derived from adult monocytes may have distinct biological roles in a given organ. Sieweke et al. (4) propose that the major difference might not be in distinct end states of differentiation but rather differences in propensity to display subtly different phenotypes that are accessible to both types of macrophages but in different proportions. This concept is in line with the notion that changes in the enhancer landscape would likely give rise to quantitative rather than qualitative differences (7). Finally, in the consideration of macrophages in disease states, tuberculosis has long been a classic example of a disease, where macrophages are cen© 2014 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd Immunological Reviews 262/2014
Randolph A macrophage revolution
trally involved. Ernst et al. (12) discuss the role of macrophages in tuberculosis extensively, even as they point out the diversity of myeloid cells that work in concert to affect the course of disease, including granulocytes and dendritic cells. Critical to the biology of macrophages in health or disease, no matter the organ where they are found, are fundamental functional properties that truly define macrophages. Focus on such functional properties and the frontier of research in these important areas sometimes gets lost in the excitement over the developing new concepts on macrophage origin. In that light, this review series contains two articles that respectively focus on the key functions of phagocytosis and migration in macrophages, providing
state-of-the-art insight into the cell biology of macrophages. Indeed, when one delves into these two articles, it becomes apparent that the classical disciplines of cell signaling and cell biology need not be forgotten in the race to understand the transcriptional control of macrophage identity, diversity, and activation. In their article on phagocytosis—arguably the core macrophage function—by Grinstein and Freeman (13), signal integration from a variety of phagocytic receptors is discussed and the point is made that signal output is influenced by multiple parameters, including cell shape and degree of adhesion. Maridonneau-Parini (14) provides a compelling article on signaling and coordination of macrophage motility. She discusses the evidence that macrophages adapt very different modes of migration, ameboid or
Fig. 1. Model for diversity in macrophages promoted by diversity in the microenvironment. The left of the figure depicts resting resident macrophages (top and lower left) in distinct microenvironments taking on distinct shapes. Packing of chromatin is schematized on the right, with lineage-determining transcription factors creating sites of chromosomal accessibility for the binding of signal-dependent transcription factors. These factors are likely distinct in different environments. Mobilization of macrophages in response to signals that include phagocytic cargo or pathogens for engulfment lead the macrophage to change its shape and overall interaction with the environment. Cytoskeletal rearrangements serve as part of the critical signaling events that occur in response to phagocytosis and migration; these new signals generate yet additional signaldependent transcription factors that work together with lineage-determining transcription factors to effect gene expression. Further reading on this concept is found throughout this volume, including imagery modeled after the figures in reviews by Gordon (3), Glass (7), Grinstein (13), and Maridonneau-Parini (14) and their colleagues. © 2014 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd Immunological Reviews 262/2014
7
Randolph A macrophage revolution
mesenchymal, depending upon the porosity and composition of the extracellular matrix. She makes a case that macrophages are uniquely adaptable in selecting between different modes of migration, compared with other cell types that exhibit particular default modes (14). Gordon et al. (3) show a figure in which the shape of brain macrophages, microglia, is strikingly different in different regions of the central nervous system, as they also point out that we know relatively little about stromal cell-macrophage interactions that likely affects the assumption of the distinct shapes observed. The upshot of this discussion is that macrophage diversity in different organs, and corresponding diversity in macrophage activities, is as much a
problem of cell biology as it is genomics and epigenomics, because the extracellular interactions between macrophages and their environment no doubt pivotally influence, if not dictate, macrophage phenotypes (Fig. 1). Like the macrophage able to integrate numerous inputs and adapt accordingly, future advances in macrophage biology will be richly found among those scientists able to integrate research on macrophages coming from diverse disciplines. This volume, containing a series of reviews that collectively covers many different aspects of macrophage biology, provides a wonderful starting point for integrative reading on macrophages across disciplines.
References 1. Scott CL, Henri S, Guilliams M. Mononuclear phagocytes of the intestine, the skin, and the lung. Immunol Rev 2014;262:9–24. 2. Haldar M, Murphy KM. Origin, development, and homeostasis of tissue-resident macrophages. Immunol Rev 2014;262:25–35. 3. Gordon S, Pl€ uddemann A, Martinez Estrada F. Macrophage heterogeneity in tissues: phenotypic diversity and functions. Immunol Rev 2014;262:36–55. 4. Gentek R, Molawi K, Sieweke MH. Tissue macrophage identity and self-renewal. Immunol Rev 2014;262:56–73. 5. Hume DA, Freeman TC. Transcriptomic analysis of mononuclear phagocyte
8
6.
7. 8.
9.
differentiation and activation. Immunol Rev 2014;262:74–84. Gautier EL, Yvan-Charvet L. Understanding macrophage diversity at the ontogenic and transcriptomics levels. Immunol Rev 2014;262:85–95. Gosselin D, Glass CK. Epigenomics of macrophages. Immunol Rev 2014;262:96–112. R€ uckerl D, Allen JE. Macrophage proliferation, provenance, and plasticity in macroparasite infection. Immunol Rev 2014;262:113–133. Hill AA, Bolus WR, Hasty AH. A decade of progress in adipose tissue macrophage biology. Immunol Rev 2014;262:134–152.
10. Colin S, Chinetti-Gbagiudi G, Staels B. Macrophage phenotypes in atherosclerosis. Immunol Rev 2014;262:153–166. 11. Dutta P, Nahrendorf M. Regulation and consequences of monocytosis. Immunol Rev 2014;262:167–178. 12. Srivastava S, Ernst JD, Desvignes L. Beyond macrophages: the diversity of mononuclear cells in tuberculosis. Immunol Rev 2014;262:179–192. 13. Freeman SA, Grinstein S. Phagocytosis: receptors, signal integration, and the cytoskeleton. Immunol Rev 2014;262:193–215. 14. Maridonneau-Parini I. Control of macrophage 3D migration: a therapeutic challenge to limit tissue infiltration. Immunol Rev 2014;262:216–231.
© 2014 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd Immunological Reviews 262/2014
