Homegrown Macrophages
Authors: Ki-Wook Kim, Nan Zhang, Kyunghee Choi, Gwendalyn J. Randolph
Journal: Immunity (2016)

Macrophages residing in different organs have diverse gene-expression programs. Mass et al., 2016Mass E. Ballesteros I. Farlik M. Halbritter F. Günther P. Crozet L. Jacome-Galarza C.E. Händler K. Klughammer J. Kobayashi Y. et al.Science. 2016; 353 (Published online August 4, 2016)https://doi.org/10.1126/science.aaf4238Crossref PubMed Scopus (463) Google Scholar propose that this diversity develops "at home"—within those organs—after the recruitment of a common precursor that had not made prior commitments to diversity. Macrophages residing in different organs have diverse gene-expression programs. Mass et al., 2016Mass E. Ballesteros I. Farlik M. Halbritter F. Günther P. Crozet L. Jacome-Galarza C.E. Händler K. Klughammer J. Kobayashi Y. et al.Science. 2016; 353 (Published online August 4, 2016)https://doi.org/10.1126/science.aaf4238Crossref PubMed Scopus (463) Google Scholar propose that this diversity develops "at home"—within those organs—after the recruitment of a common precursor that had not made prior commitments to diversity. A radically new view of macrophage identity has emerged in the last 5 years, arising from two major developments in the field. First, tissue-resident macrophages are now recognized to arise from embryonic precursors that seed tissues in pre- or peri-natal periods. These precursors give rise to locally proliferating, self-sustaining macrophages throughout all or much of adulthood (Hashimoto et al., 2013Hashimoto D. Chow A. Noizat C. Teo P. Beasley M.B. Leboeuf M. Becker C.D. See P. Price J. Lucas D. et al.Immunity. 2013; 38: 792-804Abstract Full Text Full Text PDF PubMed Scopus (1428) Google Scholar, Schulz et al., 2012Schulz C. Gomez Perdiguero E. Chorro L. Szabo-Rogers H. Cagnard N. Kierdorf K. Prinz M. Wu B. Jacobsen S.E. Pollard J.W. et al.Science. 2012; 336: 86-90Crossref PubMed Scopus (1721) Google Scholar, Yona et al., 2013Yona S. Kim K.W. Wolf Y. Mildner A. Varol D. Breker M. Strauss-Ayali D. Viukov S. Guilliams M. Misharin A. et al.Immunity. 2013; 38: 79-91Abstract Full Text Full Text PDF PubMed Scopus (1872) Google Scholar), upsetting the decades-long notion proposed by R. van Furth in the 1960s that they arise continuously from blood monocytes. Precursors for embryonically derived macrophages might include yolk sac (YS)-derived erythroid myeloid progenitors (EMP) (Gomez Perdiguero et al., 2015Gomez Perdiguero E. Klapproth K. Schulz C. Busch K. Azzoni E. Crozet L. Garner H. Trouillet C. de Bruijn M.F. Geissmann F. Rodewald H.R. Nature. 2015; 518: 547-551Crossref PubMed Scopus (1329) Google Scholar), "late" EMP-derived fetal monocytes (Hoeffel et al., 2015Hoeffel G. Chen J. Lavin Y. Low D. Almeida F.F. See P. Beaudin A.E. Lum J. Low I. Forsberg E.C. et al.Immunity. 2015; 42: 665-678Abstract Full Text Full Text PDF PubMed Scopus (662) Google Scholar), or hematopoietic stem cell (HSC)-derived fetal liver monocytes (Sheng et al., 2015Sheng J. Ruedl C. Karjalainen K. Immunity. 2015; 43: 382-393Abstract Full Text Full Text PDF PubMed Scopus (309) Google Scholar). Second, it has become clear that the diversity in gene expression of resident macrophages from different organs is substantial (Gautier et al., 2012Gautier E.L. Shay T. Miller J. Greter M. Jakubzick C. Ivanov S. Helft J. Chow A. Elpek K.G. Gordonov S. et al.Immunological Genome ConsortiumNat. Immunol. 2012; 13: 1118-1128Crossref PubMed Scopus (1390) Google Scholar). How is this diversity of macrophage populations in different organs specified? Mass et al. address this question by carefully following the seeding of organ macrophages throughout embryogenesis, and conclude that a common "pre-macrophage" enters the various organs essentially simultaneously and then locally derives unique tissue identity (Mass et al., 2016Mass E. Ballesteros I. Farlik M. Halbritter F. Günther P. Crozet L. Jacome-Galarza C.E. Händler K. Klughammer J. Kobayashi Y. et al.Science. 2016; 353 (Published online August 4, 2016)https://doi.org/10.1126/science.aaf4238Crossref PubMed Scopus (463) Google Scholar). A priori, an alternative possibility was that circulating precursors developed preliminary diversity in gene expression before homing to given tissues. Instinctively, this possibility seemed unlikely, as evidence for local specification already included a modest ability of peritoneal macrophages to convert to lung macrophages when transferred into the lung (Lavin et al., 2014Lavin Y. Winter D. Blecher-Gonen R. David E. Keren-Shaul H. Merad M. Jung S. Amit I. Cell. 2014; 159: 1312-1326Abstract Full Text Full Text PDF PubMed Scopus (1319) Google Scholar), although this repopulation ability is quite limited relative to that of fetal monocytes (van de Laar et al., 2016van de Laar L. Saelens W. De Prijck S. Martens L. Scott C.L. Van Isterdael G. Hoffmann E. Beyaert R. Saeys Y. Lambrecht B.N. Guilliams M. Immunity. 2016; 44: 755-768Abstract Full Text Full Text PDF PubMed Scopus (366) Google Scholar). In addition, adult bone marrow cells, including monocytes, can develop lung-specific macrophage phenotypes and replace embryo-derived macrophages when conditions are such that their pulmonary niche is not filled by embryo-derived macrophages (Hashimoto et al., 2013Hashimoto D. Chow A. Noizat C. Teo P. Beasley M.B. Leboeuf M. Becker C.D. See P. Price J. Lucas D. et al.Immunity. 2013; 38: 792-804Abstract Full Text Full Text PDF PubMed Scopus (1428) Google Scholar, van de Laar et al., 2016van de Laar L. Saelens W. De Prijck S. Martens L. Scott C.L. Van Isterdael G. Hoffmann E. Beyaert R. Saeys Y. Lambrecht B.N. Guilliams M. Immunity. 2016; 44: 755-768Abstract Full Text Full Text PDF PubMed Scopus (366) Google Scholar). However, if we consider how tissue-resident memory T cell diversity is specified, through distal programming by interactions with dendritic cells in specific draining lymph nodes and return to the circulation for site-specific recruitment, the idea that specified precursors might exist in the circulation is not far-fetched. To distinguish between the possibilities that tissue-resident macrophages originate from circulating precursors exhibiting preliminary diversity or a common, as-of-yet uncommitted precursor, Mass et al., 2016Mass E. Ballesteros I. Farlik M. Halbritter F. Günther P. Crozet L. Jacome-Galarza C.E. Händler K. Klughammer J. Kobayashi Y. et al.Science. 2016; 353 (Published online August 4, 2016)https://doi.org/10.1126/science.aaf4238Crossref PubMed Scopus (463) Google Scholar performed RNA sequencing analyses on cell populations isolated from different tissues as development proceeded, between embryonic day 9 (E9) and postnatal day 21 (P21). At E9, expression of c-kit on CD45+ cells in YS was used to define EMP serving as a source of macrophages. Then, as organs including kidney, brain, lung, and liver began to develop and accumulate macrophages, they sorted the expanding tissues for two cell types: (1) mature macrophages, which they defined as F4/80+ cells in each organ, and (2) CD45+ cells that had lost c-kit (thus not EMP) but also lacked F4/80 (thus not macrophages), Ter119 (thus not erythrocytes), and Gr-1 (antibody recognizing Ly6G and Ly6C on other myeloid cells), collectively removing mature erythroid and myeloid cells from the analysis and allowing the identification of a potential cell state between EMP and mature cell types. Extensive bioinformatics analyses of each sorted cell state, bulk or single cells, pointed to the conclusion that CD11b+CD45+c-kit−F4/80−Ter119−Gr1− cells were a transitional cell type between EMP and mature macrophages. Because these transitional cells were present within the various organs analyzed, the authors concluded that these "pre-macrophages," likely derived from EMP, seed all tissues before maturing to tissue resident macrophages. In addition, this pre-macrophage population exhibited the signature characteristic of the onset of macrophage differentiation previously defined in adult mice (Gautier et al., 2012Gautier E.L. Shay T. Miller J. Greter M. Jakubzick C. Ivanov S. Helft J. Chow A. Elpek K.G. Gordonov S. et al.Immunological Genome ConsortiumNat. Immunol. 2012; 13: 1118-1128Crossref PubMed Scopus (1390) Google Scholar). The study did not, however, include experimental evidence showing that isolated pre-macrophages can develop a tissue-specific macrophage phenotype. It would have been interesting to demonstrate experimentally that pre-macrophages were at least as or more efficient than fetal monocytes (van de Laar et al., 2016van de Laar L. Saelens W. De Prijck S. Martens L. Scott C.L. Van Isterdael G. Hoffmann E. Beyaert R. Saeys Y. Lambrecht B.N. Guilliams M. Immunity. 2016; 44: 755-768Abstract Full Text Full Text PDF PubMed Scopus (366) Google Scholar) in repopulating tissue macrophages in vivo in the context of an open niche. Where do pre-macrophages fit in our understanding of macrophage ontogeny? The developmental pathways likely to give rise to pre-macrophages are summarized in Figure 1. Importantly, and in conflict with other reports (Hoeffel et al., 2015Hoeffel G. Chen J. Lavin Y. Low D. Almeida F.F. See P. Beaudin A.E. Lum J. Low I. Forsberg E.C. et al.Immunity. 2015; 42: 665-678Abstract Full Text Full Text PDF PubMed Scopus (662) Google Scholar, Sheng et al., 2015Sheng J. Ruedl C. Karjalainen K. Immunity. 2015; 43: 382-393Abstract Full Text Full Text PDF PubMed Scopus (309) Google Scholar), the authors argue that the pre-macrophage is not akin to HSC-derived fetal progenitors, even though definitive hematopoiesis that could give rise to blood monocytes has begun in the embryo prior to the earliest analysis of pre-macrophages at E9.5 (Sheng et al., 2015Sheng J. Ruedl C. Karjalainen K. Immunity. 2015; 43: 382-393Abstract Full Text Full Text PDF PubMed Scopus (309) Google Scholar). The argument against derivation from definitive hematopoiesis is the use of Tnfrsf11aCre reporter mice, a reporter that was chosen solely because pre-macrophages uniformly showed high expression of Tnfrsf11a and reported its expression well. Only 15% of HSC-derived cells including Ly6C+ blood monocytes from adult mice expressed the reporter. Nonetheless, it is possible that pre-macrophages arise from the small proportion of fetal monocytes that are Tnfrsf11a+, or alternatively, that this driver is rapidly induced locally in tissues after monocytes arrive. This uncertainty is depicted in Figure 1 via the use of dotted arrows. Given that circulation is established by E8.5, prior to the time points examined by Mass et al., 2016Mass E. Ballesteros I. Farlik M. Halbritter F. Günther P. Crozet L. Jacome-Galarza C.E. Händler K. Klughammer J. Kobayashi Y. et al.Science. 2016; 353 (Published online August 4, 2016)https://doi.org/10.1126/science.aaf4238Crossref PubMed Scopus (463) Google Scholar, it is impossible to be sure that the pre-macrophages analyzed are actually located in the tissue, as they might be recovered from tiny blood vessels that perfuse each organ. Addressing this problem presents technical hurdles that, at present, seem insurmountable. Further progress in defining macrophage origins will require technological advances that will allow precision beyond that provided by current genetic and physical tools. The landmark studies that proved tissue macrophages were not replenished in an ongoing manner by blood monocytes, but instead were embryonically derived (Hashimoto et al., 2013Hashimoto D. Chow A. Noizat C. Teo P. Beasley M.B. Leboeuf M. Becker C.D. See P. Price J. Lucas D. et al.Immunity. 2013; 38: 792-804Abstract Full Text Full Text PDF PubMed Scopus (1428) Google Scholar, Schulz et al., 2012Schulz C. Gomez Perdiguero E. Chorro L. Szabo-Rogers H. Cagnard N. Kierdorf K. Prinz M. Wu B. Jacobsen S.E. Pollard J.W. et al.Science. 2012; 336: 86-90Crossref PubMed Scopus (1721) Google Scholar, Yona et al., 2013Yona S. Kim K.W. Wolf Y. Mildner A. Varol D. Breker M. Strauss-Ayali D. Viukov S. Guilliams M. Misharin A. et al.Immunity. 2013; 38: 79-91Abstract Full Text Full Text PDF PubMed Scopus (1872) Google Scholar), changed the field. However, with the limitations now before us, attention might best be turned to other important questions that loom: Do embryonically derived macrophages indeed have a distinct function in a given organ from macrophages arising from adult monocytes? What does the diversity in macrophages achieve functionally for each organ? In line with these questions, Mass et al., 2016Mass E. Ballesteros I. Farlik M. Halbritter F. Günther P. Crozet L. Jacome-Galarza C.E. Händler K. Klughammer J. Kobayashi Y. et al.Science. 2016; 353 (Published online August 4, 2016)https://doi.org/10.1126/science.aaf4238Crossref PubMed Scopus (463) Google Scholar find that the transcriptional regulator Id3 is a selective driver of Kupffer cell development. Identifying this selective mediator adds to the tools that can be exploited to discern how and why macrophage diversity in different organs is critical.