Local macrophage proliferation, ra ... | Article | H1 Connect

I found this article interesting because it clearly demonstrates that macrophages proliferate within inflamed tissues in the setting of T helper type 2 (Th2)-driven inflammation and challenges the concept that recruitment is the primary mechanism of macrophage accumulation. The authors showed that mice infected with a parasite (Litomosoides sigmodontis) develop pleural Th2-type inflammation and accumulation of M2 or 'alternatively activated' macrophages. Monocytes did not significantly increase in the pleural cavity, despite a significant increase in macrophages. This was in contrast to an increase in monocyte recruitment after thioglycollate injection, considered by the authors to be a model of more conventional inflammation. Using clodronate-liposomes to deplete blood monocytes, the authors found abrogation of macrophage accumulation in the thioglycollate model, but no effect on macrophage numbers after parasite-induced Th2 inflammation. Additionally, using fluorescence-activated cell sorting (FACS) proliferation assays, pleural macrophages proliferated only in the Th2 model. Not surprisingly, the proliferative capacity of the M2 macrophages was interleukin (IL)-4-dependent and injection of IL-4 into mice induced proliferation of macrophages in the liver, peritoneum, and pleural spaces. Congenic bone marrow chimera studies in partially irradiated mice confirmed that M2 macrophages proliferate locally in Th2-inflamed tissue and in mice injected with IL-4. Macrophage plasticity was also supported by experiments using IL-4 along with thioglycollate. These studies have elegantly shown that local proliferation of macrophages occurs in the specific Th2 models utilized. Further work with other Th2 models, such as in allergic asthma, will need to confirm whether the same process occurs.

This paper demonstrates that T helper 2 (Th2) immune responses induce an interleukin-4 (IL-4)-promoted proliferation of tissue macrophages in the absence of extravasated precursors from the blood, as seen in type-1 inflammation. Nematode infections in mice - e.g. Litomosoides sigmodontis, which is a tissue-residing mouse helminth - ellicit a type-2 inflammation characterized by an upregulation of a distinct subset of molecules. In contrast to type-1 inflammation, nematode injection causes a slow, clodronate-resistant accumulation of F4/80high macrophages in the absence of invasion of monocytes. These macrophages are tissue macrophages that proliferate in situ, driven by IL-4. Indeed, when IL-4 is injected together with the classical inducer of type-1 inflammation, thioglycolate, the monocyte-derived macrophages proliferate and become activated as type-2 macrophages.

This paper has changed our concept of where inflammatory macrophages come from. We never would have expected that proliferation could be a major contributor of the alternatively activated macrophage (AAM/M2) population. The perception that activated macrophages should come from monocytes recruited from the blood is clearly more finely nuanced than expected. One interesting possibility will be to determine if inflammatory monocytes recruited during chronic inflammation can be stimulated to proliferate by interleukin (IL)-4 in vivo and convert into AAM/M2 cells.

This is the first study to describe the mechanism of how alternatively activated macrophages (M2) accumulate in 'sterile' tissues during T-helper type 2 (Th2) inflammation. The surprising finding that M2 cells are capable of self-renewal has implications regarding their roles in health and disease. This article reveals that interleukin-4 (IL-4) augments the accumulation of M2 cells through proliferation of tissue-resident cells rather than through the recruitment of blood monocytes. This conclusion was based on three lines of evidence: the accumulation of pleural M2 cells following nematode infection was unaffected by treatment with clodronate-loaded liposomes, which block monocyte recruitment; nematode infection increased pleural M2 cell proliferation as measured by bromodeoxyuridine (BrdU) incorporation and Ki67 staining in an IL-4-dependent manner; donor bone marrow-derived macrophages did not contribute to the increased pleural M2 cell numbers following exogenous IL-4 treatment. This data is surprising considering that the accumulation of classically activated macrophages during Th1 inflammation is driven by the recruitment of blood monocytes, and is not accompanied by in situ proliferation in the pleural cavity. The authors suggest that proliferation in situ allows macrophages to perform critical functions in the absence of potentially damaging cell recruitment. Further, these data demonstrate that blocking monocyte recruitment to tissues will potentially have no effect on M2 activity. As demonstrated by their bone marrow chimera experiment (Fig. 3), these data have implications for patients receiving radiation treatment. An interesting question is how many rounds of division are tissue-resident macrophages capable of undergoing in the absence of bone marrow activity? Secondly, as suggested by the authors, does this mechanism contribute to wound healing? Thirdly, is there a way to develop a diagnostic tool based on this Th2 signature?

This exceptional paper identifies the source of M2 macrophages in tissues. Macrophages come in two flavors, classically activated (M1) and alternatively activated (M2). Prior to this paper, macrophage expansion during inflammation was thought to be driven by recruitment of blood monocytes, precursors of (some) macrophages. Here, Jenkins et al. show that in a model of helminthic infection, resident pleural macrophages incorporate bromodeoxyuridine (BrdU), express the proliferation marker Ki-67 and proliferate in situ in a process that requires interleukin-4 (IL-4), a cytokine associated with parasitic infections. These macrophages have an M2 phenotype and do not require recruitment of blood monocytes. By contrast, thioglycollate (TG) challenge (classical inflammatory response) induced recruitment of blood monocytes (Gr-1 intermediate (int)/Ly-6Chigh), which mature into F4/80 low M1 macrophages. Depletion of blood circulating monocytes using i.v. injection of clodronate-liposomes blocked the accumulation of TG-induced macrophages, but had no effect on macrophage expansion following nematode infection. The in situ proliferation of M2 macrophages appears to be driven by the T helper 2 (Th2) cytokine IL-4. IL-4 can also induce TG-elicited inflammatory M1 macrophages to differentiate into M2 macrophages.

This report of local macrophage proliferation by Jenkins et al. changes the way we view macrophages in type 2 inflammation. These results indicate that, contrary to popular belief, M2 macrophages arise from local cell proliferation rather than via monocyte recruitment from the blood. This occurs in an interleukin-4 (IL-4)-dependent manner; although, whether this is a direct or indirect effect of IL-4 on macrophages or a precursor cell remains to be determined. In addition, it will be important to confirm these results in different settings and to investigate local macrophage proliferation in human tissues. Nonetheless, these results significantly advance our understanding of macrophage accumulation in IL-4-rich settings, such as helminth infection.

This study is interesting in that it provides a nice example of in situ proliferation of tissue macrophages and provides a rationale for why this response, which is slower than recruiting inflammatory cells from the blood, may be favored under helper T cell (TH)2 immune response conditions. While anyone who has grown bone marrow-derived macrophages knows that mature macrophages can proliferate, the common understanding of the role of macrophages in an immune response is that they (or their monocytes precursors) are recruited from the blood in order to fight infection. So where does the ability to proliferate come into play during real immune responses? Jenkins et al. provide a nice example of where tissue macrophages proliferate to support a TH2-type immune response that is relatively long lived. The TH2 cytokine interleukin (IL)-4 is sufficient to drive local macrophage proliferation and the response is thought to mitigate the tissue destruction that can occur during an inflammatory response to infection.

This study presents the surprising finding that increased numbers of tissue macrophages during helminth-induced inflammation are attributed to enhanced local proliferation as opposed to recruitment from the circulation. Tissue macrophages and dendritic cells (DCs) are, in general, derived from their precursors in the circulation. Although at steady state certain cell types such as Langerhans cells and microglia can be maintained by local self-renewal, rather than repletion by blood-borne cells, it is widely believed that during infection the rapid accumulation of macrophages in infected tissues is a result of the recruitment of blood monocytes. This paper challenges this dogma by showing local proliferation is a major driver for the alternatively activated macrophage (M2) population. Following Litomosoides sigmodontis infection, which elicits a type-2 immune response, M2 macrophage accumulation is detected in the pleural cavity, while recruitment of blood monocytes is at a very low level. Unexpectedly, these macrophages express proliferation markers and incorporate BrdU. Moreover, depletion of circulation monocytes does not affect the expansion of the pleural macrophage population, suggesting that the accumulation is due to proliferation and not monocyte recruitment. The authors further demonstrate that in situ proliferation of M2 macrophages is dependent on interleukin (IL)-4. In contrast, type-1 inflammations, which are analyzed in this paper as well as many other studies, induce monocyte release from the bone marrow and subsequent recruitment into the tissues, but no macrophage proliferation in tissues. Further studies are needed to understand the costs and benefits of having local proliferation versus cell recruitment for host defence during each condition. This paper also provides insights for developing therapeutics that modulate the population of tissue macrophages to improve immune response against pathogens and wound healing or to prevent detrimental inflammations.

Traditional pathology has taught that acute inflammation is mediated by neutrophils and chronic inflammation by monocytes, with both cell types derived from the circulation. While this view remains valid for neutrophils, the active form of the monocyte, i.e. the macrophage, has long been known to also exist in normal tissues as a 'histiocyte' or tissue macrophage. In this paper, Jenkins and his colleagues show us that macrophages can accumulate in a site of inflammation by local replication. The observation is not entirely new. Over a decade ago, Gordon et al. {1} showed that the principle replicating cell in atherosclerotic plaques was the plaque macrophage. While that paper preceded current concepts of distinct differentiation states for macrophages {2}, the authors did raise the issue of whether replication of monocytes within the plaque might represent a form of cell differentiation. This issue is also relevant for the Jenkins paper, especially in regard to accumulating evidence that histiocytes themselves may arise from multiple origins other than the bone marrow. NB – I am listed as an author on ref {1}.

This exceptional study by Jenkins and colleagues reports the surprising finding that alternatively activated macrophages can locally proliferate, in an interleukin-4 (IL-4)-dependent manner, during helminth infections. We are in agreement with the previous evaluations that this is a very interesting study that changed our understanding of macrophage biology and has broad therapeutic implications in autoimmunity and anti-tumor immunotherapy where T helper cells type 2 (Th2) responses are predominant.

The results reported call for a fresh, new look at mechanisms of macrophage accumulation. In particular, it will be important to reassess the proliferative capacity of human monocyte-macrophages, also in view of the induction of proliferation-related genes during macrophage colony-stimulating factor (M-CSF)-driven differentiation. The authors investigate the mechanism of macrophage accumulation during infection by the rodent filarial nematode Litomosoides sigmodontis in the 'sterile' environment of the pleural cavity. In this model of type II inflammation, accumulation of M2 macrophages does not depend on recruitment of blood monocytes but rather on local macrophage proliferation. Macrophage proliferation is interleukin (IL)-4-dependent. These findings, together with those of Davies et al. {1}, call for a reappraisal of the role of proliferation in sustaining macrophage numbers in M2 polarized inflammation, including cancer.

This article is the first report in the literature to show that, during T helper 2 (Th2)-driven immune conditions, proliferating resident tissue macrophage populations and not influxing blood monocytes are the major source of alternatively activated macrophages. This finding will lead to a major shift in the way we think about macrophage differentiation and polarization, and the roles that these cells play during inflammation and homeostasis.