Judith E. Allen

Local Macrophage Proliferation, Rather than Recruitment from the Blood, Is a Signature of TH2 Inflammation

Proliferation in situ, rather than immune cell recruitment, drives macrophage expansion in response to parasitic infection. A defining feature of inflammation is the accumulation of innate immune cells in the tissue that are thought to be recruited from the blood. We reveal that a distinct process exists in which tissue macrophages undergo rapid in situ proliferation in order to increase population density. This inflammatory mechanism occurred during T helper 2 (TH2)–related pathologies under the control of the archetypal TH2 cytokine interleukin-4 (IL-4) and was a fundamental component of TH2 inflammation because exogenous IL-4 was sufficient to drive accumulation of tissue macrophages through self-renewal. Thus, expansion of innate cells necessary for pathogen control or wound repair can occur without recruitment of potentially tissue-destructive inflammatory cells.

Helminth parasites – masters of regulation

Summary:  Immune regulation by parasites is a global concept that includes suppression, diversion, and conversion of the host immune response to the benefit of the pathogen. While many microparasites escape immune attack by antigenic variation or sequestration in specialized niches, helminths appear to thrive in exposed extracellular locations, such as the lymphatics, bloodstream, or gastrointestinal tract. We review here the multiple layers of immunoregulation that have now been discovered in helminth infection and discuss both the cellular and the molecular interactions involved. Key events among the host cell population are dominance of the T‐helper 2 cell (Th2) phenotype and the selective loss of effector activity, against a background of regulatory T cells, alternatively activated macrophages, and Th2‐inducing dendritic cells. Increasingly, there is evidence of important effects on other innate cell types, particularly mast cells and eosinophils. The sum effect of these changes to host reactivity is to create an anti‐inflammatory environment, which is most favorable to parasite survival. We hypothesize therefore that parasites have evolved specific molecular strategies to induce this conducive landscape, and we review the foremost candidate immunomodulators released by helminths, including cytokine homologs, protease inhibitors, and an intriguing set of novel products implicated in immune suppression.

Tissue-resident macrophages

Tissue-resident macrophages are a heterogeneous population of immune cells that fulfill tissue-specific and niche-specific functions. These range from dedicated homeostatic functions, such as clearance of cellular debris and iron processing, to central roles in tissue immune surveillance, response to infection and the resolution of inflammation. Recent studies highlight marked heterogeneity in the origins of tissue macrophages that arise from hematopoietic versus self-renewing embryo-derived populations. We discuss the tissue niche-specific factors that dictate cell phenotype, the definition of which will allow new strategies to promote the restoration of tissue homeostasis. Understanding the mechanisms that dictate tissue macrophage heterogeneity should explain why simplified models of macrophage activation do not explain the extent of heterogeneity seen in vivo.

Diversity and dialogue in immunity to helminths

The vertebrate immune system has evolved in concert with a broad range of infectious agents, including ubiquitous helminth (worm) parasites. The constant pressure of helminth infections has been a powerful force in shaping not only how immunity is initiated and maintained, but also how the body self-regulates and controls untoward immune responses to minimize overall harm. In this Review, we discuss recent advances in defining the immune cell types and molecules that are mobilized in response to helminth infection. Finally, we more broadly consider how these immunological players are blended and regulated in order to accommodate persistent infection or to mount a vigorous protective response and achieve sterile immunity.

Th1-Th2: Reliable paradigm or dangerous dogma?

Abstract The identification of crossregulating T helper (Th) cells has revolutionized current understanding of the immune response to infection. While paying tribute to this revolution, Judith Allen and Rick Maizels argue that the paradigm can be dangerously oversimplified and that the interaction between host and pathogen cannot always be addressed in the context of Th1 and Th2 cells.

Removal of Regulatory T Cell Activity Reverses Hyporesponsiveness and Leads to Filarial Parasite Clearance In Vivo

Human filarial parasites cause chronic infection associated with long-term down-regulation of the host’s immune response. We show here that CD4+ T cell regulation is the main determinant of parasite survival. In a laboratory model of infection, using Litomosoides sigmodontis in BALB/c mice, parasites establish for >60 days in the thoracic cavity. During infection, CD4+ T cells at this site express increasing levels of CD25, CTLA-4, and glucocorticoid-induced TNF receptor family-related gene (GITR), and by day 60, up to 70% are CTLA-4+GITRhigh, with a lesser fraction coexpressing CD25. Upon Ag stimulation, CD4+CTLA-4+GITRhigh cells are hyporesponsive for proliferation and cytokine production. To test the hypothesis that regulatory T cell activity maintains hyporesponsiveness and prolongs infection, we treated mice with Abs to CD25 and GITR. Combined Ab treatment was able to overcome an established infection, resulting in a 73% reduction in parasite numbers (p < 0.01). Parasite killing was accompanied by increased Ag-specific immune responses and markedly reduced levels of CTLA-4 expression. The action of the CD25+GITR+ cells was IL-10 independent as in vivo neutralization of IL-10R did not restore the ability of the immune system to kill parasites. These data suggest that regulatory T cells act, in an IL-10-independent manner, to suppress host immunity to filariasis.

Alternative activation is an innate response to injury that requires CD4+ T cells to be sustained during chronic infection.

Alternatively activated macrophages (AAMΦ) are found in abundance during chronic Th2 inflammatory responses to metazoan parasites. Important roles for these macrophages are being defined, particularly in the context of Th2-mediated pathology and fibrosis. However, a full understanding of the requirements for alternative activation, particularly at the innate level, is lacking. We present evidence that alternative activation by the Th2 cytokines IL-4 and IL-13 is an innate and rapid response to tissue injury that takes place even in the absence of an infectious agent. This early response does not require CD4+ Th2 cells because it occurred in RAG-deficient mice. However, class II-restricted CD4+ T cell help is essential to maintain AAMΦ in response to infection, because AAMΦ were absent in RAG-deficient and MHC class II-deficient, but not B cell-deficient mice after chronic exposure to the nematode parasite, Brugia malayi. The absence of AAMΦ was associated with increased neutrophilia and reduced eosinophilia, suggesting that AAMΦ are involved in the clearance of neutrophils as well as the recruitment of eosinophils. Consistent with this hypothesis, AAMΦ show enhanced phagocytosis of apoptotic neutrophils, but not latex beads. Our data demonstrate that alternative activation by type 2 cytokines is an innate response to injury that can occur in the absence of an adaptive response. However, analogous to classical activation by microbial pathogens, Th2 cells are required for maintenance and full activation during the ongoing response to metazoan parasites.

Beyond Stem Cells: Self-Renewal of Differentiated Macrophages

Background Many mature cells of the body are continuously replaced, particularly in tissues that are most exposed to the environment such as cells of the immune system. The need for new cells is driven by cellular turnover during normal tissue homeostasis and is further increased upon infection. Because differentiated cells typically withdraw from the cell cycle, replacement of mature cells is generally thought to depend on differentiation of self-renewing, tissue-specific stem cells. Until recently, tissue macrophages were thought to follow such a pathway, developing from hematopoietic stem cells via bone marrow–progenitor and blood monocyte intermediates. But this view has changed of late with several observations indicating that macrophages can self-renew by local proliferation of mature differentiated cells. Macrophage origin and self-renewal. In the classical view, macrophages develop from self-renewing hematopoietic stem cells (HSC) in the bone marrow (BM) via blood monocyte intermediates. However, new data show that some adult tissue macrophage populations develop from embryonic progenitors independent of HSCs and can self-renew. Local proliferation can assure homeostatic maintenance (dotted arrows) and dramatically increase cell number (solid arrows) upon challenge. Advances Recent studies have demonstrated that in macrophages, differentiation and cell cycle withdrawal can be uncoupled by the inactivation of specific transcription factors. These cells can then be expanded indefinitely as functionally differentiated macrophages without tumorigenic transformation. At the same time, it became clear that mature macrophages could also expand massively in vivo in response to infections by local proliferation, independently of input from adult hematopoietic stem cells. Furthermore, several populations of tissue macrophages were found to be derived from embryonic progenitors, and macrophages can be self-maintained in adult tissues by local proliferation. Together, these recent data suggest that macrophages are mature differentiated cells that may be endowed with self-renewal capacity akin to that of stem cells. Outlook These findings challenge the classical view of tissue maintenance by adult tissue-specific stem cells and indicate that stem cell–like self-renewal mechanisms may be activated in mature differentiated cells. It will be important to determine whether the engaged pathways resemble those active in stem cells and whether they might be activated in other cell types as well. Furthermore, we need to understand how such self-renewal capacity differs from uncontrolled proliferation induced by oncogenic transformation. A first step will be to explore how macrophage proliferation is regulated in vivo: How do macrophages adapt their cell numbers to diverse tissue requirements, from near quiescence during homeostasis to massive expansion under challenge? Macrophages are present in nearly every tissue and serve important functions in immunity, cancer, metabolism, and tissue repair. The role of local macrophage proliferation in these processes has remained largely unexplored. It will be important to investigate how the consequences of macrophage accumulation by local proliferation differ from those of monocyte-derived macrophage recruitment under inflammatory conditions. The control of macrophage numbers independent of inflammatory signals may provide new opportunities for therapeutic intervention in many of these areas. Macrophage Makeover Macrophages are important immune cells that function in tissue repair during homeostasis and in the innate immune response. Inflammation, which can be triggered by infection, is accompanied by a massive expansion of macrophages in affected tissues. The major source of this increase in resident macrophages has been thought to be hematopoietic stem cells in the bone marrow. However, recent results have shown that the mature differentiated macrophages residing in the affected tissues can themselves proliferate to boost cell numbers. Sieweke and Allen (10.1126/science.1242974) review what we know about the origin of macrophages and outline the consequences of local macrophage proliferation for the immune response and tissue homeostasis. In many mammalian tissues, mature differentiated cells are replaced by self-renewing stem cells, either continuously during homeostasis or in response to challenge and injury. For example, hematopoietic stem cells generate all mature blood cells, including monocytes, which have long been thought to be the major source of tissue macrophages. Recently, however, major macrophage populations were found to be derived from embryonic progenitors and to renew independently of hematopoietic stem cells. This process may not require progenitors, as mature macrophages can proliferate in response to specific stimuli indefinitely and without transformation or loss of functional differentiation. These findings suggest that macrophages are mature differentiated cells that may have a self-renewal potential similar to that of stem cells.

Chitinase and Fizz Family Members Are a Generalized Feature of Nematode Infection with Selective Upregulation of Ym1 and Fizz1 by Antigen-Presenting Cells

ABSTRACT Ym1 and Fizz1 are secreted proteins that have been identified in a variety of Th2-mediated inflammatory settings. We originally found Ym1 and Fizz1 as highly expressed macrophage genes in a Brugia malayi infection model. Here, we show that their expression is a generalized feature of nematode infection and that they are induced at the site of infection with both the tissue nematode Litomosoides sigmodontis and the gastrointestinal nematode Nippostrongylus brasiliensis. At the sites of infection with N. brasiliensis, we also observed induction of other chitinase and Fizz family members (ChaFFs): acidic mammalian chitinase (AMCase) and Fizz2. The high expression of both Ym1 and AMCase in the lungs of infected mice suggests that abundant chitinase production is an important feature of Th2 immune responses in the lung. In addition to expression of ChaFFs in the tissues, Ym1 and Fizz1 expression was observed in the lymph nodes. Expression both in vitro and in vivo was restricted to antigen-presenting cells, with the highest expression in B cells and macrophages. ChaFFs may therefore be important effector or wound-repair molecules at the site of nematode infection, with potential regulatory roles for Ym1 and Fizz1 in the draining lymph nodes.

Alternatively activated macrophages induced by nematode infection inhibit proliferation via cell‐to‐cell contact

The cytokine microenvironment is thought to play an important role in the generation of immunoregulatory cells. Nematode infections are commonly associated with Th2 cytokines and hyporesponsive T cells. Here we show that IL‐4‐dependent macrophages recruited in vivo by the nematode parasite Brugia malayi actively suppress the proliferation of lymphocytes on co‐culture in vitro. These alternatively activated macrophages block proliferation by cell‐to‐cell contact, implicating a receptor‐mediated mechanism. Further, the proliferative block is reversible and is not a result of apoptosis. Suppressed cells accumulate in the G1 and G2 / M phase of the cell cycle. Interestingly, the G1 and G2 / M block correlates with increased levels of Ki‐67 protein, suggesting a mechanism that affects degradation of cell cycle proteins. We also show that, in addition to lymphocyte cell lines of murine origin, these suppressive cells can inhibit proliferation of a wide range of transformed human carcinoma lines. Our data reveal a novel mechanism of proliferative suppression induced by a parasitic nematode that acts via IL‐4‐dependent macrophages. These macrophages may function as important immune regulatory cells in both infectious and noninfectious disease contexts.