The many faces of macrophage activation

C-type lectins are the most diverse and prevalent lectin family in immunity. Particular interest has recently been attracted by the C-type lectin-like receptors on NK cells, which appear to regulate the activation/inhibitory balance of these cells, controlling cytotoxicity and cytokine production. We previously identified a human C-type lectin-like receptor, closely related to both the beta-glucan receptor and the lectin-like receptor for oxidized-LDL, named MICL (myeloid inhibitory C-type lectin-like receptor), which we had shown using chimeric analysis to function as an inhibitory receptor. Using a novel MICL-specific monoclonal antibody, we show here that human MICL is expressed primarily on myeloid cells, including granulocytes, monocytes, macrophages, and dendritic cells. Although MICL was highly N-glycosylated in primary cells, the level of glycosylation was found to vary between cell types. MICL surface expression was down-regulated during inflammatory/activation conditions in vitro, as well as during an in vivo model of acute inflammation, which we characterize here. This suggests that human MICL may be involved in the control of myeloid cell activation during inflammation. IntroductionIn order to execute their immune (and non-immune) functions, leukocytes must interact with a broad range of endogenous and exogenous ligands and must be able to respond to these interactions appropriately. This requires a wide and flexible, yet specific and regulated, repertoire of cell surface molecules. One large family of such molecules, which are particularly important to immunity, are the C-type lectin and lectin-like receptors. C-type lectin-like receptors were first identified as dimeric molecules on NK cells. On these cells, much attention has been drawn toward their ability to regulate the balance between cellular activation and inhibition, such as cytotoxicity and cytokine production. This is achieved through signalling via intracellular ITIM present in the cytoplasmic tails of these receptors or via ITAM, of which the majority are located in the cytoplasmic tails of associated molecules, such as DAP12. The study of these activationand inhibitory NK cell receptorshas generated a number of interesting hypotheses, including immune privilege [1], the recognition of 'missing self', 'non-self' and 'induced self' [2][3][4][5], as well elucidating the underlying mechanisms of some of the immune responses to viruses [6] and malignancy [7]. Others and we have demonstrated that C-type lectinlike receptors are not restricted to NK cells, but are expressed by many other cell types including myeloid cells [8][9][10]. This introduces the novel concept that the myeloid paralogs of NK cell receptors might govern myeloid cell activation/inhibition in the same manner as those on NK cells. The genes of most C-type lectin-like molecules are located within the 'natural killer complex' (NKC), but many of those expressed by myeloid and other cells are found in a distinct cluster of genes within the NKC [10]. This cluster includes Decti...

Section: Discussionsupporting

confidence: 86%

Human MICL (CLEC12A) is differentially glycosylated and is down‐regulated following cellular activation

Willment¹,

Pyż²,

Dennehy³

et al. 2006

“…We examined the effects of 48 h incubation with IFN-g, TNF-a and LPS alone and in combination as our stimuli for classical activation of macrophages [25], and used IL-4 and IL-13 as alternative activation stimuli [26]. P2X 4 R-mediated currents were measured under all these stimuli at 48 h ( Fig.…”

Section: Effects On P2x 4 R By Classical and Alternative Activation Omentioning

confidence: 99%

Dynamic regulation of the P2X₄ receptor in alveolar macrophages by phagocytosis and classical activation

Stokes

Surprenant

2009

ATP-gated P2X 4 receptors (P2X 4 R) in macrophages and microglia have been implicated in neuropathic and inflammatory pain by currently unidentified mechanisms. P2X 4 R are found predominantly in intracellular lysosomal compartments but can be rapidly trafficked to the surface membrane by procedures that induce endolysosomal secretion. We studied total and surface membrane P2X 4 R protein expression by Western blot and biotinylation assays and functional expression by whole-cell patch clamp assays in human and rat alveolar macrophages in response to phagocytosis of zymosan and opsonized zymosan bioparticles and to classical and alternative macrophage activation. Unstimulated macrophages showed high total protein expression but very low functional expression. Phagocytosis rapidly (within 4 h) increased functional P2X 4 R expression by 2-to 7-fold as did chloroquine, an agent known to induce lysosomal secretion. In contrast, classical activation of macrophage for 48 h with IFN-c and TNF-a or IFN-c and LPS reduced surface and functional P2X 4 R expression by 3-fold without altering total P2X 4 R protein levels. Alternative activation with IL-4 or IL-13 did not alter total, surface or functional expression of P2X 4 R. This is the first study of the regulation of P2X 4 R in macrophages by physiological stimuli and presents a picture whereby P2X 4 R become functional in response to initial phagocytic stimuli but return to a non-functional state during sustained activation by classical macrophage activation.Key words: Inflammation . Ion channels . Patch-clamp . Purine receptors Introduction Extracellular ATP acts as a ''danger signal'' when it is released from cells during tissue damage and inflammation; it acts on metabotropic (G-protein coupled) P2Y and ionotropic (cation channel) P2X purinergic receptors to affect several inflammatory and immune responses [1,2]. There are seven members of the P2X receptor family (P2X 1-7 R) that show widespread distribution in both neuronal and non-neuronal tissues [3]. Two of these seven members, P2X 4 R and P2X 7 R, have generated increasing interest as potential anti-inflammatory drug targets [2,4,5]. The case for P2X 7 R is now well established: Stimulation of P2X 7 R in activated monocytes, macrophages and microglia leads to rapid release of pro-inflammatory IL-1b and IL-18 cytokines, to phospholipase D activation and, if stimulation is sustained, to apopotosis [1,[3][4][5]. Classical activation of macrophages, generally induced by IFN-g and/or LPS, up-regulates P2X 7 R mRNA and protein levels and functional expression [6,7] and this correlates well with a role for P2X 7 R in bacterial killing [8,9]. Recently, highly selective and potent P2X 7 R antagonists have been identified [10] with one of these showing initial positive results in Phase II clinical trials for rheumatoid arthritis [11]. The case for P2X 4 R remains tentative, largely due to the absence of selective P2X 4 R antagonists and the complicating presence of P2X 7 R, which is generally co-expressed with P2X 4 R in ...

“…Diversity is a hallmark of mononuclear phagocytes [2,3] and the same applies to the various forms of macrophage activation [16][17][18][19][20][21] (Fig. 1).…”

Section: Activation and Adaptive Responses Of Macrophagesmentioning

confidence: 99%

From phagocyte diversity and activation to probiotics: Back to Metchnikoff

Mantovani

2008

In this issue of the European Journal of Immunology, Siamon Gordon gives a detailed account of Metchnikoff's life and his achievements (Eur. J. Immunol. 2008. 38: 3257-3264). Looking back at the roots of innate immunity stimulates reflections on open issues in the field. Here, I give a personal view of some of these issues, including myeloid-derived suppressor cells, macrophage polarization and adaptive responses of mononuclear phagocytes. Key words: Macrophage activation Á Macrophages Á M1/M2 polarization Á Probiotics See accompanying article by Gordon IntroductionIn his scholarly review [1], Siamon Gordon tracks the roots of innate immunity to Elie Metchnikoff. The essay provides a fascinating insight into Metchnikoff's scientific life and how he tackled fundamental issues in science, some of which remain with us to this day. The perspective offered by a major player in the very same field of phagocytes and innate immunity [2,3] adds flavour and fascination to this article not necessarily present in other accounts [4]. Such a reflection on a central part of modern immunology stimulates consideration of remaining open issues and there follows a personal view of some of these.Phagocyte heterogeneity: From microphage-macrophage dichotomy to myeloid-derived suppressor cellsThe fundamental distinction between polymorphonuclear leukocytes (microphages) and mononuclear phagocytes (macrophages) was a major first step in the dissection of phagocyte heterogeneity and lineage differentiation. The identification of myeloid-derived suppressor cells (MDSC) [5][6][7] raises the questions whether the classic sharp distinction between the myeloid and the monocyte-macrophage differentiation and activation pathways is appropriate. In contrast to long-held views, neutrophils express specific transcriptional programmes in response to environmental signals [8]. MDSC include immature myeloid precursors that can further differentiate, and play a key role in the suppression of adaptive immunity in diverse pathological conditions ranging from cancer to chronic infections [5][6][7]. Are we dealing with a blurring of a classic distinction or with a well-defined third phagocyte population? Using current identification and separation criteria (e.g. Gr1, CD11b, F4/80) MDSC in the blood and lymphoid organs are a mixed population, which includes myeloid cells at different stages of differentiation and mononuclear phagocytes. Thus, the definition of MDSC remains an operational one rather than that of a cell type, a reality that is frequently neglected. The issue of lineage of the actual effectors of MDSC-mediated suppression is even more relevant when their recruitment and activation in non-lymphoid tissues is considered. For instance, do MDSC retain an immature phenotype in tumour tissues or do they differentiate into tumourassociated macrophages [9][10][11] [9,12,16,17], bacterial [43][44][45][46][47][48][49] and parasitic [50][51][52][53] infections. Evidence has also accumulated that polarized macrophages are more than mere spectators of i...