We report that human galectin-1 (dGal-1), a small dimeric ␤-galactoside-binding protein, induces phosphatidylserine ( It is believed that the turnover of neutrophils and other leukocytes in tissues involves programmed cell death (apoptosis) and then phagocytosis by tissue macrophages (1-4). However, the factors regulating turnover of leukocytes are unclear. Although Fas and Fas ligand (FasL) 1 induce apoptosis of mature, circulating neutrophils in vitro (5-7), mice deficient in FasL (gld) or Fas (lpr) have essentially normal numbers of circulating mature granulocytes (8). FasL/Fas-mediated apoptosis is not essential in regulating the clearance of neutrophils during inflammation (9). Fas and FasL may promote, rather than decrease, inflammatory responses in vivo (10 -12). In transgenic mice expressing bcl-2 in mature neutrophils, apoptosis of circulating cells is inhibited, but neutrophil homeostasis is unaltered, and macrophage-mediated phagocytosis of neutrophils is normal (13). Phagocytosis is required for resolution of the inflammatory process and leukocyte homeostasis in vivo (14 -16). These results suggest that factors not yet defined may regulate leukocyte turnover in tissues. Such observations led us to explore whether the basement membrane and extracellular matrix might harbor other proteins capable of binding to leukocytes and inducing their apoptosis or phagocytic recognition. A candidate protein is the ␤-galactoside-binding protein termed galectin-1 (dGal-1), which binds to most leukocytes. dGal-1 is a widely expressed dimeric protein (subunit ϳ14.6 kDa), which is a member of the galectin family of lectins (17)(18)(19). It is secreted by many cell types, including human endothelial cells (20,21), and is found in the basement membrane and extracellular matrices around capillary walls (22,23). dGal-1 has been reported to have various biological activities, including effects on neurite outgrowth (24, 25), growth inhibition of non-neural cells (26 -28), cell growth stimulation (29,30), and apoptosis of immature thymocytes (31, 32), and to activate human T cells and T cell lines (33,34).To explore the biological activity of dGal-1 toward leukocytes, we prepared a recombinant form of dimeric human dGal-1 and a mutated, monomeric form of galectin-1 (mGal-1). We explored the interactions of these lectins with HL-60 cells, MOLT-4 cells, and both resting and activated human neutrophils. Our results show that dGal-1, but not mGal-1, rapidly enhances surface staining with Annexin V (phosphatidylserine (PS) exposure) in desialylated HL-60 cells, desialylated MOLT-4 cells, and activated, but not resting, human neutrophils. The exposure of PS is often associated with apoptosis * This work was supported by National Institutes of Health Grants AI48075 (to R. D. C.) and HL34363 and RR15577 (to R. P. M.). The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate ...

Galectin-1 (Gal-1) and galectin-3 (Gal-3) exhibit profound but unique immunomodulatory activities in animals but their molecular mechanisms are incompletely understood. Early studies suggested that Gal-1 inhibits leukocyte function by inducing apoptotic cell death and removal, but recent studies show that some galectins induce exposure of the common death signal phosphatidylserine (PS) independently of apoptosis. In this study, we report that Gal-3, but not Gal-1, induces both PS exposure and apoptosis in primary activated human T cells, whereas both Gal-1 and Gal-3 induce PS exposure in neutrophils in the absence of cell death. Gal-1 and Gal-3 bind differently to the surfaces of T cells and only Gal-3 mobilizes intracellular Ca2+ in these cells, although Gal-1 and Gal-3 bind their respective T cell ligands with similar affinities. Although Gal-1 does not alter T cell viability, it induces IL-10 production and attenuates IFN-γ production in activated T cells, suggesting a mechanism for Gal-1-mediated immunosuppression in vivo. These studies demonstrate that Gal-1 and Gal-3 induce differential responses in T cells and neutrophils, and identify the first factor, Gal-3, capable of inducing PS exposure with or without accompanying apoptosis in different leukocytes, thus providing a possible mechanism for galectin-mediated immunomodulation in vivo.

IntroductionThe physiologic causes of leukocyte turnover in homeostasis and during disease conditions are not well understood. It is believed that leukocytes are partly eliminated by programmed cell death or apoptosis 1-4 through phagocytosis by macrophages, dendritic cells, or neighboring cells. 5,6 The efficient removal of dying cells is important in homeostasis, since it limits accumulation of cellular debris that could be potentially immunogenic or toxic. [7][8][9] However, the role of apoptosis in removing large numbers of cells in inflammation and during the resolution phase remains uncertain. Although human neutrophils undergo apoptosis spontaneously when cultured in vitro, the role of apoptosis in regulating neutrophil turnover in vivo is unclear. 6,[10][11][12] Apoptosis impairs cellular functions and might impair proinflammatory functions of neutrophils. 13 However, excessive neutrophil influx with loss of membrane integrity during late apoptotic events could contribute to neutrophil-mediated injury of surrounding viable parenchymal tissue. 14 Exuberant apoptosis may therefore be proinflammatory instead of anti-inflammatory, [15][16][17] presumably due to the release of cellular contents prior to phagocytic removal. In this regard, nonapoptotic neutrophils can be cleared by phagocytosis in vivo. 18 Finally, factors known to induce or to block apoptosis, such as ligation of Fas/FasL and expression of bcl-2, respectively, do not alter neutrophil turnover in mouse models. [19][20][21][22] These studies suggest that unidentified factors may be involved in the phagocytic removal of viable, rather than apoptotic, cells.The removal of apoptotic cells occurs partly through tethering to phagocytic cells due to receptor-ligand interactions involving recognition of phosphatidylserine (PS) exposed on the surfaces of apoptotic cells. 23 Surface PS is recognized by a defined PS-receptor in macrophages 24 and by other receptors. [25][26][27] However, PS exposure in leukocytes can occur independently of apoptosis. 28,29 Thus, factors that induce PS exposure independently of apoptosis may be involved in leukocyte turnover.Recently, we showed that galectin-1 (Gal-1), a prototypical homodimeric member (subunit ϳ 14.5 kDa) of the galectin family, which has immunomodulatory functions, [30][31][32][33][34] can induce PS exposure in activated, but not resting, neutrophils, independently of cell death, while concomitantly rendering them sensitive to phagocytic recognition and removal. 35 We also found that the signaling pathway of Gal-1 in activated neutrophils is unique and involves elevations of cytosolic Ca 2ϩ and mobilization of PS through the actions of Src kinases and phospholipase C␥. 36 However, there are many conflicting reports about galectin's effects on leukocytes. Several groups have reported that Gal-1 induces apoptosis along with PS exposure in vitro of activated T lymphocytes and several T-leukemic cell lines. [37][38][39] It has also been reported that human Gal-2, a protein structurally related to Gal-1, ind...

SynopsisIn rheumatoid arthritis, cells within the inflamed synovium and pannus elaborate a variety of cytokines, including TNFα, IL-1, IL-6 and IL-17, that contribute to inflammation, and may directly impact bone. The RANKL/RANK/OPG pathway plays a critical role in regulating osteoclastogenesis in articular bone erosions in RA. Pro-inflammatory cytokines can modulate this pathway, and may also affect the ability of the osteoblast to repair bone at sites of articular erosion. In this review, we discuss the current understanding of pathogenic mechanisms of bone erosion in RA and examine current therapeutic approaches to prevent this damage.