Gr-1 antibody induces STAT signaling, macrophage marker expression and abrogation of myeloid-derived suppressor cell activity in BM cells

Steady-state neutrophil homeostasis is dependent on TLR4/TRIF signaling

Steady-state and emergency granulopoiesis are both dependent on TLR signaling.

Neonatal hyperoxia increases sensitivity of adult mice to bleomycin-induced lung fibrosis

Supplemental oxygen used to treat infants born prematurely constitutes a major risk factor for long-term deficits in lung function and host defense against respiratory infections. Likewise, neonatal oxygen exposure results in alveolar simplification in adult mice, and enhances leukocyte recruitment and fibrosis when adult mice are infected with a sublethal dose of influenza A virus. Because pulmonary fibrosis was not observed in infected adult mice exposed to room air as neonates, previous neonatal oxygen exposure may have reprogrammed how the adult lung responds to epithelial injury. By administering bleomycin to adult mice exposed to room air or hyperoxia as neonates, we tested the hypothesis that neonatal hyperoxia enhances fibrosis when the epithelium is injured by direct fibrotic stimulus. Increased sensitivity to bleomycin-induced lung fibrosis was observed in adult mice exposed to neonatal hyperoxia, and was associated with increased numbers of leukocytes and an accumulation of active transforming growth factor (TGF)-β1 in the lung. Fate mapping of the respiratory epithelium revealed that the epithelial-mesenchymal transition was not a significant source of fibroblasts in room air-exposed or oxygen-exposed mice treated with bleomycin. Instead, the treatment of mice with anti-Gr-1 antibody that depletes neutrophils and myeloid-derived suppressor cells reduced the early activation of TGF-β1 and attenuated hyperoxia-enhanced fibrosis. Because bleomycin and influenza A virus both cause epithelial injury, understanding how neonatal hyperoxia reprograms the epithelial response to these two different injurious agents could lead to new therapeutic opportunities for treating lung diseases attributed to prematurity.

Anti-Gr-1 antibody depletion fails to eliminate hepatic myeloid-derived suppressor cells in tumor-bearing mice

Recent studies show that the liver is a preferred organ for the accumulation of MDSC. In this study, we examined the effect of systemic RB6-8C5 treatment on hepatic MDSC in tumor-bearing mice. EL4 tumor-bearing mice were injected i.p. with RB6-8C5, and hepatic, splenic, and blood MDSCs were analyzed by flow cytometry. Unexpectedly, hepatic MDSC remained in the liver, although RB6-8C5 completely eliminated them from the spleen and peripheral blood 24 h after treatment. Secondary antibody staining confirmed the presence of RB6-8C5-bound MDSC in the liver of mice with s.c. tumors. Similar observations were made in two other (colon and melanoma) tumor models. Whereas RB6-8C5 injection induced cell death of hepatic MDSC, as shown by Annexin V/7-AAD staining, these cells were replaced immediately, leading to a constant, increased frequency of hepatic MDSC. Adoptively transferred MDSC migrated preferentially to the liver after RB6-8C5 treatment, suggesting that hepatic MDSCs are reconstituted rapidly after depletion. Finally, hepatic MDSC remained immunosuppressive despite RB6-8C5 injection. Our study demonstrates that RB6-8C5 is not suitable for depletion of hepatic MDSCs and analysis of their function.

Phosphatidylinositol 3-kinase-γ signaling promotes Campylobacter jejuni-induced colitis through neutrophil recruitment in mice

Crypt abscesses caused by excessive neutrophil accumulation are prominent features of human campylobacteriosis and its associated pathology. The molecular and cellular events responsible for this pathological situation are currently unknown. We investigated the contribution of PI3K-γ signaling in Campylobacter jejuni-induced neutrophil accumulation and intestinal inflammation. Germ-free and specific pathogen-free Il10(-/-) and germ-free Il10(-/-);Rag2(-/-) mice were infected with C. jejuni (10(9) CFU/mouse). PI3K-γ signaling was manipulated using either the pharmacological PI3K-γ inhibitor AS252424 (i.p. 10 mg/kg daily) or genetically using Pi3k-γ(-/-) mice. After up to 14 d, inflammation was assessed histologically and by measuring levels of colonic Il1β, Cxcl2, and Il17a mRNA. Neutrophils were depleted using anti-Gr1 Ab (i.p. 0.5 mg/mouse/every 3 d). Using germ-free Il10(-/-);Rag2(-/-) mice, we observed that innate immune cells are the main cellular compartment responsible for campylobacteriosis. Pharmacological blockade of PI3K-γ signaling diminished C. jejuni-induced intestinal inflammation, neutrophil accumulation, and NF-κB activity, which correlated with reduced Il1β (77%), Cxcl2 (73%), and Il17a (72%) mRNA accumulation. Moreover, Pi3k-γ(-/-) mice pretreated with anti-IL-10R were resistant to C. jejuni-induced intestinal inflammation compared with Wt mice. This improvement was accompanied by a reduction of C. jejuni translocation into the colon and extraintestinal tissues and by attenuation of neutrophil migratory capacity. Furthermore, neutrophil depletion attenuated C. jejuni-induced crypt abscesses and intestinal inflammation. Our findings indicate that C. jejuni-induced PI3K-γ signaling mediates neutrophil recruitment and intestinal inflammation in Il10(-/-) mice. Selective pharmacological inhibition of PI3K-γ may represent a novel means to alleviate severe cases of campylobacteriosis, especially in antibiotic-resistant strains.

Current insights into neutrophil homeostasis

Neutrophil granulocytes represent the first immunologic barrier against invading pathogens, and neutropenia predisposes to infection. However, neutrophils may also cause significant collateral inflammatory damage. Therefore, neutrophil numbers are tightly regulated by an incompletely understood homeostatic feedback loop adjusting the marrow's supply to peripheral needs. Granulocyte colony-stimulating factor (G-CSF) is accepted to be the major determinant of neutrophil production, and G-CSF levels have, soon after its discovery, been described to be inversely correlated with neutrophil counts. A neutrophil sensor, or "neutrostat," has, therefore, been postulated. The prevailing feedback hypothesis was established in adhesion molecule-deficient mice; it includes macrophages and Th17 cells, which determine G-CSF levels in response to the number of peripherally transmigrated, apoptosing neutrophils. Recent work has deepened our understanding of homeostatic regulation of neutrophil granulopoiesis, but there are still inconsistent findings and unresolved questions when it comes to a plausible hypothesis, similar to the feedback control models of red cell or platelet homeostasis.

Characterization of iNOS(+) Neutrophil-like ring cell in tumor-bearing mice

Background

Myeloid-derived Suppressor Cells (MDSC) have been identified as tumor-induced immature myeloid cells (IMC) with potent immune suppressive activity in cancer. Whereas strict phenotypic classification of MDSC has been challenging due to the highly heterogeneous nature of cell surface marker expression, use of functional markers such as Arginase and inducible nitric oxide synthase (iNOS) may represent a better categorization strategy. In this study we investigated whether iNOS could be utilized as a specific marker for the identification of a more informative homogenous MDSC subset.

Methods

Single-cell suspensions from tumors and other organs were prepared essentially by enzymatic digestion. Flow cytometric analysis was performed on a four-color flow cytometer. Morphology, intracellular structure and localization of iNOS⁺ ring cells in the tumor were determined by cytospin analysis, immunofluorescence microscopy and immunohistochemistry, respectively. For functional analysis, iNOS⁺ ring subset were sorted and tested in vitro cell culture experiments. Pharmacologic inhibition of iNOS was performed both in vivo and in vitro.

Results

The results showed that intracellular iNOS staining distinguished a granular iNOS⁺ SSC^hi CD11b⁺ Gr-1^dim F4/80⁺ subset with ring-shaped nuclei (ring cells) among the CD11b⁺ Gr-1⁺ cell populations found in tumors. The intensity of the ring cell infiltrate correlated with tumor size and these cells constituted the second major tumor-infiltrating leukocyte subset found in established tumors. Although phenotypic analysis demonstrated that ring cells shared characteristics with tumor-associated macrophages (TAM), morphological analysis revealed a neutrophil-like appearance as detected by cytospin and immunofluorescence microscopy analysis. The presence of distinct iNOS filled granule-like structures located next to the cell membrane suggested that iNOS was stored in pre-formed vesicles and available for rapid release upon activation. Tumor biopsies showed large areas with infiltrating ring cells primarily surrounding necrotic areas. Importantly, these cells significantly impaired CD8⁺ T-cell proliferation and induced apoptotic death. The intratumoral accumulation and suppressive activity of ring cells could be blocked through pharmacologic inhibition of iNOS, demonstrating the critical role of this enzyme in mediating both the differentiation and the activity of these cells.

Conclusions

In this study, iNOS expression was linked to a homogeneous subset; ring cells with a particular phenotype and immune suppressive function, in a common and well-established murine tumor model; 4T-1. Since the absence of a Gr-1 homolog in humans has made the identification of MDSC much more challenging, use of iNOS as a functional marker of MDSC may also have clinical importance.

Complement regulates conventional DC-mediated NK-cell activation by inducing TGF-β1 in Gr-1+ myeloid cells

Complement activation modulates DC-mediated T-cell activation, but whether complement affects DC-mediated priming of NK cells is unknown. Here, we demonstrated that conventional DCs (cDCs) from C3(-/-) and C5aR(-/-) mice are hyperresponsive to polyI:C, a TLR3 ligand, leading to enhanced NK-cell activation. We found that cDCs lack C5a receptor (C5aR) and do not respond to C5a directly. Depletion of Gr-1(+) myeloid cells augments polyI:C-induced cDC activation in WT but not in C3(-/-) or C5aR(-/-) mice, indicating that the effect of complement activation on cDCs is indirectly mediated through C5aR-expressing Gr-1(+) myeloid cells. We further demonstrated that the mechanism by which Gr-1(+) myeloid cells regulate the activity of cDCs involves C5a-dependent TGF-β1 production in Gr-1(+) myeloid cells. C5a enhances and blocking C5aR decreases TGF-β1 production in cultured bone marrow Gr-1(+) CD11b(+) cells. C5aR deficiency is associated with reduced circulating TGF-β1 levels, while depleting Gr-1(+) myeloid cells abrogates this difference between WT and C5aR(-/-) mice. Lastly, we showed that enhanced cDC-NK-cell activity in C3(-/-) mice led to delayed melanoma tumor growth. Thus, complement activation indirectly regulates cDC-NK-cell activation in response to inflammatory stimuli such as TLR3 by promoting TGF-β1 production in Gr-1(+) myeloid cells at steady state.

Ly6G ligation blocks recruitment of neutrophils via a β2-integrin-dependent mechanism

Ly6G is a glycosylphosphatidylinositol (GPI)-anchored protein of unknown function that is commonly targeted to induce experimental neutrophil depletion in mice. In the present study, we found that doses of anti-Ly6G Abs too low to produce sustained neutropenia remained capable of inhibiting experimental arthritis, leaving joint tissues free of infiltrating neutrophils. Thioglycollate-stimulated peritonitis was also attenuated. No alteration in neutrophil apoptosis was observed, implicating impaired recruitment. Indeed, Ly6G ligation abrogated neutrophil migration toward LTB(4) and other chemoattractants in a transwell system. Exploring the basis for this blockade, we identified colocalization of Ly6G and β2-integrins by confocal microscopy and confirmed close association by both coimmunoprecipitation and fluorescence lifetime imaging microscopy. Anti-Ly6G Ab impaired surface expression of β2-integrins in LTB(4)-stimulated neutrophils and mimicked CD11a blockade in inhibiting both ICAM-1 binding and firm adhesion to activated endothelium under flow conditions. Correspondingly, migration of β2-integrin-deficient neutrophils was no longer inhibited by anti-Ly6G. These results demonstrate that experimental targeting of Ly6G has functional effects on the neutrophil population and identify a previously unappreciated role for Ly6G as a modulator of neutrophil migration to sites of inflammation via a β2-integrin-dependent mechanism.

Control of murine Ly6C(high) monocyte traffic and immunosuppressive activities by atypical chemokine receptor D6

The atypical chemokine receptor D6 is a decoy and scavenger receptor for most inflammatory CC chemokines and prevents the development of exacerbated inflammatory reactions. Here we report that mice lacking D6 expression in the nonhematopoietic compartment have a selective increase in the number of Ly6C(high) monocytes in the circulation and in secondary lymphoid tissues. Under inflammatory conditions, Ly6C(high) monocytes accumulate in increased number in secondary lymphoid organs of D6(-/-) mice in a CCR2-dependent manner. Ly6C(high) monocytes derived from D6(-/-) mice have enhanced immunosuppressive activity, inhibit the development of adaptive immune responses, and partially protect mice from the development of GVHD. Thus, control of CCR2 ligands by D6 regulates the traffic of Ly6C(high) monocytes and controls their immunosuppressive potential.

Neutrophil depletion impairs natural killer cell maturation, function, and homeostasis

Neutropenia in mice and humans results in the generation of NK cells with an immature and hyporesponsive phenotype.

Natural killer (NK) cells are bone marrow (BM)–derived granular lymphocytes involved in immune defense against microbial infections and tumors. In an N-ethyl N-nitrosourea (ENU) mutagenesis strategy, we identified a mouse mutant with impaired NK cell reactivity both in vitro and in vivo. Dissection of this phenotype showed that mature neutrophils were required both in the BM and in the periphery for proper NK cell development. In mice lacking neutrophils, NK cells displayed hyperproliferation and poor survival and were blocked at an immature stage associated with hyporesponsiveness. The role of neutrophils as key regulators of NK cell functions was confirmed in patients with severe congenital neutropenia and autoimmune neutropenia. In addition to their direct antimicrobial activity, mature neutrophils are thus endowed with immunoregulatory functions that are conserved across species. These findings reveal novel types of cooperation between cells of the innate immune system and prompt examination of NK cell functional deficiency in patients suffering from neutropenia-associated diseases.

Injury-induced GR-1+ macrophage expansion and activation occurs independently of CD4 T-cell influence

Burn injury initiates an enhanced inflammatory condition referred to as the systemic inflammatory response syndrome or the two-hit response phenotype. Prior reports indicated that macrophages respond to injury and demonstrate a heightened reactivity to Toll-like receptor stimulation. Since we and others observed a significant increase in splenic GR-1 F4/80 CD11b macrophages in burn-injured mice, we wished to test if these macrophages might be the primary macrophage subset that shows heightened LPS reactivity. We report here that burn injury promoted higher level TNF-α expression in GR-1, but not GR-1 macrophages, after LPS activation both in vivo and ex vivo. We next tested whether CD4 T cells, which are known to suppress injury-induced inflammatory responses, might control the activation and expansion of GR-1 macrophages. Interestingly, we found that GR-1 macrophage expansion and LPS-induced TNF-α expression were not significantly different between wild-type and CD4 T cell-deficient CD4(-/-) mice. However, further investigations showed that LPS-induced TNF-α production was significantly influenced by CD4 T cells. Taken together, these data indicate that GR-1 F4/80 CD11b macrophages represent the primary macrophage subset that expands in response to burn injury and that CD4 T cells do not influence the GR-1 macrophage expansion process, but do suppress LPS-induced TNF-α production. These data suggest that modulating GR-1 macrophage activation as well as CD4 T cell responses after severe injury may help control the development of systemic inflammatory response syndrome and the two-hit response phenotype.

Neutrophils, not monocyte/macrophages, are the major splenic source of postburn IL-10

Burn induces myeloid-derived suppressor cells (MDSCs), a heterogeneous population of immature polymorphonuclear neutrophils (PMNs) and monocytes, which protect against infection. Previous work from our laboratory demonstrated that inflammatory monocytes (iMos) were the major MDSC source of TNF-α in the postburn spleen, and we hypothesized that they were also the major source of postburn IL-10. To test this hypothesis, we examined cytokine production by postburn CCR2 knockout (KO) mice, which have fewer iMos than burn wild-type (WT) splenocytes, but equal numbers of PMNs and F4/80 macrophages. Using cell sorting and/or intracellular cytokine techniques, we examined IL-10 production by postburn PMNs and iMos. Finally, we compared IL-10 production by postburn PMNs and iMos with culture-derived MDSCs. Splenocytes from postburn CCR2 KO mice produced less IL-6 and TNF-α than WT burn splenocytes in response to LPS, but KO and WT burn splenocytes produced equal amounts of IL-10 in response to peptidoglycan. Depletion of PMNs from postburn splenocytes led to reductions in IL-10 and increases in IL-6 and TNF-α in response to peptidoglycan, but not in response to LPS. Sorting or intracellular cytokine techniques gave consistent results: Burn PMNs made more IL-10 than sham PMNs and also more IL-10 than burn or sham iMos. Polymorphonuclear neutrophil and iMos subpopulations from culture-derived MDSCs produced the same cytokine profiles in response to LPS and peptidoglycan as did the PMNs and iMos from postburn spleens: PMNs made IL-10, whereas iMos made IL-6. Finally, LPS-induced mortality of burn mice was made worse by anti-Gr-1 depletion of all PMNs and 66% of iMos from burn mice. This suggests that PMNs play a primarily anti-inflammatory role in vitro and in vivo.

Acquisition of antigen-presenting functions by neutrophils isolated from mice with chronic colitis

Active episodes of the inflammatory bowel diseases are associated with the infiltration of large numbers of myeloid cells including neutrophils, monocytes, and macrophages. The objective of this study was to systematically characterize and define the different populations of myeloid cells generated in a mouse model of chronic gut inflammation. Using the T cell transfer model of chronic colitis, we found that induction of disease was associated with enhanced production of myelopoietic cytokines (IL-17 and G-CSF), increased production of neutrophils and monocytes, and infiltration of large numbers of myeloid cells into the mesenteric lymph nodes (MLNs) and colon. Detailed characterization of these myeloid cells revealed three major populations including Mac-1(+)Ly6C(high)Gr-1(low/neg) cells (monocytes), Mac-1(+)Ly6C(int)Gr-1(+) cells (neutrophils), and Mac-1(+)Ly6C(low/neg)Gr-1(low/neg) leukocytes (macrophages, dendritic cells, and eosinophils). In addition, we observed enhanced surface expression of MHC class II and CD86 on neutrophils isolated from the inflamed colon when compared with neutrophils obtained from the blood, the MLNs, and the spleen of colitic mice. Furthermore, we found that colonic neutrophils had acquired APC function that enabled these granulocytes to induce proliferation of OVA-specific CD4(+) T cells in an Ag- and MHC class II-dependent manner. Finally, we observed a synergistic increase in proinflammatory cytokine and chemokine production following coculture of T cells with neutrophils in vitro. Taken together, our data suggest that extravasated neutrophils acquire APC function within the inflamed bowel where they may perpetuate chronic gut inflammation by inducing T cell activation and proliferation as well as by enhancing production of proinflammatory mediators.

Immune suppression: the hallmark of myeloid derived suppressor cells

Myeloid derived suppressor cells (MDSC) are heterogeneous cell population consisting of myeloid progenitor cells and immature myeloid cells. These cells have essential immunoregulatory role in tumor bearing hosts and under different inflammatory conditions. No specific marker has been described to identify MDSC, which leaves their suppressor activity as their only hallmark function. In this review, we discuss the current in vivo and in vitro developed assays for elucidation of MDSC function and describe the discrepancies between murine and human MDSC in regard to their suppressor function. We also discuss antigen specificity of MDSC function and approaches to determine the effector function of these cells in vivo. Finally, we summarize different approaches currently being employed to target MDSC with the aim to enhance immune based therapies.

CD11d Antibody Treatment Improves Recovery in Spinal Cord-Injured Mice

Acute administration of a monoclonal antibody (mAb) raised against the CD11d subunit of the leukocyte CD11d/CD18 integrin after spinal cord injury (SCI) in the rat greatly improves neurological outcomes. This has been chiefly attributed to the reduced infiltration of neutrophils into the injured spinal cord in treated rats. More recently, treating spinal cord-injured mice with a Ly-6G neutrophil-depleting antibody was demonstrated to impair neurological recovery. These disparate results could be due to different mechanisms of action utilized by the two antibodies, or due to differences in the inflammatory responses between mouse and rat that are triggered by SCI. To address whether the anti-CD11d treatment would be effective in mice, a CD11d mAb (205C) or a control mAb (1B7) was administered intravenously at 2, 24, and 48 h after an 8-g clip compression injury at the fourth thoracic spinal segment. The anti-CD11d treatment reduced neutrophil infiltration into the injured mouse spinal cord and was associated with increased white matter sparing and reductions in myeloperoxidase (MPO) activity, reactive oxygen species, lipid peroxidation, and scar formation. These improvements in the injured spinal cord microenvironment were accompanied by increased serotonin (5-HT) immunoreactivity below the level of the lesion and improved locomotor recovery. Our results with the 205C CD11d mAb treatment complement previous work using this anti-integrin treatment in a rat model of SCI.

Enhanced T-cell-independent antitumor effect of cyclophosphamide combined with anti-CD40 mAb and CpG in mice

We have earlier demonstrated T-cell-independent antitumor effects of a combination of anti-CD40 monoclonal antibody (mAb) and CpG oligodeoxynucleotides (CpG) which involved macrophages. As some immunotherapeutic treatments can be potentiated by chemotherapy, we tested if cyclophosphamide (CY) would enhance the antitumor effect of anti-CD40 mAb+CpG. Treatment of B16 melanoma-bearing mice with CY and anti-CD40 mAb+CpG resulted in a significant reduction in tumor growth in immunocompetent mice compared with either CY alone or anti-CD40 mAb with CpG. This enhanced antitumor effect was maintained in severe combined immunodeficiency mice, as measured by both tumor growth and overall survival. Natural killer cells were not required for this antitumor effect as it was also observed in severe combined immunodeficiency/beige mice. Moreover, although CY treatment of immunocompetent mice suppressed natural killer cell activity, it did not negatively affect the antitumor activity of their macrophages when assayed in vitro. Depletion of macrophages in vivo reduced the antitumor effect of CY and anti-CD40 mAb+CpG. These results suggest that therapeutic strategies to activate macrophages may have potential for clinical application in cancer patients receiving chemotherapy.

Measurement of myeloid cell immune suppressive activity

This unit presents simple methods to assess the immunosuppressive properties of immunoregulatory cells of myeloid origin, such as myeloid-derived suppressor cells (MDSCs), both in vitro and in vivo. These methods are general and could be adapted to test the impact of different suppressive populations on T cell activation, proliferation, and cytotoxic activity; moreover they could be useful to assess the influence exerted on immune suppressive pathways by genetic modifications, chemical inhibitors, and drugs.

Myeloid-derived suppressor cells--their role in haemato-oncological malignancies and other cancers and possible implications for therapy

Myeloid-derived suppressor cells (MDSCs) are a heterogeneous population of immature myeloid cells at different stages of maturation that play a role in cancer tolerance and function as an immune-suppressive cell subpopulation. They utilize different mechanisms to block both innate and adaptive arms of anti-tumour immunity, mostly through inhibition of T cell activation and expansion. Further advances in our understanding of this cell population in both murine models and humans has enabled more accurate characterization of their phenotype and the recognition of two major classes of MDSCs: granulocytic and monocytic. Recently, the mechanism of action and clinical importance of MDSCs has been more clearly defined and their interactions with cancer cells have been shown to be among the factors influencing tumour development and induction of tolerance. Most of the earlier studies were performed using murine models, but recent clinical investigations have shown their potential role in human cancers. Here, we review the origin of MDSCs, their mechanisms of action, the factors influencing their production and related signalling pathways. We focus on their role in human solid tumours and haemato-oncological malignancies, and relate to possible novel therapeutic approaches targeting MDSCs which could be considered together with other anticancer strategies in the not too distant future.

Contribution of myeloid-derived suppressor cells to tumor-induced immune suppression, angiogenesis, invasion and metastasis

Growing evidence suggests that myeloid-derived suppressor cells (MDSCs), which have been named "immature myeloid cells" or "myeloid suppressor cells" (MSCs), play a critical role during the progression of cancer in tumor-bearing mice and cancer patients. As their name implies, these cells are derived from bone marrow and have a tremendous potential to suppress immune responses. Recent studies indicated that these cells also have a crucial role in tumor progression. MDSCs can directly incorporate into tumor endothelium. They secret many pro-angiogenic factors as well. In addition, they play an essential role in cancer invasion and metastasis through inducing the production of matrix metalloproteinases (MMPs), chemoattractants and creating a pre-metastatic environment. Increasing evidence supports the idea that cancer stem cells (CSCs) are responsible for tumorigenesis, resistance to therapies, invasion and metastasis. Here, we hypothesize that CSCs may "hijack" MDSCs for use as alternative niche cells, leading to the maintenance of stemness and enhanced chemo- and radio-therapy resistance. The countermeasure that directly targets to MDSCs may be useful for against angiogenesis and preventing cancer from invasion and metastasis. Therefore, the study of MDSCs is important to understand tumor progression and to enhance the therapeutic efficacy against cancer.

The biology of myeloid-derived suppressor cells: the blessing and the curse of morphological and functional heterogeneity

Myeloid-derived suppressor cells (MDSC) play an important role in the cellular network regulating immune responses in cancer, chronic infectious diseases, autoimmunity, and in other pathological conditions. Morphological, phenotypic and functional heterogeneity is a hallmark of MDSC. This heterogeneity demonstrates the plasticity of this immune suppressive myeloid compartment, and shows how various tumors and infectious agents can have similar biological effects on myeloid cells despite the differences in the factors that they produce to influence the immune system; however, such a heterogeneity creates ambiguity in the definition of MDSC as well as confusion regarding the origin and fate of these cells. In this review, we will discuss recent findings that help to better clarify these issues and to determine the place of MDSC within the myeloid cell lineage.

Subsets, expansion and activation of myeloid-derived suppressor cells

Tumor cells and microorganisms manipulate the immune system to minimize any counter response in order to survive. Myeloid-derived suppressor cells (MDSC) in the mouse represent activated Gr-1(+) CD11b(+) myeloid precursor cells. Activation may occur through endogenous or exogenous factors leading to the suppression of immune responses. Under steady state conditions the same precursors differentiate into dendritic cells, macrophages and neutrophils. Their linkage to tumor progression and several suppression mechanisms employing the arginine metabolism are well documented, but knowledge of their role in chronic infections, autoimmune diseases and graft-versus-host reactions is just emerging. Several factors have been described to promote MDSC expansion and activation in bone marrow, spleen and tumor sites. New evidence suggests that the Gr-1 antibody itself may differentially trigger myelopoiesis under steady state conditions or induce apoptosis in inflammatory situations after binding to a common epitope expressed on Ly-6C and Ly-6G molecules, respectively. Moreover, two subsets of neutrophil- and monocyte-related MDSC have been described in tumor-bearing and healthy mice. In the present review, we summarize some early work leading to recent findings on these two MDSC subsets, the factors supporting MDSC expansion and activation, as well as novel insights on Gr-1 antibody functions.