Myeloid-derived suppressor cells as regulators of the immune system

Tim-3/galectin-9 pathway: regulation of Th1 immunity through promotion of CD11b+Ly-6G+ myeloid cells

IFN-gamma plays a central role in antitumor immunity. T cell Ig and mucin domain (Tim-3) is expressed on IFN-gamma-producing Th1 cells; on interaction with its ligand, galectin-9, Th1 immunity is terminated. In this study, we show that transgenic overexpression of Tim-3 on T cells results in an increase in CD11b(+)Ly-6G(+) cells and inhibition of immune responses. Molecular characterization of CD11b(+)Ly-6G(+) cells reveals a phenotype consistent with granulocytic myeloid-derived suppressor cells. Accordingly, we find that modulation of the Tim-3/galectin-9 (Gal-9) pathway impacts on tumor growth. Similarly, overexpression of Tim-3 ligand, Gal-9, results in an increase in CD11b(+)Ly-6G(+) cells and inhibition of immune responses. Loss of Tim-3 restores normal levels of CD11b(+)Ly-6G(+) cells and normal immune responses in Gal-9 transgenic mice. Our data uncover a novel mechanism by which the Tim-3/Gal-9 pathway regulates immune responses and identifies this pathway as a therapeutic target in diseases where myeloid-derived suppressor cells are disadvantageous.

Myeloperoxidase-rich Ly-6C+ myeloid cells infiltrate allografts and contribute to an imaging signature of organ rejection in mice

Rates of graft rejection are high among recipients of heart transplants. The onset and progression of clinically significant heart transplant rejection are currently monitored by serial biopsy, but this approach is highly invasive and lacks sensitivity. Here, we have developed what we believe to be a new technique to measure organ rejection noninvasively that involves the exploration of tissue-infiltrating leukocytes as biomarker sources for diagnostic imaging. Specifically, we profiled the myeloid response in a murine model of heart transplantation with the aim of defining and validating an imaging signature of graft rejection. Ly-6Chi monocytes, which promote inflammation, accumulated progressively in allografts but only transiently in isografts. Ly-6Clo monocytes, which help resolve inflammation, did not accumulate, although they composed the majority of the few remaining monocytes in isografts. The persistence of Ly-6Chi monocytes in allografts prompted us to screen for a Ly-6Chi monocyte-associated imaging marker. Low-density array data revealed that Ly-6Chi monocytes express 10-fold higher levels of myeloperoxidase (MPO) than Ly-6Clo monocytes. Noninvasive magnetic resonance imaging of MPO with an MPO-activatable Gd-chelate revealed a spatially defined T1-weighted signal in rejected allografts but not in isografts or MPO-deficient allograft recipients. Flow cytometry, enzymography, and histology validated the approach by mapping MPO activity to Ly-6Chi monocytes and neutrophils. Thus, MPO imaging represents a potential alternative to the current invasive clinical standard by which transplants are monitored.

Different tumor microenvironments contain functionally distinct subsets of macrophages derived from Ly6C(high) monocytes

Tumor-associated macrophages (TAM) form a major component of the tumor stroma. However, important concepts such as TAM heterogeneity and the nature of the monocytic TAM precursors remain speculative. Here, we show for the first time that mouse mammary tumors contained functionally distinct subsets of TAMs and provide markers for their identification. Furthermore, in search of the TAM progenitors, we show that the tumor-monocyte pool almost exclusively consisted of Ly6C(hi)CX(3)CR1(low) monocytes, which continuously seeded tumors and renewed all nonproliferating TAM subsets. Interestingly, gene and protein profiling indicated that distinct TAM populations differed at the molecular level and could be classified based on the classic (M1) versus alternative (M2) macrophage activation paradigm. Importantly, the more M2-like TAMs were enriched in hypoxic tumor areas, had a superior proangiogenic activity in vivo, and increased in numbers as tumors progressed. Finally, it was shown that the TAM subsets were poor antigen presenters, but could suppress T-cell activation, albeit by using different suppressive mechanisms. Together, our data help to unravel the complexities of the tumor-infiltrating myeloid cell compartment and provide a rationale for targeting specialized TAM subsets, thereby optimally "re-educating" the TAM compartment.

Induction of anti-glioma natural killer cell response following multiple low-dose intracerebral CpG therapy

PURPOSE

Stimulation of toll-like receptor-9 by CpG oligodeoxynucleotides (CpG-ODN) has been shown to counteract the immunosuppressive microenvironment and to inhibit tumor growth in glioma models. These studies, however, have used high doses of CpG-ODN, which can induce toxicity in a clinical setting. The goal of this study was to evaluate the antitumor efficacy of multiple low-dose intratumoral CpG-ODN in a glioma model.

EXPERIMENTAL DESIGN

Mice bearing 4-day-old intracranial GL261 gliomas received a single or multiple (two or four) intratumoral injections of CpG-ODN (3 microg) every 4 days. Tumor growth was measured by bioluminescent imaging, brain histology, and animal survival. Flow cytometry and cytotoxicity assays were used to assess anti-glioma immune response.

RESULTS

Two and four intracranial injections of low-dose CpG-ODN, but not a single injection, eradicated gliomas in 70% of mice. Moreover, surviving animals exhibited durable tumor-free remission (> 3 months) and were protected from intracranial rechallenge with GL261 gliomas, showing the capacity for long-term antitumor immunity. Although most inflammatory cells seemed to increase, activated natural killer (NK) cells (i.e., NK(+)CD107a(+)) were more frequent than CD8(+)CD107a(+) in the brains of rechallenged CpG-ODN-treated animals and showed a stronger in vitro cytotoxicity against GL261 target cells. Leukocyte depletion studies confirmed that NK cells played an important role in the initial CpG-ODN antitumor response, but both CD8 and NK cells were equally important in long-term immunity against gliomas.

CONCLUSIONS

These findings suggest that multiple low-dose intratumoral injections of CpG-ODN can eradicate intracranial gliomas possibly through mechanisms involving NK-mediated effector function.

Tolerogenic dendritic cells and myeloid-derived suppressor cells: potential for regulation and therapy of liver auto- and alloimmunity

Organ transplantation is now established as an accepted treatment for end-stage liver disease, acute fulminant hepatic liver failure and hepatocellular carcinoma. While early graft acceptance rates have increased markedly due to improved immunosuppressive drug regimens, rates of late graft failure remain largely unchanged. Recent findings suggest that in addition to alloimmunity, chronic rejection of liver allografts may also reflect de novo autoimmune hepatitis or recurrence of pre-existing hepatic autoimmune disease. Dendritic cell (DC)- based therapy is a promising experimental approach to promotion of transplant tolerance and the treatment of autoimmune diseases. Newly emerging evidence also demonstrates the potential efficacy of myeloid-derived suppressor cells (MDSC) in the antigen (Ag)-specific regulation of T-cell responses. Herein, we discuss current understanding of liver autoimmunity post-transplantation, along with current approaches for the development of tolerogenic DC, and the potential use of MDSC for the development of stable, Ag-specific tolerance.

A novel nanoparticle drug delivery system: the anti-inflammatory activity of curcumin is enhanced when encapsulated in exosomes

Monocyte-derived myeloid cells play vital roles in inflammation-related autoimmune/inflammatory diseases and cancers. Here, we report that exosomes can deliver anti-inflammatory agents, such as curcumin, to activated myeloid cells in vivo. This technology provides a means for anti-inflammatory drugs, such as curcumin, to target the inflammatory cells as well as to overcome unwanted off-target effects that limit their utility. Using exosomes as a delivery vehicle, we provide evidence that curcumin delivered by exosomes is more stable and more highly concentrated in the blood. We show that the target specificity is determined by exosomes, and the improvement of curcumin activity is achieved by directing curcumin to inflammatory cells associated with therapeutic, but not toxic, effects. Furthermore, we validate the therapeutic relevance of this technique in a lipopolysaccharide (LPS)-induced septic shock mouse model. We further show that exosomes, but not lipid alone, are required for the enhanced anti-inflammatory activity of curcumin. The specificity of using exosomes as a drug carrier creates opportunities for treatments of many inflammation-related diseases without significant side effects due to innocent bystander or off-target effects.

Proton dynamics in cancer

Cancer remains a leading cause of death in the world today. Despite decades of research to identify novel therapeutic approaches, durable regressions of metastatic disease are still scanty and survival benefits often negligible. While the current strategy is mostly converging on target-therapies aimed at selectively affecting altered molecular pathways in tumor cells, evidences are in parallel pointing to cell metabolism as a potential Achilles' heel of cancer, to be disrupted for achieving therapeutic benefit. Critical differences in the metabolism of tumor versus normal cells, which include abnormal glycolysis, high lactic acid production, protons accumulation and reversed intra-extracellular pH gradients, make tumor site a hostile microenvironment where only cancer cells can proliferate and survive. Inhibiting these pathways by blocking proton pumps and transporters may deprive cancer cells of a key mechanism of detoxification and thus represent a novel strategy for a pleiotropic and multifaceted suppression of cancer cell growth.

Research groups scattered all over the world have recently started to investigate various aspects of proton dynamics in cancer cells with quite encouraging preliminary results. The intent of unifying investigators involved in this research line led to the formation of the "International Society for Proton Dynamics in Cancer" (ISPDC) in January 2010. This is the manifesto of the newly formed society where both basic and clinical investigators are called to foster translational research and stimulate interdisciplinary collaboration for the development of more specific and less toxic therapeutic strategies based on proton dynamics in tumor cell biology.

Tumor-conditioned macrophages secrete migration-stimulating factor: a new marker for M2-polarization, influencing tumor cell motility

Tumor-associated macrophages (TAMs) are key orchestrators of the tumor microenvironment directly affecting neoplastic cell growth, neoangiogenesis, and extracellular matrix remodeling. In turn, the tumor milieu strongly influences maturation of TAMs and shapes several of their features. To address the early macrophage (M) differentiation phase in a malignant context, we mimicked a tumor microenvironment by in vitro coculturing human blood monocytes with conditioned media from different cancer cell lines. Only 2 out of 16 tumor cell lines induced M differentiation due to secreted M-CSF isoforms, including high molecular mass species. A global gene profiling of tumor-conditioned M was performed. Comparison with other datasets (polarized M1-M, M2-M, and TAMs isolated from human tumors) highlighted the upregulation of several genes also shared by TAM and M2-polarized M. The most expressed genes were selenoprotein 1, osteoactivin, osteopontin, and, interestingly, migration-stimulating factor (MSF), a poorly studied oncofoetal isoform of fibronectin. MSF (present in fetal/cancer epithelial and stromal cells but not in healthy tissues) was never identified in M. MSF production was confirmed by immunohistochemistry in human TAMs. MSF was induced by M-CSF, IL-4, and TGFbeta but not by proinflammatory stimuli. RNA and protein analysis clearly demonstrated that it is specifically associated with the M2 polarization of M. Tumor-conditioned M-derived MSFs strongly stimulated tumor cell migration, thus contributing to the motile phenotype of neoplastic cells. In conclusion, MSF is a new molecule associated with the M2 polarization of M and expressed by TAMs. Its biological function may contribute to M-mediated promotion of cancer cell invasion and metastasis.

CD49d is a new marker for distinct myeloid-derived suppressor cell subpopulations in mice

Myeloid-derived suppressor cells (MDSCs) are a heterogenous population of cells that negatively regulate the immune response during tumor progression, inflammation, and infection. In this study, through gene-expression analysis, we have identified a new marker, CD49d, which is expressed exclusively on CD11b(+)Gr-1(dull/int.) MDSCs. We have characterized two subpopulations of MDSCs based on CD49d expression in two different settings, a mouse model of inflammatory bowel disease and tumor-bearing mice. The CD49d(+) subset of MDSCs was mainly monocytic and strongly suppressed Ag-specific T cell proliferation in an NO-dependent mechanism similar to Gr-1(dull/int.) MDSCs. Alternatively, CD49d(-) cells were granulocytic and poorly inhibited T cell proliferation compared with CD11b(+)Gr-1(high) cells. Both mouse models showed preferential expansion of the granulocytic CD49d(-) subset. We suggest that CD49d can be used as an alternative marker for Gr-1 to differentiate between the subpopulations of MDSCs together with CD11b, which will ultimately help in understanding the mechanisms of immune suppression by MDSCs.

Tumor-induced tolerance and immune suppression depend on the C/EBPbeta transcription factor

Tumor growth is associated with a profound alteration in myelopoiesis, leading to recruitment of immunosuppressive cells known as myeloid-derived suppressor cells (MDSCs). We showed that among factors produced by various experimental tumors, the cytokines GM-CSF, G-CSF, and IL-6 allowed a rapid generation of MDSCs from precursors present in mouse and human bone marrow (BM). BM-MDSCs induced by GM-CSF+IL-6 possessed the highest tolerogenic activity, as revealed by the ability to impair the priming of CD8(+) T cells and allow long term acceptance of pancreatic islet allografts. Cytokines inducing MDSCs acted on a common molecular pathway and the immunoregulatory activity of both tumor-induced and BM-derived MDSCs was entirely dependent on the C/EBPbeta transcription factor. Adoptive transfer of tumor antigen-specific CD8(+) T lymphocytes resulted in therapy of established tumors only in mice lacking C/EBPbeta in the myeloid compartment, suggesting that C/EBPbeta is a critical regulator of the immunosuppressive environment created by growing cancers.

TGF-beta and immune cells: an important regulatory axis in the tumor microenvironment and progression

Transforming growth factor beta (TGF-beta) plays an important role in tumor initiation and progression, functioning as both a suppressor and a promoter. The mechanisms underlying this dual role of TGF-beta remain unclear. TGF-beta exerts systemic immune suppression and inhibits host immunosurveillance. Neutralizing TGF-beta enhances CD8+ T-cell- and NK-cell-mediated anti-tumor immune responses. It also increases neutrophil-attracting chemokines resulting in recruitment and activation of neutrophils with an antitumor phenotype. In addition to its systemic effects, TGF-beta regulates infiltration of inflammatory/immune cells and cancer-associated fibroblasts in the tumor microenvironment causing direct changes in tumor cells. Understanding TGF-beta regulation at the interface of tumor and host immunity should provide insights into developing effective TGF-beta antagonists and biomarkers for patient selection and efficacy of TGF-beta antagonist treatment.

Immature immunosuppressive CD14+HLA-DR-/low cells in melanoma patients are Stat3hi and overexpress CD80, CD83, and DC-sign

Myeloid-derived suppressor cells (MDSC) have emerged as key immune modulators in various tumor models and human malignancies, but their characteristics in humans remain to be unequivocally defined. In this study, we have examined circulating CD14(+)HLA-DR(-/low) MDSC in 34 advanced malignant melanoma (MM) patients. Their frequency is significantly increased and associated with disease activity. Contrary to the common notion that MDSC are a heterogeneous population of exclusively immature cells, we find the coexpression of markers associated with mature phenotype. We show for the first time the overexpression of CD80, CD83, and DC-Sign in human MDSC. Further, increased levels of signal transducer and activator of transcription 3 (Stat3), an important regulator in MDSC development and function, were noted in MM-MDSC. Stat3 was altered toward an active, phosphorylated state in the HLA-DR(-) population of CD14(+) cells and was more reactive to activating stimuli in patients. Importantly, inhibition of Stat3 abolished their suppressive activity almost completely. The described MM-MDSC use arginase in conjunction with other yet undefined mechanisms to suppress CD4(+) and CD8(+) T cells. Several observations suggest a redox imbalance in MDSC and indicate an important role of Stat3-dependent oxidative stress in MDSC-mediated T-cell suppression. These results emphasize the diversity of MDSC in human cancer and provide potential targets for therapeutic interventions.

Immune regulation of cancer

Innate and adaptive immune system cells play a major role in regulating the growth of cancer. Although it is commonly thought that an immune response localized to the tumor will inhibit cancer growth, it is clear that some types of inflammation induced in a tumor may also lead to cancer proliferation, invasion, and dissemination. Recent evidence suggests, however, that some patients with cancer can mount an antitumor immune response that has the potential to control or eliminate cancer. Indeed, a so-called "immune response" signature has been described in malignancy that is associated with improved outcomes in several tumor types. Moreover, the presence of specific subsets of T cells, which have the capability to penetrate tumor stroma and infiltrate deep into the parenchyma, identifies patients with an improved prognosis. Immune-based therapies have the potential to modulate the tumor microenvironment by eliciting immune system cells that will initiate acute inflammation that leads to tissue destruction.

Inflammation and colon cancer

The connection between inflammation and tumorigenesis is well-established and in the last decade has received a great deal of supporting evidence from genetic, pharmacological, and epidemiological data. Inflammatory bowel disease is an important risk factor for the development of colon cancer. Inflammation is also likely to be involved with other forms of sporadic as well as heritable colon cancer. The molecular mechanisms by which inflammation promotes cancer development are still being uncovered and could differ between colitis-associated and other forms of colorectal cancer. Recent work has elucidated the role of distinct immune cells, cytokines, and other immune mediators in virtually all steps of colon tumorigenesis, including initiation, promotion, progression, and metastasis. These mechanisms, as well as new approaches to prevention and therapy, are discussed in this review.

Thyroid cancer and inflammation

Some cancer types are strongly associated with chronic inflammatory or infectious diseases whereas others are not, but an inflammatory component is present in most human neoplastic lesions. This review focuses on various aspects of thyroid cancer and inflammation. The incidence of thyroid cancer, in particular of well-differentiated papillary thyroid carcinomas (PTCs), is increased in autoimmune thyroid diseases such as Hashimoto's thyroiditis. Thyroid cancer often has an inflammatory cell infiltrate, which includes lymphocytes, macrophages, dendritic cells and mast cells, whose role in thyroid cancer is still not completely understood. However, most experimental evidence suggests these cells exert a protumorigenic function. Moreover, oncoproteins typically expressed in human PTCs, such as RET/PTC, RAS, and BRAF, trigger a proinflammatory programme in thyreocytes. These data suggest that inflammatory molecules are promising targets for thyroid cancer therapy.

Alternative activation of macrophages: mechanism and functions

The concept of an alternative pathway of macrophage activation has stimulated interest in its definition, mechanism, and functional significance in homeostasis and disease. We assess recent research in this field, argue for a restricted definition, and explore pathways by which the T helper 2 (Th2) cell cytokines interleukin-4 (IL-4) and IL-13 mediate their effects on macrophage cell biology, their biosynthesis, and responses to a normal and pathological microenvironment. The stage is now set to gain deeper insights into the role of alternatively activated macrophages in immunobiology.

Salmonella inhibits monocyte differentiation into CD11c hi MHC-II hi cells in a MyD88-dependent fashion

Monocytes and DCs originate from a shared precursor in the bone marrow, and steady-state DCs in lymphoid organs develop directly from the precursor rather than via a monocyte intermediate. However, monocytes can differentiate into DCs in tissues such as the lung and gut mucosa and into macrophages in most tissues. As Ly6C hi monocytes accumulate in lymphoid organs during oral Salmonella infection, we investigated their ability to develop into potential DCs, identified as CD11c hi MHC-II hi cells, in infected hosts. Ly6C hi monocytes, isolated from the blood of Salmonella-infected mice, developed into CD11c hi MHC-II hi cells after culture with GM-CSF or Flt3L. In contrast, the same monocytes cultured in the presence of GM-CSF and heat-killed Salmonella did not differentiate into CD11c hi MHC-II hi cells. The bacteria-induced differentiation block was dependent on TLRs, as monocytes from MyD88-/- mice converted into CD11c hi MHC-II hi cells even in the presence of bacteria. We hypothesized that Salmonella-activated wild-type monocytes secreted mediators that inhibited differentiation of MyD88-/--derived monocytes. However, IL-6, IL-10, TNF-alpha, or IL-12p70 did not account for the inhibition. Finally, monocyte-derived CD11c hi MHC-II hi cells pulsed with OVA peptide or protein did not induce proliferation of antigen-specific CD4+ T cells but rather, suppressed the ability of DCs to activate CD4+ T cells. Overall, the data show that Ly6C hi monocytes from Salmonella-infected mice develop into CD11c hi MHC-II hi cells with poor antigen-presentation capacity when cultured ex vivo, and that monocyte exposure to Salmonella inhibits their differentiation into CD11c hi MHC-II hi cells in a MyD88-dependent fashion.

Local and distant immunity induced by intralesional vaccination with an oncolytic herpes virus encoding GM-CSF in patients with stage IIIc and IV melanoma

BACKGROUND

An oncolytic herpes simplex virus engineered to replicate selectively in tumor cells and to express granulocyte-macrophage colony-stimulating factor (GMCSF) was tested as a direct intralesional vaccination in melanoma patients. The work reported herein was performed to better characterize the effect of vaccination on local and distant antitumor immunity.

METHODS

Metastatic melanoma patients with accessible lesions were enrolled in a multicenter 50-patient phase II clinical trial of an oncolytic herpesvirus encoding GM-CSF (Oncovex(GM-CSF)). An initial priming dose of 10(6) pfu vaccine was given by intratumoral injection, followed by 10(8) pfu every 2 weeks to 24 total doses. Peripheral blood and tumor tissue were collected for analysis of effector T cells, CD4(+)FoxP3(+) regulatory T cells (Treg), CD8(+)FoxP3(+) suppressor T cells (Ts), and myeloid-derived suppressive cells (MDSC).

RESULTS

Phenotypic analysis of T cells derived from tumor samples suggested distinct differences from peripheral blood T cells. There was an increase in melanomaassociated antigen recognized by T cells (MART-1)-specific T cells in tumors undergoing regression after vaccination compared with T cells derived from melanoma patients not treated with vaccine. There was also a significant decrease in Treg and Ts cells in injected lesions compared with noninjected lesions in the same and different melanoma patients. Similarly MDSC were increased in melanoma lesions but underwent a significant decrease only in vaccinated lesions.

CONCLUSIONS

Melanoma patients present with elevated levels of Tregs, Ts, and MDSC within established tumors. Direct injection of Oncovex(GM-CSF) induces local and systemic antigen-specific T cell responses and decreases Treg, Ts, and MDSC in patients exhibiting therapeutic responses.

Myeloid cells in the tumor microenvironment: modulation of tumor angiogenesis and tumor inflammation

Myeloid cells are a heterogeneous population of bone marrow-derived cells that play a critical role during growth and metastasis of malignant tumors. Tumors exhibit significant myeloid cell infiltrates, which are actively recruited to the tumor microenvironment. Myeloid cells promote tumor growth by stimulating tumor angiogenesis, suppressing tumor immunity, and promoting metastasis to distinct sites. In this review, we discuss the role of myeloid cells in promoting tumor angiogenesis. Furthermore, we describe a subset of myeloid cells with immunosuppressive activity (known as myeloid-derived suppressor cells). Finally, we will comment on the mechanisms regulating myeloid cell recruitment to the tumor microenvironment and on the potential of myeloid cells as new targets for cancer therapy.

Autoreactive natural killer T cells: promoting immune protection and immune tolerance through varied interactions with myeloid antigen-presenting cells

Natural killer T (NKT) cells are innate T lymphocytes that are restricted by CD1d antigen-presenting molecules and recognize lipids and glycolipids as antigens. NKT cells have attracted attention for their potent immunoregulatory effects. Like other types of regulatory lymphocytes, a high proportion of NKT cells appear to be autoreactive to self antigens. Thus, as myeloid antigen-presenting cells (APCs) such as monocytes, dendritic cells (DCs) and myeloid-derived suppressor cells (MDSCs) constitutively express CD1d, NKT cells are able to interact with these APCs not only during times of immune activation but also in immunologically quiescent periods. The interactions of NKT cells with myeloid APCs can have either pro-inflammatory or tolerizing outcomes, and a central question is how the ensuing response is determined. Here we bring together published results from a variety of model systems to highlight three critical factors that influence the outcome of the NKT-APC interaction: (i) the strength of the antigenic signal delivered to the NKT cell, as determined by antigen abundance and/or T-cell receptor (TCR) affinity; (ii) the presence or absence of cytokines that costimulate NKT cells [e.g. interleukin (IL)-12, IL-18 and interferon (IFN)-alpha]; (iii) APC intrinsic factors such as differentiation state (e.g. monocyte versus DC) and Toll-like receptor (TLR) stimulation. Together with recent findings that demonstrate new links between NKT cell activation and endogenous lipid metabolism, these results outline a picture in which the functions of NKT cells are closely attuned to the existing biological context. Thus, NKT cells may actively promote tolerance until a critical level of danger signals arises, at which point they switch to activating pro-inflammatory immune responses.

Myeloid-derived suppressor cells: more mechanisms for inhibiting antitumor immunity

Myeloid-derived suppressor cells (MDSC) accumulate in most cancer patients and experimental animals with cancer. They accumulate in response to pro-inflammatory mediators and they use a variety of mechanisms to block both innate and adaptive antitumor immunity. Because of their critical role in obstructing immune responses, MDSC are a strategic obstacle to immunotherapies that require activation of the host's cell-mediated and innate immune responses. Following a brief description of the factors that induce MDSC accumulation, this article reviews two newly discovered mechanisms that MDSC use to suppress the activation of CD4(+) and CD8(+) T cells. The first mechanism is MDSC sequestration of cysteine, an amino acid that T cells are unable to synthesize de novo and that they require for activation. The second mechanism is MDSC-mediated down-regulation of L: -selectin. T cells must have an L: -selectin(high) phenotype to home to lymph nodes and inflammatory sites where they encounter antigen and are activated. By down-regulating L: -selectin on T cells, MDSC perturb T cell trafficking patterns and thereby inhibit T cell activation. Given the complexity of conditions that regulate MDSC accumulation and the variety of suppressive mechanisms used by MDSC, it is essential to understand which conditions and mechanisms are dominant so MDSC accumulation and/or activity can be targeted in individual patients to minimize MDSC-induced immune suppression.

Macrophage diversity enhances tumor progression and metastasis

There is persuasive clinical and experimental evidence that macrophages promote cancer initiation and malignant progression. During tumor initiation, they create an inflammatory environment that is mutagenic and promotes growth. As tumors progress to malignancy, macrophages stimulate angiogenesis, enhance tumor cell migration and invasion, and suppress antitumor immunity. At metastatic sites, macrophages prepare the target tissue for arrival of tumor cells, and then a different subpopulation of macrophages promotes tumor cell extravasation, survival, and subsequent growth. Specialized subpopulations of macrophages may represent important new therapeutic targets.

Early exposure of high-dose interleukin-4 to tumor stroma reverses myeloid cell-mediated T-cell suppression

Myeloid-derived suppressor cells (MDSCs) inhibit T-cell activity and promote tumor growth in tumor-bearing hosts. We sought to determine how to prevent the generation of these cells and modulate anti-tumor immunity at different times during tumor growth. Interleukin-4 (IL-4), a cytokine closely associated with the differentiation of myeloid cells, was expressed locally at the tumor site with its dose and expression time tightly regulated by a tet-off system. Early exposure of high-dose IL-4 to the tumor stromal cells effectively prevented the generation of myeloid suppressor cells and led to a T-cell-mediated tumor rejection. However, IL-4 had no effect a few days after tumor growth, when myeloid suppressor cells had been generated and T cells were tolerized. Importantly, coinoculation of IL-4 receptor (IL-4R)-deficient tumor cells with IL-4R competent, but not IL-4R-deficient myeloid cells led to IL-4-mediated tumor regression in IL-4R-deficient mice, indicating that IL-4 acts directly on myeloid cells. These results show a novel way to prevent T cells from MDSC-induced suppression, with important indications for cancer therapy.

Molecularly defined vaccines for cancer immunotherapy, and protective T cell immunity

Malignant cells are frequently recognized and destroyed by T cells, hence the development of T cell vaccines against established tumors. The challenge is to induce protective type 1 immune responses, with efficient Th1 and CTL activation, and long-term immunological memory. These goals are similar as in many infectious diseases, where successful immune protection is ideally induced with live vaccines. However, large-scale development of live vaccines is prevented by their very limited availability and vector immunogenicity. Synthetic vaccines have multiple advantages. Each of their components (antigens, adjuvants, delivery systems) contributes specifically to induction and maintenance of T cell responses. Here we summarize current experience with vaccines based on proteins and peptide antigens, and discuss approaches for the molecular characterization of clonotypic T cell responses. With carefully designed step-by-step modifications of innovative vaccine formulations, T cell vaccination can be optimized towards the goal of inducing therapeutic immune responses in humans.

Direct and differential suppression of myeloid-derived suppressor cell subsets by sunitinib is compartmentally constrained

The antiangiogenic drug sunitinib is a receptor tyrosine kinase inhibitor with significant, yet not curative, therapeutic effects in metastatic renal cell carcinoma (RCC). Sunitinib is also an immunomodulator, potently reversing myeloid-derived suppressor cell (MDSC) accumulation and T-cell inhibition in the blood even of nonresponder RCC patients. We observed that sunitinib similarly prevented MDSC accumulation and restored normal T-cell function to the spleens of tumor-bearing mice, independent of the capacity of sunitinib to inhibit tumor progression (RENCA>CT26>4T1). Both monocytic and neutrophilic splenic MDSC were highly repressible by sunitinib. In contrast, MDSC within the microenvironment of 4T1 tumors or human RCC tumors proved highly resistant to sunitinib and ambient T-cell function remained suppressed. Proteomic analyses comparing tumor to peripheral compartments showed that granulocyte macrophage colony-stimulating factor (GM-CSF) predicted sunitinib resistance and recombinant GM-CSF conferred sunitinib resistance to MDSC in vivo and in vitro. MDSC conditioning with GM-CSF uniquely inhibited signal transducers and activators of transcription (STAT3) and promoted STAT5 activation. STAT5ab(null/null) MDSC were rendered sensitive to sunitinib in the presence of GM-CSF in vitro. We conclude that compartment-dependent GM-CSF exposure in resistant tumors may account for the regionalized effect of sunitinib upon host MDSC modulation and hypothesize that ancillary strategies to decrease such regionalized escape will enhance the potency of sunitinib as an immunomodulator and a cancer therapy.

Tumor-initiated inflammation overrides protective adaptive immunity in an induced melanoma model in mice

We studied the effect of the immune system on two differentially aggressive melanomas developing in mice on conditional deletion of the INK4A/ARF tumor suppressor gene, with concomitant expression of oncogene H-Ras(G12V) and a natural cancer-germline tumor antigen (TA). "Slow progressor" melanomas contained no activated T lymphocytes (TL). In contrast, "aggressive" melanomas were infiltrated by activated TLs lacking effector molecules and expressing high levels of PD-1, indicating an exhausted phenotype. Aggressive melanomas were also infiltrated by immature myeloid cells (IMC). Infiltration was associated with local inflammation and systemic Th2/Th17-oriented chronic inflammation that seemed to impair further activation of TLs, as tumor-specific T cells adoptively transferred into mice bearing aggressive melanomas were poorly activated and failed to infiltrate the melanoma. This immunosuppression also led to the incapacity of these mice to reject inoculated TA-positive tumors, in contrast to slow-progressing melanoma-bearing mice, which were responsive. To test the role of adaptive immunity in tumor progression, we induced melanomas in immunodeficient RagKO compound mice. These mice developed aggressive but not slow-progressing melanomas at a higher frequency and with a shorter latency than immunocompetent mice. Immunodeficient mice also developed abnormal inflammation and infiltration of IMCs in a manner similar to immunocompetent mice, indicating that this phenotype was not dependent on adaptive immunity. Therefore, tumor-intrinsic factors distinguishing the two melanoma types control the initiation of inflammation, which was independent of adaptive immunity. The latter delayed development of aggressive melanomas but was overridden by inflammation.

Myeloid cells in cancer progression: unique subtypes and their roles in tumor growth, vascularity, and host immune suppression

Leukocytic infiltrates, particularly myeloid cells, can stimulate an anti-tumor immune response, but more often they stimulate tumor development, including promoting invasion, tumor growth, angiogenesis, and metastasis. Distinct myeloid phenotypes are being characterized that have been shown to promote tumor growth, angiogenesis, and metastasis. This review provides an overview of myeloid differentiation and spotlights specific pro-tumorogenic myeloid populations and their role in cancer progression. Efforts to characterize these pro-tumorogenic myeloid cell immunophenotypes may lead to novel targets for cancer therapy.

Vaccines based on abnormal self-antigens as tumor-associated antigens: immune regulation

Abnormal expression of "self" antigens on tumors compared with normal cells provides opportunities and challenges for development of cancer vaccines. We review recent work in pre-clinical transgenic mouse models and in clinical trials that has elucidated multiple regulatory mechanisms that interfere with the induction of effective immunity. We discuss these as being either part of the normal function of the immune system or being driven by the tumor microenvironment. Collectively this work shows that it is possible to design vaccines based on tumor-associated antigens and elicit effective immunity against abnormal expression of these antigens on tumors without causing autoimmunity.

Combination therapy of established tumors by antibodies targeting immune activating and suppressing molecules

The blockade of immune suppression against antitumor responses is a particularly attractive strategy when combined with agents that promote tumor-specific CTLs. In this study, we have attempted to further improve the CTL induction and potent antitumor efficacy of a combination mAb-based therapy (termed "trimAb therapy") that comprises tumor cell death-inducing anti-death receptor 5 mAb and immune activating anti-CD40 and anti-CD137 mAbs. Among trimAb-treated tumors, the infiltration of CD4(+) Foxp3(+) cells was greater in progressing tumors compared with stable tumors. Blockade of CTLA-4 (CD152)-mediated signals by an antagonistic mAb substantially increased the tumor rejection rate of trimAb therapy, although the immune responses of draining lymph node cells were not augmented. Interestingly, by comparison, additional treatment with agonistic anti-glucocorticoid-induced TNF receptor mAb, antagonistic anti-programmed death-1 (CD279) mAb, or agonistic anti-OX40 (CD134) mAb significantly augmented immune responses of draining lymph node cells, but did not augment the therapeutic effect of trimAb. CD4 T cell depletion reduced the antitumor effect of anti-CTLA-4 mAb treatment alone, but did not reduce the tumor rejection rate of trimAb in conjunction with anti-CTLA-4 mAb. Thus, the blockade of the CTLA-4-mediated inhibitory signal in tumor infiltrating CTL may be the most effective strategy to augment the effect of immune therapies that generate tumor-specific CTL.

The myeloid 7/4-antigen defines recently generated inflammatory macrophages and is synonymous with Ly-6B

This study aimed to identify the inflammation-associated 7/4-antigen, which is highly expressed on neutrophils, inflammatory monocytes, some activated macrophages, as well as on bone marrow myeloid-restricted progenitors. The high expression on inflammatory cells is suggestive of a role in inflammation and makes the 7/4-antigen a potential target for the manipulation of inflammatory cells. Consistent with this, the 7/4-antibody mediates specific depletion of 7/4-expressing neutrophils and monocytes. We have identified the 7/4-antigen as a 25- to 30-kDa GPI-anchored glycoprotein synonymous with the Ly-6B.2 alloantigen. We characterized the expression of Ly-6B during the inflammatory reaction induced by zymosan. During the later stages of an experimental, acute, self-resolving inflammatory response, we found that Ly-6B is differentially expressed on macrophages. Ly-6B-expressing macrophages also express more MHCII, CIITA, CCR2, Ly-6C, and CD62L than the Ly-6B-negative macrophages, which in turn, express more of the resident tissue macrophage marker SIGN-R1 and higher CD11b and F4/80. Ly-6B-expressing macrophages incorporate more BrdU than their Ly-6B-negative contemporaries when fed during the resolution phase of the acute inflammatory response. Thus, Ly-6B expression on mature macrophages defines a subset of recently generated inflammatory macrophages that retain monocytic markers and is hence a surrogate marker of macrophage turnover in inflammatory lesions. The definition of the 7/4:Ly-6B antigen will allow further characterization and specific modulation of Ly-6B-expressing cells in vivo.

5-Fluorouracil selectively kills tumor-associated myeloid-derived suppressor cells resulting in enhanced T cell-dependent antitumor immunity

Myeloid-derived suppressor cells (MDSC) accumulate in the spleen and tumor bed during tumor growth. They contribute to the immune tolerance of cancer notably by inhibiting the function of CD8(+) T cells. Thus, their elimination may hamper tumor growth by enhancing antitumor T-cell functions. We have previously reported that some anticancer agents relied on T cell-dependent anticancer responses to achieve maximal efficacy. However, the effect of anticancer agents on MDSC has remained largely unexplored. In this study, we observed that gemcitabine and 5-fluorouracil (5FU) were selectively cytotoxic on MDSC. In vivo, the treatment of tumor-bearing mice with 5FU led to a major decrease in the number of MDSC in the spleens and tumor beds of animals whereas no significant effect on T cells, natural killer cells, dendritic cells, or B cells was noted. Interestingly, 5FU showed a stronger efficacy over gemcitabine to deplete MDSC and selectively induced MDSC apoptotic cell death in vitro and in vivo. The elimination of MDSC by 5FU increased IFN-gamma production by tumor-specific CD8(+) T cells infiltrating the tumor and promoted T cell-dependent antitumor responses in vivo. Altogether, these findings suggest that the antitumor effect of 5FU is mediated, at least in part, by its selective cytotoxic action on MDSC.

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.

Nonresolving inflammation

Nonresolving inflammation is a major driver of disease. Perpetuation of inflammation is an inherent risk because inflammation can damage tissue and necrosis can provoke inflammation. Nonetheless, multiple mechanisms normally ensure resolution. Cells like macrophages switch phenotypes, secreted molecules like reactive oxygen intermediates switch impact from pro- to anti-inflammatory, and additional mediators of resolution arise, including proteins, lipids, and gasses. Aside from persistence of initiating stimuli, nonresolution may result from deficiencies in these mechanisms when an inflammatory response begins either excessively or subnormally. This greatly complicates the development of anti-inflammatory therapies. The problem calls for conceptual, organizational, and statistical innovations.

Immunity, inflammation, and cancer

Inflammatory responses play decisive roles at different stages of tumor development, including initiation, promotion, malignant conversion, invasion, and metastasis. Inflammation also affects immune surveillance and responses to therapy. Immune cells that infiltrate tumors engage in an extensive and dynamic crosstalk with cancer cells, and some of the molecular events that mediate this dialog have been revealed. This review outlines the principal mechanisms that govern the effects of inflammation and immunity on tumor development and discusses attractive new targets for cancer therapy and prevention.

Tumor vaccines in 2010: need for integration

Induction of tumor-specific immunity is an attractive approach to cancer therapy, however to date every major pivotal trial has resulted in failure. While the phenomena of tumor-mediated immune suppression has been known for decades, only recently have specific molecular pathways been elucidated, and for the first time, rationale means of intervening and observing results of intervention have been developed. In this review we describe major advances in our understanding of tumor escape from immunological pressure and provide some possible therapeutic scenarios for enhancement of efficacy in future cancer vaccine trials.

3-Methylcholanthrene-induced transforming growth factor-beta-producing carcinomas, but not sarcomas, are refractory to regulatory T cell-depletion therapy

Regulatory T cells (Tregs) are major immunosuppressors in tumor-bearing hosts. Although Treg-depletion therapy has been shown to induce a complete cure in tumor-bearing mice, this treatment is not always successful. Using 3-methylcholanthrene-induced primary mouse tumors, we examined the distinct regulation of Treg-mediated immunosuppression between carcinomas and sarcomas. We showed that the number of Tregs was greatly increased in squamous cell carcinoma (SCC)-bearing mice compared with sarcoma-bearing mice. This appeared to be because SCC produced higher levels of active transforming growth factor (TGF)-beta, which is essential for inducing Tregs, compared with sarcoma. Moreover, SCC, but not sarcomas, were refractory to Treg-depletion therapy by treatment with anti-CD25 mAb. The refractoriness of SCC against Treg-depletion therapy was due to the rapid recovery of Tregs in SCC-bearing mice compared with sarcoma-bearing mice. However, combination treatment of anti-TGF-beta mAb with anti-CD25 mAb caused a significant reduction in Treg recovery and induced a complete cure in SCC-bearing mice. Thus, we showed the refractoriness of mouse carcinoma against Treg-depletion therapy using anti-CD25 mAb treatment. We also proposed a novel Treg-blocking combination therapy using anti-CD25 mAb and anti-TGF-beta mAb to induce a complete cure of tumor-bearing hosts.

Contribution of MyD88 to the tumor exosome-mediated induction of myeloid derived suppressor cells

In this study we observed that mice pretreated with tumor exosomes had a significant acceleration of tumor metastasis in the lung. Tumor metastasis correlated significantly with an increase in recruitment of more Myeloid-derived suppressor cells (MDSCs) in the lung of C57BL/6j (B6) mice pretreated with tumor exosomes. These effects were blunted when MyD88 knockout (KO) mice were pretreated with tumor exosomes. MDSCs induced by tumor exosomes and isolated from wild-type B6 mice also more potently inhibited T cell activation and induction of interleukin-6 and tumor necrosis factor-alpha than MDSCs isolated from the lung of MyD88 KO mice. In vitro, addition of tumor exosomes to bone marrow-derived CD11b(+)Gr-1(+) cells isolated from wild-type B6 mice resulted in more cytokine production, including tumor necrosis factor-alpha, interleukin-6, and the chemokine CCL2, than CD11b(+)Gr-1(+) cells isolated from MyD88 KO mice. Moreover, lower levels of CCL2 were observed in the lungs in MyD88 KO mice pretreated with tumor exosomes than that in wild-type mice. Together these data demonstrate a pivotal role for MyD88 in tumor exosome-mediated expansion of MDSCs and tumor metastasis.

Critical roles of lysosomal acid lipase in myelopoiesis

Lysosomal acid lipase (LAL) is a key enzyme that cleaves cholesteryl esters and triglycerides to generate free fatty acids and cholesterol in lysosomes. Genetic ablation of the lal gene (lal(-/-)) in mice has resulted in a systemic increase of macrophages and neutrophils, causing severe inflammation and pathogenesis in multiple organs. We hypothesized that aberrant growth and differentiation of myeloid cells in lal(-/-) mice arises from dysregulated production of progenitor cells in the bone marrow. Indeed, lal(-/-) mice displayed increased numbers of primitive lin(-)Sca-1(+)c-Kit(+) (LSK) cells and granulocyte-macrophage precursors (GMP). Increased high proliferative potential colony-forming cells (HPP-CFC) were enumerated from cultured lal(-/-) bone marrow cells, as were significantly more CFU-GM, CFU-G, and CFU-M colonies. As a consequence, lal(-/-) mice developed significant myeloid infiltration, particularly with CD11b+/Gr-1+ myeloid-derived suppressive cells in multiple organs. Both decreased apoptosis and increased proliferation contribute to the systemic increase of myeloid cells in lal(-/-) myeloid cells. These lal(-/-) CD11b(+)/Gr-1(+) cells displayed suppressive activity on T cell proliferation and function in vitro. Bone marrow chimeras confirmed that the myeloproliferative disorder in lal(-/-) mice was primarily attributable to autonomous defects in myeloid progenitor cells, although the hematopoietic microenvironment in the lal(-/-) mice did not support hematopoiesis normally. These results provide evidence that LAL is an important regulator of myelopoiesis during hematopoietic development, differentiation, and homeostasis.

Homeostatic and pathogenic extramedullary hematopoiesis

Extramedullary hematopoiesis (EH) is defined as hematopoiesis occurring in organs outside of the bone marrow; it occurs in diverse conditions, including fetal development, normal immune responses, and pathological circumstances. During fetal development, before formation of mature marrow, EH occurs in the yolk sac, fetal liver, and spleen. EH also occurs during active immune responses to pathogens. Most frequently, this response occurs in the spleen and liver for the production of antigen-presenting cells and phagocytes. EH also occurs when the marrow becomes inhabitable for stem and progenitor cells in certain pathological conditions, including myelofibrosis, where marrow cells are replaced with collagenous connective tissue fibers. Thus, EH occurs either actively or passively in response to diverse changes in the hematopoietic environment. This article reviews the key features and regulators of the major types of EH.

Vaccination with human pluripotent stem cells generates a broad spectrum of immunological and clinical responses against colon cancer

The history of immunizing with embryonic materials to generate an antitumor immune response dates back to a century ago. The premise is that cancer cells share the expression of oncofetal antigens with embryonic materials and that the immune response against these antigens in the embryonic tissues is cross-protective against cancer. However, such a practice has never advanced beyond experimental animal settings, because of lack of uniformed source tissues and ethical challenges. With the availability of well-characterized human pluripotent stem cells, it is now possible to ask whether tumor protective immunity could indeed be elicited with stem cells. Herein, we investigated whether vaccination with defined human embryonic stem cells (hESCs) or induced pluripotent stem (iPS) cells was effective against a colon carcinoma. We discovered that vaccination of mice with hESC line H9 generated consistent cellular and humoral immune responses against CT26 colon carcinoma. Protection correlated strongly with the expansion of tumor-responsive and interferon-gamma-producing cells and the profound loss of CD11b(+)Gr-1(+) myeloid-derived suppressor cells in the spleen. No evidence of autoimmunity was observed. We also compared the immunogenicity against colon cancer between a hESC line CT2 and an iPS cell line TZ1 that were generated in the same stem cell facility. We found that the iPS cell line was inferior to the hESC line in conferring tumor protection, suggesting that there is heterogeneity of expression of oncofetal antigens by hESCs and iPS cells. We conclude that the hESC-based vaccine is a promising modality for immunotherapy of cancer.

The immunosuppressive surface ligand CD200 augments the metastatic capacity of squamous cell carcinoma

CD200 (OX-2) is a cell surface glycoprotein that imparts immune privileges by suppressing alloimmune and autoimmune responses through its receptor, CD200R, expressed primarily on myeloid cells. The ability of CD200 to suppress myeloid cell activation is critical for maintaining normal tissue homeostasis but may also enhance the survival of migratory neoplastic cells. We show that CD200 expression is largely absent in well-differentiated primary squamous cell carcinoma (SCC) of the skin, but is highly induced in SCC metastases to the lymph node and other solid tissues. CD200 does not influence the proliferative or invasive capacity of SCC cells or their ability to reconstitute primary skin tumors. However, loss of CD200 impairs the ability of SCC cells to metastasize and seed secondary tumors, indicating that the survival of CD200(+) SCC cells may depend on their ability to interact with CD200R(+) immune cells. The predominant population of CD200R(+) stromal cells was CD11b(+)Gr-1(+) myeloid-derived suppressor cells, which release elevated levels of granulocyte colony-stimulating factor and granulocyte macrophage colony-stimulating factor when in the presence of SCC cells in a CD200-dependent manner. Collectively, our findings implicate CD200 as a hallmark of SCC metastasis and suggest that the ability of CD200(+) SCC keratinocytes to directly engage and modulate CD200R(+) myeloid-derived suppressor cells is essential to metastatic survival.

Targeting immune suppressing myeloid-derived suppressor cells in oncology

Emerging data suggests that host immune cells with a suppressive phenotype represent a significant hurdle to successful therapy for metastatic cancer. Among the suppressor cells, T regulatory cells (Treg) and myeloid-derived suppressor cells (MDSC) are significantly increased in hosts with advanced malignancies. MDSC mediate the suppression of the tumor antigen-specific T cell response through the induction of T cell anergy and the development of Treg in tumor-bearing mice. These results provide robust evidence of an in vivo immunoregulatory function of MDSC in the establishment of tumor antigen-specific tolerance and the development of Treg in tumor-bearing hosts. To achieve effective anti-tumor immunity, tumor-induced immunosuppression must be reversed. Our preliminary results indicate that c-kit ligand (stem cell factor) expressed by tumor cells may be required for MDSC accumulation in tumor-bearing mice, and that blocking the c-kit ligand/c-kit receptor interaction can prevent the development of Treg and reverse immune tolerance induced by MDSC. Since c-kit can be readily inhibited by several small molecule inhibitors including imatinib, sunitinib and dasatinib, targeting immune suppressing cells can be readily accomplished in the clinic.

Azithromycin alters macrophage phenotype and pulmonary compartmentalization during lung infection with Pseudomonas

Infection with mucoid strains of Pseudomonas aeruginosa in chronic inflammatory diseases of the airway is difficult to eradicate and can cause excessive inflammation. The roles of alternatively activated and regulatory subsets of macrophages in this pathophysiological process are not well characterized. We previously demonstrated that azithromycin induces an alternatively activated macrophage-like phenotype in vitro. In the present study, we tested whether azithromycin affects the macrophage activation status and migration in the lungs of P. aeruginosa-infected mice. C57BL/6 mice received daily doses of oral azithromycin and were infected intratracheally with a mucoid strain of P. aeruginosa. The properties of macrophage activation, immune cell infiltration, and markers of pulmonary inflammation in the lung interstitial and alveolar compartments were evaluated postinfection. Markers of alternative macrophage activation were induced by azithromycin treatment, including the surface expression of the mannose receptor, the upregulation of arginase 1, and a decrease in the production of proinflammatory cytokines. Additionally, azithromycin increased the number of CD11b(+) monocytes and CD4(+) T cells that infiltrated the alveolar compartment. A predominant subset of CD11b(+) cells was Gr-1 positive (Gr-1(+)), indicative of a subset of cells that has been shown to be immunoregulatory. These differences corresponded to decreases in neutrophil influx into the lung parenchyma and alteration of the characteristics of peribronchiolar inflammation without any change in the clearance of the organism. These results suggest that the immunomodulatory effects of azithromycin are associated with the induction of alternative and regulatory macrophage activation characteristics and alteration of cellular compartmentalization during infection.

IFN-gamma receptor signaling regulates memory CD8+ T cell differentiation

IFN-gamma regulates multiple processes in the immune system. Although its antimicrobial effector functions are well described, less is known about the mechanisms by which IFN-gamma regulates CD8(+) T cell homeostasis. With the help of adoptive T cell transfers, we show in this study that IFN-gammaR signaling in CD8(+) T cells is dispensable for expansion, contraction, and memory differentiation in response to peptide vaccination. In contrast, host IFN-gammaR signaling counterregulates CD8(+) T cell responses and the generation of effector memory T cell processes, which are partially regulated by CD11b(+) cells. Similar to vaccination-induced proliferation, host IFN-gammaR signaling limits the expansion of naive CD8(+) T cells and their differentiation into effector memory-like T cells in lymphopenic mice. In contrast to peptide vaccination, IFN-gammaR signaling in CD8(+) T cells contributes to memory fate decision in response to lymphopenia, an effect that is fully reversed by high-affinity TCR ligands. In conclusion, we show that host IFN-gammaR signaling controls the magnitude of CD8(+) T cell responses and subsequent memory differentiation under lymphopenic and nonlymphopenic conditions. In contrast, IFN-gammaR signaling in CD8(+) T cells does not affect cell numbers under either condition, but it directs memory fate decision in response to weak TCR ligands.

Chemotherapy enhances tumor cell susceptibility to CTL-mediated killing during cancer immunotherapy in mice

Cancer immunotherapy faces a serious challenge because of low clinical efficacy. Recently, a number of clinical studies have reported the serendipitous finding of high rates of objective clinical response when cancer vaccines are combined with chemotherapy in patients with different types of cancers. However, the mechanism of this phenomenon remains unclear. Here, we tested in mice several cancer vaccines and an adoptive T cell transfer approach to cancer immunotherapy in combination with several widely used chemotherapeutic drugs. We found that chemotherapy made tumor cells more susceptible to the cytotoxic effect of CTLs through a dramatic perforin-independent increase in permeability to GrzB released by the CTLs. This effect was mediated via upregulation of mannose-6-phosphate receptors on the surface of tumor cells and was observed in mouse and human cells. When combined with chemotherapy, CTLs raised against specific antigens were able to induce apoptosis in neighboring tumor cells that did not express those antigens. These data suggest that small numbers of CTLs could mediate a potent antitumor effect when combined with chemotherapy. In addition, these results provide a strong rationale for combining these modalities for the treatment of patients with advanced cancers.

Mechanism of T cell tolerance induced by myeloid-derived suppressor cells

Ag-specific T cell tolerance plays a critical role in tumor escape. Recent studies implicated myeloid-derived suppressor cells (MDSCs) in the induction of CD8(+) T cell tolerance in tumor-bearing hosts. However, the mechanism of this phenomenon remained unclear. We have found that incubation of Ag-specific CD8(+) T cells, with peptide-loaded MDSCs, did not induce signaling downstream of TCR. However, it prevented subsequent signaling from peptide-loaded dendritic cells. Using double TCR transgenic CD8(+) T cells, we have demonstrated that MDSC induced tolerance to only the peptide, which was presented by MDSCs. T cell response to the peptide specific to the other TCR was not affected. Incubation of MDSCs with Ag-specific CD8(+) T cells caused nitration of the molecules on the surface of CD8(+) T cells, localized to the site of physical interaction between MDSC and T cells, which involves preferentially only TCR specific for the peptide presented by MDSCs. Postincubation with MDSCs, only nitrotyrosine-positive CD8(+) T cells demonstrated profound nonresponsiveness to the specific peptide, whereas nitrotyrosine-negative CD8(+) T cells responded normally to that stimulation. MDSCs caused dissociation between TCR and CD3zeta molecules, disrupting TCR complexes on T cells. Thus, these data describe a novel mechanism of Ag-specific CD8(+) T cell tolerance in cancer.