Nitric oxide-producing CD11b+Ly-6G(Gr-1)+CD31(ER-MP12)+cells in the spleen of cyclophosphamide–treated mice: implications for T-cell responses in immunosuppressed mice

[Myeloid-derived suppressor cells as important factors and potential targets for breast cancer progression]

Recurrence and metastasis remain the leading cause of death in breast cancer patients due to the lack of effective treatment. A microenvironment suitable for cancer cell growth, referred to as pre-metastatic niche (PMN), is formed in distant organs before metastasis occurs. Myeloid-derived suppressor cells (MDSCs) are a heterogenous population of immature myeloid cells with immunosuppressive effects. They can expand in large numbers in breast cancer patients and participate in the formation of PMN. MDSCs can remodel the extracellular matrix of pulmonary vascular endothelial cells and recruit cancer stem cells to promote the lung metastasis of breast cancer. Furthermore, MDSCs facilitate immune evasion of breast cancer cells to impact the efficacy of immunotherapy. It is proposed that MDSCs represent a potential therapeutic target for the inhibition of recurrence and metastasis in breast cancer. Therapeutic strategies targeting MDSCs have shown promising efficacy in preclinical studies and clinical trials. This review presents a summary of the principal factors involved in the recruitment and activation of MDSCs during the formation of PMN, and outlines MDSCs functions such as immunosuppression and the current targeted therapies against MDSCs, aiming to provide new ideas for the treatment of distant metastases in breast cancer.

TGF-β Enhances Immunosuppression of Myeloid-Derived Suppressor Cells to Induce Transplant Immune Tolerance Through Affecting Arg-1 Expression

Myeloid-derived suppressor cells (MDSCs) are a class of heterogeneous myeloid cells, which play an important role in immunosuppression. We intended to find an effective method that can produce MDSCs with significantly better efficiency and promote immune tolerance for transplant rejection through cell therapy. It has been reported that granulocyte and macrophage colony-stimulating factor (GM-CSF) could induce MDSCs in vitro to cause immunosuppression. In the present study, transforming growth factor β (TGF-β) was added to the induction system, and flow cytometry analysis was used to detect the phenotypes of induced MDSCs. Their potential immunosuppressive function and mechanisms were determined by co-culturing MDSCs with stimulated T cells in vitro and transferring MDSCs to the skin grafted C57BL/6J mouse models in vivo. It was found that the addition of TGF-β could effectively cause bone marrow cells to differentiate into a group of cells with stronger immunosuppressive functions, thereby inhibiting the proliferation of stimulated T cells. The population of CD11b+Gr-1+ MDSCs also increased significantly as compared with GM-CSF alone treatment. While detecting for immunosuppressive effectors, we found that expression of arginase 1 (Arg-1) was significantly upregulated in these MDSCs, and inhibitor of Arg-1 significantly suppressed their immunosuppressive capabilities. Moreover, an adoptive transfer of these cells significantly prolonged survival of allo-skin and improved immune tolerance in vivo. These findings indicated that TGF-β + GM-CSF could serve as an effective and feasible method to induce powerful immunosuppressive MDSCs in vitro. Thus, TGF-β + GM-CSF–induced MDSCs may have a promising role in prevention of the graft rejection.

Immunoenhancement Effects of the Herbal Formula Hemomine on Cyclophosphamide-Induced Immunosuppression in Mice

Hemomine is an herbal blend comprising Angelicae Gigantis Radix and other herbs known to have immunomodulatory effects. We examined the immunopotentiating effect of this herbal blend on cyclophosphamide (CPA)-induced immunosuppression. Male mice were assigned to one of six groups: the intact control and five CPA treatment groups (one control, one reference (β-glucan), and three with the application of hemomine at different concentrations; 4, 2, or 1 mL/kg; n = 10 per group). Mice were injected with CPA to induce myelosuppression and immunosuppression, after which they received one of the experimental treatments. In immunosuppressed mice, hemomine treatment alleviated the noticeable reductions in body, spleen, and submandibular lymph node weights caused by CPA; caused changes in hematological markers; induced the reduced levels of serum IFN-γ and spleen TNF-α, IL-1β, and IL-10 by CPA; improved natural killer cell activities in the spleen and peritoneal cavity; and also improved lymphoid organ atrophy in a dose-dependent manner. We demonstrate that hemomine, a mixture of six immunomodulatory herbs, is an effective immunomodulatory agent, with the potential to enhance immunity.

Early recovery of myeloid-derived suppressor cells after allogeneic hematopoietic cells: comparison of post-transplantation cyclophosphamide to standard graft-versus-host disease prophylaxis

BACKGROUND

Allogeneic hematopoietic cell transplantation (alloHCT) using haploidentical donors (haploHCT) with post-transplantation cyclophosphamide (PTCy) for augmented graft-versus-host disease (GVHD) prophylaxis has emerged as a robust platform to expand donor options with acceptable levels of GVHD and graft failure. The mechanism by which PTCy mitigates GVHD risk is partly explained by preferential cytotoxicity based on aldehyde dehydrogenase levels and up-regulation of regulatory T cells, but is incompletely understood. Myeloid-derived suppressor cells are important mediators of T-cell function and are up-regulated by cyclophosphamide exposure.

OBJECTIVES

We hypothesized that this cell type may play a role in GVHD protection in children undergoing haploHCT/PTCy.

STUDY DESIGN

We prospectively collected samples in the first month after alloHCT from children undergoing standard of care (SOC) alloHCT with matched donors and tacrolimus-based GVHD prophylaxis (N=11) and PTCy recipients (N=11). MDSC recovery was compared using flow cytometry, and MDSC suppressive function was assessed at the peak of MDSC quantitative recovery post-alloHCT.

RESULTS

Groups were well matched for conditioning regimen and stem cell source. PTCy recipients exhibited more robust MDSC recovery, particularly polymorphonuclear-MDSCs than SOC recipients, with preservation of T-cell suppressive function. This corresponded to significantly lower incidence of Grade II-IV acute GVHD (9.1% versus 27.3%) and moderate/severe chronic GVHD (0% versus 27.3%) in PTCy recipients. Patients who developed GVHD had decreased MDSC-mediated T-cell suppression, as well as higher levels of IL-10, a cytokine closely linked to GVHD biology.

CONCLUSION

Overall, these findings provide support for the role of MDSCs in mediating GVHD protection after PTCy-based haploHCT.

Role of myeloid-derived suppressor cells in viral respiratory infections; Hints for discovering therapeutic targets for COVID-19

The expansion of myeloid-derived suppressor cells (MDSCs), known as heterogeneous population of immature myeloid cells, is enhanced during several pathological conditions such as inflammatory or viral respiratory infections. It seems that the way MDSCs behave in infection depends on the type and the virulence mechanisms of the invader pathogen, the disease stage, and the infection-related pathology. Increasing evidence showing that in correlation with the severity of the disease, MDSCs are accumulated in COVID-19 patients, in particular in those at severe stages of the disease or ICU patients, contributing to pathogenesis of SARS-CoV2 infection. Based on the involved subsets, MDSCs delay the clearance of the virus through inhibiting T-cell proliferation and responses by employing various mechanisms such as inducing the secretion of anti-inflammatory cytokines, inducible nitric oxide synthase (iNOS)-mediated hampering of IFN-γ production, or forcing arginine shortage. While the immunosuppressive characteristic of MDSCs may help to preserve the tissue homeostasis and prevent hyperinflammation at early stages of the infection, hampering of efficient immune responses proved to exert significant pathogenic effects on severe forms of COVID-19, suggesting the targeting of MDSCs as a potential intervention to reactivate T-cell immunity and thereby prevent the infection from developing into severe stages of the disease. This review tried to compile evidence on the roles of different subsets of MDSCs during viral respiratory infections, which is far from being totally understood, and introduce the promising potential of MDSCs for developing novel diagnostic and therapeutic approaches, especially against COVID-19 disease.

Evaluation of Pseudomonas aeruginosa pathogenesis and therapeutics in military-relevant animal infection models

Modern combat-related injuries are often associated with acute polytrauma. As a consequence of severe combat-related injuries, a dysregulated immune response results in serious infectious complications. The gram-negative bacterium Pseudomonas aeruginosa is an opportunistic pathogen that often causes life-threatening bloodstream, lung, bone, urinary tract, and wound infections following combat-related injuries. The rise in the number of multidrug-resistant P. aeruginosa strains has elevated its importance to civilian clinicians and military medicine. Development of novel therapeutics and treatment options for P. aeruginosa infections is urgently needed. During the process of drug discovery and therapeutic testing, in vivo testing in animal models is a critical step in the bench-to-bedside approach, and required for Food and Drug Administration approval. Here, we review current and past literature with a focus on combat injury-relevant animal models often used to understand infection development, the interplay between P. aeruginosa and the host, and evaluation of novel treatments. Specifically, this review focuses on the following animal infection models: wound, burn, bone, lung, urinary tract, foreign body, and sepsis.

The Monocytes That Repopulate in Mice After Cyclophosphamide Treatment Acquire a Neutrophil Precursor Gene Signature and Immunosuppressive Activity

Cyclophosphamide (CTX) is a major component of the chemotherapy conditioning regimens used in the clinic to prepare cancer patients for hematopoietic stem cell transplantation or adoptive T cell therapy. Previous studies have shown that CTX given at nonmyeloablative doses in mice and patients leads to expansion of myeloid cells within which the monocytic subset exhibits immunosuppressive activity. However, the ontogeny and gene expression signature of these CTX-induced monocytes are not well-defined. Here, we report that the expansion of myeloid cells is a default process intrinsic to hematopoietic recovery after chemotherapy. During this process, the monocytes repopulated in mice acquire immunosuppressive activity, which can persist long after cessation of chemotherapy. Moreover, monocytes acquire a gene signature characteristic of neutrophil precursors, marked by increased proliferative capability and elevated expressions of multiple primary and secondary granules. We provide evidence that CTX-induced myeloid cell expansion is regulated by DNA methyltransferase 1 (Dnmt1) and dependent on chemotherapy-induced microbial translocation. These findings help advance our understanding of the differentiation, heterogeneity, and function of myeloid cells repopulating after chemotherapy.

Reactive myelopoiesis and the onset of myeloid-mediated immune suppression: Implications for adoptive cell therapy

Adoptive T cell therapy (ACT) in combination with lymphodepleting chemotherapy is an effective strategy to induce the eradication of cancer, providing long-term regressions in patients. However, only a minority of patients that receive ACT with tumor-specific T cells exhibit durable benefit. Thus, there is an urgent need to characterize mechanisms of resistance and define strategies to alleviate immunosuppression in the context of ACT in cancer. This article reviews the importance of lymphodepleting regimens in promoting the optimal engraftment and expansion of T cells in hosts after adoptive transfer. In addition, we discuss the role of concomitant immunosuppression and the accumulation of myeloid derived suppressor cells (MDSCs) during immune recovery after lymphodepleting regimens and mobilization regimens.

Heterologous Prime Boost Vaccination Induces Protective Melanoma-Specific CD8+ T Cell Responses

Cancer vaccination aims at inducing an adaptive immune response against tumor-derived antigens. In this study, we utilize recombinant human adenovirus serotype 5 (rAd5) and recombinant lymphocytic choriomeningitis virus (rLCMV)-based vectors expressing the melanocyte differentiation antigen gp100. In contrast to single or homologous vaccination, a heterologous prime boost vaccination starting with a rAd5-gp100 prime immunization followed by a rLCMV-gp100 boost injection induces a high magnitude of polyfunctional gp100-specific CD8⁺ T cells. Our data indicate that an optimal T cell induction is dependent on the order and interval of the vaccinations. A prophylactic prime boost vaccination with rAd5- and rLCMV-gp100 protects mice from a B16.F10 melanoma challenge. In the therapeutic setting, combination of the vaccination with low-dose cyclophosphamide showed a synergistic effect and significantly delayed tumor growth. Our findings suggest that heterologous viral vector prime boost immunizations can mediate tumor control in a mouse melanoma model.

Reactive Myelopoiesis Triggered by Lymphodepleting Chemotherapy Limits the Efficacy of Adoptive T Cell Therapy

Adoptive T cell therapy (ACT) in combination with lymphodepleting chemotherapy is an effective strategy to induce the eradication of tumors, providing long-term regression in cancer patients. Despite that lymphodepleting regimens condition the host for optimal engraftment and expansion of adoptively transferred T cells, lymphodepletion concomitantly promotes immunosuppression during the course of endogenous immune recovery. In this study, we have identified that lymphodepleting chemotherapy initiates the mobilization of hematopoietic progenitor cells that differentiate to immunosuppressive myeloid cells, leading to a dramatic increase of peripheral myeloid-derived suppressor cells (MDSCs). In melanoma and lung cancer patients, MDSCs rapidly expanded in the periphery within 1 week after completion of a lymphodepleting regimen and infusion of autologous tumor-infiltrating lymphocytes (TILs). This expansion was associated with disease progression, poor survival, and reduced TIL persistence in melanoma patients. We demonstrated that the interleukin 6 (IL-6)-driven differentiation of mobilized hematopoietic progenitor cells promoted the survival and immunosuppressive capacity of post-lymphodepletion MDSCs. Furthermore, the genetic abrogation or therapeutic inhibition of IL-6 in mouse models enhanced host survival and reduced tumor growth in mice that received ACT. Thus, the expansion of MDSCs in response to lymphodepleting chemotherapy may contribute to ACT failure, and targeting myeloid-mediated immunosuppression may support anti-tumor immune responses.

p53-Reactive T Cells Are Associated with Clinical Benefit in Patients with Platinum-Resistant Epithelial Ovarian Cancer After Treatment with a p53 Vaccine and Gemcitabine Chemotherapy

Purpose: To conduct a phase I trial of a Modified Vaccinia Ankara vaccine delivering wild-type human p53 (p53MVA) in combination with gemcitabine chemotherapy in patients with platinum-resistant ovarian cancer.Experimental Design: Patients received gemcitabine on days 1 and 8 and p53MVA vaccine on day 15, during the first 3 cycles of chemotherapy. Toxicity was classified using the NCI Common Toxicity Criteria and clinical response assessed by CT scan. Peripheral blood samples were collected for immunophenotyping and monitoring of anti-p53 immune responses.Results: Eleven patients were evaluated for p53MVA/gemcitabine toxicity, clinical outcome, and immunologic response.

TOXICITY

there were no DLTs, but 3 of 11 patients came off study early due to gemcitabine-attributed adverse events (AE). Minimal AEs were attributed to p53MVA vaccination. Immunologic and clinical response: enhanced in vitro recognition of p53 peptides was detectable after immunization in both the CD4⁺ and CD8⁺ T-cell compartments in 5 of 11 and 6 of 11 patients, respectively. Changes in peripheral T regulatory cells (Tregs) and myeloid-derived suppressor cells (MDSC) did not correlate significantly with vaccine response or progression-free survival (PFS). Patients with the greatest expansion of p53-reactive T cells had significantly longer PFS than patients with lower p53-reactivity after therapy. Tumor shrinkage or disease stabilization occurred in 4 patients.Conclusions: p53MVA was well tolerated, but gemcitabine without steroid pretreatment was intolerable in some patients. However, elevated p53-reactive CD4⁺ and CD8⁺ T-cell responses after therapy correlated with longer PFS. Therefore, if responses to p53MVA can be enhanced with alternative agents, superior clinical responses may be achievable. Clin Cancer Res; 24(6); 1315-25. ©2018 AACR.

mTOR masters monocytic myeloid-derived suppressor cells in mice with allografts or tumors

CD11b(+) Gr1(+) myeloid-derived suppressor cells (MDSCs) play critical roles in controlling the processes of tumors, infections, autoimmunity and graft rejection. Immunosuppressive drug rapamycin (RPM), targeting on the key cellular metabolism molecule mTOR, is currently used in clinics to treat patients with allo-grafts, autoimmune diseases and tumors. However, the effect of RPM on MDSCs has not been studied. RPM significantly decreases the cell number and the immunosuppressive ability on T cells of CD11b(+) Ly6C(high) monocytic MDSCs (M-MDSCs) in both allo-grafts-transplanted and tumor-bearing mice respectively. Mice with a myeloid-specific deletion of mTOR have poor M-MDSCs after grafting with allo-skin tissue or a tumor. Grafting of allo-skin or tumors significantly activates glycolysis pathways in myeloid precursor cells in bone marrow, which is inhibited by RPM or mTOR deletion. 2-deoxyglucose (2-DG), an inhibitor of the glycolytic pathway, inhibits M-MDSC differentiation from precursors, while enhancing glycolysis by metformin significantly rescues the RPM-caused deficiency of M-MDSCs. Therefore, we offer evidence supporting that mTOR is an intrinsic factor essential for the differentiation and immunosuppressive function of M-MDSCs and that these metabolism-relevant medicines may impact MDSCs-mediated immunosuppression or immune tolerance induction, which is of considerable clinical importance in treating graft rejection, autoimmune diseases and cancers.

G-CSF mobilizes CD34 + regulatory monocytes that inhibit graft-versus-host disease

G-SCF–mobilized CD34 + monocytes inhibit graft-versus-host disease by the production of nitric oxide and the induction of regulatory T cells.

Immunomodulatory Effects of Ethanolic Extract of Thyphonium flagelliforme (Lodd) Blume in Rats Induced by Cyclophosphamide

The present study aimed to examine the immunomodulatory effect of ethanolic extract of Typhonium flagelliforme (Lodd) Blume in cyclophosphamide-treated rats. The immunomodulatory effects were determined by lymphocytes proliferation, phagocytic activity of macrophages, plasma cytokines of tumor necrosis factor-α, interleukin-1α, interleukin-10 levels, and killer T cells (CD8+ T cells) counts. The results showed that the administration of ethanolic extract of T flagelliforme reduced immunosupessive effect on lymphocyte proliferation, increase the number and phagocytic activity of macrophages in cyclophosphamide-treated rats. Moreover, the ethanolic extract of T flagelliforme also significantly ( P < .05) improved the immune system activities especially the proliferation of CD8+T cells and reduced the suppressive effects on cytokines such as tumor necrosis factor-α and interleukin-1α. In conclusion, the ethanolic extract of T flagelliforme has immunomodulatory properties in cyclophosphamide-treated rats. The results suggest that T flagelliforme can reduce immunosuppresive effect caused by a chemotherapeutic agent.

Immunosuppressive myeloid cells induced by chemotherapy attenuate antitumor CD4+ T-cell responses through the PD-1-PD-L1 axis

In recent years, immune-based therapies have become an increasingly attractive treatment option for patients with cancer. Cancer immunotherapy is often used in combination with conventional chemotherapy for synergistic effects. The alkylating agent cyclophosphamide (CTX) has been included in various chemoimmunotherapy regimens because of its well-known immunostimulatory effects. Paradoxically, cyclophosphamide can also induce suppressor cells that inhibit immune responses. However, the identity and biologic relevance of these suppressor cells are poorly defined. Here we report that cyclophosphamide treatment drives the expansion of inflammatory monocytic myeloid cells (CD11b(+)Ly6C(hi)CCR2(hi)) that possess immunosuppressive activities. In mice with advanced lymphoma, adoptive transfer (AT) of tumor-specific CD4(+) T cells following cyclophosphamide treatment (CTX+CD4 AT) provoked a robust initial antitumor immune response, but also resulted in enhanced expansion of monocytic myeloid cells. These therapy-induced monocytes inhibited long-term tumor control and allowed subsequent relapse by mediating functional tolerization of antitumor CD4(+) effector cells through the PD-1-PD-L1 axis. PD-1/PD-L1 blockade after CTX+CD4 AT therapy led to persistence of CD4(+) effector cells and durable antitumor effects. Depleting proliferative monocytes by administering low-dose gemcitabine effectively prevented tumor recurrence after CTX+CD4 AT therapy. Similarly, targeting inflammatory monocytes by disrupting the CCR2 signaling pathway markedly potentiated the efficacy of cyclophosphamide-based therapy. Besides cyclophosphamide, we found that melphalan and doxorubicin can also induce monocytic myeloid suppressor cells. These findings reveal a counter-regulation mechanism elicited by certain chemotherapeutic agents and highlight the importance of overcoming this barrier to prevent late tumor relapse after chemoimmunotherapy.

Radiation combined with thermal injury induces immature myeloid cells

The continued development of nuclear weapons and the potential for thermonuclear injury necessitates the further understanding of the immune consequences after radiation combined with injury (RCI). We hypothesized that sublethal ionization radiation exposure combined with a full-thickness thermal injury would result in the production of immature myeloid cells. Mice underwent either a full-thickness contact burn of 20% total body surface area or sham procedure followed by a single whole-body dose of 5-Gy radiation. Serum, spleen, and peripheral lymph nodes were harvested at 3 and 14 days after injury. Flow cytometry was performed to identify and characterize adaptive and innate cell compartments. Elevated proinflammatory and anti-inflammatory serum cytokines and profound leukopenia were observed after RCI. A population of cells with dual expression of the cell surface markers Gr-1 and CD11b were identified in all experimental groups, but were significantly elevated after burn alone and RCI at 14 days after injury. In contrast to the T-cell-suppressive nature of myeloid-derived suppressor cells found after trauma and sepsis, myeloid cells after RCI augmented T-cell proliferation and were associated with a weak but significant increase in interferon γ and a decrease in interleukin 10. This is consistent with previous work in burn injury indicating that a myeloid-derived suppressor cell-like population increases innate immunity. Radiation combined injury results in the increase in distinct populations of Gr-1CD11b cells within the secondary lymphoid organs, and we propose these immature inflammatory myeloid cells provide innate immunity to the severely injured and immunocompromised host.

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.

CpG blocks immunosuppression by myeloid-derived suppressor cells in tumor-bearing mice

PURPOSE

The Toll-like receptor (TLR) 9 ligand CpG has been used successfully for the immunotherapy of cancer. Chronic CpG application in tumor-free hosts leads, however, to the expansion of myeloid-derived suppressor cells (MDSC), which can cause T-cell suppression and may thus hamper the development of an effective immune response. Here, we investigated the effect of TLR9 activation on the function of MDSC in tumor-bearing mice.

EXPERIMENTAL DESIGN

We investigated the effect of CpG treatment on the number, phenotype, and function of MDSC in mice bearing subcutaneous C26 tumors and in CEA424-TAg mice bearing autochthonous gastric tumors.

RESULTS

CpG treatment blocks the suppressive activity of MDSC on T-cell proliferation in both tumor models. Inhibition of MDSC function by CpG was particularly pronounced for a highly suppressive Ly6G(hi) polymorphonuclear subset of MDSC. We further show that TLR9 activation by CpG promotes maturation and differentiation of MDSC and strongly decreases the proportion of Ly6G(hi) MDSC in both tumor-bearing and tumor-free mice. We demonstrate that IFN-α produced by plasmacytoid dendritic cells upon CpG stimulation is a key effector for the induction of MDSC maturation in vitro and show that treatment of mice with recombinant IFN-α is sufficient to block MDSC suppressivity.

CONCLUSIONS

We show here for the first time that TLR9 activation inhibits the regulatory function of MDSC in tumor-bearing mice and define a role for the antitumoral cytokine IFN-α in this process.

A myeloid cell population induced by Freund adjuvant suppresses T-cell-mediated antitumor immunity

Although adjuvants are important components of vaccines, few studies have been conducted to establish the criteria on adjuvant selection and to investigate mechanisms of adjuvant actions during vaccination. Here we found that complete Freund adjuvant (CFA) induced a CD11b cell population in a B-cell independent manner. This cell population exhibited strong ability to inhibit T-cell-mediated rejection of tumor transplants. In vitro studies indicated that these cells induced T-cell apoptosis and down-regulated interferon-gamma production. Nitric oxide (NO) played important roles to achieve these effects. Plenty of NO was produced by these CFA-induced CD11b cells. The addition of N-nitro-L-arginine-methyl ester, an inhibitor of NO synthase, rescued T cells from apoptosis and partially abrogated the detrimental effects of CFA in cancer vaccines. Incomplete Freund adjuvant, one of the adjuvants still being used in clinical trials, also induced a similar cell population. Our results reveal a previously unknown mechanism in which the myeloid cell population induced by Freund adjuvant impairs antitumor immunity, and highlight the importance of adjuvant selection during tumor vaccination.

Administration of cyclophosphamide changes the immune profile of tumor-bearing mice

Cyclophosphamide (CTX) is often used to create a "window" for more effective therapeutic tumor vaccination. According to a commonly applied protocol, we injected 2 mg CTX intraperitoneally to mice with small (2 to 3 mm diameter) or large (5 to 7 mm, and in one experiment 8 to 10 mm diameter) subcutaneously growing tumors from the SW1 clone of the K1735 melanoma, euthanized the mice 4 days later and studied the composition of lymphoid cells by flow cytometry in both spleens and tumors. Administration of CTX increased the percentage of CD3, CD4, and CD8 cells with the increase in tumors being significantly greater than in spleens, and it also increased the percentage of B cells in spleens and tumors. Furthermore, CTX dramatically increased the frequency of tumor-infiltrating CD4 and CD8 cells containing interferon gamma, of cells expressing NK1.1, and of cells expressing the dendritic cell markers CD11c, CD80, and CD86, with the greatest increases seen among tumor-infiltrating lymphoid cells (TIL) from mice with small tumors. Although CTX decreased the percentage of TIL that expressed CD4 or CD8 together with CD25 and FoxP3 and were therefore considered to be regulatory T cells, it increased the frequency of TIL that stained for Gr1/CD11b, a marker for myeloid-derived suppressor cells. We conclude that the administration of CTX can favorably impact several cell populations that are involved in tumor rejection. However, since CTX has a limited effect on TIL from tumors larger than a few millimeter in diameter and in view of an increased percentage of myeloid-derived suppressor cells among TIL from mice given CTX there is a need for more effective ways to improve tumor vaccination.

Conditions that diminish myeloid-derived suppressor cell activities stimulate cross-protective immunity

Immunity conferred by conventional vaccines is restricted to a narrow range of closely related strains, highlighting the unmet medical need for the development of vaccines that elicit protection against multiple pathogenic serotypes. Here we show that a Salmonella bivalent vaccine comprised of strains that lack and overproduce DNA adenine methylase (Dam) conferred cross-protective immunity to salmonella clinical isolates of human and animal origin. Protective immunity directly correlated with increased levels of cross-reactive opsonizing antibodies and memory T cells and a diminished expansion of myeloid-derived suppressor cells (MDSCs) that are responsible for the immune suppression linked to several conditions of host stress, including chronic microbial infections, traumatic insults, and many forms of cancer. Further, aged mice contained increased numbers of MDSCs and were more susceptible to Salmonella infection than young mice, suggesting a role for these cells in the immune declines associated with the natural aging process. These data suggest that interventions capable of reducing MDSC presence and activities may allow corresponding increases in B- and T-cell stimulation and benefit the ability of immunologically diverse populations to be effectively vaccinated as well as reducing the risk of susceptible individuals to infectious disease.

Recovery of humoral immunity is critical for successful antiviral therapy in disseminated mouse adenovirus type 1 infection

Severe adenovirus infections in transplant recipients undergoing immunosuppressive therapy are of increasing concern. Controversy exists on the contribution of antiviral therapy and the host immune response to recovery from these infections. Here, we established a systemic mouse adenovirus type 1 (MAV-1) infection in cyclophosphamide (CyP)-treated BALB/c mice. CyP was administered at 100 mg per kg of body weight every other day for 2, 3, or 4 weeks, thereby inducing general but reversible leukopenia, with a major suppression of the B-cell numbers and functionality that was more pronounced than that seen with T cells. The outcome of MAV-1 infection was dependent on the duration of CyP therapy, as the mice with the most severe immunosuppression were the most vulnerable to MAV-1-induced hemorrhagic enteritis and mortality. The protective effect of concomitant antiviral therapy with cidofovir depended on the level of immunosuppression. The combination of cidofovir treatment with the withdrawal of immunosuppression was the most successful regimen for increasing survival rates. Survival was clearly correlated with the clearance of virus and increased titers of MAV-1-specific antibodies in sera. In addition, the passive transfer of MAV-1-specific immunoglobulin G into MAV-1-infected SCID BALB/c mice caused a marked delay in mortality, the extent of the delay being dependent on the titer of MAV-1-specific antibodies. Based on the critical role of the humoral immune response in the early defense against disseminated adenovirus infection, the concomitant use of adenovirus-specific immunoglobulins and antiviral therapy should be considered for transplant patients at risk for severe adenovirus infections.

Defining the ability of cyclophosphamide preconditioning to enhance the antigen-specific CD8+ T-cell response to peptide vaccination: creation of a beneficial host microenvironment involving type I IFNs and myeloid cells

Although cyclophosphamide (CTX) has been clearly shown to enhance active specific and adoptive immunotherapies, the mechanism(s) underlying these beneficial effects have not been clearly defined. To define the impact of CTX preconditioning on the antigen-specific CD8 T-cell response to peptide vaccination, we used an adoptive transfer model based on the OT-1 T-cell receptor transgenic mouse. CTX preconditioning dramatically enhanced the antigen-specific CD8 T-cell response to peptide vaccination. Specifically, CTX significantly enhanced the expansion and function of responding CD8 T cells as demonstrated by flow cytometry and cytokine production. In parallel experiments, we attempted to define the mechanism(s) underlying these beneficial effects of CTX therapy. CTX therapy increased the relative number and activation status of myeloid dendritic cells, and was associated with the induction of significant levels of the inflammatory cytokines interferon-alpha, monocyte chemoattractant protein-1, and IL-6. Adoptive transfer experiments into type I IFNR-/- and CR3-/- mice confirmed that the beneficial effects of CTX were at least partially dependent on type I interferons and myeloid cells. Adoptive transfer of up to 150x10(6) naive spleen cells at the time of antigen-specific CD8 T-cell transfer did not abrogate the effects of CTX therapy, suggesting that the creation of a niche in the immune system may not be required. CTX decreased the absolute, but not relative number of CD4+CD25+ Treg cells, consistent with the possibility that regulatory T cells may be targeted by CTX therapy. Of note, combination therapy with CTX and a synthetic TLR3 agonist further enhanced the antigen-specific CD8+ T-cell response. Taken together, our data suggest that CTX modulates specific components of the innate immune system resulting in a beneficial host microenvironment. Specific targeting of these components may enhance the effectiveness of CTX preconditioning for adoptive immunotherapy.

In vitro induction of inhibitory macrophage differentiation by granulocyte-macrophage colony-stimulating factor, stem cell factor and interferon-gamma from lineage phenotypes-negative c-kit-positive murine hematopoietic progenitor cells

CD11b+Gr-1+ inhibitory macrophages (iMacs) were implicated in profound depression of T cell functions sometimes observed during cyclophosphamide treatments and overwhelming infections, through a secretion of nitric oxide (NO). Myeloid origin and maturation stages of iMacs are still unknown. As tumor necrosis factor-alpha (TNF-alpha) and interferon-gamma (IFN-gamma) contributed crucially to the activation of inducible NO synthase (iNOS) gene transcription and to the differentiation of macrophages, we tested their roles in the induction of iMacs differentiation from bone marrow hematopoietic progenitor cells (HPC) of uncompromised mice. Lineage phenotypes-negative (lin)) c-kit+ cells of Balb/c mice were cultured 6 days with granulocyte-macrophage colony-stimulating factor (GM-CSF), stem cell factor (SCF, c-kit ligand) in presence or not of TNF-alpha or IFN-gamma. CD11b+Gr-1+ cells only derived in presence of [GM-CSF + SCF + TNF-alpha] or [GM-CSF + SCF + IFN-gamma] could express iNOS upon in vitro stimulation with [IFN-gamma + TNF-alpha] or [IFN-gamma + LPS] known to boost iNOS expression in murine macrophages. However, whereas [GM-CSF + SCF + TNF-alpha] induced only weakly iMacs generation and contributed also to the differentiation of CD11b+Gr-1-CD11c+ myeloid dendritic cells, [GM-CSF + SCF + IFN-gamma] induced exclusively and importantly iMacs differentiation. Moreover [GM-CSF + SCF + IFN-gamma]-generated iMacs were more mature than [GM-CSF + SCF + TNF-alpha]-derived iMacs since IFN-gamma increased more strongly CD11b+Gr-1+ cells expressing Ly-6C and generated lesser cells expressing MHC class II and CD86 molecules. Finally [GM-CSF + SCF + IFN-gamma]-generated CD11b+ cells showing a powerful suppressive activity on T cell proliferations, correlated with NO secretion. In conclusion, our study showed, for the first time, that IFN-gamma induced very efficiently the differentiation of functional iMacs from lin- c-kit+ murine HPC in vitro, and indicated clearly that iMacs progenitors may be present in bone marrow of naïve mice.

Derangement of immune responses by myeloid suppressor cells

In tumor-bearing mice and cancer patients, tumor progression is often associated with altered hematopoiesis leading to the accumulation of myeloid cells. Extensive studies in preclinical models indicate that these cells share the CD11b and the Gr-1 markers, possess a mixed mature-immature myeloid phenotype, and are responsible for the induction of T-cell dysfunctions, both tumor-specific and nonspecific. Moreover, CD11b(+)Gr-1(+) myeloid cells are described under different unrelated situations associated with temporary impairment of the T-lymphocyte reactivity. This review examines recent findings on the nature, properties, and mechanisms of action of these myeloid suppressor cells (MSCs).

Central role for interleukin-4 in regulating nitric oxide-mediated inhibition of T-cell proliferation and gamma interferon production in schistosomiasis

Schistosoma mansoni-infected wild-type (WT) mice develop a Th2 response and chronic disease. In contrast, infected interleukin-4 double-deficient (IL-4(-/-)) mice develop a Th1-like response and an acute, lethal syndrome. Disease severity in these animals correlates with excessive and prolonged production of nitric oxide (NO) associated with enhanced antigen-driven gamma interferon (IFN-gamma) production in the absence of IL-4. Strikingly, splenic lymphocytes from infected IL-4(-/-) mice failed to proliferate as well as those from infected WT mice following stimulation in vitro with antigen or anti-CD3 antibody. Contrary to antigen-driven IFN-gamma responses, anti-CD3 antibody stimulation of splenocytes resulted in significantly less IFN-gamma being produced by CD8 cells from infected IL-4(-/-) mice than by those from infected WT mice or normal mice. NO is largely responsible for the impaired T-cell functions in infected IL-4(-/-) mice, as inhibition of iNOS significantly enhanced proliferation and IFN-gamma production.

Identification of a CD11b+/Gr-1+/CD31+ myeloid progenitor capable of activating or suppressing CD8+T cells

Apoptotic death of CD8+ T cells can be induced by a population of inhibitory myeloid cells that are double positive for the CD11b and Gr-1 markers. These cells are responsible for the immunosuppression observed in pathologies as dissimilar as tumor growth and overwhelming infections, or after immunization with viruses. The appearance of a CD11b+/Gr-1+ population of inhibitory macrophages (iMacs) could be attributed to high levels of granulocyte-macrophage colony-stimulating factor (GM-CSF) in vivo. Deletion of iMacs in vitro or in vivo reversed the depression of CD8+ T-cell function. We isolated iMacs from the spleens of immunocompromised mice and found that these cells were positive for CD31, ER-MP20 (Ly-6C), and ER-MP58, markers characteristic of granulocyte/monocyte precursors. Importantly, although iMacs retained their inhibitory properties when cultured in vitro in standard medium, suppressive functions could be modulated by cytokine exposure. Whereas culture with the cytokine interleukin 4 (IL-4) increasediMac inhibitory activity, these cells could be differentiated into a nonadherent population of fully mature and highly activated dendritic cells when cultured in the presence of IL-4and GM-CSF. A common CD31+/CD11b+/Gr-1+ progenitor can thus give rise to cells capable of either activating or inhibiting the function of CD8+ T lymphocytes, depending on the cytokinemilieu that prevails during antigen-presenting cell maturation.