Carmela De Santo

Tumors induce a subset of inflammatory monocytes with immunosuppressive activity on CD8+ T cells

Active suppression of tumor-specific T lymphocytes can limit the efficacy of immune surveillance and immunotherapy. While tumor-recruited CD11b+ myeloid cells are known mediators of tumor-associated immune dysfunction, the true nature of these suppressive cells and the fine biochemical pathways governing their immunosuppressive activity remain elusive. Here we describe a population of circulating CD11b+IL-4 receptor alpha+ (CD11b+IL-4Ralpha+), inflammatory-type monocytes that is elicited by growing tumors and activated by IFN-gamma released from T lymphocytes. CD11b+IL-4Ralpha+ cells produced IL-13 and IFN-gamma and integrated the downstream signals of these cytokines to trigger the molecular pathways suppressing antigen-activated CD8+ T lymphocytes. Analogous immunosuppressive circuits were active in CD11b+ cells present within the tumor microenvironment. These suppressor cells challenge the current idea that tumor-conditioned immunosuppressive monocytes/macrophages are alternatively activated. Moreover, our data show how the inflammatory response elicited by tumors had detrimental effects on the adaptive immune system and suggest novel approaches for the treatment of tumor-induced immune dysfunctions.

IL-4-Induced Arginase 1 Suppresses Alloreactive T Cells in Tumor-Bearing Mice

We previously demonstrated that a specialized subset of immature myeloid cells migrate to lymphoid organs as a result of tumor growth or immune stress, where they suppress B and T cell responses to Ags. Although NO was required for suppression of mitogen activation of T cells by myeloid suppressor cells (MSC), it was not required for suppression of allogenic responses. In this study, we describe a novel mechanism used by MSC to block T cell proliferation and CTL generation in response to alloantigen, which is mediated by the enzyme arginase 1 (Arg1). We show that Arg1 increases superoxide production in myeloid cells through a pathway that likely utilizes the reductase domain of inducible NO synthase (iNOS), and that superoxide is required for Arg1-dependent suppression of T cell function. Arg1 is induced by IL-4 in freshly isolated MSC or cloned MSC lines, and is therefore up-regulated by activated Th2, but not Th1, cells. In contrast, iNOS is induced by IFN-γ and Th1 cells. Because Arg1 and iNOS share l-arginine as a common substrate, our results indicate that l-arginine metabolism in myeloid cells is a potential target for selective intervention in reversing myeloid-induced dysfunction in tumor-bearing hosts.

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).

Invariant NKT cells reduce the immunosuppressive activity of influenza A virus–induced myeloid-derived suppressor cells in mice and humans

Infection with influenza A virus (IAV) presents a substantial threat to public health worldwide, with young, elderly, and immunodeficient individuals being particularly susceptible. Inflammatory responses play an important role in the fatal outcome of IAV infection, but the mechanism remains unclear. We demonstrate here that the absence of invariant NKT (iNKT) cells in mice during IAV infection resulted in the expansion of myeloid-derived suppressor cells (MDSCs), which suppressed IAV-specific immune responses through the expression of both arginase and NOS, resulting in high IAV titer and increased mortality. Adoptive transfer of iNKT cells abolished the suppressive activity of MDSCs, restored IAV-specific immune responses, reduced IAV titer, and increased survival rate. The crosstalk between iNKT and MDSCs was CD1d- and CD40-dependent. Furthermore, IAV infection and exposure to TLR agonists relieved the suppressive activity of MDSCs. Finally, we extended these results to humans by demonstrating the presence of myeloid cells with suppressive activity in the PBLs of individuals infected with IAV and showed that their suppressive activity is substantially reduced by iNKT cell activation. These findings identify what we believe to be a novel immunomodulatory role of iNKT cells, which we suggest could be harnessed to abolish the immunosuppressive activity of MDSCs during IAV infection.

B cell receptor-mediated uptake of CD1d-restricted antigen augments antibody responses by recruiting invariant NKT cell help in vivo.

Highly regulated activation of B cells is required for the production of specific antibodies necessary to provide protection from pathogen infection. This process is initiated by specific recognition of antigen through the B cell receptor (BCR), leading to early intracellular signaling followed by the late recruitment of T cell help. In this study we demonstrate that specific BCR uptake of CD1d-restricted antigens represents an effective means of enhancing invariant natural killer T (iNKT)-dependent B cell responses in vivo. This mechanism is effective over a wide range of antigen affinities but depends on exceeding a tightly regulated avidity threshold necessary for BCR-mediated internalization and CD1d-dependent presentation of particulate antigenic lipid. Subsequently, iNKT cells provide the help required for stimulating B cell proliferation and differentiation. iNKT-stimulated B cells develop within extrafollicular foci and mediate the production of high titers of specific IgM and early class-switched antibodies. Thus, we have demonstrated that in response to particulate antigenic lipids iNKT cells are recruited for the assistance of B cell activation, resulting in the enhancement of specific antibody responses. We propose that such a mechanism may operate to potentiate adaptive immune responses against pathogens in vivo.

In vivo Administration of Artificial Antigen-Presenting Cells Activates Low-Avidity T Cells for Treatment of Cancer

The development of effective antitumor immune responses is normally constrained by low-avidity, tumor-specific CTLs that are unable to eradicate the tumor. Strategies to rescue antitumor activity of low-avidity melanoma-specific CTLs in vivo may improve immunotherapy efficacy. To boost the in vivo effectiveness of low-avidity CTLs, we immunized mice bearing lung melanoma metastases with artificial antigen-presenting cells (aAPC), made by covalently coupling (pep)MHC-Ig dimers and B7.1-Ig molecules to magnetic beads. aAPC treatment induced significant tumor reduction in a mouse telomerase antigen system, and complete tumor eradication in a mouse TRP-2 antigen system, when low-avidity CTLs specific for these antigens were adoptively transferred. In addition, in an in vivo treatment model of subcutaneous melanoma, aAPC injection also augmented the activity of adoptively transferred CTLs and significantly delayed tumor growth. In vivo tumor clearance due to aAPC administration correlated with in situ proliferation of the transferred CTL. In vitro studies showed that aAPC effectively stimulated cytokine release, enhanced CTL-mediated lysis, and TCR downregulation in low-avidity CTLs. Therefore, in vivo aAPC administration represents a potentially novel approach to improve cancer immunotherapy.

Leukocyte infiltration in cancer creates an unfavorable environment for antitumor immune responses: a novel target for therapeutic intervention.

The interaction between tumor cells and the nearby environment is being actively investigated to explore how this interplay affects the initiation and progression of cancer. Host-tumor relationship results in the production of pro-inflammatory cytokines and chemokines that promote the recruitment of leukocytes within and around developing neoplasms. Cancer cells, together with newly recruited tumor-infiltrating cells, can also activate fibroblast and vascular responses, thus resulting in a chronic microenvironment perturbation. In this complex scenario, interactions between innate and adaptive immune cells can be disturbed, leading to a failure of immune-mediated tumor recognition and destruction. On the basis of the recent awareness about tumor promotion and immune deregulation by immune/inflammatory cells, novel anti-cancer strategies can be exploited.

Effective Genetic Vaccination with a Widely Shared Endogenous Retroviral Tumor Antigen Requires CD40 Stimulation during Tumor Rejection Phase

Endogenous retrovirus (ERV) products are recognized by T lymphocytes in mice and humans. As these Ags are preferentially expressed by neoplastic tissues, they might represent an ideal target for active immunization by genetic vaccination. However, i.m. inoculation of plasmid DNA encoding mouse gp70 or p15E, two products of the env gene of an endogenous murine leukemia virus, elicited a weak Ag-specific T lymphocyte response and resulted in partial protection from challenge with mouse tumors possessing these Ags. Depletion experiments showed that CD8+, but not CD4+, T lymphocytes were crucial for the antitumor activity of the vaccines. Systemic administration of agonistic anti-CD40 mAb increased the therapeutic potential of genetic vaccination, but only when given during the tumor rejection phase and not at the time of immunization. This effect correlated with a dramatic increase in the number of ERV-specific CD8+ T lymphocytes. Adjuvant activity of CD40 agonists thus seems to be relevant to enhance the CD8+ T cell-dependent response in tumor-bearing hosts, suggesting that sustaining tumor-specific T lymphocyte survival in subjects undergoing vaccination might be a key event in the successful vaccination with weak tumor Ags.

Therapeutic Effectiveness of Recombinant Cancer Vaccines Is Associated with a Prevalent T-Cell Receptor α Usage by Melanoma-specific CD8+ T Lymphocytes

Definition of immune variables that correlate with the antitumor activity of cancer vaccines is critical for monitoring immunotherapy protocols. To define surrogate end points predictive of the therapeutic efficacy of recombinant vaccines based on melanoma antigen tyrosinase-related protein (TRP)-2, we evaluated several properties of antigen-specific CD8+ T lymphocytes in single mice undergoing either prophylactic or therapeutic immunization. Predictive markers for the efficacy of genetic vaccination were identified in the prophylactic model used. Interestingly, the number of tetramer+ CD8+ T lymphocytes expanded in vitro after a single cycle of stimulation with the immunodominant TRP-2 peptide was of the highest predictive value. In the therapeutic model, no variable examined at a single mouse level predicted the long-term therapeutic effect. Mice that survived did not show the highest expansion of antigen-specific lymphocytes or the more functionally active effectors, ex vivo or after in vitro culture with the peptide antigen. Successful therapy correlated strictly with the skewing of the T-cell receptor repertoire of tetramer-sorted, TRP-2–specific CD8+ T lymphocytes, which showed a preferential α chain usage with a common CDR3 region.

Interaction between invariant NKT cells and myeloid-derived suppressor cells in cancer patients: evidence and therapeutic opportunities.

Despite advances in therapeutic strategies, the ability of cancer cells to evade destruction remains a significant obstacle to the development of effective anticancer treatment. In recent years a subset of immune cells, myeloid-derived suppressor cells (MDSCs), has been shown to play a key role in evasion of the patient’s immune system by tumor cells. A number of different tumor types are associated with increased numbers of circulating MDSCs in cancer patients, suppressing the immune response and permitting continued tumor cell proliferation. Invariant NKT (iNKT) cells have recently been defined as a unique subset of immune cells that are able to act as a link between the innate and adaptive arms of the immune system. iNKT cells have the ability to carry out immune surveillance of tumor cells and control proliferation of malignant cells. Recently, we presented evidence that iNKT cells are able to interact with and decrease the numbers of circulating MDSCs in melanoma patients. This review discusses the evidence for MDSCs in tumor progression and the implication that iNKT cells could be developed as a potent therapeutic strategy.