Interim results of brentuximab vedotin in combination with nivolumab in patients with relapsed or refractory Hodgkin lymphoma.

In this phase 1/2 study, brentuximab vedotin (BV) and nivolumab (Nivo) administered in combination were evaluated as initial salvage therapy in patients with relapsed or refractory (R/R) classical Hodgkin lymphoma (HL). Patients received up to 4 cycles of combination treatment, with BV administered on day 1 and Nivo on day 8 of the first cycle. For cycles 2 to 4, BV and Nivo were both administered on day 1. After study treatment, responses were evaluated by investigators per the 2014 Lugano classification, and patients could proceed to autologous stem cell transplantation (ASCT). Sixty-two patients were enrolled; the complete response rate among all treated patients (n = 61) was 61%, with an objective response rate of 82%. Before ASCT, adverse events (AEs) occurred in 98% of patients, mostly grades 1 and 2. Infusion-related reactions (IRRs) occurred in 44% of patients overall, with 41% of patients experiencing an IRR during at least 1 infusion of BV. Five patients (8%) were treated with systemic steroids for immune-related AEs. A reduction of peripheral T-cell subsets including regulatory T cells was observed after the first dose of BV, and reduced serum levels of thymus- and activation-regulated chemokine concurrent with an increase in proinflammatory cytokines and chemokines were seen after the first BV plus Nivo infusions. The combination of BV plus Nivo was an active and well-tolerated first salvage regimen, potentially providing patients with R/R HL an alternative to traditional chemotherapy. This trial was registered at www.clinicaltrials.gov as #NCT02572167.

Transient Receptor Potential Melastatin 2 (TRPM2) ion channel is required for innate immunity against Listeria monocytogenes.

The generation of reactive oxygen species (ROS) is inherent to immune responses. ROS are crucially involved in host defense against pathogens by promoting bacterial killing, but also as signaling agents coordinating the production of cytokines. Transient Receptor Potential Melastatin 2 (TRPM2) is a Ca(2+)-permeable channel gated via binding of ADP-ribose, a metabolite formed under conditions of cellular exposure to ROS. Here, we show that TRPM2-deficient mice are extremely susceptible to infection with Listeria monocytogenes (Lm), exhibiting an inefficient innate immune response. In a comparison with IFNγR-deficient mice, TRPM2(-/-) mice shared similar features of uncontrolled bacterial replication and reduced levels of inducible (i)NOS-expressing monocytes, but had intact IFNγ responsiveness. In contrast, we found that levels of cytokines IL-12 and IFNγ were diminished in TRPM2(-/-) mice following Lm infection, which correlated with their reduced innate activation. Moreover, TRPM2(-/-) mice displayed a higher degree of susceptibility than IL-12-unresponsive mice, and supplementation with recombinant IFNγ was sufficient to reverse the unrestrained bacterial growth and ultimately the lethal phenotype of Lm-infected TRPM2(-/-) mice. The severity of listeriosis we observed in TRPM2(-/-) mice has not been reported for any other ion channel. These findings establish an unsuspected role for ADP-ribose and ROS-mediated cation flux for innate immunity, opening up unique possibilities for immunomodulatory intervention through TRPM2.

CX3CL1/fractalkine is released from apoptotic lymphocytes to stimulate macrophage chemotaxis.

Cells undergoing apoptosis are efficiently located and engulfed by phagocytes. The mechanisms by which macrophages, the professional scavenging phagocytes of apoptotic cells, are attracted to sites of apoptosis are poorly defined. Here we show that CX3CL1/fractalkine, a chemokine and intercellular adhesion molecule, is released rapidly from apoptotic lymphocytes, via caspase- and Bcl-2-regulated mechanisms, to attract macrophages. Effective chemotaxis of macrophages to apoptotic lymphocytes is dependent on macrophage fractalkine receptor, CX3CR1. CX3CR1 deficiency caused diminished recruitment of macrophages to germinal centers of lymphoid follicles, sites of high-rate B-cell apoptosis. These results provide the first demonstration of chemokine/chemokine-receptor activity in the navigation of macrophages toward apoptotic cells and identify a mechanism by which macrophage infiltration of tissues containing apoptotic lymphocytes is achieved.

Persistence of apoptotic cells without autoimmune disease or inflammation in CD14-/- mice.

Interaction of macrophages with apoptotic cells involves multiple steps including recognition, tethering, phagocytosis, and anti-inflammatory macrophage responses. Defective apoptotic cell clearance is associated with pathogenesis of autoimmune disease. CD14 is a surface receptor that functions in vitro in the removal of apoptotic cells by human and murine macrophages, but its mechanism of action has not been defined. Here, we demonstrate that CD14 functions as a macrophage tethering receptor for apoptotic cells. Significantly, CD14(-/-) macrophages in vivo are defective in clearing apoptotic cells in multiple tissues, suggesting a broad role for CD14 in the clearance process. However, the resultant persistence of apoptotic cells does not lead to inflammation or increased autoantibody production, most likely because, as we show, CD14(-/-) macrophages retain the ability to generate anti-inflammatory signals in response to apoptotic cells. We conclude that CD14 plays a broad tethering role in apoptotic cell clearance in vivo and that apoptotic cells can persist in the absence of proinflammatory consequences.

Role of complement and other innate immune mechanisms in the removal of apoptotic cells.

The complement system is regarded as an ancient host defense mechanism that helps to promote phagocytosis and/or killing of foreign microorganisms. Less well known is the facilitatory role that complement and other closely related molecules of the innate immune system play in the removal of dying cells. In this chapter, we review the complement system and the mechanisms of complement activation that include natural antibodies and acute phase proteins. The effects of spontaneous and genetically engineered mutations on function of these proteins and their relationship to autoimmune diseases such as lupus are discussed. We also review the known function of non-complement receptors and their roles in recognition and removal of dying cells in normal cellular homeostasis and in inflammation.

IGM is required for efficient complement mediated phagocytosis of apoptotic cells in vivo.

A variety of complement components have been detected on apoptotic cells and proposed to facilitate recognition and/or ingestion by phagocytes. The triggers for complement activation remain uncertain. To determine the role of IgM in classical pathway activation and clearance of apoptotic cells in vitro and in vivo, we quantified these parameters in mice deficient in serum IgM (sIgM). Phagocytosis by bone marrow-derived macrophages of apoptotic cells incubated with serum deficient in sIgM was markedly reduced, similar to apoptotic cells incubated with C1q deficient serum in vitro. Similarly, intraperitoneal clearance of apoptotic cells and cellular C3 deposition were significantly reduced in mice deficient in sIgM compared to wild-type mice. Clearance and C3 deposition were reconstituted by addback of IgM. In mice deficient in both sIgM and Clq, addback of both serum factors was required for restoration of clearance. These findings indicate that, on a quantitative basis, sIgM is a potent factor required for intraperitoneal phagocytosis of apoptotic cells, and further demonstrate that IgM and C1q work in concert to activate complement, resulting in C3 deposition on the apoptotic cell surface and ultimately, efficient clearance of the apoptotic cell by macrophages.

Discovering Molecules That Regulate Efferocytosis Using Primary Human Macrophages and High Content Imaging.

Defective clearance of apoptotic cells can result in sustained inflammation and subsequent autoimmunity. Macrophages, the "professional phagocyte" of the body, are responsible for efficient, non-phlogistic, apoptotic cell clearance. Controlling phagocytosis of apoptotic cells by macrophages is an attractive therapeutic opportunity to ameliorate inflammation. Using high content imaging, we have developed a system for evaluating the effects of antibody treatment on apoptotic cell uptake in primary human macrophages by comparing the Phagocytic Index (PI) for each antibody. Herein we demonstrate the feasibility of evaluating a panel of antibodies of unknown specificities obtained by immunization of mice with primary human macrophages and show that they can be distinguished based on individual PI measurements. In this study ~50% of antibodies obtained enhance phagocytosis of apoptotic cells while approximately 5% of the antibodies in the panel exhibit some inhibition. Though the specificities of the majority of antibodies are unknown, two of the antibodies that improved apoptotic cell uptake recognize recombinant MerTK; a receptor known to function in this capacity in vivo. The agonistic impact of these antibodies on efferocytosis could be demonstrated without addition of either of the MerTK ligands, Gas6 or ProS. These results validate applying the mechanism of this fundamental biological process as a means for identification of modulators that could potentially serve as therapeutics. This strategy for interrogating macrophages to discover molecules regulating apoptotic cell uptake is not limited by access to purified protein thereby increasing the possibility of finding novel apoptotic cell uptake pathways.

Oncogenic properties of apoptotic tumor cells in aggressive B cell lymphoma.

BACKGROUND: Cells undergoing apoptosis are known to modulate their tissue microenvironments. By acting on phagocytes, notably macrophages, apoptotic cells inhibit immunological and inflammatory responses and promote trophic signaling pathways. Paradoxically, because of their potential to cause death of tumor cells and thereby militate against malignant disease progression, both apoptosis and tumor-associated macrophages (TAMs) are often associated with poor prognosis in cancer. We hypothesized that, in progression of malignant disease, constitutive loss of a fraction of the tumor cell population through apoptosis could yield tumor-promoting effects. RESULTS: Here, we demonstrate that apoptotic tumor cells promote coordinated tumor growth, angiogenesis, and accumulation of TAMs in aggressive B cell lymphomas. Through unbiased "in situ transcriptomics" analysis-gene expression profiling of laser-captured TAMs to establish their activation signature in situ-we show that these cells are activated to signal via multiple tumor-promoting reparatory, trophic, angiogenic, tissue remodeling, and anti-inflammatory pathways. Our results also suggest that apoptotic lymphoma cells help drive this signature. Furthermore, we demonstrate that, upon induction of apoptosis, lymphoma cells not only activate expression of the tumor-promoting matrix metalloproteinases MMP2 and MMP12 in macrophages but also express and process these MMPs directly. Finally, using a model of malignant melanoma, we show that the oncogenic potential of apoptotic tumor cells extends beyond lymphoma. CONCLUSIONS: In addition to its profound tumor-suppressive role, apoptosis can potentiate cancer progression. These results have important implications for understanding the fundamental biology of cell death, its roles in malignant disease, and the broader consequences of apoptosis-inducing anti-cancer therapy.

Macrophage chemotaxis to apoptotic Burkitt's lymphoma cells in vitro: role of CD14 and CD36.

In Burkitt's lymphoma (BL), apoptosis occurs at high frequency alongside uncontrolled proliferation. Macrophages infiltrate these tumours in large numbers and engage in the phagocytic clearance of apoptotic cells in situ. Here we tested the hypothesis that apoptosis of BL cells may provide a mechanism for recruitment of macrophages to these tumours. We show that monocytes and macrophages, but not neutrophils, preferentially migrated to apoptotic BL cells in vitro. Transfection of BL cells with the anti-apoptotic gene bcl-2 both prevented apoptosis and abolished macrophage chemotaxis. Macrophage migration to BL populations correlated well with the number of apoptotic BL cells present (the Pearson correlation r = 0.81, p<0.0001). Chemoattraction of murine macrophages to apoptotic human BL cells demonstrated that the mechanism was conserved across these species. In an attempt to identify the macrophage receptors involved in this process, we investigated whether CD14 and CD36, two receptors important in the phagocytic clearance of apoptotic cells, were also involved in the chemotactic macrophage response. We found that bone marrow-derived macrophages from CD14-/- and CD36-/- mice moved as well as wild-type macrophages in chemotaxis assays towards apoptotic BL cells. Migrating macrophages were found to be up-regulated in their expression of CD14, however, suggesting that, although this receptor does not appear to be required for 'sensing' apoptotic cells, it may be up-regulated on the surface of the migrating macrophage in readiness for apoptotic corpse clearance.

Innate and adaptive immune response to apoptotic cells.

The immune system is constantly exposed to dying cells, most of which arise during central tolerance and from effete circulating immune cells. Under homeostatic conditions, phagocytes (predominantly macrophages and dendritic cells) belonging to the innate immune system, rapidly ingest cells and their debris. Apoptotic cell removal requires recognition of altered self on the apoptotic membrane, a process which is facilitated by natural antibodies and serum opsonins. Recognition, may be site and context specific. Uptake and ingestion of apoptotic cells promotes an immunosuppressive environment that avoids inflammatory responses to self-antigens. However, it does not preclude a T cell response and it is likely that constant exposure to self-antigen, particularly by immature dendritic cells, leads to T cell tolerance. Tolerance occurs by several different mechanisms including anergy and deletion (for CD8+T cells) and induction of T regulatory cells (for CD4+T cells). Failed apoptotic cell clearance promotes immune responses to self-antigens, especially when the cellular contents are leaked from the cell (necrosis). Inflammatory responses may be induced by nucleic acid stimulation of Toll like receptors and other immune sensors, specific intracellular proteins and non-protein (uric acid) stimulation of inflammasomes.

Enhanced apoptotic cell clearance capacity and B cell survival factor production by IL-10-activated macrophages: implications for Burkitt's lymphoma.

Burkitt's lymphoma (BL) is typified by frequent tumor cell apoptosis and significant macrophage infiltration. Since BL cells have an inherent tendency to undergo apoptosis at a high rate, we reasoned that macrophages in BL are functionally enhanced in at least two activities that have implications for tumor pathogenesis: 1) engulfment of apoptotic cells, an anti-inflammatory process known to suppress immune responses, and 2) production of BL cell survival factors that limit the extent of tumor cell apoptosis. In this study, we show that the microenvironment of BL is rich in the pleiotropic cytokine IL-10, which can be produced by both tumor cells and macrophages, and that IL-10-activated human macrophages have enhanced capacity to engulf apoptotic cells in vitro. This was found to be dependent on the macrophage tethering receptor of apoptotic cells, CD14. Furthermore, IL-10-activated macrophages were found to produce markedly higher levels of the B cell survival factor, B cell-activating factor of the TNF family/B lymphocyte stimulator (BAFF/BLyS) than macrophages matured in the absence of IL-10. Coculture of macrophages with BL cells further enhanced BAFF secretion. Significantly, we show that enhancement of BL cell survival by IL-10-activated macrophages is mediated by a BAFF-dependent component and that BAFF is produced at high levels by tumor-associated macrophages in situ. These results indicate that macrophages, regulated by IL-10, have the potential to promote BL pathogenesis, first, through suppression of antitumor immunity following enhanced engulfment of apoptotic tumor cells and, second, through increased production of tumor cell growth/survival factors.

Role of surfactant proteins A, D, and C1q in the clearance of apoptotic cells in vivo and in vitro: calreticulin and CD91 as a common collectin receptor complex.

Removal of cells dying by apoptosis is essential to normal development, maintenance of tissue homeostasis, and resolution of inflammation. Surfactant protein A (SP-A) and surfactant protein D (SP-D) are high abundance pulmonary collectins recently implicated in apoptotic cell clearance in vitro. Other collectins, such as mannose-binding lectin and the collectin-like C1q, have been shown to bind to apoptotic cells and drive ingestion through interaction with calreticulin and CD91 on the phagocyte in vitro. However, only C1q has been shown to enhance apoptotic cell uptake in vivo. We sought to determine the relative importance of SP-A, SP-D, and C1q in pulmonary clearance of apoptotic cells using knockout and overexpressing mice, and to determine the role of calreticulin and CD91 in this process. SP-A, SP-D, and C1q all enhanced apoptotic cell ingestion by resident murine and human alveolar macrophages in vitro. However, only SP-D altered apoptotic cell clearance from the naive murine lung, suggesting that SP-D plays a particularly important role in vivo. Similar to C1q and mannose-binding lectin, SP-A and SP-D bound to apoptotic cells in a localized, patchy pattern and drove apoptotic cell ingestion by phagocytes through a mechanism dependent on calreticulin and CD91. These results suggest that the entire collectin family of innate immune proteins (including C1q) works through a common receptor complex to enhance removal of apoptotic cells, and that collectins are integral, organ-specific components of the clearance machinery.