Killer dendritic cells: IKDC and the others

Theoretical premises of a "three in one" therapeutic approach to treat immunogenic and nonimmunogenic cancers: a narrative review

Objective

We describe experimental and theoretical premises of a powerful cancer therapy based on the combination of three approaches. These include (I) in situ vaccination (intratumoral injections of CpG oligonucleotides and anti-OX40 antibody); (II) chronometric or metronomic low-dose cyclophosphamide (CMLD CP)-based chemotherapy; (III) cancer stem cell-eradicating therapy referred to as Karanahan (from the Sanskrit kāraṇa [“source”] + han [“to kill”]).

Background

In murine models, the first two approaches are particularly potent in targeting immunogenic tumors for destruction. In situ vaccination activates a fully fledged anticancer immune response via an intricate network of ligand–receptor–cytokine interactions. CMLD CP-based chemotherapy primarily targets the suppressive tumor microenvironment and activates tumor-infiltrating effectors. In contrast, Karanahan technology, being aimed at replicative machinery of tumor cells (both stem-like and committed), does not depend on tumor immunogenicity. With this technology, mice engrafted with ascites and/or solid tumors can be successfully cured. There is a significant degree of mechanistic and therapeutic overlap between these three approaches. For instance, the similarities shared between in situ vaccination and Karanahan technology include the therapeutic procedure, the cell target [antigen-presenting cells (APC) and dendritic cells (DC)], and the use of DNA-based preparations (CpG and DNAmix). Features shared between CMLD CP-based chemotherapy and Karanahan technology are the timing and the dose of the cytostatic drug administration, which lead to tumor regression.

Methods

The following keywords were used to search PubMed for the latest research reporting successful eradication of transplantable cancers in animal models that relied on approaches distinct from those used in the Karanahan technology: eradication of malignancy, cure cancer, complete tumor regression, permanently eradicating advanced mouse tumor, metronomic chemotherapy, in situ vaccination, immunotherapy, and others.

Conclusion

We hypothesize, therefore, that very potent anticancer activity can be achieved once these three therapeutic modalities are combined into a single approach. This multimodal approach is theoretically curative for any type of cancer that depends on the presence of tumor-inducing cancer stem cells, provided that the active therapeutic components are efficiently delivered into the tumor and the specific biological features of a given patient’s tumor are properly addressed. We expect this multimodal approach to be primarily applicable to late-stage or terminal cancer patients who have exhausted all treatment options as well as patients with inoperable tumors.

Drug delivery strategies in maximizing anti-angiogenesis and anti-tumor immunity

Metronomic chemotherapy has been shown to elicit anti-tumor immune response and block tumor angiogenesis distinct from that observed with maximal tolerated dose (MTD) therapy. This review delves into the mechanisms behind anti-tumor immunity and seeks to identify the differential effect of dosing regimens, including daily low-dose and medium-dose intermittent chemotherapy (MEDIC), on both innate and adaptive immune populations involved in observed anti-tumor immune response. Given reports of VEGF/VEGFR blockade antagonizing anti-tumor immunity, drug choice, dose, and selective delivery determined by advanced formulations/vehicles are highlighted as potential sources of innovation for identifying anti-angiogenic modalities that may be combined with metronomic regimens without interrupting key immune players in the anti-tumor response. Engineered drug delivery mechanisms that exhibit extended and local release of anti-angiogenic agents both alone and in combination with chemotherapeutic treatments have also been demonstrated to elicit a potent and potentially systemic anti-tumor immune response, favoring tumor regression and stasis over progression. This review examines this interplay between various cancer models, the host immune response, and select anti-cancer agents depending on drug dosing, scheduling/regimen, and delivery modality.

Cytotoxic and antigen presenting functions of T helper-1-activated dendritic cells

Although primarily defined by their cardinal antigen-presenting function, dendritic cells (DCs) are also equipped with cytotoxic properties. We have recently reported that DCs activated by IFNγ-secreting Th-1 lymphocytes can kill cancer cells and subsequently present the acquired tumor-derived antigens to T lymphocytes both in vitro and in vivo.

Natural killer cells in cancer biology and therapy

The tumor microenvironment is highly complex, and immune escape is currently considered an important hallmark of cancer, largely contributing to tumor progression and metastasis. Named for their capability of killing target cells autonomously, natural killer (NK) cells serve as the main effector cells toward cancer in innate immunity and are highly heterogeneous in the microenvironment. Most current treatment options harnessing the tumor microenvironment focus on T cell-immunity, either by promoting activating signals or suppressing inhibitory ones. The limited success achieved by T cell immunotherapy highlights the importance of developing new-generation immunotherapeutics, for example utilizing previously ignored NK cells. Although tumors also evolve to resist NK cell-induced cytotoxicity, cytokine supplement, blockade of suppressive molecules and genetic engineering of NK cells may overcome such resistance with great promise in both solid and hematological malignancies. In this review, we summarized the fundamental characteristics and recent advances of NK cells within tumor immunometabolic microenvironment, and discussed potential application and limitations of emerging NK cell-based therapeutic strategies in the era of presicion medicine.

Regulatory IFN-γ-producing killer dendritic cells are enhanced in B6.MLR-Faslpr /J lupus-prone mice

A novel cell population denominated IFN-γ-producing killer dendritic cells (IKDCs) have been recently described. These cells are lymphocytes lacking B- or T- receptors, but they can be identified by the presence of B220⁺ CD38⁺ CD49b⁺ and low CD11c, among other cell surface markers. The main characteristics of IKDCs are the production of IFN-γ and the ability to spontaneously kill tumor cells. We found that this population increases in B6.MLR-Fas^lpr /J mice. Interestingly, IKDCs increase with age and are more abundant in mice older than 6 months onward. To analyze whether these cells have any role in the induction of the lupus-like phenotype in the B6.MLR-Fas^lpr /J mice, IKDCs were purified and transferred into 6-month-old B6.MRL-Fas^lpr /J mice, then the presence of anti-nuclear antibodies (ANAS) and anti-dsDNA antibodies were analyzed 2 and 4 months after the transfer. The results showed a reduction in the levels of these autoantibodies and increased survival of these mice, indicating that these cells may have a regulatory function. In vitro assays demonstrated that IKDCs reduced the proliferation of both autoreactive B and T cells, suggesting that these may be the mechanisms used by these cells to ameliorate the lupus-like phenotype in the B6.MRL-Fas^lpr /J mice.

A pancreatic tumor-specific biomarker characterized in humans and mice as an immunogenic onco-glycoprotein is efficient in dendritic cell vaccination

Oncofetal fucose-rich glycovariants of the pathological bile salt-dependent lipase (pBSDL) appear during human pancreatic oncogenesis and are detected by themonoclonal antibody J28 (mAbJ28). We aimed to identify murine counterparts onpancreatic ductal adenocarcinoma (PDAC) cells and tissue and investigate the potential of dendritic cells (DC) loaded with this unique pancreatic tumor antigen to promote immunotherapy in preclinical trials. Pathological BSDLs purified from pancreatic juices of patients with PDAC were cleaved to generate glycosylated C-terminal moieties (C-ter) containing mAbJ28-reactive glycoepitopes. Immunoreactivity of the murine PDAC line Panc02 and tumor tissue to mAbJ28 was detected by immunohistochemistry and flow cytometry. C-ter-J28⁺ immunization promoted Th1-dominated immune responses. In vitro C-ter-J28⁺-loaded DCskewed CD3⁺ T-cells toward Th1 polarization. C-ter-J28⁺-DC-vaccinations selectively enhanced cell immunoreactivity to Panc02, as demonstrated by CD4⁺- and CD8⁺-T-cell activation, increased percentages of CD4⁺- and CD8⁺-T-cells and NK1.1⁺ cells expressing granzyme B, and T-cell cytotoxicity. Prophylactic and therapeutic C-ter-J28⁺-DC-vaccinations reduced ectopic Panc02-tumor growth, provided long-lasting protection from Panc02-tumor development in 100% of micebut not from melanoma, and attenuated progression of orthotopic tumors as revealed by MRI. Thusmurine DC loaded with pancreatic tumor-specific glycoepitope C-ter-J28⁺ induce efficient anticancer adaptive immunity and represent a potential adjuvant therapy for patients afflicted with PDAC.

Trypanosoma cruzi extracts elicit protective immune response against chemically induced colon and mammary cancers

Trypanosoma cruzi, the protozoan parasite that causes Chagas' disease, has anticancer effects mediated, at least in part, by parasite-derived products which inhibit growth of tumor cells. We investigated whether immunity to T. cruzi antigens could induce antitumor activity, using two rat models which reproduce human carcinogenesis: colon cancer induced by 1,2-dimethylhydrazine (DMH), and mammary cancer induced by N-nitroso-N-methylurea (NMU). We found that vaccination with T. cruzi epimastigote lysates strongly inhibits tumor development in both animal models. Rats immunized with T. cruzi antigens induce activation of both CD4(+) and CD8(+) T cells and splenocytes from these animals showed higher cytotoxic responses against tumors as compared to rats receiving adjuvant alone. Tumor-associated immune responses included increasing number of CD11b/c(+) His48(-) MHC II(+) cells corresponding to macrophages and/or dendritic cells, which exhibited augmented NADPH-oxidase activity. We also found that T. cruzi lysate vaccination developed antibodies specific for colon and mammary rat cancer cells, which were capable of mediating antibody-dependent cellular cytotoxicity (ADCC) in vitro. Anti-T. cruzi antibodies cross-reacted with human colon and breast cancer cell lines and recognized 41/60 (68%) colon cancer and 38/63 (60%) breast cancer samples in a series of 123 human tumors. Our results suggest that T. cruzi antigens can evoke an integrated antitumor response involving both the cellular and humoral components of the immune response and provide novel insights into the understanding of the intricate relationship between parasite infection and tumor growth.

Downregulation of STAT3 phosphorylation enhances tumoricidal effect of IL-15-activated dendritic cell against doxorubicin-resistant lymphoma and leukemia via TNF-α

Although disputed by some, increasing evidence suggests that TNF-α synergies with traditional chemotherapeutic drugs to exert a heightened antitumor effect. The present study investigated the antitumor efficacy of recombinant IL-15 in combination with the STAT3 inhibitor cucurbitacin-I in a doxorubicin-resistant murine lymphoma model. The significance of the work is to understand and design effective strategies in doxorubicin resistant lymphomas. TNF-α is downregulated in dendritic cells from mice with Dalton's lymphoma and shows an inverse relationship with disease progression. Doxorubicin-resistant DL cells have elevated levels of Bcl-2 and Mcl-1 and increased phosphorylation of STAT3. These cells are refractory to dendritic cell derived TNF-α. Doxorubicin resistant Dalton's lymphoma is susceptible to dendritic cell derived TNF-α upon stimulation with the STAT3 inhibitor cucurbitacin-I, which downregulates STAT3 and other survival molecules. The combined treatment of low dose of cucurbitacin-I and rIL-15 is ineffective in mice with doxorubicin resistant Dalton's lymphoma, but a similar therapy prolongs the survival of mice transplanted with parental Dalton's lymphoma. Doxorubicin resistant Dalton's lymphoma responds to therapy with high doses of cucurbitacin-I and rIL-15. Dendritic cell derived from mice responded positively to the therapy and regained their tumoricidal properties with respect to growth inhibition and killing of DL tumor cells. Similar to DL, DC derived from CML patients are impaired in TNF-α expression and are unable to restrict the growth of drug-resistant lymphoma and leukemia cells. This combination approach could be used as a new therapeutic strategy for aggressive and highly metastatic doxorubicin resistant lymphoma.

Anti-tumor innate immunity activated by intermittent metronomic cyclophosphamide treatment of 9L brain tumor xenografts is preserved by anti-angiogenic drugs that spare VEGF receptor 2

Background

Metronomic cyclophosphamide given on an intermittent, 6-day repeating schedule, but not on an exposure dose-equivalent daily schedule, activates an anti-tumor innate immune response that leads to major regression of large implanted gliomas, without anti-angiogenesis.

Methods and approach

Mice bearing implanted 9L gliomas were used to investigate the effects of this 6-day repeating, immunogenic cyclophosphamide schedule on myeloid-derived suppressor cells, which are pro-angiogenic and can inhibit anti-tumor immunity, and to elucidate the mechanism whereby the innate immune cell-dependent tumor regression response to metronomic cyclophosphamide treatment is blocked by several anti-angiogenic receptor tyrosine kinase inhibitors.

Results

Intermittent metronomic cyclophosphamide scheduling strongly increased glioma-associated CD11b⁺ immune cells but not CD11b⁺Gr1⁺ myeloid-derived suppressor cells, while bone marrow and spleen reservoirs of the suppressor cells were decreased. The inhibition of immune cell recruitment and tumor regression by anti-angiogenic receptor tyrosine kinase inhibitors, previously observed in several brain tumor models, was recapitulated in the 9L tumor model with the VEGFR2-specific inhibitory monoclonal antibody DC101 (p < 0.01), implicating VEGFR2 signaling as an essential step in metronomic cyclophosphamide-stimulated immune cell recruitment. In contrast, sorafenib, a multi-receptor tyrosine kinase inhibitor with comparatively weak VEGF receptor phosphorylation inhibitory activity, was strongly anti-angiogenic but did not block metronomic cyclophosphamide-induced innate immunity or tumor regression (p > 0.05).

Conclusions

The interference by receptor tyrosine kinase inhibitors in the immunogenic actions of intermittent metronomic chemotherapy is not a consequence of anti-angiogenesis per se, as demonstrated in an implanted 9L tumor model. Furthermore, this undesirable interaction with tyrosine kinase inhibitors can be avoided by using anti-angiogenic drugs that spare the VEGFR2 pathway.

PIAS1 and STAT-3 impair the tumoricidal potential of IFN-γ-stimulated mouse dendritic cells generated with IL-15

Primarily defined by their antigen-presenting property, dendritic cells (DCs) are being implemented as cancer vaccines in immunotherapeutic interventions. DCs can also function as direct tumor cell killers. How DC cytotoxic activity can be efficiently harnessed and the mechanisms controlling this nonconventional property are not fully understood. We report here that the tumoricidal potential of mouse DCs generated from myeloid precursors with GM-CSF and IL-15 (IL-15 DCs) can be triggered with the Toll-like receptor (TLR) 4 ligand lipopolysaccharide to a similar extent compared with that of their counterparts, conventionally generated with IL-4 (IL-4 DCs). The mechanism of tumor cell killing depends on the induction of iNOS expression by DCs. In contrast, interferon (IFN)-γ induces the cytotoxic activity of IL-4 but not IL-15 DCs. Although the IFN-γ-STAT-1 signaling pathway is overall functional in IL-15 DCs, IFN-γ fails to induce iNOS expression in these cells. iNOS expression is negatively controlled in IFN-γ-stimulated IL-15 DCs by the cooperation between the E3 SUMO ligase PIAS1 and STAT-3, and can be partially restored with PIAS1 siRNA and STAT-3 inhibitors.

Interleukin-17D mediates tumor rejection through recruitment of natural killer cells

The process of cancer immunoediting generates a repertoire of cancer cells that can persist in immune-competent hosts. In its most complex form, this process begins with the elimination of highly immunogenic unedited tumor cells followed by the escape of less immunogenic edited cells. Although edited tumors can release immunosuppressive factors, it is unknown whether unedited tumors produce cytokines that enhance antitumor function. Utilizing gene microarray analysis, we found the cytokine interleukin 17D (IL-17D) was highly expressed in certain unedited tumors but not in edited mouse tumor cell lines. Moreover, forced expression of IL-17D in edited tumor cells induced rejection by stimulating MCP-1 production from tumor endothelial cells, leading to the recruitment of natural killer (NK) cells. NK cells promoted M1 macrophage development and adaptive immune responses. IL-17D expression was also decreased in certain high-grade and metastatic human tumors, suggesting that it can be targeted for tumor immune therapy.

Immature, Semi-Mature, and Fully Mature Dendritic Cells: Toward a DC-Cancer Cells Interface That Augments Anticancer Immunity

Dendritic cells (DCs) are the sentinel antigen-presenting cells of the immune system; such that their productive interface with the dying cancer cells is crucial for proper communication of the "non-self" status of cancer cells to the adaptive immune system. Efficiency and the ultimate success of such a communication hinges upon the maturation status of the DCs, attained following their interaction with cancer cells. Immature DCs facilitate tolerance toward cancer cells (observed for many apoptotic inducers) while fully mature DCs can strongly promote anticancer immunity if they secrete the correct combinations of cytokines [observed when DCs interact with cancer cells undergoing immunogenic cell death (ICD)]. However, an intermediate population of DC maturation, called semi-mature DCs exists, which can potentiate either tolerogenicity or pro-tumorigenic responses (as happens in the case of certain chemotherapeutics and agents exerting ambivalent immune reactions). Specific combinations of DC phenotypic markers, DC-derived cytokines/chemokines, dying cancer cell-derived danger signals, and other less characterized entities (e.g., exosomes) can define the nature and evolution of the DC maturation state. In the present review, we discuss these different maturation states of DCs, how they might be attained and which anticancer agents or cell death modalities (e.g., tolerogenic cell death vs. ICD) may regulate these states.

Tumoricidal activity of human dendritic cells

•

Human DC subsets can exert tumoricidal activity.

•

Killer DCs exploit several mechanisms for direct killing of target cells, including TRAIL and granzyme B.

•

Antigen presentation and/or IFN production are important additional effector functions.

•

Killer DCs are promising targets for immunotherapeutic strategies.

Dendritic cells (DCs) are a family of professional antigen-presenting cells (APCs) that are able to initiate innate and adaptive immune responses against pathogens and tumor cells. The DC family is heterogeneous and is classically divided into two main subsets, each with its unique phenotypic and functional characteristics: myeloid DCs (mDCs) and plasmacytoid DCs (pDCs). Recent results have provided intriguing evidence that both DC subsets can also function as direct cytotoxic effector cells; in particular, against cancer cells. In this review, we delve into this understudied function of human DCs and discuss why these so-called killer DCs might become important tools in future cancer immunotherapies.

Dendritic cell-based immunotherapy for glioma: multiple regimens and implications in clinical trials

High grade glioma is a highly invasive brain tumor and recurrence is almost inevitable, even after radical resection of the tumor mass. Cytotoxic immune responses and immunological memory induced by immunotherapy might prevent tumor recurrence. Dendritic cells (DCs) are professional antigen-presenting cells of the innate immune system with the potential to generate robust antigen-specific T cell immune responses. DC-based immunotherapeutic strategies have been intensively studied in both preclinical and clinical settings. Although advances have been made in the experimental use of DCs, there are still considerable challenges that need to be addressed for clinical translation. In this review, we describe the variability of regimens currently available for DC-based immunotherapy and then review strategies to optimize DC therapeutic efficacy against glioma.

Dendritic cells in human Pneumovirus and Metapneumovirus infections

Lung dendritic cells (DC) play a fundamental role in sensing invading pathogens, as well as in the control of tolerogenic responses in the respiratory tract. Their strategic localization at the site of pathogen entry makes them particularly susceptible to initial viral invasion. Human respiratory syncytial virus (hRSV) and human metapneumovirus (hMPV) belong to the Paramyxoviridae family, within the Pneumovirus and Metapneumovirus genera, respectively. hRSV and hMPV are significant human respiratory pathogens that cause similar clinical manifestations and affect many of the same subpopulations. However, they differentially activate the host immune response, including DC, which represents a fundamental link between the innate and adaptive immune response. In this review, the role of DC in the immune response against hRSV and hMPV infections, as well as the inhibitory effects of these paramyxoviruses on the DC immunity will be discussed.

An efferocytosis-induced, IL-4-dependent macrophage-iNKT cell circuit suppresses sterile inflammation and is defective in murine CGD

Efferocytosis of apoptotic neutrophils by macrophages following tissue injury is fundamental to the resolution of inflammation and initiation of tissue repair. Using a sterile peritonitis model in mice, we identified interleukin (IL)-4-producing efferocytosing macrophages in the peritoneum that activate invariant natural killer T (iNKT) cells to produce cytokines including IL-4, IL-13, and interferon-γ. Importantly, IL-4 from macrophages contributes to alternative activation of peritoneal exudate macrophages and augments type 2 cytokine production from NKT cells to suppress inflammation. The increased peritonitis in mice deficient in IL-4, NKT cells, or IL-4Rα expression on myeloid cells suggested that each is a key component for resolution of sterile inflammation. The reduced NAD phosphate oxidase is also critical for this model, because in mice with X-linked chronic granulomatous disease (X-CGD) that lack oxidase subunits, activation of iNKT cells by X-CGD peritoneal exudate macrophages was impaired during sterile peritonitis, resulting in enhanced and prolonged inflammation in these mice. Therefore, efferocytosis-induced IL-4 production and activation of IL-4-producing iNKT cells by macrophages are immunomodulatory events in an innate immune circuit required to resolve sterile inflammation and promote tissue repair.

Human plasmacytoid dendritic cells are equipped with antigen-presenting and tumoricidal capacities

Human plasmacytoid dendritic cells (pDCs) represent a highly specialized naturally occurring dendritic-cell subset and are the main producers of type I interferons (IFNs) in response to viral infections. We show that human pDCs activated by the preventive vaccine FSME specifically up-regulate CD56 on their surface, a marker that was thought to be specific for NK cells and associated with cytolytic effector functions. We observed that FSME-activated pDCs specifically lysed NK target cells and expressed cytotoxic molecules, such as tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) and granzyme B. Elevated levels of these molecules coincided with the expression of CD56, indicative for skewing human pDCs toward an interferon-producing killer DC subset. Detailed phenotypical and functional analysis revealed that pDCs attained a mature phenotype, secreted proinflammatory cytokines, and had the capacity to present antigens and stimulate T cells. Here, we report on the generation of CD56(+) human interferon producing killer pDCs with the capacity to present antigens. These findings aid in deciphering the role for pDCs in antitumor immunity and present a promising prospect of developing antitumor therapy using pDCs.

Essential roles of insulin expression in Aire+ tolerogenic dendritic cells in maintaining peripheral self-tolerance of islet β-cells

Anti-insulin autoimmunity is one of the primary forces in initiating and progressing β-cell destruction in type 1 diabetes. While insulin expression in thymic medullary epithelial cells has been shown to be essential for establishing β-cell central tolerance, the function of insulin expression in antigen-presenting cells (APCs) of hematopoietic lineage remains elusive. With a Cre-lox reporter approach, we labeled Aire-expressing cells with enhanced yellow fluorescent proteins, and found that insulin expression in the spleen was restricted predominantly to a population of Aire(+)CD11c(int)B220(+) dendritic cells (DCs). Targeted insulin deletion in APCs failed to induce anti-islet autoimmunity in B6 mice. In contrast, elevated levels of T cell infiltration into islets were observed in B6(g7) congenic mice when insulin was specifically deleted in their CD11c-expressing DCs (B6(g7)·CD11c-ΔIns mice). Thus, insulin expression in BM-derived, Aire(+) tolerogenic DCs may play an essential role to prevent the activation and expansion of insulin-reactive T cells in the periphery.

VEGF receptor inhibitors block the ability of metronomically dosed cyclophosphamide to activate innate immunity-induced tumor regression

In metronomic chemotherapy, frequent drug administration at lower than maximally tolerated doses can improve activity while reducing the dose-limiting toxicity of conventional dosing schedules. Although the antitumor activity produced by metronomic chemotherapy is attributed widely to antiangiogenesis, the significance of this mechanism remains somewhat unclear. In this study, we show that a 6-day repeating metronomic schedule of cyclophosphamide administration activates a potent antitumor immune response associated with brain tumor recruitment of natural killer (NK) cells, macrophages, and dendritic cells that leads to marked tumor regression. Tumor regression was blocked in nonobese diabetic/severe combined immunodeficient (NOD/SCID-γ) mice, which are deficient or dysfunctional in all these immune cell types. Furthermore, regression was blunted by NK cell depletion in immunocompetent syngeneic mice or in perforin-deficient mice, which are compromised for NK, NKT, and T-cell cytolytic functions. Unexpectedly, we found that VEGF receptor inhibitors blocked both innate immune cell recruitment and the associated tumor regression response. Cyclophosphamide administered at a maximum tolerated dose activated a transient, weak innate immune response, arguing that persistent drug-induced cytotoxic damage or associated cytokine and chemokine responses are required for effective innate immunity-based tumor regression. Together, our results reveal an innate immunity-based mechanism of tumor regression that can be activated by a traditional cytotoxic chemotherapy administered on a metronomic schedule. These findings suggest the need to carefully evaluate the clinical effects of combination chemotherapies that incorporate antiangiogenesis drugs targeting VEGF receptor.

Breaking T cell tolerance to beta cell antigens by merocytic dendritic cells

In type 1 diabetes (T1D), a break in central and peripheral tolerance results in antigen-specific T cells destroying insulin-producing, pancreatic beta cells. Herein, we discuss the critical sub-population of dendritic cells responsible for mediating both the cross-presentation of islet antigen to CD8(+) T cells and the direct presentation of beta cell antigen to CD4(+) T cells. These cells, termed merocytic dendritic cells (mcDC), are more numerous in non-obese diabetic (NOD), and antigen-loaded mcDC rescue CD8(+) T cells from peripheral anergy and deletion, and stimulate islet-reactive CD4(+) T cells. When purified from the pancreatic lymph nodes of overtly diabetic NOD mice, mcDC can break peripheral T cell tolerance to beta cell antigens in vivo and induce rapid onset T cell-mediated T1D in young NOD mouse. Thus, the mcDC subset appears to represent the long-sought critical antigen-presenting cell responsible for breaking peripheral tolerance to beta cell antigen in vivo.

Membrane-bound IL-15 stimulation on peripheral blood natural kiler progenitors leads to the generation of an adherent subset co-expressing dendritic cells and natural kiler functional markers

Human peripheral blood natural killer progenitors represent a flexible, heterogeneous population whose phenotype and function are controlled by their membrane-bound IL-15. Indeed, reciprocal membrane-bond IL-15 trans-presentation commits these cells into NK differentiation, while membrane-bound IL-15 stimulation with its soluble ligand (sIL-15Rα) triggers a reverse signal (pERK1/2 and pFAK) that modifies the developmental program of at least two subsets of PB-NKPs. This treatment generates: i) the expansion of an immature NK subset growing in suspension; ii) the appearance of an unprecedented adherent non-proliferative subset with a dendritic morphology co-expressing marker, cytokines and functions typical of myeloid dendritic cells (CD1a(+)/BDCA1(+)/IL-12(+)) and NK cells (CD3-/NKp46(+)/ CD56(+)/IFNγ(+)). The generation of these putative NK/DCs is associated to the rapid inhibition of negative regulators of myelopoiesis (the transcription factors STAT6 and GATA-3) followed by the transient upregulation of inducers of myeloid development, such as the transcription factors (PU.1, GATA-1) and the anti-apoptotic molecule (MCL-1).

Killer dendritic cells and their potential for cancer immunotherapy

Known for years as the principal messengers of the immune system, dendritic cells (DC) represent a heterogeneous population of antigen presenting cells critically located at the nexus between innate and adaptive immunity. DC play a central role in the initiation of tumor-specific immune responses as they are endowed with the unique ability to take up, process and present tumor antigens to naïve CD4(+) or CD8(+) effector T lymphocytes. By virtue of the cytokines they produce, DC also regulate the type, strength and duration of T cell immune responses. In addition, they can participate in anti-tumoral NK and NKT cell activation and in the orchestration of humoral immunity. More recent studies have documented that besides their primary role in the induction and regulation of adaptive anti-tumoral immune responses, DC are also endowed with the capacity to directly kill cancer cells. This dual role of DC as killers and messengers may have important implications for tumor immunotherapy. First, the direct killing of malignant cells by DC may foster the release and thereby the immediate availability of specific tumor antigens for presentation to cytotoxic or helper T lymphocytes. Second, DC may participate in the effector phase of the immune response, potentially augmenting the diversity of the killing mechanisms leading to tumor elimination. This review focuses on this non-conventional cytotoxic function of DC as it relates to the promotion of cancer immunity and discusses the potential application of killer DC (KDC) in tumor immunotherapy.

Aglycosylated IgG variants expressed in bacteria that selectively bind FcgammaRI potentiate tumor cell killing by monocyte-dendritic cells

The N-linked glycan of immunoglobulin G (IgG) is indispensable for the interaction of the Fc domain with Fcgamma receptors on effector cells and the clearance of target cells via antibody dependent cell-mediated cytotoxicity (ADCC). Escherichia coli expressed, aglycosylated Fc domains bind effector FcgammaRs poorly and cannot elicit ADCC. Using a novel bacterial display/flow cytometric library screening system we isolated Fc variants that bind to FcgammaRI (CD64) with nanomolar affinity. Binding was critically dependent on amino acid substitutions (E382V, and to a lesser extent, M428I) distal to the putative FcgammaRI binding epitope within the CH3 domain. These mutations did not adversely affect its pH-dependent interaction with FcRn in vitro nor its serum persistence in vivo. Remarkably, the anti-Her2 IgG trastuzumab containing the E382V, M428I substitutions and expressed in E. coli exhibited highly selective binding to FcgammaRI but not to the other activating receptors (FcgammaRIIa, FcgammaRIIIa) nor to the inhibitory receptor, FcgammaRIIb. In contrast, the glycosylated version of trastuzumab (E382V, M428I) purified from HEK293T cells bound to all Fcgamma receptors in a manner similar to that of clinical grade trastuzumab. E. coli-purified trastuzumab (E382V, M428I), but not glycosylated trastuzumab (E382V, M428I) or clinical grade trastuzumab, was capable of potentiating the killing of Her2 overexpressing tumor cells with dendritic cells (DCs) as effectors. These results indicate that aglycosylated IgGs can be engineered to display unique FcgammaR selectivity profiles that, in turn, mediate ADCC via mechanisms that are not normally displayed by glycosylated monoclonal antibodies.

Both conventional and interferon killer dendritic cells have antigen-presenting capacity during influenza virus infection

Natural killer cells are innate effector cells known for their potential to produce interferon-gamma and kill tumour and virus-infected cells. Recently, B220(+)CD11c(int)NK1.1(+) NK cells were found to also have antigen-presenting capacity like dendritic cells (DC), hence their name interferon-producing killer DC (IKDC). Shortly after discovery, it has already been questioned if IKDC really represent a separate subset of NK cells or merely represent a state of activation. Despite similarities with DCs, in vivo evidence that they behave as bona fide APCs is lacking. Here, using a model of influenza infection, we found recruitment of both conventional B220(-) NK cells and IKDCs to the lung. To study antigen-presenting capacity of NK cell subsets and compare it to cDCs, all cell subsets were sorted from lungs of infected mice and co-cultured ex vivo with antigen specific T cells. Both IKDCs and conventional NK cells as well as cDCs presented virus-encoded antigen to CD8 T cells, whereas only cDCs presented to CD4 T cells. The absence of CD4 responses was predominantly due to a deficiency in MHCII processing, as preprocessed peptide antigen was presented equally well by cDCs and IKDCs. In vivo, the depletion of NK1.1-positive NK cells and IKDCs reduced the expansion of viral nucleoprotein-specific CD8 T cells in the lung and spleen, but did finally not affect viral clearance from the lung. In conclusion, we found evidence for APC function of lung NK cells during influenza infection, but this is a feature not exclusive to the IKDC subset.

The dendritic cell-like functions of IFN-producing killer dendritic cells reside in the CD11b+ subset and are licensed by tumor cells

IFN producing killer dendritic cells (IKDC) were originally defined as CD11c(int) B220(+)NK1.1(+) (or CD49b(+)) cells that exert a potent tumoricidal activity in animals lacking B, T, and conventional natural killer effectors. MHC class II expression on tumor infiltrating IKDC prompted us to investigate their putative antigen presenting function. Here, we show that tumor cells license IKDC to acquire the properties of antigen presenting cells, i.e., expression of MHC class II and costimulatory CD86 molecules. We show that the CD11b(+) subset of IKDC are able to prime naïve CD4(+) T cells and cross-prime naïve CD8(+) T lymphocytes. Licensing of IKDC by tumor cells was mandatory for the full differentiation of T cells into polarized effectors. IKDC could engulf and process soluble Ova protein in a CD206-dependent manner. Finally, we show that CD11b(+)IKDC is selectively endowed with CTLA4Ig-inhibitable antigen presenting capacities and that targeting this subset with the detoxified adenylate cyclase toxin of Bordetella pertussis fused to antigen resulted in efficient cross-presentation of antigen by IKDC to specific TCR transgenic CD8(+)T cells in vivo. Collectively, our data indicate that upon exposure to tumor cells, IKDC subserve DC-like functions.

Subversion of pulmonary dendritic cell function by paramyxovirus infections

Lower respiratory tract infections caused by the paramyxoviruses human metapneumovirus (hMPV) and respiratory syncytial virus (RSV) are characterized by short-lasting virus-specific immunity and often long-term airway morbidity, both of which may be the result of alterations in the Ag-presenting function of the lung which follow these infections. In this study, we investigated whether hMPV and RSV experimental infections alter the phenotype and function of dendritic cell (DC) subsets that are recruited to the lung. Characterization of lung DC trafficking demonstrated a differential recruitment of plasmacytoid DC (pDC), conventional DC (cDC), and IFN-producing killer DC to the lung and draining lymph nodes after hMPV and RSV infection. In vitro infection of lung DC indicated that in pDC, production of IFN-alpha, TNF-alpha, and CCL5 was induced only by hMPV, whereas CCL3 and CCL4 were induced by both viruses. In cDC, a similar repertoire of cytokines was induced by hMPV and RSV, except for IFN-beta, which was not induced by RSV. The function of lung pDC was altered following hMPV or RSV infection in vivo, as we demonstrated a reduced capacity of lung pDC to produce IFN-alpha as well as other cytokines including IL-6, TNF-alpha, CCL2, CCL3, and CCL4 in response to TLR9 stimulation. Moreover, we observed an impaired capacity of cDC from infected mice to present Ag to CD4(+) T cells, an effect that lasted beyond the acute phase of infection. Our findings suggest that acute paramyxovirus infections can alter the long-term immune function of pulmonary DC.

Introduction: the immune response against dying cells

The innate immune system is charged with the daunting task to discriminate harmless cell death events, as they occur in normal tissue homeostasis, and potentially harmful cell deaths, as they are elicited by infectious microorganisms or as a byproduct of malignant transformation. The distinction between harmful and harmless cell death relies on subtle biochemical differences that precede or accompany cell death and that act on a series of receptors, including pattern recognition receptors that are present on cells of the innate immune system, particularly dendritic cells. The present series of articles provides an up-to-date compendium on the molecular crosstalk between cell death and immune cells.