Activating Fc γ receptors contribute to the antitumor activities of immunoregulatory receptor-targeting antibodies

Principles of antibody-mediated TNF receptor activation

From the beginning of research on receptors of the tumor necrosis factor (TNF) receptor superfamily (TNFRSF), agonistic antibodies have been used to stimulate TNFRSF receptors in vitro and in vivo. Indeed, CD95, one of the first cloned TNFRSF receptors, was solely identified as the target of cell death-inducing antibodies. Early on, it became evident from in vitro studies that valency and Fcγ receptor (FcγR) binding of antibodies targeting TNFRSF receptors can be of crucial relevance for agonistic activity. TNFRSF receptor-specific antibodies of the IgM subclass and secondary cross-linked or aggregation prone dimeric antibodies typically display superior agonistic activity compared with dimeric antibodies. Likewise, anchoring of antibodies to cell surface-expressed FcγRs potentiate their ability to trigger TNFRSF receptor signaling. However, only recently has the relevance of oligomerization and FcγR binding for the in vivo activity of antibody-induced TNFRSF receptor activation been straightforwardly demonstrated in vivo. This review discusses the crucial role of oligomerization and/or FcγR binding for antibody-mediated TNFRSF receptor stimulation in light of current models of TNFRSF receptor activation and especially the overwhelming relevance of these issues for the rational development of therapeutic TNFRSF receptor-targeting antibodies.

Synergistic antitumor responses by combined GITR activation and sunitinib in metastatic renal cell carcinoma

Sunitinib, a multitargeted tyrosine kinase inhibitor, is the frontline therapy for renal and gastrointestinal cancers. In view of its well-documented proapoptotic and immunoadjuvant properties, we speculate that combination of Sunitinib and immunotherapy would provide a synergistic antitumor effect. Here, we report that a remarkably synergistic antitumor responses elicited by the combined treatment of Sunitinib and an agonistic antibody against glucocorticoid-induced TNFR related protein (GITR) in a model of metastatic renal cell carcinoma. Sunitinib significantly increased the infiltration, activation, and proliferation and/or cytotoxicity of CD8(+) T cells and NK cells in liver metastatic foci when combined with the anti (α)-GITR agonist, which was associated with treatment-induced prominent upregulation of Th1-biased immune genes in the livers from mice receiving combined therapy versus single treatment. Sunitinib/α-GITR treatment also markedly promoted the maturation, activation and cytokine production of liver-resident macrophages and DCs compared with that achieved by α-GITR or Sunitinib treatment alone in mice. Cell depletion experiments demonstrated that CD8(+) T cells, NK cells and macrophage infiltrating liver metastatic foci all contribute to the antitumor effect induced by combined treatment. Furthermore, mechanistic investigation revealed that Sunitinib treatment reprograms tumor-associated macrophages toward classically activated or "M1" polarization upon GITR stimulation and consequently mounts an antitumor CD8(+) T and NK cell response via inhibiting STAT3 activity. Thus, our findings provide a proof of concept that Sunitinib can synergize with α-GITR treatment to remodel the tumor immune microenvironment to trigger regressions of an established metastatic cancer.

Glucocorticoid-induced tumor necrosis factor receptor-related protein co-stimulation facilitates tumor regression by inducing IL-9-producing helper T cells

T cell stimulation via glucocorticoid-induced tumor necrosis factor receptor (TNFR)-related protein (GITR) elicits antitumor activity in various tumor models; however, the underlying mechanism of action remains unclear. Here we demonstrate a crucial role for interleukin (IL)-9 in antitumor immunity generated by the GITR agonistic antibody DTA-1. IL-4 receptor knockout (Il4ra(-/-)) mice, which have reduced expression of IL-9, were resistant to tumor growth inhibition by DTA-1. Notably, neutralization of IL-9 considerably impaired tumor rejection induced by DTA-1. In particular, DTA-1-induced IL-9 promoted tumor-specific cytotoxic T lymphocyte (CTL) responses by enhancing the function of dendritic cells in vivo. Furthermore, GITR signaling enhanced the differentiation of IL-9-producing CD4(+) T-helper (TH9) cells in a TNFR-associated factor 6 (TRAF6)- and NF-κB-dependent manner and inhibited the generation of induced regulatory T cells in vitro. Our findings demonstrate that GITR co-stimulation mediates antitumor immunity by promoting TH9 cell differentiation and enhancing CTL responses and thus provide a mechanism of action for GITR agonist-mediated cancer immunotherapies.

Co-Stimulatory Blockade of the CD28/CD80-86/CTLA-4 Balance in Transplantation: Impact on Memory T Cells?

CD28 and CTLA-4 are prototypal co-stimulatory and co-inhibitory cell surface signaling molecules interacting with CD80/86, known to be critical for immune response initiation and regulation, respectively. Initial “bench-to-beside” translation, two decades ago, resulted in the development of CTLA4-Ig, a biologic that targets CD80/86 and prevents T-cell costimulation. In spite of its proven effectiveness in inhibiting allo-immune responses, particularly in murine models, clinical experience in kidney transplantation with belatacept (high-affinity CTLA4-Ig molecule) reveals a high incidence of acute, cell-mediated rejection. Originally, the etiology of belatacept-resistant graft rejection was thought to be heterologous immunity, i.e., the cross-reactivity of the pool of memory T cells from pathogen-specific immune responses with alloantigens. Recently, the standard view that memory T cells arise from effector cells after clonal contraction has been challenged by a “developmental” model, in which less differentiated memory T cells generate effector cells. This review delineates how this shift in paradigm, given the differences in co-stimulatory and co-inhibitory signal depending on the maturation stage, could profoundly affect our understanding of the CD28/CD80-86/CTLA-4 blockade and highlights the potential advantages of selectively targeting CD28, instead of CD80/86, to control post-transplant immune responses.

Evolving synergistic combinations of targeted immunotherapies to combat cancer

Immunotherapy has now been clinically validated as an effective treatment for many cancers. There is tremendous potential for synergistic combinations of immunotherapy agents and for combining immunotherapy agents with conventional cancer treatments. Clinical trials combining blockade of cytotoxic T lymphocyte-associated antigen 4 (CTLA4) and programmed cell death protein 1 (PD1) may serve as a paradigm to guide future approaches to immuno-oncology combination therapy. In this Review, we discuss progress in the synergistic design of immune-targeting combination therapies and highlight the challenges involved in tailoring such strategies to provide maximal benefit to patients.

A Threshold Level of Intratumor CD8+ T-cell PD1 Expression Dictates Therapeutic Response to Anti-PD1

Despite successes, thus far, a significant proportion of the patients treated with anti-PD1 antibodies have failed to respond. We use mouse tumor models of anti-PD1 sensitivity and resistance and flow cytometry to assess tumor-infiltrating immune cells immediately after therapy. We demonstrate that the expression levels of T-cell PD1 (PD1(lo)), myeloid, and T-cell PDL1 (PDL1(hi)) in the tumor microenvironment inversely correlate and dictate the efficacy of anti-PD1 mAb and function of intratumor CD8(+) T cells. In sensitive tumors, we reveal a threshold for PD1 downregulation on tumor-infiltrating CD8(+) T cells below which the release of adaptive immune resistance is achieved. In contrast, PD1(hi) T cells in resistant tumors fail to be rescued by anti-PD1 therapy and remain dysfunctional unless intratumor PDL1(lo) immune cells are targeted. Intratumor Tregs are partly responsible for the development of anti-PD1-resistant tumors and PD1(hi) CD8(+) T cells. Our analyses provide a framework to interrogate intratumor CD8(+) T-cell PD1 and immune PDL1 levels and response in human cancer.

Immunotherapy of Ovarian Cancer: The Role of Checkpoint Inhibitors

Ovarian cancer is the most important cause of gynecological cancer-related mortality, with the majority of women presenting with advanced disease. Although surgery and chemotherapy can improve survival rates, it is necessary to integrate alternative strategies to improve the outcomes. Advances in understanding the role of immune system in the pathogenesis of cancer have led to the rapid evolvement of immunotherapy, which might establish a sustained immune system response against recurring cancer cells. Recently, it has emerged that powerful immunologic effector cells may be blocked by inhibitory regulatory pathways controlled by specific molecules often called “immune checkpoints,” which turn off the immune system. Similarly, cancer cells are able to use these checkpoints to avoid immune control and rejection. Inhibition of these inhibitory pathways represents a potent strategy in the fight against cancer and is currently under investigation with encouraging results in some cancers, such as melanoma. In ovarian cancer researches are still in an early phase, but with promising results. In this review we will explore the rationale of immunotherapy in ovarian cancer with a special focus on these emerging molecules.

Control of CD8 T-Cell Infiltration into Tumors by Vasculature and Microenvironment

CD8 T-cells are a critical brake on the initial development of tumors. In established tumors, the presence of CD8 T-cells is correlated with a positive patient prognosis, although immunosuppressive mechanisms limit their effectiveness and they are rarely curative without manipulation. Cancer immunotherapies aim to shift the balance back to dominant antitumor immunity through antibody blockade of immunosuppressive signaling pathways, vaccination, and adoptive transfer of activated or engineered T-cells. These approaches have yielded striking responses in small subsets of patients with solid tumors, most notably those with melanoma. Importantly, the subset of patients who respond to vaccination or immunosuppression blockade therapies are those with CD8 T-cells present in the tumor prior to initiating therapy. While current adoptive cell therapy approaches can be dramatically effective, they require infusion of extremely large numbers of T-cells, but the number that actually infiltrates the tumor is very small. Thus, poor representation of CD8 T-cells in tumors is a fundamental hurdle to successful immunotherapy, over and above the well-established barrier of immunosuppression. In this review, we discuss the factors that determine whether immune cells are present in tumors, with a focus on the representation of cytotoxic CD8 T-cells. We emphasize the critically important role of tumor-associated vasculature as a gateway that enables the active infiltration of both effector and naïve CD8 T-cells that exert antitumor activity. We also discuss strategies to enhance the gateway function and extend the effectiveness of immunotherapies to a broader set of cancer patients.

CD28 co-stimulation in T-cell homeostasis: a recent perspective

T-cells play a key role within the adaptive immune system mediating cellular immunity and orchestrating the immune response as a whole. Their activation requires not only recognition of antigen/major histocompatibility complexes by the T-cell receptor but in addition co-stimulation via the CD28 molecule through binding to CD80, CD86, or as recently discovered, inducible co-stimulator ligand expressed by antigen-presenting cells. Apart from tight control of the co-stimulatory signal by the T-cell receptor complex, expression of the inhibitory receptor cytotoxic T-lymphocyte antigen-4 (CTLA-4) sharing its ligands with CD28 is required to avoid inappropriate or prolonged T-cell activation. CD4(+) Foxp3(+) regulatory T (Treg) cells, which are crucial inhibitors of autoimmunity, add another level of complexity in that they differ from conventional non-regulatory CD4(+) T-cells by strongly depending on CD28 signaling for their generation and homeostasis. Moreover, CTLA-4 is constitutively expressed by Treg cells where it serves as a key mediator of suppression, while conventional CD4(+) T-cells express CTLA-4 only after activation. Here, we discuss recent insights into the molecular events underlying CD28-mediated co-stimulation, its impact on gene regulation, and the differential role of CD28 expression on Treg cells versus conventional CD4(+) and CD8(+) T-cells. Moreover, we summarize the exciting therapeutic options which have arisen from our current understanding of T-cell co-stimulation. Some of these have already been translated into the clinic, while others are expected to follow soon due to promising preclinical results. In particular, we discuss the failed 2006 trial of the CD28 superagonist TGN1412, and the return of this potent T-cell activator to clinical development.

Brucella spp. Lumazine Synthase Induces a TLR4-Mediated Protective Response against B16 Melanoma in Mice

Brucella Lumazine Synthase (BLS) is a highly immunogenic decameric protein which can accept the fusion of foreign proteins at its ten N-termini. These chimeras are very efficient to elicit systemic and oral immunity without adjuvants. BLS signaling via Toll-Like Receptor 4 (TLR4) regulates innate and adaptive immune responses, inducing dendritic cell maturation and CD8+ T-cell cytotoxicity. In this work we study the effect induced by BLS in TLR4-expressing B16 melanoma. In order to evaluate the effectiveness of BLS as a preventive vaccine, C57BL/6J mice were immunized with BLS or BLS-OVA, and 35 days later were subcutaneously inoculated with B16-OVA melanoma. BLS or BLS-OVA induced a significant inhibition of tumor growth, and 50% of mice immunized with the highest dose of BLS did not develop visible tumors. This effect was not observed in TLR4-deficient mice. For treatment experiments, mice were injected with BLS or BLS-OVA 2 days after the inoculation of B16 cells. Both treatments induced significant and equal tumor growth delay and increased survival. Moreover, BLS and BLS-OVA stimulation were also effective in TLR4-deficient mice. In order to study whether BLS has a direct effect on tumor cells, B16 cells were preincubated with BLS, and after 48h, cells were inoculated. Tumors induced by BLS-stimulated cells had inhibited growth and survival was increased. In the BLS group, 40% of mice did not develop tumors. This effect was abolished by the addition of TLR4/MD2 blocking antibody to cells before BLS stimulation. Our work demonstrates that BLS immunization induces a preventive antitumor response that depends on mice TLR4. We also show that BLS generates a therapeutic effect in mice inoculated with B16 cells. Our results show that BLS acts directly in cultured tumor cells via TLR4, highly suggesting that BLS elicits its therapeutic effects acting on the TLR4 from B16 melanoma cells.

Ipilimumab-dependent cell-mediated cytotoxicity of regulatory T cells ex vivo by nonclassical monocytes in melanoma patients

Enhancing immune responses with immune-modulatory monoclonal antibodies directed to inhibitory immune receptors is a promising modality in cancer therapy. Clinical efficacy has been demonstrated with antibodies blocking inhibitory immune checkpoints such as cytotoxic T lymphocyte-associated antigen 4 (CTLA-4) or PD-1/PD-L1. Treatment with ipilimumab, a fully human CTLA-4-specific mAb, showed durable clinical efficacy in metastatic melanoma; its mechanism of action is, however, only partially understood. This is a study of 29 patients with advanced cutaneous melanoma treated with ipilimumab. We analyzed peripheral blood mononuclear cells (PBMCs) and matched melanoma metastases from 15 patients responding and 14 not responding to ipilimumab by multicolor flow cytometry, antibody-dependent cell-mediated cytotoxicity (ADCC) assay, and immunohistochemistry. PBMCs and matched tumor biopsies were collected 24 h before (i.e., baseline) and up to 4 wk after ipilimumab. Our findings show, to our knowledge for the first time, that ipilimumab can engage ex vivo FcγRIIIA (CD16)-expressing, nonclassical monocytes resulting in ADCC-mediated lysis of regulatory T cells (Tregs). In contrast, classical CD14(++)CD16(-) monocytes are unable to do so. Moreover, we show that patients responding to ipilimumab display significantly higher baseline peripheral frequencies of nonclassical monocytes compared with nonresponder patients. In the tumor microenvironment, responders have higher CD68(+)/CD163(+) macrophage ratios at baseline and show decreased Treg infiltration after treatment. Together, our results suggest that anti-CTLA-4 therapy may target Tregs in vivo. Larger translational studies are, however, warranted to substantiate this mechanism of action of ipilimumab in patients.

Cytotoxic T lymphocyte antigen 4 expression in human breast cancer: implications for prognosis

To examine the relationship between cytotoxic T lymphocyte antigen 4 (CTLA-4) expression and breast cancer prognosis, CTLA-4 expression was immunohistochemically detected in paraffin-embedded specimens of primary tumors from 130 patients with breast cancer who had a mean follow-up period of 112 months. CTLA-4 expressed in cytoplasm of breast cancer cells and in cytoplasm and cell membranes of interstitial lymphocytes. Univariate analysis (log-rank) associated higher density of interstitial CTLA-4(+) lymphocytes with longer DFS and OS, but higher tumor CTLA-4 expression with shorter OS. After controlling for age, clinical stage, Scarff-Bloom-Richardson grade, tumor thrombus, ER, PR, HER2 and Ki-67, multivariate analysis (Cox) showed that density of interstitial CTLA-4(+) lymphocytes independently predicted longer DFS (HR 0.315, P = 0.002) and OS (HR 0.313, P = 0.005), whereas tumor CTLA-4 expression independently predicted shorter DFS (HR 2.176, P = 0.029) and OS (HR 2.820, P = 0.007), i.e., patients with high CTLA-4(+) lymphocyte density and CTLA-4(low) tumor cells had the best prognoses. These results indicated that CTLA-4 expression in lymphocytes was associated with better prognosis, but that in tumor cells was associated with worse prognosis. Patients' CTLA-4 profiles might thus be used to predict the benefits and toxicity of CTLA-4 blockade.

Glucocorticoid-induced tumour necrosis factor receptor-related protein: a key marker of functional regulatory T cells

Glucocorticoid-induced tumour necrosis factor receptor-related protein (GITR, TNFRSF18, and CD357) is expressed at high levels in activated T cells and regulatory T cells (Tregs). In this review, we present data from mouse and human studies suggesting that GITR is a crucial player in the differentiation of thymic Tregs (tTregs), and expansion of both tTregs and peripheral Tregs (pTregs). The role of GITR in Treg expansion is confirmed by the association of GITR expression with markers of memory T cells. In this context, it is not surprising that GITR appears to be a marker of active Tregs, as suggested by the association of GITR expression with other markers of Treg activation or cytokines with suppressive activity (e.g., IL-10 and TGF-β), the presence of GITR(+) cells in tissues where Tregs are active (e.g., solid tumours), or functional studies on Tregs. Furthermore, some Treg subsets including Tr1 cells express either low or no classical Treg markers (e.g., FoxP3 and CD25) and do express GITR. Therefore, when evaluating changes in the number of Tregs in human diseases, GITR expression must be evaluated. Moreover, GITR should be considered as a marker for isolating Tregs.

CTLA-4⁺ Regulatory T Cells Increased in Cetuximab-Treated Head and Neck Cancer Patients Suppress NK Cell Cytotoxicity and Correlate with Poor Prognosis

The EGFR-targeted antibody cetuximab is effective against head and neck cancer (HNSCC), but in only 15% to 20% of patients, and the variability and extent of cetuximab-mediated cellular immunity is not fully understood. We hypothesized that regulatory T cells (Treg) may exert a functional and clinical impact on antitumor immunity in cetuximab-treated individuals. The frequency, immunosuppressive phenotype, and activation status of Treg and natural killer (NK) cells were analyzed in the circulation and tumor microenvironment of cetuximab-treated patients with HNSCC enrolled in a novel neoadjuvant, single-agent cetuximab clinical trial. Notably, cetuximab treatment increased the frequency of CD4(+)FOXP3(+) intratumoral Treg expressing CTLA-4, CD39, and TGFβ. These Treg suppressed cetuximab-mediated antibody-dependent cellular cytotoxicity (ADCC) and their presence correlated with poor clinical outcome in two prospective clinical trial cohorts. Cetuximab expanded CTLA-4(+)FOXP3(+) Treg in vitro, in part, by inducing dendritic cell maturation, in combination with TGFβ and T-cell receptor triggering. Importantly, cetuximab-activated NK cells selectively eliminated intratumoral Treg but preserved effector T cells. In ex vivo assays, ipilimumab targeted CTLA-4(+) Treg and restored cytolytic functions of NK cells mediating ADCC. Taken together, our results argue that differences in Treg-mediated suppression contribute to the clinical response to cetuximab treatment, suggesting its improvement by adding ipilimumab or other strategies of Treg ablation to promote antitumor immunity.

Haematological malignancies: at the forefront of immunotherapeutic innovation

The recent successes of cancer immunotherapies have stimulated interest in the potential widespread application of these approaches; haematological malignancies have provided both initial proofs of concept and an informative testing ground for various immune-based therapeutics. The immune-cell origin of many of the blood malignancies provides a unique opportunity both to understand the mechanisms of cancer immune responsiveness and immune evasion, and to exploit these mechanisms for therapeutic purposes.

Immunotherapy in Cancer: A Combat between Tumors and the Immune System; You Win Some, You Lose Some

Cancer immunotherapy has emerged as a treatment modality, mainly as the result of discoveries in the immune response regulation, including mechanisms that turn off immune responses. Immunogenic cutaneous melanoma is a canonical model for therapeutic immunotherapy studies. “Passive” immunotherapy with monoclonal antibodies (mAbs) has outpaced “active” immunotherapy with anti-tumor vaccines, and mAbs that antagonize the off responses have been recently introduced in clinical practice. Despite these recent successes, many unresolved practical and theoretical questions remain. Notably unknown are the identity of the lymphocytes that eliminate tumor cells, which white cells enter into tumors, through which endothelium, in what order, and how they perform their task. The parameters of size and location that could be used to determine in which tumors the immune response may be sufficient to eradicate the tumor are yet unknown. Immunotherapy has been so far more efficient to treat solid and hematologic tumors located outside the central nervous system, than primary brain tumors and brain metastases. In contrast to recent advances with mAbs, anti-tumor vaccine development has been lagging behind. The multiplicity of antigens that must be targeted to achieve significant clinical response is partially responsible for this lag, especially in melanoma, one of the most mutated tumors. Further hampering vaccination results is the fact that tumor elimination by the immune system is the result of a race between tumors with different growth rates and the relatively slow development of the adaptive immune response. The enhancement of the native arm of the immune response or the administration of targeted chemotherapy to slow tumor development, are approaches that should be studied. Finally, criteria used to analyze patient response to immunotherapeutic treatments must be perfected, and the patient populations that could benefit the most from this approach must be better defined.

Immune checkpoint protein inhibition for cancer: preclinical justification for CTLA-4 and PD-1 blockade and new combinations

Over the last two decades, our understanding of the molecular basis of immunity has revealed the complexity of regulatory pathways involved in immune responses to cancer. A significant body of data support the critical importance of immune checkpoints in the control of the adaptive immune response to malignancy, and suggest that inhibitors of those checkpoints might have significant utility in treating cancer. This has been borne out by the recent US Food and Drug Administration (FDA) approvals of two different antibodies, one against cytotoxic T-lymphocyte antigen-4 (CTLA-4) and one against programmed death-1 (PD-1). Here, we provide a comprehensive review of the literature regarding the preclinical justification for the use of CTLA-4 and PD-1 blockade as monotherapy, and as combination therapy in the treatment of cancer. The animal data strongly supported the use of these drugs in patients, and in many cases suggested strategies that directly led to successful registration trials. In contrast, many of the toxicities, and some of the unusual response patterns seen in patients with these drugs, were not predicted by the preclinical work that we cite, highlighting the importance of early-phase trials with patients to inform future drug development. In addition, we review herein the preclinical data surrounding emerging immune checkpoint proteins, including BTLA, VISTA, CD160, LAG3, TIM3, and CD244 as potential targets for inhibition. The current comprehensive review of the literature regarding CTLA-4 and PD-1, as well as a number of novel checkpoint proteins demonstrates a strong preclinical basis for the use of these antibodies singly and in combination to overcome checkpoint inhibition in the treatment of cancer. We also suggest that the use of these antibodies may augment the efficacy of other activating immune antibodies, cytokines, radiation, and adoptive cell therapy in human cancer.

Gene expression of mesothelioma in vinylidene chloride-exposed F344/N rats reveal immune dysfunction, tissue damage, and inflammation pathways

A majority (∼80%) of human malignant mesotheliomas are asbestos-related. However, non-asbestos risk factors (radiation, chemicals, and genetic factors) account for up to 30% of cases. A recent 2-year National Toxicology Program carcinogenicity bioassay showed that male F344/N rats exposed to the industrial toxicant vinylidene chloride (VDC) resulted in a marked increase in malignant mesothelioma. Global gene expression profiles of these tumors were compared to spontaneous mesotheliomas and the F344/N rat mesothelial cell line (Fred-PE) in order to characterize the molecular features and chemical-specific profiles of mesothelioma in VDC-exposed rats. As expected, mesotheliomas from control and VDC-exposed rats shared pathways associated with tumorigenesis, including cellular and tissue development, organismal injury, embryonic development, inflammatory response, cell cycle regulation, and cellular growth and proliferation, while mesotheliomas from VDC-exposed rats alone showed overrepresentation of pathways associated with pro-inflammatory pathways and immune dysfunction such as the nuclear factor kappa-light-chain-enhancer of activated B cells signaling pathway, interleukin (IL)-8 and IL-12 signaling, interleukin responses, Fc receptor signaling, and natural killer and dendritic cells signaling, as well as overrepresentation of DNA damage and repair. These data suggest that a chronic, pro-inflammatory environment associated with VDC exposure may exacerbate disturbances in oncogene, growth factor, and cell cycle regulation, resulting in an increased incidence of mesothelioma.

Conformation of the human immunoglobulin G2 hinge imparts superagonistic properties to immunostimulatory anticancer antibodies

Monoclonal antibody (mAb) drugs that stimulate antitumor immunity are transforming cancer treatment but require optimization for maximum clinical impact. Here, we show that, unlike other immunoglobulin isotypes, human IgG2 (h2) imparts FcγR-independent agonistic activity to immune-stimulatory mAbs such as anti-CD40, -4-1BB, and -CD28. Activity is provided by a subfraction of h2, h2B, that is structurally constrained due its unique arrangement of hinge region disulfide bonds. Agonistic activity can be transferred from h2 to h1 by swapping their hinge and CH1 domains, and substitution of key hinge and CH1 cysteines generates homogenous h2 variants with distinct agonistic properties. This provides the exciting opportunity to engineer clinical reagents with defined therapeutic activity regardless of FcγR expression levels in the local microenvironment.

Improved survival with T cell clonotype stability after anti-CTLA-4 treatment in cancer patients

Cytotoxic T lymphocyte-associated antigen-4 (CTLA-4) blockade can promote antitumor T cell immunity and clinical responses. The mechanism by which anti-CTLA-4 antibodies induces antitumor responses is controversial. To determine the effects of CTLA-4 blockade on the T cell repertoire, we used next-generation deep sequencing to measure the frequency of individual rearranged T cell receptor β (TCRβ) genes, thereby characterizing the diversity of rearrangements, known as T cell clonotypes. CTLA-4 blockade in patients with metastatic castration-resistant prostate cancer and metastatic melanoma resulted in both expansion and loss of T cell clonotypes, consistent with a global turnover of the T cell repertoire. Overall, this treatment increased TCR diversity as reflected in the number of unique TCR clonotypes. The repertoire of clonotypes continued to evolve over subsequent months of treatment. Whereas the number of clonotypes that increased with treatment was not associated with clinical outcome, improved overall survival was associated with maintenance of high-frequency clones at baseline. In contrast, the highest-frequency clonotypes fell with treatment in patients with short overall survival. Stably maintained clonotypes included T cells having high-avidity TCR such as virus-reactive T cells. Together, these results suggest that CTLA-4 blockade induces T cell repertoire evolution and diversification. Moreover, improved clinical outcomes are associated with less clonotype loss, consistent with the maintenance of high-frequency TCR clonotypes during treatment. These clones may represent the presence of preexisting high-avidity T cells that may be relevant in the antitumor response.

Confusing signals: recent progress in CTLA-4 biology

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Cell extrinsic and cell intrinsic mechanisms of action of CTLA-4 are unclear.

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Data suggest that the extracellular domain of CTLA4 elicits regulatory function.

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The function of CTLA-4 tail may lie in regulating localization rather than signaling.

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Membrane levels of CTLA-4 directly impact access of CD28 to shared ligands.

The mechanism of action of cytotoxic T-lymphocyte-associated protein 4 (CTLA-4) remains surprisingly unclear. Regulatory T (Treg) cells can use CTLA-4 to elicit suppression; however, CTLA-4 also operates in conventional T cells, reputedly by triggering inhibitory signals. Recently, interactions mediated via the CTLA-4 cytoplasmic domain have been shown to preferentially affect Treg cells, yet other evidence suggests that the extracellular domain of CTLA-4 is sufficient to elicit suppression. Here, we discuss these paradoxical findings in the context of CTLA-4-mediated ligand regulation. We propose that the function of CTLA-4 cytoplasmic domain is not to transmit inhibitory signals but to precisely control the turnover, cellular location, and membrane delivery of CTLA-4 to facilitate its central function: regulating the access of CD28 to their shared ligands.

Induced expression of FcγRIIIa (CD16a) on CD4+ T cells triggers generation of IFN-γhigh subset

Whether or not CD4(+) T-cells express low affinity receptor FcγRIIIa (CD16a) in disease pathology has not been examined in great detail. In this study, we show that a subset of activated CD4(+) T-cells in humans express FcγRIIIa. The ligation of FcγRIIIa by immune complexes (ICs) in human CD4(+) T-cells produced co-stimulatory signal like CD28 that triggered IFN-γ production. The induced expression of FcγRIIIa on CD4(+) helper T-cells is an important finding since these receptors via ITAM contribute to intracellular signaling. The induced expression of FcγRIIIa on CD4(+) T helper cells and their ability to co-stimulate T-cell activation are important and novel findings that may reveal new pathways to regulate adaptive immune responses during inflammation and in autoimmunity.

Selective cell elimination in vitro and in vivo from tissues and tumors using antibodies conjugated with a near infrared phthalocyanine

Cell cultures and tissues often contain cellular subpopulations that potentially interfere with or contaminate other cells of interest. However, it is difficult to eliminate unwanted cells without damaging the very cell population one is seeking to protect, especially established tissue. Here, we report a method of eliminating a specific subpopulation of cells from a mixed 2D or 3D cell culture in vitro and a mixed-population in vivo tumor model by using antibody-photosensitizer conjugates (APC) with a near infrared (NIR) phthalocyanine-derivative (IRdye700DX, IR700) combined with NIR light exposure with minimal damage to non-targeted cells. Thus, APC combined with NIR light exposure holds promise as a method of removing specific cells from mixed cell cultures and tumors.

The New Era of Cancer Immunotherapy: Manipulating T-Cell Activity to Overcome Malignancy

Using the immune system to control cancer has been investigated for over a century. Yet it is only over the last several years that therapeutic agents acting directly on the immune system have demonstrated improved overall survival for cancer patients in phase III clinical trials. Furthermore, it appears that some patients treated with such agents have been cured of metastatic cancer. This has led to increased interest and acceleration in the rate of progress in cancer immunotherapy. Most of the current immunotherapeutic success in cancer treatment is based on the use of immune-modulating antibodies targeting critical checkpoints (CTLA-4 and PD-1/PD-L1). Several other immune-modulating molecules targeting inhibitory or stimulatory pathways are being developed. The combined use of these medicines is the subject of intense investigation and holds important promise. Combination regimens include those that incorporate targeted therapies that act on growth signaling pathways, as well as standard chemotherapy and radiation therapy. In fact, these standard therapies have intrinsic immune-modulating properties that can support antitumor immunity. In the years ahead, adoptive T-cell therapy will also be an important part of treatment for some cancer patients. Other areas which are regaining interest are the use of oncolytic viruses that immunize patients against their own tumors and the use of vaccines against tumor antigens. Immunotherapy has demonstrated unprecedented durability in controlling multiple types of cancer and we expect its use to continue expanding rapidly.

Cutting edge: epigenetic regulation of Foxp3 defines a stable population of CD4+ regulatory T cells in tumors from mice and humans

CD4(+) regulatory T cells (Tregs) are critical for maintaining self-tolerance and function to prevent autoimmune disease. High densities of intratumoral Tregs are generally associated with poor patient prognosis, a correlation attributed to their broad immune-suppressive features. Two major populations of Tregs have been defined, thymically derived natural Tregs (nTregs) and peripherally induced Tregs (iTregs). However, the relative contribution of nTregs versus iTregs to the intratumoral Treg compartment remains controversial. Demarcating the proportion of nTregs versus iTregs has important implications in the design of therapeutic strategies to overcome their antagonistic effects on antitumor immune responses. We used epigenetic, phenotypic, and functional parameters to evaluate the composition of nTregs versus iTregs isolated from mouse tumor models and primary human tumors. Our findings failed to find evidence for extensive intratumoral iTreg induction. Rather, we identified a population of Foxp3-stable nTregs in tumors from mice and humans.

Tim-3: an emerging target in the cancer immunotherapy landscape

The cancer immunotherapy field has grown exponentially in the past few years, largely driven by the success of immune checkpoint blockade. Therapies targeting the immune checkpoint molecules CTLA-4 and PD-1 have achieved objective responses in melanoma, renal cancer, and lung cancer; however, a large number of patients are still suffering with these cancers that are not benefiting from these therapies. Moreover, several cancers have proved to be largely refractory to therapies that target CTLA-4 and PD-1. This has catalyzed interest in targeting novel immune checkpoint receptors with the goal of realizing the full potential of checkpoint blockade for treating cancer. In this regard, the immune checkpoint receptor Tim-3 exhibits several unique features that make it an intriguing candidate for the next wave of therapies that target immune checkpoints in cancer.

Role of crosslinking for agonistic CD40 monoclonal antibodies as immune therapy of cancer

Agonists of the TNF superfamily of receptors hold promise as novel therapy for cancer. Recent data on agonistic anti-murine TNF receptors (TNFR) such as CD40 suggest that the specific engagement of Fc-receptor (FcR) is required for optimal antitumor effects, prompting calls to engineer anti-human CD40 and other TNFR mAb accordingly. CP-870,893 is a fully human anti-CD40 mAb, selected in part because it is an IgG2 which is presumed to have poor reactivity with FcR; however, CP-870,893 has been evaluated in multiple clinical trials with beneficial activity in patients with melanoma, pancreatic and other cancers. Here, we confirmed that the activity of anti-murine CD40 mAb was dependent on FcγRIIB engagement, was decreased significantly in FcγRIIB (-/-) mice, and upon Fc-crosslinking anti-mouse CD40 mAb enhanced the activation of antigen presenting cells. In contrast, the CP-870,893-mediated activation of human B cells was not enhanced with anti-IgG-crosslinking nor abrogated when used as an F(ab)'2 reagent. Crosslinking of CP-870,893 using the CD32-expressing K562 cells yielded an Fc-dependent modest increase in the expression of some activation markers relative to that of the soluble CP-870,893 mAb. Classic Fc-dependent functions such as antibody-dependent cellular cytotoxicity (ADCC) and complement-mediated cytotoxicity (CMC) were minimal for CP-870,893 as compared to the IgG1 anti-CD20 mAb rituximab, which mediated both ADCC and CMC in parallel assays. Anti-mouse CD40 mAb competed for the CD40 ligand binding site, but CP-870,893 did not. Thus, Fc-crosslinking is not an essential requirement for agonistic anti-human CD40 mAb, whose potency is more dependent on the CD40 epitope recognized and the strength of the signal achieved.

"In vitro" 3D models of tumor-immune system interaction

Interaction between cancer cells and immune system critically affects development, progression and treatment of human malignancies. Experimental animal models and conventional "in vitro" studies have provided a wealth of information on this interaction, currently used to develop immune-mediated therapies. Studies utilizing three-dimensional culture technologies have emphasized that tumor architecture dramatically influences cancer cell-immune system interaction by steering cytokine production and regulating differentiation patterns of myeloid cells, and decreasing the sensitivity of tumor cells to lymphocyte effector functions. Hypoxia and increased production of lactic acid by tumor cells cultured in 3D architectures appear to be mechanistically involved. 3D culture systems could be further developed to (i) include additional cell partners potentially influencing cancer cell-immune system interaction, (ii) enable improved control of hypoxia, and (iii) allow the use of freshly derived clinical cancer specimens. Such advanced models will represent new tools for cancer immunobiology studies and for pre-clinical assessment of innovative treatments.

The immunoreceptor TIGIT regulates antitumor and antiviral CD8(+) T cell effector function

Tumors constitute highly suppressive microenvironments in which infiltrating T cells are "exhausted" by inhibitory receptors such as PD-1. Here we identify TIGIT as a coinhibitory receptor that critically limits antitumor and other CD8(+) T cell-dependent chronic immune responses. TIGIT is highly expressed on human and murine tumor-infiltrating T cells, and, in models of both cancer and chronic viral infection, antibody coblockade of TIGIT and PD-L1 synergistically and specifically enhanced CD8(+) T cell effector function, resulting in significant tumor and viral clearance, respectively. This effect was abrogated by blockade of TIGIT's complementary costimulatory receptor, CD226, whose dimerization is disrupted upon direct interaction with TIGIT in cis. These results define a key role for TIGIT in inhibiting chronic CD8(+) T cell-dependent responses.

The development of immunomodulatory monoclonal antibodies as a new therapeutic modality for cancer: the Bristol-Myers Squibb experience

The discovery and increased understanding of the complex interactions regulating the immune system have contributed to the pharmacologic activation of antitumor immunity. The activity of effector cells, such as T and NK cells, is regulated by an array of activating and attenuating receptors and ligands. Agents that target these molecules can modulate immune responses by exerting antagonistic or agonistic effects. Several T- or NK-cell modulators have entered clinical trials, and two have been approved for use. Ipilimumab (Yervoy®, Bristol-Myers Squibb) and nivolumab (OPDIVO, Ono Pharmaceutical Co., Ltd./Bristol-Myers Squibb) were approved for the treatment of metastatic melanoma, in March 2011 in the United States, and in July 2014 in Japan, respectively. The clinical activity of these two antibodies has not been limited to tumor types considered sensitive to immunotherapy, and promising activity has been reported in other solid and hematologic tumors. Clinical development of ipilimumab and nivolumab has presented unique challenges in terms of safety and efficacy, requiring the establishment of new evaluation criteria for adverse events and antitumor effects. Guidelines intended to help oncologists properly manage treatment in view of these non-traditional features have been implemented. The introduction of this new modality of cancer treatment, which is meant to integrate with or replace the current standards of care, requires additional efforts in terms of optimization of treatment administration, identification of biomarkers and application of new clinical trial designs. The availability of immune modulators with different mechanisms of action offers the opportunity to establish immunological combinations as new standards of care.

Intratumoral immunization: a new paradigm for cancer therapy

Immune cell infiltration in the tumor microenvironment is of prognostic and therapeutic import. These immune cell subsets can be heterogeneous and are composed of mature antigen-presenting cells, helper and effector cytotoxic T cells, toleragenic dendritic cells, tumor-associated macrophages, and regulatory T cells, among other cell types. With the development of novel drugs that target the immune system rather than the cancer cells, the tumor immune microenvironment is not only prognostic for overall patient outcome, but also predictive for likelihood of response to these immune-targeted therapies. Such therapies aim to reverse the cancer immunotolerance and trigger an effective antitumor immune response. Two major families of immunostimulatory drugs are currently in clinical development: pattern recognition receptor agonists (PRRago) and immunostimulatory monoclonal antibodies (ISmAb). Despite their immune-targeted design, these agents have so far been developed clinically as if they were typical anticancer drugs. Here, we review the limitations of this conventional approach, specifically addressing the shortcomings of the usual schedules of intravenous infusions every 2 or 3 weeks. If the new modalities of immunotherapy target specific immune cells within the tumor microenvironment, it might be preferable to deliver them locally into the tumor rather than systemically. There is preclinical and clinical evidence that a therapeutic systemic antitumor immune response can be generated upon intratumoral immunomodulation. Moreover, preclinical results have shown that therapeutic synergy can be obtained by combining PRRagos and ISmAbs to the local tumor site.

Too much of a good thing? Tim-3 and TCR signaling in T cell exhaustion

T cell exhaustion is thought to be a natural mechanism for limiting immune pathology, although it may be desirable to circumvent this mechanism to help eliminate viral reservoirs or tumors. Although there are no definitive markers, a fingerprint for exhausted T cells has been described that includes the transmembrane proteins PD-1, LAG3, and Tim-3. However, apart from the recruitment of tyrosine phosphatases to PD-1, little is known about the biochemical mechanisms by which these proteins contribute to the development or maintenance of exhaustion. Tim-3 contains no known motifs for the recruitment of inhibitory phosphatases, but it may actually increase signaling downstream of TCR/CD3, at least under acute conditions. Other studies showed that T cell exhaustion results from chronic stimulation that extends the effector phase of T cell activation, at the expense of T cell memory. We suggest that Tim-3 may contribute to T cell exhaustion by enhancing TCR-signaling pathways.

GITR+ regulatory T cells in the treatment of autoimmune diseases

Autoimmune diseases decrease life expectancy and quality of life for millions of women and men. Although treatments can slow disease progression and improve quality of life, all currently available drugs have adverse effects and none of them are curative; therefore, requiring patients to take immunosuppressive drugs for the remainder of their lives. A curative therapy that is safe and effective is urgently needed. We believe that therapies promoting the in vivo expansion of regulatory T cells (Tregs) or injection of in vitro expanded autologous/heterologous Tregs (cellular therapy) can alter the natural history of autoimmune diseases. In this review, we present data from murine and human studies suggesting that 1) glucocorticoid-induced tumor necrosis factor receptor-related protein (GITR) plays a crucial role in thymic Treg (tTreg) differentiation and expansion; 2) GITR plays a crucial role in peripheral Treg (pTreg) expansion; 3) in patients with Sjögren syndrome and systemic lupus erythematosus, CD4(+)GITR(+) pTregs are expanded in patients with milder forms of the disease; and 4) GITR is superior to other cell surface markers to differentiate Tregs from other CD4(+) T cells. In this context, we consider two potential new approaches for treating autoimmune diseases consisting of the in vivo expansion of GITR(+) Tregs by GITR-triggering drugs and in vitro expansion of autologous or heterologous GITR(+) Tregs to be infused in patients. Advantages of such an approach, technical problems, and safety issues are discussed.

Type I and type II Fc receptors regulate innate and adaptive immunity

Antibodies produced in response to a foreign antigen are characterized by polyclonality, not only in the diverse epitopes to which their variable domains bind but also in the various effector molecules to which their constant regions (Fc domains) engage. Thus, the antibody's Fc domain mediates diverse effector activities by engaging two distinct classes of Fc receptors (type I and type II) on the basis of the two dominant conformational states that the Fc domain may adopt. These conformational states are regulated by the differences among antibody subclasses in their amino acid sequence and by the complex, biantennary Fc-associated N-linked glycan. Here we discuss the diverse downstream proinflammatory, anti-inflammatory and immunomodulatory consequences of the engagement of type I and type II Fc receptors in the context of infectious, autoimmune, and neoplastic disorders.

The path to reactivation of antitumor immunity and checkpoint immunotherapy

Cancer immunology has recently made major therapeutic inroads that represent clinical application of basic insights into mechanisms that govern immunity against tumors. Research into fundamental elements of T-cell and natural killer-cell biology, including the basis of antigen recognition, activation, proliferation, and survival, has informed the design of new therapeutic approaches to augment the body's natural anticancer immune response. Here, we describe some of the key steps that have provided the foundation for current strategies of immunotherapy.

The perspective of immunotherapy: new molecules and new mechanisms of action in immune modulation

PURPOSE OF REVIEW

Targeting CTLA-4, the patriarch of immune checkpoint modulators, is currently the only immunotherapeutic approach that has achieved significant clinical benefit in melanoma phase III trials. In this review, recent new ideas about the mechanism of action of anti-CTLA antibodies, other new molecules to target, and rationales for combination therapies will be discussed.

RECENT FINDINGS

Although the clinical efficacy of the anti-CTLA-4 monoclonal antibody (mAb) ipilimumab is meanwhile without doubt, its mechanism of action is still not fully understood. Recent data indicate that, besides modulation of the TCR signal, CTLA-4 mAbs can mediate regulatory T-cell depletion in an Fc gamma receptor dependent manner.Blockade of the molecules PD-1 and PD-L1 has given promising clinical responses (and this beyond melanoma), whereas their complex expression and interaction pattern makes a clear statement about the mechanism of action challenging.Additional other co-inhibitory and co-stimulatory molecules have been identified recently. Combinations of immune checkpoint modulators themselves or with other therapies, such as adoptive cell therapy, targeted therapy or radiotherapy, will improve the outcomes further.

SUMMARY

Immune checkpoint blockade is currently the most promising systemic therapeutic approach to achieve long-lasting responses or even cure in melanoma and other malignancies.

Disruption of the immune-checkpoint VISTA gene imparts a proinflammatory phenotype with predisposition to the development of autoimmunity

V domain-containing Ig suppressor of T-cell activation (VISTA) is a negative checkpoint regulator that suppresses T cell-mediated immune responses. Previous studies using a VISTA-neutralizing monoclonal antibody show that VISTA blockade enhances T-cell activation. The current study describes a comprehensive characterization of mice in which the gene for VISTA has been deleted. Despite the apparent normal hematopoietic development in young mice, VISTA genetic deficiency leads to a gradual accumulation of spontaneously activated T cells, accompanied by the production of a spectrum of inflammatory cytokines and chemokines. Enhanced T-cell responsiveness was also observed upon immunization with neoantigen. Despite the presence of multiorgan chronic inflammation, aged VISTA-deficient mice did not develop systemic or organ-specific autoimmune disease. Interbreeding of the VISTA-deficient mice with 2D2 T-cell receptor transgenic mice, which are predisposed to the development of experimental autoimmune encephalomyelitis, drastically enhanced disease incidence and intensity. Disease development is correlated with the increase in the activation of encephalitogenic T cells in the periphery and enhanced infiltration into the CNS. Taken together, our data suggest that VISTA is a negative checkpoint regulator whose loss of function lowers the threshold for T-cell activation, allowing for an enhanced proinflammatory phenotype and an increase in the frequency and intensity of autoimmunity under susceptible conditions.

Anti-CTLA-4 therapy broadens the melanoma-reactive CD8+ T cell response

Anti-CTLA-4 treatment improves the survival of patients with advanced-stage melanoma. However, although the anti-CTLA-4 antibody ipilimumab is now an approved treatment for patients with metastatic disease, it remains unknown by which mechanism it boosts tumor-specific T cell activity. In particular, it is unclear whether treatment amplifies previously induced T cell responses or whether it induces new tumor-specific T cell reactivities. Using a combination ultraviolet (UV)-induced peptide exchange and peptide-major histocompatibility complex (pMHC) combinatorial coding, we monitored immune reactivity against a panel of 145 melanoma-associated epitopes in a cohort of patients receiving anti-CTLA-4 treatment. Comparison of pre- and posttreatment T cell reactivities in peripheral blood mononuclear cell samples of 40 melanoma patients demonstrated that anti-CTLA-4 treatment induces a significant increase in the number of detectable melanoma-specific CD8 T cell responses (P = 0.0009). In striking contrast, the magnitude of both virus-specific and melanoma-specific T cell responses that were already detected before start of therapy remained unaltered by treatment (P = 0.74). The observation that anti-CTLA-4 treatment induces a significant number of newly detected T cell responses-but only infrequently boosts preexisting immune responses-provides strong evidence for anti-CTLA-4 therapy-enhanced T cell priming as a component of the clinical mode of action.

The anti-lymphoma activities of anti-CD137 monoclonal antibodies are enhanced in FcγRIII(-/-) mice

Agonistic monoclonal antibodies (mAbs) directed against the co-signaling molecule CD137 (4-1BB) elicit potent anti-tumor immunity in mice. This anti-tumor immunity has traditionally been thought to result from the ability of the Fab portion of anti-CD137 to function as an analog for CD137L. Although binding of CD137 by anti-CD137 mAbs has the potential to cross-link the Fc fragments, enabling Fc engagement of low to moderate affinity Fc gamma receptors (FcγR), the relative import of such Fc-FcγR interactions in mediating anti-CD137 associated anti-tumor immunity is unknown. We studied the ability of a rat anti-mouse CD137 mAb (2A) to mediate the anti-tumor response against the EL4E7 lymphoma in WT and FcγR(-/-) strains. 2A-treated FcRγ(-/-) mice had improved anti-tumor immunity against EL4E7, which could be completely recapitulated in FcγRIII(-/-) animals. These improved anti-tumor responses were associated with increased splenic CD8β T cell and dendritic cell (DC) populations. Furthermore, there was an increase in the number of DCs expressing high levels of the CD40, CD80, and CD86 molecules that are associated with more effective antigen presentation. Our results demonstrate an unexpected inhibitory role for FcγRIII in the anti-tumor function of anti-CD137 and underscore the need to consider antibody isotype when engineering therapeutic mAbs.

Interplay between regulatory T cells and PD-1 in modulating T cell exhaustion and viral control during chronic LCMV infection

T reg cells effectively maintain CD8 T cell exhaustion during chronic LCMV infection, but blockade of PD-1 is critical for elimination of infected cells.

Regulatory T (T reg) cells are critical for preventing autoimmunity mediated by self-reactive T cells, but their role in modulating immune responses during chronic viral infection is not well defined. To address this question and to investigate a role for T reg cells in exhaustion of virus-specific CD8 T cells, we depleted T reg cells in mice chronically infected with lymphocytic choriomeningitis virus (LCMV). T reg cell ablation resulted in 10–100-fold expansion of functional LCMV-specific CD8 T cells. Rescue of exhausted CD8 T cells was dependent on cognate antigen, B7 costimulation, and conventional CD4 T cells. Despite the striking recovery of LCMV-specific CD8 T cell responses, T reg cell depletion failed to diminish viral load. Interestingly, T reg cell ablation triggered up-regulation of the molecule programmed cell death ligand-1 (PD-L1), which upon binding PD-1 on T cells delivers inhibitory signals. Increased PD-L1 expression was observed especially on LCMV-infected cells, and combining T reg cell depletion with PD-L1 blockade resulted in a significant reduction in viral titers, which was more pronounced than that upon PD-L1 blockade alone. These results suggest that T reg cells effectively maintain CD8 T cell exhaustion, but blockade of the PD-1 inhibitory pathway is critical for elimination of infected cells.

New insights into the mechanism of action of immune checkpoint antibodies

Preclinical models have been developed and applied to predict the clinical efficacy of immune checkpoint antibodies. Now these models can be used to dissect the mechanisms by which such immunotherapeutic antibodies work and to build the rationale for combining immune checkpoint-targeting antibodies with potential synergistic activity in cancer patients.

Therapeutic use of anti-CTLA-4 antibodies

Targeting CTLA-4 represents a new type of immunotherapeutic approach, namely immune checkpoint inhibition. Blockade of CTLA-4 by ipilimumab was the first strategy to achieve a significant clinical benefit for late-stage melanoma patients in two phase 3 trials. These results fueled the notion of immunotherapy being the breakthrough strategy for oncology in 2013. Subsequently, many trials have been set up to test various immune checkpoint modulators in malignancies, not only in melanoma. In this review, recent new ideas about the mechanism of action of CTLA-4 blockade, its current and future therapeutic use, and the intensive search for biomarkers for response will be discussed. Immune checkpoint blockade, targeting CTLA-4 and/or PD-1/PD-L1, is currently the most promising systemic therapeutic approach to achieve long-lasting responses or even cure in many types of cancer, not just in patients with melanoma.

Fcγ receptor dependency of agonistic CD40 antibody in lymphoma therapy can be overcome through antibody multimerization

Immunomodulatory mAbs, led by the anti-CTLA4 mAb ipilimumab, are an exciting new class of drugs capable of promoting anticancer immunity and providing durable control of some tumors. Close analysis of a number of agents has revealed a critical yet variable role for Fcγ receptors in their efficacy. In this article, we reveal that agonistic anti-CD40 mAbs have an absolute requirement for cross-linking by inhibitory FcγRIIB when used systemically to treat established BCL1 syngeneic lymphoma, and therapy is lost when using a mouse IgG2a mAb not cross-linked by FcγRIIB. Furthermore, in FcγRIIB-deficient mice the lymphoma itself can provide FcγRIIB to cross-link anti-CD40 on neighboring cells, and only when this is blocked does therapy fail. The dependence on FcγRIIB for immunostimulatory activity was not absolute, however, because when anti-CD40 mAbs were administered systemically with the TLR3 agonist polyinosinic:polycytidylic acid or were given subcutaneously, activatory FcγR could also provide cross-linking. Using this mechanistic insight, we designed multimeric forms of anti-CD40 mAb with intrinsic FcγR-independent activity that were highly effective in the treatment of lymphoma-bearing mice. In conclusion, FcγR-independent anti-CD40 activation is a viable strategy in vivo. These findings have important translational implications, as humans, unlike mice, do not have IgG that binds strongly to FcγRIIB; therefore FcγR-independent derivatives represent an attractive therapeutic option.

Galectin-9 controls the therapeutic activity of 4-1BB-targeting antibodies

Biologics to TNF family receptors are prime candidates for therapy of immune disease. Whereas recent studies have highlighted a requirement for Fcγ receptors in enabling the activity of CD40, TRAILR, and GITR when engaged by antibodies, other TNFR molecules may be controlled by additional mechanisms. Antibodies to 4-1BB (CD137) are currently in clinical trials and can both augment immunity in cancer and promote regulatory T cells that inhibit autoimmune disease. We found that the action of agonist anti-4-1BB in suppressing autoimmune and allergic inflammation was completely dependent on Galectin-9 (Gal-9). Gal-9 directly bound to 4-1BB, in a site distinct from the binding site of antibodies and the natural ligand of 4-1BB, and Gal-9 facilitated 4-1BB aggregation, signaling, and functional activity in T cells, dendritic cells, and natural killer cells. Conservation of the Gal-9 interaction in humans has important implications for effective clinical targeting of 4-1BB and possibly other TNFR superfamily molecules.

Antibody therapies for melanoma: new and emerging opportunities to activate immunity (Review)

The interface between malignant melanoma and patient immunity has long been recognised and efforts to treat this most lethal form of skin cancer by activating immune responses with cytokine, vaccine and also antibody immunotherapies have demonstrated promise in limited subsets of patients. In the present study, we discuss different antibody immunotherapy approaches evaluated in the context of melanoma, each designed to act on distinct targets and to employ different mechanisms to restrict tumour growth and spread. Monoclonal antibodies recognising melanoma-associated antigens such as CSPG4/MCSP and targeting elements of tumour-associated vasculature (VEGF) have constituted long-standing translational approaches aimed at reducing melanoma growth and metastasis. Recent insights into mechanisms of immune regulation and tumour-immune cell interactions have helped to identify checkpoint molecules on immune (CTLA4, PD-1) and tumour (PD-L1) cells as promising therapeutic targets. Checkpoint blockade with antibodies to activate immune responses and perhaps to counteract melanoma-associated immunomodulatory mechanisms led to the first clinical breakthrough in the form of an anti-CTLA4 monoclonal antibody. Novel modalities to target key mechanisms of immune suppression and to redirect potent effector cell subsets against tumours are expected to improve clinical outcomes and to provide previously unexplored avenues for therapeutic interventions.