Accumulation of immunosuppressive CD11b+ myeloid cells correlates with the failure to prevent tumor growth in the anterior chamber of the eye

Uveal melanoma: progress in molecular biology and therapeutics

Uveal melanoma (UM) is the most common intraocular malignancy in adults. So far, no systemic therapy or standard treatment exists to reduce the risk of metastasis and improve overall survival of patients. With the increased knowledge regarding the molecular pathways that underlie the oncogenesis of UM, it is expected that novel therapeutic approaches will be available to conquer this disease. This review provides a summary of the current knowledge of, and progress made in understanding, the pathogenesis, genetic mutations, epigenetics, and immunology of UM. With the advent of the omics era, multi-dimensional big data are publicly available, providing an innovation platform to develop effective targeted and personalized therapeutics for UM patients. Indeed, recently, a great number of therapies have been reported specifically for UM caused by oncogenic mutations, as well as other etiologies. In this review, special attention is directed to advancements in targeted therapies. In particular, we discuss the possibilities of targeting: GNAQ/GNA11, PLCβ, and CYSLTR2 mutants; regulators of G-protein signaling; the secondary messenger adenosine diphosphate (ADP)-ribosylation factor 6 (ARF6); downstream pathways, such as those involving mitogen-activated protein kinase/MEK/extracellular signal-related kinase, protein kinase C (PKC), phosphoinositide 3-kinase/Akt/mammalian target of rapamycin (mTOR), Trio/Rho/Rac/Yes-associated protein, and inactivated BAP1; and immune-checkpoint proteins cytotoxic T-lymphocyte antigen 4 and programmed cell-death protein 1/programmed cell-death ligand 1. Furthermore, we conducted a survey of completed and ongoing clinical trials applying targeted and immune therapies for UM. Although drug combination therapy based on the signaling pathways involved in UM has made great progress, targeted therapy is still an unmet medical need.

Immunobiology of Uveal Melanoma: State of the Art and Therapeutic Targets

Uveal Melanoma (UM) represents the most common primary intraocular malignant tumor in adults. Although it originates from melanocytes as cutaneous melanoma, it shows significant clinical and biological differences with the latter, including high resistance to immune therapy. Indeed, UM can evade immune surveillance via multiple mechanisms, such as the expression of inhibitory checkpoints (e.g., PD-L1, CD47, CD200) and the production of IDO-1 and soluble FasL, among others. More in-depth understanding of these mechanisms will suggest potential targets for the design of novel and more effective management strategies for UM patients.

WO3/Pt nanoparticles are NADPH oxidase biomimetics that mimic effector cells in vitro and in vivo

To provide a means of delivering an artificial immune effector cell-like attack on tumor cells, we report the tumoricidal ability of inorganic WO3/Pt nanoparticles that mimic a leukocyte's functional abilities. These nanoparticles route electrons from organic structures and electron carriers to form hydroxyl radicals within tumor cells. During visible light exposure, WO3/Pt nanoparticles manufacture hydroxyl radicals, degrade organic compounds, use NADPH, trigger lipid peroxidation, promote lysosomal membrane disruption, promote the loss of reduced glutathione, and activate apoptosis. In a model of advanced breast cancer metastasis to the eye's anterior chamber, we show that WO3/Pt nanoparticles prolong the survival of 4T1 tumor-bearing Balb/c mice. This new generation of inorganic photosensitizers do not photobleach, and therefore should provide an important therapeutic advance in photodynamic therapy. As biomimetic nanoparticles destroy targeted cells, they may be useful in treating ocular and other forms of cancer.

Circulating tumor-associated neutrophils (cTAN) contribute to circulating tumor cell survival by suppressing peripheral leukocyte activation

During malignant progression, primary tumors rebuild leukocyte profile and suppress the host anti-tumor immune response. Tumor-associated neutrophils (TAN) increased in the cancer patients and emerged as an important participant and regulator of immune responses. The aim of this study is to investigate the role of circulating TAN (cTAN) in the metastatic process of advanced malignancy. We tested circulating neutrophils from patients (n = 180) with various types of cancer using flow cytometry analyses. We also used B16F10 cell-implanted C57BL/6 tumor-bearing mice model to simulate the advanced malignancy. Peripheral neutrophils were isolated by ficoll density gradient centrifugation, and in vitro tumor-leukocyte co-culture model was used to test tumor cell survival under leukocyte challenge condition. Here, we showed that neutrophils increased in the peripheral blood under the pathological condition of advanced malignancy both in cancer patients and in tumor-bearing mice. In mouse model, the malignantly increased neutrophils were identified as TAN according to the gene transcriptional analyses. We also showed that cTAN enhance tumor metastasis and cTAN could inhibit the activation of the peripheral leukocytes and rescue tumor cells from leukocyte challenge. In conclusion, our finding suggests that the abundance of cTAN in advanced cancer patients contributes to the circulating tumor cell survival by suppressing peripheral leukocyte activation.

Splenectomy restores tumoricidal activity to promote elimination of intraocular tumors

We recently demonstrated that splenectomy restores an interaction between CD8⁺ T cells and macrophages necessary for intraocular tumor elimination. Taking into consideration other studies indicating that intraocular tumor growth does not induce tumor-specific CD8⁺ T-cell tolerance, our data suggest that splenectomy may influence the phenotype of tumor-associated macrophages.

Splenectomy promotes indirect elimination of intraocular tumors by CD8+ T cells that is associated with IFNγ- and Fas/FasL-dependent activation of intratumoral macrophages

Ocular immune privilege (IP) limits the immune surveillance of intraocular tumors as certain immunogenic tumor cell lines (P815, E.G7-OVA) that are rejected when transplanted in the skin grow progressively when placed in the anterior chamber of the eye. As splenectomy (SPLNX) is known to terminate ocular IP, we characterized the immune mechanisms responsible for rejection of intraocular tumors in SPLNX mice as a first step toward identifying how to restore tumoricidal activity within the eye. CD8(+) T cells, IFNγ, and FasL, but not perforin, or TNFα were required for the elimination of intraocular E.G7-OVA tumors that culminated in destruction of the eye (ocular phthisis). IFNγ and FasL did not target tumor cells directly as the majority of SPLNX IFNγR1(-/-) mice and Fas-defective lpr mice failed to eliminate intraocular E.G7-OVA tumors that expressed Fas and IFNγR1. Bone marrow chimeras revealed that IFNγR1 and Fas expression on immune cells was most critical for rejection, and SPLNX increased the frequency of activated macrophages (Mϕ) within intraocular tumors in an IFNγ- and Fas/FasL-dependent manner, suggesting an immune cell target of IFNγ and Fas. As depletion of Mϕs limited CD8 T cell-mediated rejection of intraocular tumors in SPLNX mice, our data support a model in which IFNγ- and Fas/FasL-dependent activation of intratumoral Mϕs by CD8(+) T cells promotes severe intraocular inflammation that indirectly eliminates intraocular tumors by inducing phthisis, and suggests that immunosuppressive mechanisms that maintain ocular IP interfere with the interaction between CD8(+) T cells and Mϕs to limit the immunosurveillance of intraocular tumors.

Biology of advanced uveal melanoma and next steps for clinical therapeutics

Uveal melanoma is the most common intraocular malignancy although it is a rare subset of all melanomas. Uveal melanoma has distinct biology relative to cutaneous melanoma, with widely divergent patient outcomes. Patients diagnosed with a primary uveal melanoma can be stratified for risk of metastasis by cytogenetics or gene expression profiling, with approximately half of patients developing metastatic disease, predominately hepatic in location, over a 15-yr period. Historically, no systemic therapy has been associated with a clear clinical benefit for patients with advanced disease, and median survival remains poor. Here, as a joint effort between the Melanoma Research Foundation's ocular melanoma initiative, CURE OM and the National Cancer Institute, the current understanding of the molecular and immunobiology of uveal melanoma is reviewed, and on-going laboratory research into the disease is highlighted. Finally, recent investigations relevant to clinical management via targeted and immunotherapies are reviewed, and next steps in the development of clinical therapeutics are discussed.

Good news-bad news: the Yin and Yang of immune privilege in the eye

The eye and the brain are prototypical tissues manifesting immune privilege (IP) in which immune responses to foreign antigens, particularly alloantigens are suppressed, and even completely inhibited. Explanations for this phenomenon are numerous and mostly reflect our evolving understanding of the molecular and cellular processes underpinning immunological responses generally. IP is now viewed as a property of many tissues and the level of expression of IP varies not only with the tissue but with the nature of the foreign antigen and changes in the limited conditions under which privilege can operate as a mechanism of immunological tolerance. As a result, IP functions normally as a homeostatic mechanism preserving normal function in tissues, particularly those with highly specialized function and limited capacity for renewal such as the eye and brain. However, IP is relatively easily bypassed in the face of a sufficiently strong immunological response, and the privileged tissues may be at greater risk of collateral damage because its natural defenses are more easily breached than in a fully immunocompetent tissue which rapidly rejects foreign antigen and restores integrity. This two-edged sword cuts its swathe through the eye: under most circumstances, IP mechanisms such as blood-ocular barriers, intraocular immune modulators, induction of T regulatory cells, lack of lymphatics, and other properties maintain tissue integrity; however, when these are breached, various degrees of tissue damage occur from severe tissue destruction in retinal viral infections and other forms of uveoretinal inflammation, to less severe inflammatory responses in conditions such as macular degeneration. Conversely, ocular IP and tumor-related IP can combine to permit extensive tumor growth and increased risk of metastasis thus threatening the survival of the host.

Influence of CD8+ T regulatory cells on intraocular tumor development

The interior of the eye, or uvea, is a site of immune privilege where certain immune responses are attenuated or completely excluded to protect non-regenerating tissues essential for vision. One consequence of this immunoregulation is compromised immune mediated elimination of intraocular tumors. For example, certain murine tumor cell lines which are rejected by host immune responses when transplanted in the skin grow progressively when placed in the anterior chamber (a.c.) of the eye. Progressive ocular tumor growth occurs despite induction of tumor-specific CD8+ T cell responses capable of eliminating a subsequent tumor challenge in the skin or opposite eye. Why these CD8+ T effectors fail to eliminate established ocular tumors is not known. It is well appreciated that growth of tumors in the a.c. induces the generation of immunosuppressive CD8+ T regulatory (Treg) cells. However, the contribution of CD8+ Treg in ocular tumor progression remains unclear. Several studies indicate that these CD8+ Treg target responding CD4+ T cells to inhibit their induction of macrophage-dependent delayed type hypersensitivity (DTH) responses to tumor antigens (Ags). However, induction of tumor-specific CD4+ T cell responses does not assure intraocular tumor elimination. This review is focused on how CD8+ Treg could influence the tumoricidal activity of ocular tumor-specific CD8+ T effector cells.

Uveal melanoma: the inflammatory microenvironment

Uveal melanoma is a highly malignant intraocular tumor with quite homogeneous tumor tissue and a diffuse leukocytic infiltration. In contrast with many other malignancies, the presence of infiltrating macrophages and T cells is associated with a poor prognosis rather than a good one. The clear link between inflammation and cancer in this malignancy provides a paradigm for macrophage plasticity and function. Macrophages in uveal melanoma have an M2-like phenotype and are associated with the loss of one specific chromosome - monosomy 3. The central players involved in this process and discussed in this review include macrophages, T lymphocytes, chemokines and cytokines, including the macrophage-attraction molecules. When a tumor acquires the ability to release significant amounts of macrophage-attraction molecules it causes the expansion of a population of myeloid immature cells that may not only help the tumor to suppress immune reactions but also aid in the construction of new blood vessels for tumor growth. A better understanding of the molecular basis of a local myelomonocytic cell population will bring a better understanding of the immunopathology of this disease and will lead to therapeutic interventions in uveal melanoma. This review focuses on the roles of the local inflammatory microenvironment in the development and progression of uveal melanoma.

Effects of interleukin-1 receptor antagonist on tumor stroma in experimental uveal melanoma

PURPOSE

In contrast to many malignancies showing evidence that interleukin-1 (IL-1) promotes progression through effects on tumor vascularity and myeloid suppressor cell populations, in uveal melanoma there is evidence that IL-1 can inhibit progression.

METHODS

The effects of the IL-1 receptor antagonist IL-1ra against the aggressive/invasive MUM2B and the nonaggressive/noninvasive OCM1 uveal melanoma models were examined in vitro and in vivo in mouse xenografts. Vascularity and myeloid suppressor cell populations and their regulators were assessed.

RESULTS

In vitro, IL-1, and IL-1ra did not affect the proliferation of the uveal melanoma cells or their production of IL-1, IL-6, transforming growth factor (TGF) β, or VEGF. In vivo, IL-1ra treatment resulted in substantial growth inhibition of MUM2B tumors; less inhibition was observed against OCM1 tumors. Periodic acid-Schiff loops and CD11b⁺ macrophages within the tumor stroma decreased in vivo; CD31⁺ blood vessels were not altered. IL-1ra treatment in vivo did not affect tumor-derived IL-1, IL-6, TGF-β, or VEGF. In contrast, host IL-1β, IL-6, and tumor necrosis factor decreased. Host VEGF was not altered. Intratumoral IL-12(p40) and CXCL10, markers of host M1 polarization, increased, and intratumoral arginase and CD206, markers of myeloid-derived suppressor cells (MDSC) and M2 macrophage polarization, decreased. IL-1ra treatment in vivo also reduced splenic CD11b⁺Gr1⁺ MDSC.

CONCLUSIONS

IL-1 may play a role in promoting uveal melanoma progression. Inhibiting IL-1 with IL-1ra inhibits tumor growth in vivo but not in vitro. Tumor stroma is modified, myeloid suppressor cells are reduced, and M1 macrophage polarization is increased in vivo.

CTL induction of tumoricidal nitric oxide production by intratumoral macrophages is critical for tumor elimination

To characterize mechanisms of CTL inhibition within an ocular tumor microenvironment, tumor-specific CTLs were transferred into mice with tumors developing within the anterior chamber of the eye or skin. Ocular tumors were resistant to CTL transfer therapy whereas skin tumors were sensitive. CTLs infiltrated ocular tumors at higher CTL/tumor ratios than in skin tumors and demonstrated comparable ex vivo effector function to CTLs within skin tumors indicating that ocular tumor progression was not due to decreased CTL accumulation or inhibited CTL function within the eye. CD11b(+)Gr-1(+)F4/80(-) cells predominated within ocular tumors, whereas skin tumors were primarily infiltrated by CD11b(+)Gr-1(-)F4/80(+) macrophages (Ms), suggesting that myeloid derived suppressor cells may contribute to ocular tumor growth. However, CD11b(+) myeloid cells isolated from either tumor site suppressed CTL activity in vitro via NO production. Paradoxically, the regression of skin tumors by CTL transfer therapy required NO production by intratumoral Ms indicating that NO-producing intratumoral myeloid cells did not suppress the effector phase of CTL. Upon CTL transfer, tumoricidal concentrations of NO were only produced by skin tumor-associated Ms though ocular tumor-associated Ms demonstrated comparable expression of inducible NO synthase protein suggesting that NO synthase enzymatic activity was compromised within the eye. Correspondingly, in vitro-activated Ms limited tumor growth when co-injected with tumor cells in the skin but not in the eye. In conclusion, the decreased capacity of Ms to produce NO within the ocular microenvironment limits CTL tumoricidal activity allowing ocular tumors to progress.

A caveat for T cell transfer studies: generation of cytotoxic anti-Thy1.2 antibodies in Thy1.1 congenic mice given Thy1.2+ tumors or T cells

Thy1.1 congenic B6.PL mice were used to simultaneously monitor Thy1.2+ E.G7-OVA tumors transplanted in the a.c. of the eye and i.v.-transferred tumor-specific Thy1.2+ CTLs to determine mechanisms that inhibit the tumoricidal activity of CTL responses in mice with established ocular tumors. Transferred CTLs were systemically deleted in mice with established ocular tumors. However, this deletion was not a unique mechanism of immune evasion by ocular tumors. Rather, development of Thy1.2+ tumors in the eye or skin of B6.PL mice generated cytotoxic anti-Thy1.2 antibodies that eliminated a subsequent Thy1.2+ T cell transfer. Anti-Thy1.2 immune responses in B6.PL mice were influenced by the route of antigen administration, as the serum concentration of cytotoxic anti-Thy1.2 antibodies was 92-fold greater in mice with eye tumors in comparison with mice with skin tumors. In addition, anti-Thy1.2 immune responses were detected in B6.PL mice given naïve Thy1.2+ T cells i.p. but not i.v. Anti-Thy1.2 responses were augmented in B6.PL mice with ocular Thy1.2+ EL-4 tumors that did not express OVA, suggesting immunodominance of OVA antigen over Thy1.2. Thy1.1+ T cells given i.p. was not immunogenic in Thy1.2 congenic mice. These data reaffirm that the introduction of antigens in the a.c. induces robust antibody responses. Experimentation using allotypic differences in Thy1 between donor cells and recipient mice must consider cytotoxic anti-Thy1 antibody generation in the interpretation of results.

Influence of immune privilege on ocular tumor development

Mechanisms that maintain ocular immune privilege may contribute to ocular tumor progression by inhibiting tumoricidal immune responses. Consistent with that notion are observations from transplantable tumor models in mice demonstrating that the tumoricidal activity of CD8(+) cytolytic T lymphocytes (CTL) may be inhibited directly by interfering with CTL effector function in the eye or indirectly by abrogating the effector function of CD8+ T cell-activated intratumoral macrophages that are critical for ocular tumor rejection. In addition, epigenetic gene regulation by factors within the ocular tumor environment favors the generation of tumor variants that are resistant to CD8(+) CTL. Intratumoral macrophages may be essential for eliminating these variants because, unlike CTL, their tumoricidal activity is nonspecific. Hence, the inhibition of macrophage effector function within the eye, presumably to preserve immune privilege by minimizing ocular immunopathology, may hasten the outgrowth of tumor escape variants which contributes to ocular tumor progression.

Immune escape mechanisms of intraocular tumors

The notion that the immune system might control the growth of tumors was suggested over 100 years ago by the eminent microbiologist Paul Ehrlich. This concept was refined and expanded by Burnet and Thomas 50 years later with their articulation of the "immune surveillance" hypothesis. In its simplest form, the immune surveillance hypothesis suggests that neoplasms arise spontaneously and express novel antigens that are recognized by the immune system, which either eliminates the tumors or restrains their growth. Within the eye, immune responses are controlled and sometimes profoundly inhibited - a condition known as immune privilege. Immune privilege in the eye is the result of a complex array of anatomical, physiological, and immunoregulatory mechanisms that prevent the induction and expression of many immune responses. Tumors arising in the eye would seem to have an advantage in evading immune surveillance due to ocular immune privilege. Uveal melanoma, the most common and malignant intraocular tumor in adults, not only benefits from the immune privilege of the eye but also has adopted many of the mechanisms that contribute to ocular immune privilege as a strategy for protecting uveal melanoma cells once they leave the sanctuary of the eye and are disseminated systemically in the form of metastases. Although the immune system possesses a battery of effector mechanisms designed to rid the body of neoplasms, tumors are capable of rapidly evolving and countering even the most sophisticated immunological effector mechanisms. To date, tumors seem to be winning this arms race, but an increased understanding of these mechanisms should provide insights for designing immunotherapy that was envisioned over half a century ago, but has failed to materialize to date.

Differential effects of imatinib mesylate against uveal melanoma in vitro and in vivo

Uveal melanoma is refractory to chemotherapy. The receptor tyrosine kinase inhibitor, imatinib mesylate, has demonstrated antiproliferative effects against uveal melanoma cells in vitro. The effects of imatinib mesylate, alone and combined with the alklyating agent, temozolomide, were examined in vivo as well as in vitro. Proliferation and angiogenic factor production of human uveal melanoma cell lines in response to imatinib mesylate and temozolomide were examined in vitro. Tumor growth, angiogenic factor production, tumor interstitial fluid pressure, and stroma constituents in response to imatinib mesylate and temozolomide were examined in vivo in mice bearing human uveal melanoma xenografts. Imatinib mesylate in vitro antagonized the antiproliferative effects of temozolomide and increased the production of angiogenic factors. In contrast, pretreatment with imatinib mesylate in vivo could improve the antitumor activity of temozolomide. Imatinib mesylate in vivo decreased the production of angiogenic factors in the tumor stroma and tumor interstitial fluid pressure. These effects were transient. Increases in angiogenic factors, interstitial fluid pressure, and tumor infiltrating macrophages were observed with continued imatinib mesylate treatment in vivo. The antitumor effects of imatinib mesylate can vary in vivo when compared with in vitro. Imatinib mesylate can both positively and negatively modify host-tumor interactions in uveal melanoma.

Activated CD11b+ CD15+ granulocytes increase in the blood of patients with uveal melanoma

PURPOSE

To determine whether activated CD11b(+) CD15(+) granulocytes increase in the blood of patients with uveal melanoma.

METHODS

Peripheral blood mononuclear cells (PBMCs) were isolated by density gradient centrifugation from the blood of patients with primary choroidal/ciliochoroidal uveal melanomas (six women, four men; age range, 46-91 years) and healthy control donors (14 women, 10 men; age range, 50-81 years). The expression of CD15 and CD68 on CD11b(+) myeloid cells within PBMCs and primary uveal melanomas was evaluated by flow cytometry. CD3zeta chain expression by CD3epsilon(+) T cells in PBMCs and within primary uveal melanomas was measured as an indirect indication of T-cell function.

RESULTS

The percentage of CD11b(+) cells in PBMCs of patients with uveal melanoma increased 1.8-fold in comparison to healthy donors and comprised three subsets: CD68 negative CD15(+) granulocytes, which increased 4.1-fold; CD68(-) CD15(-) cells, which increased threefold; and CD68(+) CD15(low) cells, which were unchanged. A significant (2.7-fold) reduction in CD3zeta chain expression on CD3epsilon(+) T cells, a marker of T-cell dysfunction, was observed in PBMCs of patients with uveal melanoma in comparison with healthy control subjects and correlated significantly with the percentage of CD11b(+) cells in PBMCs. CD3zeta chain expression on T cells within primary tumors was equivalent to CD3zeta expression in PBMCs of the same patient in four of five patients analyzed.

CONCLUSIONS

Activated CD11b(+) CD15(+) granulocytes expand in the blood of patients with uveal melanoma and may contribute to immune evasion by ocular tumors by inhibiting T-cell function via decreasing CD3zeta chain expression.

CD8+ T cells circumvent immune privilege in the eye and mediate intraocular tumor rejection by a TNF-alpha-dependent mechanism

Although intraocular tumors reside in an immune-privileged environment, T cells can circumvent immune privilege and mediate tumor rejection without inducing damage to normal ocular tissue. In this study, we used a well-characterized tumor, Ad5E1 (adenovirus type 5 early region 1), to analyze the role of CD8+ T cells in the pristine rejection of intraocular tumors. It has been previously documented that Ad5E1 tumor rejection can occur in the absence of CD8+ T cells. However, here we find that CD8+ T cells infiltrated intraocular Ad5E1 tumors in C57BL/6 mice. Surprisingly, CD8+ T cells from tumor-rejector mice could mediate intraocular tumor rejection following adoptive transfer to SCID mice. In determining the mechanisms behind CD8+ T cell-mediated tumor rejection, we discovered that antitumor CTL activity was neither observed nor necessary for rejection of the intraocular tumors. CD8+ T cells from rejector mice did not produce IFN-gamma in response to Ad5E1 tumor Ags or use FasL to mediate intraocular tumor rejection. Also, CD8+ T cells did not use perforin or TRAIL, as CD8+ T cells from perforin knockout (KO) and TRAIL KO mice conferred protection to SCID recipient mice following adoptive transfer. We discovered that CD8+ T cells used TNF-alpha to mediate tumor rejection, because Ad5E1 tumor cells were highly sensitive to TNF-alpha-induced apoptosis and CD8+ T cells from TNF-alpha KO mice did not protect SCID mice from progressive Ad5E1 tumor growth. The results indicate that CD8+ T cells circumvent immune privilege and mediate intraocular tumor rejection by a TNF-alpha-dependent manner while leaving the eye intact and vision preserved.