Effective CpG immunotherapy of breast carcinoma prevents but fails to eradicate established brain metastasis

Immunotherapy in the context of immune-specialized environment of brain metastases

Brain metastases (BrM) develop in 20-40% of patients with advanced cancer. They mainly originate from lung cancer, melanoma, breast cancer, and renal cell carcinoma, and are associated with a poor prognosis. While patients with BrM traditionally lack effective treatment options, immunotherapy is increasingly gaining in importance in this group of patients, with clinical trials in the past decade demonstrating the efficacy and safety of immune checkpoint blockade in BrM originating from specific tumor types, foremost melanoma. The brain is an immune-specialized environment with several unique molecular, cellular, and anatomical features that affect immune responses, including those against tumors. In this review we discuss the potential role that some of these unique characteristics may play in the efficacy of immunotherapy, mainly focusing on the lymphatic drainage in the brain and the role of systemic anti-tumor immunity that develops due to the presence of concurrent extracranial disease in addition to BrM.

The progress of microenvironment-targeted therapies in brain metastases

The incidence of brain metastases (BrM) has become a growing concern recently. It is a common and often fatal manifestation in the brain during the end-stage of many extracranial primary tumors. Increasing BrM diagnoses can be attributed to improvements in primary tumor treatments, which have extended patients’ lifetime, and allowed for earlier and more efficient detection of brain lesions. Currently, therapies for BrM encompass systemic chemotherapy, targeted therapy, and immunotherapy. Systemic chemotherapy regimens are controversial due to their associated side effects and limited efficacy. Targeted and immunotherapies have garnered significant attention in the medical field: they target specific molecular sites and modulate specific cellular components. However, multiple difficulties such as drug resistance and low permeability of the blood-brain barrier (BBB) remain significant challenges. Thus, there is an urgent need for novel therapies. Brain microenvironments consist of cellular components including immune cells, neurons, endothelial cells as well as molecular components like metal ions, nutrient molecules. Recent research indicates that malignant tumor cells can manipulate the brain microenvironment to change the anti-tumoral to a pro-tumoral microenvironment, both before, during, and after BrM. This review compares the characteristics of the brain microenvironment in BrM with those in other sites or primary tumors. Furthermore, it evaluates the preclinical and clinical studies of microenvironment-targeted therapies for BrM. These therapies, due to their diversity, are expected to overcome drug resistance or low permeability of the BBB with low side effects and high specificity. This will ultimately lead to improved outcomes for patients with secondary brain tumors.

Brain metastasis models: What should we aim to achieve better treatments?

Brain metastasis is emerging as a unique entity in oncology based on its particular biology and, consequently, the pharmacological approaches that should be considered. We discuss the current state of modelling this specific progression of cancer and how these experimental models have been used to test multiple pharmacologic strategies over the years. In spite of pre-clinical evidences demonstrating brain metastasis vulnerabilities, many clinical trials have excluded patients with brain metastasis. Fortunately, this trend is getting to an end given the increasing importance of secondary brain tumors in the clinic and a better knowledge of the underlying biology. We discuss emerging trends and unsolved issues that will shape how we will study experimental brain metastasis in the years to come.

Immune Checkpoint Blockade - How Does It Work in Brain Metastases?

Immune checkpoints restrain the immune system following its activation and their inhibition unleashes anti-tumor immune responses. Immune checkpoint inhibitors revolutionized the treatment of several cancer types, including melanoma, and immune checkpoint blockade with anti-PD-1 and anti-CTLA-4 antibodies is becoming a frontline therapy in metastatic melanoma. Notably, up to 60% of metastatic melanoma patients develop metastases in the brain. Brain metastases (BrM) are also very common in patients with lung and breast cancer, and occur in ∼20-40% of patients across different cancer types. Metastases in the brain are associated with poor prognosis due to the lack of efficient therapies. In the past, patients with BrM used to be excluded from immune-based clinical trials due to the assumption that such therapies may not work in the context of "immune-specialized" environment in the brain, or may cause harm. However, recent trials in patients with BrM demonstrated safety and intracranial activity of anti-PD-1 and anti-CTLA-4 therapy. We here discuss how immune checkpoint therapy works in BrM, with focus on T cells and the cross-talk between BrM, the immune system, and tumors growing outside the brain. We discuss major open questions in our understanding of what is required for an effective immune checkpoint inhibitor therapy in BrM.

Prophylactic TLR9 stimulation reduces brain metastasis through microglia activation

Brain metastases are prevalent in various types of cancer and are often terminal, given the low efficacy of available therapies. Therefore, preventing them is of utmost clinical relevance, and prophylactic treatments are perhaps the most efficient strategy. Here, we show that systemic prophylactic administration of a toll-like receptor (TLR) 9 agonist, CpG-C, is effective against brain metastases. Acute and chronic systemic administration of CpG-C reduced tumor cell seeding and growth in the brain in three tumor models in mice, including metastasis of human and mouse lung cancer, and spontaneous melanoma-derived brain metastasis. Studying mechanisms underlying the therapeutic effects of CpG-C, we found that in the brain, unlike in the periphery, natural killer (NK) cells and monocytes are not involved in controlling metastasis. Next, we demonstrated that the systemically administered CpG-C is taken up by endothelial cells, astrocytes, and microglia, without affecting blood-brain barrier (BBB) integrity and tumor brain extravasation. In vitro assays pointed to microglia, but not astrocytes, as mediators of CpG- C effects through increased tumor killing and phagocytosis, mediated by direct microglia-tumor contact. In vivo, CpG-C-activated microglia displayed elevated mRNA expression levels of apoptosis-inducing and phagocytosis-related genes. Intravital imaging showed that CpG-C-activated microglia cells contact, kill, and phagocytize tumor cells in the early stages of tumor brain invasion more than nonactivated microglia. Blocking in vivo activation of microglia with minocycline, and depletion of microglia with a colony-stimulating factor 1 inhibitor, indicated that microglia mediate the antitumor effects of CpG-C. Overall, the results suggest prophylactic CpG-C treatment as a new intervention against brain metastasis, through an essential activation of microglia.

Therapeutic Injection of a C-Type CpG ODN Induced an Antitumor Immune Response in C57/BL6 Mice of Orthotopically Transplanted Hepatocellular Carcinoma

Synthetic CpG oligodeoxynucleotides (ODNs), as TLR9 agonists, have been found to play a possible role in antitumor effect. In order to determine the effect of YW002, known as a C-type CpG ODN, on the treatment of hepatocellular carcinoma (HCC), which is one of the most aggressive carcinomas, we chose to inject YW002 at the doses of 12.5 µg and 25 µg per mouse 7 days post-tumor challenge. The survival rate of mice was recorded every day. On day 14 postinjection, five mice in each group were bled and randomly sacrificed. The level of IFN-γ or TNF-α in the serum was detected and lymphocyte infiltration in the tumor tissue; the ratios of CD8⁺ T cells and CD4⁺ T cells in the spleen of mice were also analyzed. The results indicated that treatment with YW002 could raise the survival rate and delay tumor growth in the mice with orthotopically transplanted HCC. Furthermore, the treatment improved the antitumor immune response through increasing the T-cell infiltration in tumor and the ratio of CD4⁺, CD8⁺, and NK cells in the spleen. In addition, the concentration of IFN-γ was raised, and the level of TGF-β was depressed. Our data suggested that CpG ODN might be a proper medicament in a monotherapeutic regimen for treatment of HCC.

Emerging strategies for treating brain metastases from breast cancer

Brain metastasis is an end stage in breast cancer progression. Traditional treatment options have minimal efficacy, and overall survival is on the order of months. The incidence of brain metastatic disease is increasing with the improved management of systemic disease and prolongation of survival. Unfortunately, the targeted therapies that control systemic disease have diminished efficacy against brain lesions. There are reasons to be optimistic, however, as emerging therapies have shown promise in preclinical and early clinical settings. This review discusses recent advances in breast cancer brain metastasis therapy and potential approaches for successful treatment.

CD200 in CNS tumor-induced immunosuppression: the role for CD200 pathway blockade in targeted immunotherapy

BACKGROUND

Immunological quiescence in the central nervous system (CNS) is a potential barrier to immune mediated anti-tumor response. One suppressive mechanism results from the interaction of parenchyma-derived CD200 and its receptor on myeloid cells. We suggest that CD200/CD200R interactions on myeloid cells expand the myeloid-derived suppressor cell (MDSC) population and that blocking tumor-derived CD200 will enhance the efficacy of immunotherapy.

METHODS

CD200 mRNA expression levels in human brain tumor tissue samples were measured by microarray. The amount of circulating CD200 protein in the sera of patients with brain tumors was determined by ELISA and, when corresponding peripheral blood samples were available, was correlated quantitatively with MDSCs. CD200-derived peptides were used as competitive inhibitors in a mouse model of glioblastoma immunotherapy.

RESULTS

CD200 mRNA levels were measured in human brain tumors, with different expression levels being noted among the sub groups of glioblastoma, medulloblastoma and ependymoma. Serum CD200 concentrations were highest in patients with glioblastoma and correlated significantly with MDSC expansion. Similarly, in vitro studies determined that GL261 cells significantly expanded a MDSC population. Interestingly, a CD200R antagonist inhibited the expansion of murine MDSCs in vitro and in vivo. Moreover, inclusion of CD200R antagonist peptide in glioma tumor lysate-derived vaccines slowed tumor growth and significantly enhanced survival.

CONCLUSION

These data suggest that CNS-derived tumors can evade immune surveillance by engaging CD200. Because of the homology between mouse and human CD200, our data also suggest that blockade of CD200 binding to its receptor will enhance the efficacy of immune mediated anti-tumor strategies for brain tumors.

Toll-like receptor 9 in breast cancer

Toll-like receptor 9 (TLR9) is a cellular DNA receptor of the innate immune system. DNA recognition via TLR9 results in an inflammatory reaction, which eventually also activates a Th1-biased adaptive immune attack. In addition to cells of the immune system, TLR9 mRNA and protein are also widely expressed in breast cancer cell lines and in clinical breast cancer specimens. Although synthetic TLR9-ligands induce cancer cell invasion in vitro, the role of TLR9 in cancer pathophysiology has remained unclear. In the studies conducted so far, tumor TLR9 expression has been shown to have prognostic significance only in patients that have triple-negative breast cancer (TNBC). Specifically, high tumor TLR9 expression predicts good prognosis among TNBC patients. Pre-clinical studies suggest that TLR9 expression may affect tumor immunophenotype and contribute to the immunogenic benefit of chemotherapy. In this review, we discuss the possible contribution of tumor TLR9 to the pathogenesis and treatment responses in breast cancer.

Role of the systemic immune system in brain metastasis

Metastatic disease in the central nervous system (CNS) is a cause of increasing mortality amongst cancer patients. As with other types of cancer, cells of the systemic immune system play a range of important roles in the development of metastatic lesions in the CNS, both repressing and promoting tumour growth. Recent advances in immunotherapy have changed the emphasis in cancer treatment away from conventional chemotherapy and radiotherapy for certain tumour types. Despite this, our understanding of systemic immune system involvement in CNS metastases remains poor. The blood-brain barrier prevents the majority of diagnostic and therapeutic agents from crossing into the brain parenchyma until the late stages of metastatic disease. Thus, the development of immunotherapy for CNS pathologies is particularly desirable. This review draws together our current understanding in the relationships between CNS metastases and circulating systemic immune cells. We discuss the roles that circulating systemic immune cells may play in the homing of metastatic cells to the perivascular space, and the pro-metastatic and antagonistic roles that infiltrating systemic immune cells may play at sites of metastasis. This article is part of a Special Issue entitled 'Neuroinflammation in neurodegeneration and neurodysfunction'.

Immunologic privilege in the central nervous system and the blood-brain barrier

The brain is in many ways an immunologically and pharmacologically privileged site. The blood-brain barrier (BBB) of the cerebrovascular endothelium and its participation in the complex structure of the neurovascular unit (NVU) restrict access of immune cells and immune mediators to the central nervous system (CNS). In pathologic conditions, very well-organized immunologic responses can develop within the CNS, raising important questions about the real nature and the intrinsic and extrinsic regulation of this immune privilege. We assess the interactions of immune cells and immune mediators with the BBB and NVU in neurologic disease, cerebrovascular disease, and intracerebral tumors. The goals of this review are to outline key scientific advances and the status of the science central to both the neuroinflammation and CNS barriers fields, and highlight the opportunities and priorities in advancing brain barriers research in the context of the larger immunology and neuroscience disciplines. This review article was developed from reports presented at the 2011 Annual Blood-Brain Barrier Consortium Meeting.

Active efflux of Dasatinib from the brain limits efficacy against murine glioblastoma: broad implications for the clinical use of molecularly targeted agents

The importance of the blood-brain barrier in preventing effective pharmacotherapy of glioblastoma has been controversial. The controversy stems from the fact that vascular endothelial cell tight junctions are disrupted in the tumor, allowing some systemic drug delivery. P-glycoprotein (Pgp) and breast cancer resistance protein (BCRP) efflux drugs from brain capillary endothelial cells into the blood. We tested the hypothesis that although the tight junctions are "leaky" in the core of glioblastomas, active efflux limits drug delivery to tumor-infiltrated normal brain and consequently, treatment efficacy. Malignant gliomas were induced by oncogene transfer into wild-type (WT) mice or mice deficient for Pgp and BCRP (knockout, KO). Glioma-bearing mice were orally dosed with dasatinib, a kinase inhibitor and dual BCRP/PgP substrate that is being currently tested in clinical trials. KO mice treated with dasatinib survived for twice as long as WT mice. Microdissection of the tumor core, invasive rim, and normal brain revealed 2- to 3-fold enhancement in dasatinib brain concentrations in KO mice relative to WT. Analysis of signaling showed that poor drug delivery correlated with the lack of inhibition of a dasatinib target, especially in normal brain. A majority of human glioma xenograft lines tested expressed BCRP or PgP and were sensitized to dasatinib by a dual BCRP/Pgp inhibitor, illustrating a second barrier to drug delivery intrinsic to the tumor itself. These data show that active efflux is a relevant obstacle to treating glioblastoma and provide a plausible mechanistic basis for the clinical failure of numerous drugs that are BCRP/Pgp substrates.

An in vivo immunotherapy screen of costimulatory molecules identifies Fc-OX40L as a potent reagent for the treatment of established murine gliomas

PURPOSE

We tested the combination of a tumor lysate vaccine with a panel of costimulatory molecules to identify an immunotherapeutic approach capable of curing established murine gliomas.

EXPERIMENTAL DESIGN

Glioma-bearing mice were primed with a tumor lysate vaccine, followed by systemic administration of the following costimulatory ligands: OX40L, CD80, 4-1BBL, and GITRL, which were fused to the Fc portion of human immunoglobulin. Lymphocytes and mRNA were purified from the brain tumor site for immune monitoring studies. Numerous variations of the vaccine and Fc-OX40L regimen were tested alone or in combination with temozolomide.

RESULTS

Lysate vaccinations combined with Fc-OX40L led to the best overall survival, yielding cure rates of 50% to 100% depending on the timing, regimen, and combination with temozolomide. Cured mice that were rechallenged with glioma cells rejected the challenge, showing immunologic memory. Lymphocytes isolated from the draining lymph nodes of vaccine/Fc-OX40L-treated mice had superior tumoricidal function relative to all other groups. Vaccine/Fc-OX40L-treated mice exhibited a significant increase in proliferation of brain-infiltrating CD4 and CD8 T cells, as indicated by Ki67 staining. Fc-OX40L had single-agent activity in transplanted and spontaneous glioma models, and the pattern of inflammatory gene expression in the tumor predicted the degree of therapeutic response.

CONCLUSIONS

These data show that Fc-OX40L has unique and potent activity against experimental gliomas and warrants further testing.

Simultaneous TLR2 inhibition and TLR9 activation synergistically suppress tumor metastasis in mice

AIM

To develop a rational immunotherapy against tumor metastasis by combining a Toll-like-receptor 2 (TLR2)-neutralizing antibody with a TLR9 agonist CpG ODN, and then investigate the mechanism of action for this combinational regimen.

METHODS

After mouse melanoma B16-F10 cell inoculation, female C57BL/6 mice were treated with either CpG ODN (0.5 mg/kg) or the anti-TLR2 antibody (200 μg/kg), or with a combination of the two agents. Pulmonary metastases were evaluated by counting metastatic nodes on the lung surface using anatomical microscopy. Flow cytometry was used to evaluate the cytotoxicity of the immune cells in tumor-draining lymph nodes, the cell population in the spleen, and the infiltration of immune cells within the lungs. Cytokine and enzyme expression in the lung tissue was evaluated using ELISA or immunostaining.

RESULTS

Anti-metastatic effects were detected in mice treated with either CpG ODN or the anti-TLR2 antibody alone. However, treatment with CpG ODN plus the anti-TLR2 antibody synergistically suppressed the metastasis as compared with treatment with either single agent. The combinational treatment resulted in enhanced infiltration of natural killer cells and cytotoxic T cells, reduced recruitment of type 2 macrophages and Tregs, and decreased expression of immunosuppressive factors including TGF-β1, cyclooxygenase-2 and indoleamine 2,3-dioxygenase, thus stimulated tumor cytotoxicity and suppressed metastasis. The anti-metastatic effect of the combinational regimen was further confirmed in spontaneous metastatic mouse model of Lewis lung carcinoma.

CONCLUSION

Our studies suggest that combining a TLR9 agonist with an anti-TLR2 antibody, which eliminates immunosuppressive factors from the tumor environment, is critical for an effective anti-metastatic immunotherapy.

Topical imiquimod has therapeutic and immunomodulatory effects against intracranial tumors

Topical imiquimod cream (trade name: Aldara) is a Toll-like receptor (TLR) 7 agonist that is approved for the treatment of cutaneous tumors. Aldara is also used as a vaccine adjuvant in clinical trials in patients with glioma and other tumors. The main mechanism of action ascribed to Aldara has been the local activation of TLR7(+) cells near the application site. Here we report the unexpected finding that Aldara has therapeutic and immunomodulatory activity as a single agent in mice bearing intracranial tumors. Repeated administration of Aldara onto the skin significantly increased the survival of mice bearing intracranial GL261 glioma and EMT6 breast carcinoma. Aldara treatment was associated with a reduction in the number CD4(+)Foxp3(+) regulatory T cells in the blood and brain tumor site. Mice treated with Aldara exhibited a generalized lymphopenia in the blood amidst an increase in brain tumor infiltrating CD4(+) and CD8(+) T cells and dendritic cells. Brain-infiltrating CD8(+) T cells were tumor reactive as demonstrated by degranulation and interferon-γ secretion in a GL261-dependent manner. In addition, soluble imiquimod directly inhibited the proliferation of GL261 cells in a TLR7-independent manner. This is the first report demonstrating that topical application of imiquimod can enhance T-cell responses to intracranial tumors as a single agent. The results must be interpreted with caution considering anatomical and biological differences between mice and humans. Nevertheless, Aldara that is being used as a vaccine adjuvant in clinical trials may have direct antitumor effects that are independent of exogenous antigen. Further studies in humans are warranted.

Clinical variables and primary tumor characteristics predictive of the development of melanoma brain metastases and post-brain metastases survival

BACKGROUND

Melanoma patients who develop brain metastases (B-Met) have limited survival and are excluded from most clinical trials. In the current study, the authors attempted to identify primary tumor characteristics and clinical features predictive of B-Met development and post-B-Met survival.

METHODS

A prospectively accrued cohort of 900 melanoma patients was studied to identify clinicopathologic features of primary melanoma (eg, thickness, ulceration, mitotic index, and lymphovascular invasion) that are predictive of B-Met development and survival after a diagnosis of B-Met. Associations between clinical variables present at the time of B-Met diagnosis (eg, extracranial metastases, B-Met location, and the presence of neurological symptoms) and post-B-Met survival were also assessed. Univariate associations were analyzed using Kaplan-Meier survival analysis, and the effect of independent predictors was assessed using multivariate Cox proportional hazards regression analysis.

RESULTS

Of the 900 melanoma patients studied, 89 (10%) developed B-Met. Ulceration and site of the primary tumor on the head and neck were found to be independent predictors of B-Met development on multivariate analysis (P = .001 and P = .003, respectively). Clinical variables found to be predictive of post-B-Met survival on multivariate analysis included the presence of neurological symptoms (P = .008) and extracranial metastases (P = .04). Ulceration was the only primary tumor characteristic that remained a significant predictor of post-B-Met survival on multivariate analysis (P = .04).

CONCLUSIONS

Primary tumor ulceration was found to be the strongest predictor of B-Met development and remained an independent predictor of decreased post-B-Met survival in a multivariate analysis inclusive of primary tumor characteristics and clinical variables. The results of the current study suggest that patients with ulcerated primary tumors should be prospectively studied to determine whether heightened surveillance for B-Met can improve clinical outcome.

Superior efficacy of tumor cell vaccines grown in physiologic oxygen

PURPOSE

Atmospheric oxygen (∼20% O(2)) has been the universal condition employed to culture tumor cells used as vaccine antigen. We tested the hypothesis that reducing oxygen tension would increase the efficacy of tumor cell lysate vaccines.

EXPERIMENTAL DESIGN

GL261 glioma cells and EMT6 breast carcinoma cells were grown in 5% or 20% O(2). Syngeneic tumor-bearing mice were vaccinated with these tumor cell lysates mixed with CpG oligodeoxynucleotides as an adjuvant. Tumor infiltrating T cells and apoptotic GL261 cells were quantified by immunohistochemistry. Tumor-reactive immunoglobulin was detected by Western blot. Ovalbumin and gp100-derived peptides were mixed with GL261 lysates as marker antigens to detect changes in presentation of exogenous antigen on MHC class I in vitro, and in vivo following adoptive transfer of gp100-specific CD8(+) T cells.

RESULTS

Mice bearing orthotopic glioma and breast carcinoma survived significantly longer when vaccinated with 5% O(2) lysates. Antigen-specific CTL activation was significantly enhanced following stimulation with lysates derived from GL261 cells grown in 5% O(2) versus 20% O(2) through a mechanism that involved enhanced cross-presentation of exogenous antigen on MHC I. Vaccination with 5% O(2) GL261 cell lysates caused a significant increase in CTL proliferation, tumoricidal function, and trafficking into brain tumor sites, whereas 20% O(2) lysate vaccines predominantly evoked an antibody response.

CONCLUSIONS

Tissue culture oxygen functions as an "immunologic switch" by dictating the cellular and humoral immune responses elicited by tumor cell lysates. These results have profound implications for cancer vaccines that utilize tumor cells as the source of antigen.

Improvement of rituximab efficiency in chronic lymphocytic leukemia by CpG-mediated upregulation of CD20 expression independently of PU.1

Lipopolysaccharide (LPS), CpG-containing phosphothioate oligonucleotides (CpG) and various cytokines impact chronic lymphocytic leukemia (CLL) B cells. For example, they influence cell cycle entry, expression of co-receptors, and CD20. Rituximab (RTX), for which CD20 molecule is the target, proved to be less efficient in CLL than in lymphoma. This is accounted for by a lower CD20 level in the former than in the latter B lymphocytes. CD20 transcription is mediated by four transcription factors, of which only purine-rich box-1 (PU.1) is reduced in CLL. We thus examined the effects of LPS, CpG, tumor necrosis factor-alpha, interferon-alpha, interleukin (IL)-3, IL-4, IL-21, granulocyte macrophage-colony stimulating factor (CSF), and granulocyte-CSF on the transcription of PU.1, and the subsequent expression of CD20. It appeared that CpG was unique in that it raised the membrane expression of CD20 on malignant B cells, owing to a PU.1 independent increase in its gene transcription. Moreover, RTX-induced complement-mediated lysis was also ameliorated. Thus, CpG accelerates the transcription of CD20 independently of PU.1, and thereby improves the efficacy of RTX in CLL.