Emerging Principles of Brain Immunology and Immune Checkpoint Blockade in Brain Metastases

Jawad Fares; Ilya Ulasov; Peter Timashev; Maciej S. Lesniak


Brain. 2021;144(4):1046-1066. 

In This Article


Patients with brain metastases continue to be excluded from clinical trials due to their dismal outcomes and poor prognoses.[98] Many prospective and retrospective immunotherapeutic studies exclude patients with brain metastases despite increasing data that point to potential efficacy against intracranial metastases.[180] In a multivariate analysis, brain metastases were not associated with poorer survival in patients treated with immune checkpoint inhibitors in NSCLC.[181] Stable patients with brain metastases without baseline corticosteroids and a good diagnosis-specific graded prognostic assessment (DS-GPA) classification have the best prognosis.[181] Combining different immune checkpoint inhibitors together or with radiation, chemotherapy, targeted therapy, antiangiogenic therapy and/or neurosurgery seems to potentiate their effect in the setting of brain metastases. Therefore, randomized controlled trials for patients with brain metastases are needed to fully understand the exact clinical benefit of immunotherapy as monotherapy or in combination. In addition, the exact biological mechanism that governs the function of immune checkpoint inhibitors in the immunosuppressive environment of the brain needs to be determined.

Immunotherapy has not yet been brought to the brain metastasis space in patients with breast cancer. Despite clinical benefit being achieved in patients with brain metastases from melanoma and NSCLC,[23,117] breast cancer remains out of bounds. Preclinical studies in metastatic breast cancer have illustrated a lower immune content in brain metastases.[182] The development of immune strategies to understand and alter the complex brain immune microenvironment is needed. Few clinical trials are exploring this avenue (Table 4). One trial is studying the effect of atezolizumab in combination with HER2-targeting agents in HER2-positive breast cancer, and atezolizumab with doxorubicin and cyclophosphamide in HER2-negative breast cancer (NCT02605915). Another trial is exploring the combination of atezolizumab with stereotactic radiotherapy in patients with brain metastases from triple-negative breast cancer (NCT03483012). Radiotherapy can be an important tool to induce the immunogenicity of 'cold' tumours, such as breast cancer brain metastases. Doses and fractionation that trigger inflammatory responses in combination with immune checkpoint inhibitors can trigger the effector immune cells in the brain tumour microenvironment.[186] Radiation has the ability to upregulate MHC-I expression on tumour cell surfaces and to allow antigen presentation of tumour-specific peptides for recognition by cytotoxic T cells.[187] Furthermore, DNA damage induced by radiation can lead to the production of new antigens that can be recognized by the immune system.[188] In addition, radiation activates the stimulator of interferon genes (STING) pathway, which plays a central role in anti-tumour immunity.[189,190] Nevertheless, radiotherapy can also lead to pro-tumorigenic effects. DNA damage induced by radiotherapy may increase tumour resistance and aggressiveness. In addition, brain metastatic cancer cells can employ the STING pathway and produce factors that activate the STAT1 and NF-κB pathways to support tumour growth and resistance.[81] As such, more research is warranted to acquire a comprehensive biological insight into the immune mechanisms induced by radiotherapy. In clinical trials, the combination of radiotherapy and immunotherapy shows signs of promising intracranial response but marked improvement in overall survival is yet to be reported. Tumour-instigated immunosuppression remains dominant over attempts to instigate anti-tumour immunities. Thus, modalities that aim to deplete the immune microenvironment of TAMs and inhibit the activity of suppressive cytokines in the immune milieu can lead to better activation of effector T cells.[191,192]

There is a dire need for predictive biomarkers and prognostic scales that permit prospective evaluation of patients on immune checkpoint inhibitors.[193] Diagnostic clinical trials in patients with brain metastases are scarce and suffer from poor trial design and follow-up.[194] The characterization of circulating tumour cells and cell-free DNA can help reveal the molecular foundations of brain metastases and offers the chance to observe possible changes in response to immunotherapy. The integration of machine learning in the assessment and classification of patients with brain metastases seems inevitable to identify tumour-infiltrated diagnostic regions and assess the response of metastatic lesions faster and more accurately.[195] In addition, future studies should continue to explore the genetic, phenotypic and immune regulatory differences between primary tumours and brain metastases.

There are many opportunities to improve the efficacy of immune checkpoint inhibitors in light of reported observations. The timing and sequence of administration of immunotherapy alongside other regimens can be crucial to the overall clinical outcome. In addition, the receptor expression patterns on T cells differ at different anatomical locations.[96] Therefore, enhancing organ-specific T-cell trafficking to the brain is an important goal to achieve. Furthermore, NK cells have been shown to be increased in response to immune checkpoint inhibitors; however, the mechanism of action and ways to potentiate this response remain to be elucidated.

The role of B cells in enhancing the response to immune checkpoint blockade remains unclear. Cell depletion studies reveal that NK cells and CD8+ T cells are required for intracranial anti-PD-1/anti-CTLA4 efficacy.[96] Nevertheless, new studies show that the incidence of B cells in human tumours, such as melanoma and sarcomas, is associated with better response to immunotherapy.[196–198] In primary brain tumours, it was revealed that TAMs can transfer functional PD-L1 via microvesicles to confer on regulatory B cells the ability to suppress CD8+ T-cell activation.[199] Similar mechanisms may be occurring in the setting of brain metastases. In glioma, a B cell-based vaccine, with CD40 agonism and IFNγ stimulation, migrates to key secondary lymphoid organs and is proficient at antigen cross-presentation, which promotes both the survival and the functionality of CD8+ T cells. Combining this vaccine with radiation and PD-L1 blockade conferred tumour eradication in 80% of treated tumour-bearing animals.[200] As such, designing strategic immune therapies that can address B cell responses in brain metastases is essential for more effective treatments.

Brain metastatic cancer cells arising from primary tumours have the propensity and the necessary metabolic properties to colonize the brain. The brain is the organ with the highest energy demand and the ability to rewire its metabolism in response to varying stressors.[201] It poses metabolic challenges to a colonizing cancer cell, ranging from fuel and oxygen availability to oxidative stress. As such, brain metastases display a remarkable metabolic flexibility by utilizing acetate, glutamine and branched-chain amino acids as alternative sources of fuel.[202] Brain metastases upregulate the synthesis of acetyl-CoA synthetase enzyme 2, which allows them to fuel the tricarboxylic acid cycle and convert acetate to acetyl-CoA for energy metabolism.[203] Furthermore, brain metastases oxidize branched-chain amino acids and glutamine to survive and proliferate in the absence of glucose.[204] In addition, brain metastases can produce glucose by upregulating fructose-1,6-bisphosphatase 2 for gluconeogenesis.[204] Comparing the molecular profiles and gene expression patterns of melanoma brain metastases with patient-matched extracranial metastases identified significant immunosuppression and enrichment of oxidative phosphorylation in brain metastases, which was confirmed by direct metabolic profiling and [U-[13]C]-glucose tracing.[15] Increased oxidative phosphorylation leads to oxygen deprivation and hypoxia in the immune microenvironment, hampering immune cell function. Effector T cells suffer from metabolic insufficiency and dysfunction due to nutrient depletion.[205] Though oxygen metabolism is less vital for T-cell effector function, oxygen consumption is necessary for memory formation and T-cell proliferation.[206] As such, the inhibition of oxidative phosphorylation in the immune microenvironment of brain metastases can help lift the immunosuppression and lead to better immune activity against brain metastatic cells. Metabolic interventions in combination with immune checkpoint inhibitors can further enhance the therapeutic effect against brain metastases.

Delivery of immune therapies and the trafficking of cytotoxic T cells to the brain are hampered by the presence of the blood–brain barrier. Overcoming this barrier is essential to achieve therapeutic benefit akin to what is achieved in extracranial disease. A permeable blood–brain barrier may facilitate the presentation of tumour-associated antigens and enhance the effects of immune cells in the tumour microenvironment. Therefore, strategies that involve physical disruption of the blood–brain barrier can improve antigen presentation, enable immune checkpoint inhibitor transmigration, and increase immune cell trafficking into the immune microenvironment to further sensitize brain metastases to immunotherapies. In recent years, a number of strategies have been explored to hijack barriers posed by the blood–brain barrier. Recent preclinical developments with focused ultrasound allow the delivery of therapeutic agents to the brain that were once deemed undeliverable.[207] In a clinical trial on patients with recurrent glioblastoma, pulsed ultrasound in combination with systemically injected microbubbles showed that the approach is safe and well tolerated.[208] Through focused ultrasound sonication, engineered NK cells against the HER2 receptor were capable of crossing the blood–brain barrier in the setting of breast cancer brain metastases.[209] The use of whole-brain radiation therapy has shown conflicting results in regard to its capability of enhancing the delivery of therapeutic molecules across the blood–brain barrier. In a self-controlled pilot study, whole-brain radiation therapy failed to boost intracerebral gefitinib concentration in patients with brain metastatic NSCLC who were treated with the drug for 14 days.[210] Yet, the permeability of gefitinib after treatment for 30 days increased in accordance with the escalated dose of whole-brain radiation therapy in NSCLC brain metastases.[211] Intravital microscopy and computational modelling show that a single, low dose of radiation therapy can induce transient, dynamic and localized vascular bursting, as well as enlarge blood vessel volume. Increased permeability can facilitate extravasation of immune checkpoint inhibitors from blood vessels in tumours.[212] Nevertheless, the downfalls of whole-brain radiation therapy are its side effects. Cerebral oedema may be induced or worsened after the initiation of whole-brain radiation therapy. As a result, whole-brain radiation therapy is usually preceded by corticosteroid therapy, which may affect the efficacy of immune checkpoint blockade.[213] Therefore, finding the right balance between the safety profile and the therapeutic benefits of interventions, such as focused ultrasound and whole-brain radiation therapy, is vital in assessing ways to enhance the penetration of immune checkpoint inhibitors into the tumour microenvironment of brain metastases. Moreover, the translation of some of these preclinical modalities to the brain metastatic clinical setting is necessary for proper evaluation of efficacy and outcomes.

Exploring new models for the induction of immune checkpoint blockade is necessary to potentiate the effects of immunotherapy. Preclinically, nanotherapy in conjunction with photothermal induction and immune checkpoint blockade has been shown to be effective in brain tumours.[214] Targeted nanoscale immunoconjugates presented on a natural biopolymer scaffold, poly(β-L-malic acid), with covalently attached anti-CTLA4 or anti-PD-1 antibodies for systemic delivery across the blood–brain barrier and activation of local brain anti-tumour immune response resulted in an increase in CD8+ T cells, NK cells and macrophages along with a decrease in regulatory T cells in the tumour microenvironment of gliomas.[215] It also significantly increased survival when compared to animals treated with single checkpoint inhibitor-bearing nanoscale immunoconjugates or free anti-CTLA4 or anti-PD-1 antibodies.[215] Moreover, therapeutic targeting of PD-L1 on TAMs using nano-immunotherapy synergizes with radiation in primary brain tumours.[216] Similar strategies can be used against brain metastases. Furthermore, preclinical data demonstrate that neural stem cells can deliver systemic agents that target brain metastases from melanoma and breast cancer.[217,218] In addition, delivering apoptosis-promoting genes to the immunosuppressive TAMs in brain metastases can be done through haematopoietic stem cells.[26] Thus, utilizing the inherent ability of stem cells to cross the blood–brain barrier and rocket towards tumours can help in increasing immune targeting of brain metastases. Moreover, oncolytic viruses have shown promise in targeting primary brain tumours and potentiating the immune response against tumour cells.[219] The clinical delivery of oncolytic viruses to brain metastases has yet to be done. Preclinical data show that intravenous delivery of an oncolytic Orthoreovirus can immunologically prime brain metastases against PD-1 blockade.[220] In addition, the utilization of CAR T cells to target HER2-positive breast cancer brain metastases offers a new immunotherapeutic medium for exploration.[221]

The rise of cancer neuroscience as a field opens a window to explore how the immune microenvironment interacts with its neural counterpart in the setting of brain metastases. Given how astrocytes affect the immune response towards tumour cells, it is rational to believe that the nervous tissue contributes in one way or another to the immunogenicity of brain metastases. Recent discoveries reveal that excitatory neurotransmission enables NMDAR signalling in brain metastases.[95] Breast cancer cells that have metastasized to the brain upregulate neurotransmitter receptor expression and extend perisynaptic processes to receive neuronal activity-dependent neurotransmitter signals that trigger a receptor-mediated signalling cascade, induce inward currents in the malignant cells, and drive growth of breast cancer brain metastases.[95] In gliomas, glutamatergic synaptic input and electrochemical communication drives brain tumour progression.[222,223] Dissecting the neural–immune–cancer interactions is necessary to obtain a comprehensive picture of how brain metastases progress through immune checkpoint evasion. In fact, these interactions may be contributing to the effect of immunotherapies in the brain.