**13. Immune checkpoint inhibitors**

Several studies have aimed to characterize the interactions between MNs and the immune system. Specifically, studies of the immune microenvironment in MNs have revealed that NY-ESO-1, PD-L1, PD-L2, B7-H3, and CTLA-4 are expressed in MNs and may be at least partly responsible for the suppression of the anti-tumor immune response [170, 171]. PD-L1 is expressed in MNs, and expression levels are higher for higher-grade tumors [172]. The expression of these proteins has been associated with tumor progression, recurrence, and poor survival outcomes. Fang et al. extensively characterized the immune infiltrate in MNs and found that the immune cells infiltrating MNs are mainly antigen-experienced T cells and B cells [58]. In their study, B cells were activated and underwent immunoglobulin class switching, somatic hypermutation, and clonal expansion. T-cells demonstrated evidence of antigen exposure and increased expression of PD-1 and TIM-3, which can be a sign of an exhausted phenotype. Tumor-infiltrating lymphocytes in MNs are mainly T-cells. Interestingly in anaplastic MNs, the number of CD4 and CD8 T-cells is low. At the same time, the proportion of Tregs is increased [59]. These data support the notion that an immunosuppressive microenvironment in MNs may contribute to tumor progression.

In a mouse model of meningioma, infusion of anti-PD1 antibody avelumab plus highly-active NK cells (HaNK) led to increased survival, showing the importance of innate NK cell activity [173]. Currently there are two case reports on PD-L1 checkpoint inhibition for recurrent MNs [174, 175]. The cases report disease-free recurrence for >2 years in one patient and > 6 months in another patient, with both having reductions in tumor volume, cerebral edema, and patient-reported symptoms following nivolumab treatment. Based on the existing evidence on PD-L1 expression in recurrent MNs, five clinical trials are enrolling patients with to receive anti-PD1 antibodies nivolumab, avelumab, or pembrolizumab. An ongoing phase II trial is designed to compare nivolumab alone to combination therapy with the anti-CTLA-4 antibody ipilimumab (NCT02648997). A phase Ib trial will investigate the preoperative use of avelumab in combination with hypo-fractionated proton radiotherapy for 3 months to evaluate its effect on the size of unresected MNs (NCT03267836). The other trials are recruiting patients with recurrent MNs to receive adjuvant immunotherapy as PD1 blockade.

#### **14. CAR-T cell therapy**

Chimeric Antigen Receptor (CAR) T cell therapies are a novel therapeutic approach to cancer. The standard treatment consists in the leukapheresis of autologous peripheral blood mononuclear cells from the patient bearing the tumor. After successful leukapheresis, T cell isolation is performed. T cells are then grown in culture and are further transduced with a lentiviral vector carrying an integrative plasmid that encodes the CAR, which is essentially a fusion protein containing a single-chain variable fragment derived from a full antibody, plus a transmembrane domain and different array of intracellular co-receptor and co-stimulatory domains that will trigger the intracellular signaling necessary for T cell activation [176].

CAR-T cell therapies were initially approved in 2017 (axi-cel and tisa-cel) for the treatment of relapsed/refractory diffuse large B cell lymphoma and relapsed/refractory B-cell acute lymphoblastic leukemia [177]. Unfortunately, the landscape of CAR-T cell therapies in solid tumors has not been promising, mainly due to different resistance from typical features of the tumor microenvironment like high acidity, immune effector exhaustion induction and the extracellular matrix. Different workaround strategies have been explored to address these problems and currently, highly engineered cells and very complex therapies (CAR-Ts in combination with checkpoint inhibition, or small molecules, or chemotherapy, or immunomodulators) are under study in different clinical trials [178].

Brain tumors have not been an exception in CAR-T development, with glioblastoma being the most attacked condition. Tang et al. reported a case of a patient with an anaplastic MN that underwent three surgical resections and had an Ommaya device implanted. IHC from her tumor sample showed a high expression of B7-H3, also known as CD276 ([179], p. 3). The researchers prepared CAR-Ts from autologous PBMCs, and during CAR-T development patient recur and CAR-Ts were administered in three doses via the Ommaya device. A fourth surgical treatment was performed as patient was progressing quickly, and unfortunately the patient died one day after surgery. Post-mortem analysis of the tumor sample showed that CAR-T indeed penetrated the tumor and successfully targeted some cells expressing B7-H3, however, as not all the tumor was expressing this molecule, antigen loss and selection of other cells with a different transcriptome occurred [180]. Even though results were not as expected, this case marks an important step toward the development of cell therapies of different natures, to treat brain tumors, especially those of high recurrency and aggressiveness.

### **15. Conclusions**

Treatment in MN has remained similar since some decades ago. Major improvements in survival are achieved mainly by surgery and radiation therapy. Most cases of MN will respond to these conventional therapies, however, transformation of low-grade MN to high-grade MN, or de novo high-grade MN are highly recurrent and impose a very low survivability. For these tumors, surgery and radiation therapy are less than enough. With the era of genomic analysis and a better understanding of the genetic basis of cancer, different molecular targets and new therapeutic approaches have been studied for high-grade MN treatment. In this review we went through the main critical advancements in evidence that suggests that molecular targeting might be the future of high-grade MN treatment. To the date, all these molecular approaches are still under study, a conventional management is still the mainstay, but we hope in the following years, new evidence of the clinical relevance of these therapies is available and introduction of them into the therapeutic arsenal could be a true.
