New insights into the low-grade glioma tumor microenvironment for improved patient management

The interplay between tumor cells and the tumor microenvironment has been recognized over the past decade as a critical component of tumor development, maintenance, and growth. Low-grade gliomas (LGG), the most common brain tumors in children, are comprised of between 20% and 50% glioma-associated microglia/macrophages (GAM), with the greatest increase seen in patients who have the hereditary cancer predisposition syndrome neurofibromatosis type 1 (NF1), caused by NF1 mutation.1 Genetically engineered mouse models harboring mutant Nf1 develop optic pathway gliomas (Nf1-OPGs), the most common glioma seen in children with NF1, and are an excellent resource to study the interaction between GAMs and tumor cells. Studies using these models have shown that reduction of CX3CR1 expression, a chemokine receptor involved in microglia migration, and treatment with microglia inhibitors (ie, minocycline) reduces and/or delays Nf1-OPG formation.2,3 Additionally, RNA sequencing of GAMs in Nf1-OPGs shows that these cells secrete CCL5, which is a mitogen for glioma cells, and neutralization or lack of CCL5 blocks tumor growth.4 In the current issue of Neuro-Oncology, Costa et al5 sought to identify novel GAM biomarkers from existing microarray data and demonstrate that these biomarkers are therapeutic targets. Using a combination of RNA sequencing, Nf1-deficient genetically engineered mouse models, and human biospecimens, they identified ITGAL, which encodes cell membrane protein CD11a, as a potential GAM marker important for LGG biology. Existing RNA expression datasets from mouse models of amyotrophic lateral sclerosis, autism spectrum disorder, and glioma were initially normalized to control samples and used to generate a list of differentially expressed genes, including Itgal. The authors then showed that microglia isolated from Nf1-mutant mice with optic gliomas had increased Itgal expression when compared to mice lacking gliomas. Elevated expression of Itgal corresponded with increased CD11a on immunohistochemistry in 4 different Nf1-OPG models, and increased ITGAL expression was confirmed in human pilocytic astrocytomas (PAs). Since tumor cells are made up of a variety of cells, the authors next used 3 experiments to show that CD11a+ cells were in fact microglia cells. First, they used RNAscope along with immunolabeling of specific cell populations to show that the CD11a+ cells most commonly co-localized with Iba+ microglia cells. As a second confirmation, they used a series of experiments to deplete microglia and T cells in order to show that CD11a+ loss is most strongly associated with microglia loss, rather than T-cell depletion. Finally, they showed that CD11a expression is more enriched in GAM than intact tumors in both the NF1-deficient OPGs and human sporadic PAs. To determine the functional relevance of CD11a expression in microglia, the authors used a CD11a knockout mouse to study the effects of CD11a loss on microglia content in the optic nerves and cell surface proteins involved in microglia homeostasis (Tmem119) and migration (Ccr2, Ccr4, Cx3cr1). Although CD11a loss did not have an effect on microglia density or Tmem119, Ccr2, and Ccr4 expression, the CD11a-deficient mice had reduced Cx3cr1 expression and decreased migration to CX3CL1, the ligand for CX3CR1. CD11a loss in the microglia also led to decreased Ccl5 expression and impaired phagocytosis relative to wildtype (WT) counterparts, consistent with prior studies demonstrating the role of Ccl5 in NF1-deficient OPGs. Most importantly, Nf1-OPG mice receiving treatment with anti-CD11a antibodies had reduced optic nerve volumes and proliferation, demonstrating that CD11a inhibition attenuates NF1-OPG growth. In the discussion, the authors described differential survival based on ITGAL/CD11a tumor expression in children with sporadic or NF1-associated PAs. They were not able to find a survival difference in this population, but did find a small trend toward worse survival in patients with a grade 3 glioma and high ITGAL expression. Although there may be other factors in play, as the authors pointed out, this is likely due to the generally excellent overall survival in patients with PAs and another outcome measure (such as progression-free survival, response to treatment, or clinical morbidities) may be better to look at in this population, as these have more significance in patients with LGGs. The authors conclude that microglia diversity not only has implications in disease-specific contexts but will also be important with respect to microglia-directed therapies. They propose that CD11a operates to regulate microglia migration and NF1-OPG growth factor production to generate a supportive LGG microenvironment, providing novel insight into the role microglia cells may play in LGG tumor development. Based on promising pre-clinical evidence, clinical trials of minocycline and PLX3397 (colony-stimulating factor 1 receptor inhibitor; expressed on microglia) have already been conducted, but thus far have not shown survival advantage over standard-of-care therapies.6,7 This may relate to the population of patients chosen for these trials—adults with glioblastoma multiforme as opposed to pediatric LGG. An improved understanding of when to treat, which tumors are most susceptible, and what types of therapeutic combinations should be used will be required to translate the findings of this and other studies to the clinic. This study will serve as a stepping stone for researchers to use in order to develop better therapies and improved outcomes for patients with LGGs. None. Conflict of interest statement. M.B. has served as a consultant for AstraZeneca but has no conflict related to this article. Authorship statement. M.B.: concepts, writing, and editing. The text is the sole product of the author. No third party had input or gave support to its writing.