In the 2016 classification, only histological features such as mitotic activity, necrosis, and florid microvascular proliferation were used for assigning malignancy grade to adult-type diffuse gliomas. More recently, cIMPACT-NOW presented a compilation of the evidence that particular molecular characteristics substantially improve the prognostic impact of grading of these neoplasms.65,66 According to the 2021 classification, even in the absence of, for example, microvascular proliferation and necrosis, an IDH-mutant astrocytoma with cyclin-dependent kinase inhibitor 2A/B (CDKN2A/B) homozygous deletion is considered as grade 4. Similarly, in adult patients with a histologically low(er)-grade, IDH-wildtype diffuse glioma, the presence of one or more of the following three genetic characteristics is now sufficient for a diagnosis of glioblastoma, IDH-wildtype (grade 4): telomerase reverse transcriptase (TERT) promoter mutation, epithelial growth factor receptor (EGFR) gene amplification, and +7/-10 (i.e., the gain of whole chromosome 7 and loss of whole chromosome 10).10 These developments in classification necessitated updated guidelines for the clinical management of patients (Table 1).67
Glioblastoma can be further classified into molecular subtypes, informing disease progression and clinical practice.  Wang et al. identified three subtypes based on molecular features: pro-neural, mesenchymal, and classical.68 Neftel et al. utilized single-cell RNA-sequencing to identify four cellular states within glioblastoma69, showing intra-tumoral heterogeneity. The signatures of individual tumour cells were categorized into neural-progenitor-like, oligodendrocyte-progenitor-like, astrocyte-like, and mesenchymal-like.68
Although molecular knowledge has enabled more precise clinical diagnosis, the translation into more effective therapeutic approaches is lagging behind. Further elucidation of the pathobiology of these tumours through single-cell expression profiling studies70, longitudinal multiplatform analyses71 and of inherited genetic aspects72,73 may be helpful in this regard.74
3.2 Immune response and biomarkers
Gliomas are characterized by a complex immune tumour microenvironment (TME), with up to 50% of tumour composition consisting of immune cells (mainly microglia and glioma-associated macrophages). In much lower quantities, tumour-infiltrating lymphocytes, monocytes, MCs, eosinophils and neutrophils are also present.75,76 These innate and adaptive immune cell types directly and indirectly interact with tumour cells and resident glial cells, neurons and vascular cells. The immune landscape of gliomas is highly immunosuppressive with immunologically quiet macrophages and sparse lymphocytic infiltration, with a shift to Th2.77,78 IDH-mutant gliomas display even less lymphocytes than IDH-wildtype tumors.79 The complex dynamic interplay of the various cell types involves the expression of a multitude of immunoregulatory factors, partly with tumour-promoting features, such as cytokines and chemokines (e.g. IL-4, IL-10, TGF-β), colony stimulating factor 1 (CSF-1), immune-checkpoint molecules (e.g. programmed cell death (PD)-1 / PD ligand 1 (PD-L1) dependent signalling, CD39) among others.75,80 The inflammatory cytokine profile of glioblastoma (diagnosed via histopathological features) is a valuable prognostic indicator, with increased levels of immunosuppressive molecules such as TGF-β related to decreased survival.81 Genotypes of Th2-related IL-4Ra and IL-13 have also been related to prognosis.82
Angiogenic (vascular endothelial growth factor (VEGF)), metabolic (e.g. IDO, ARG1) and dietary factors83 influence the inflammatory microenvironment and immunogenicity of glial brain tumours. Presumably, the local immune reaction in gliomas is connected to the systemic immune system via meningeal lymphatic vessels and lymphatic CNS drainage to cervical lymph nodes84-87 (Figure 4). CNS immunological dysregulation in the context of allergy and/or glioma has been examined in multiple studies but remains poorly characterized (Figure 5 and Box 1).
Figure 4: The border-associated immune compartments of the brain