Table 1.

Principal cells of the tumour microenvironment of gliomas.

Cell Type	Function within the Tumour Microenvironment (TME)	References
Glioma cells	Secrete immunosuppressive cytokines Downregulate major histocompatibility complex (MHC) class I expression Upregulate programmed death-ligand 1 (PD-L1) expression Remodel the extracellular matrix Release growth factors that promote angiogenesis, proliferation, invasion and immune evasion	[13,14,15]
Tumour-associated macrophages and microglia (TAMs)	Mostly M2 phenotype promoting glioma growth and immune suppression Release interleukin 10 (IL-10), tumour growth factor beta (TGF-β) and IL-12 Suppress T-cell and NK-cell activity	[13,14,15]
Regulatory T (Treg) cells	Inhibit effector T-cell activity and promote immune evasion Increase cytotoxic T-lymphocyte-associated protein 4 (CTLA-4) and programme cell death protein 1 (PD-1) expression, suppressing anti-tumour pathways	[13,14,15]
Natural killer (NK) cells	Recognise and kill glioma cells Produce interferon gamma (IFN-γ), tumour necrosis factor alpha (TNF-α) and IL-12, promoting anti-tumour immune responses	[13,14,15]
Dendritic cells (DCs)	Antigen-presenting cells (APCs) that can active T cells and initiate anti-tumour immune response	[13,14,15]
Myeloid-derived suppressor cells (MDSCs)	Immunosuppressive cells that inhibit the activity of T cells and NK cells, promoting immune evasion	[13,14,15]