Proteins of the cancer cell secretome induce the protumoral microenvironment of diffuse intrinsic pontine glioma.

BACKGROUND: The microenvironment of diffuse intrinsic pontine glioma (DIPG) is devoid of infiltrating lymphocytes and immune checkpoint molecules, with the exception of B7-H3. Here, we studied whether the cancer secretome is a determinant of such tumor phenotype. METHODS: We quantified immune histologic markers in paraffin-embedded DIPG samples and healthy brainstem controls. We identified and quantified cytokines in frozen tissue samples, DIPG culture supernatants, cerebrospinal fluid (CSF) and plasma from patients and controls. We studied the phenotype of mesenchymal cells, brain microvascular endothelial cells and macrophages following their exposure to DIPG secretomes. RESULTS: We found profuse infiltration of anti-inflammatory CD163(+) microglia/macrophages in the brains of 23 DIPG patients, compared to low levels in 5 controls. In DIPG, B7-H3 was predominantly expressed in cells of mesenchymal origin (CD90(+)) transformed to pericytes (PDGFRβ(+)). In frozen samples from 14 patients with DIPG and 4 controls, we identified a common secretome pattern, with osteopontin and chitinase-3-like 1 (CHI3L1) overexpressed in DIPG. Such proteins were abundant in the culture supernatants of 7 DIPG models. Osteopontin and CHI3L1 concentrations in the CSF of 18 patients were significantly higher than in 18 controls (P†<†0.0001). In vitro, DIPG supernatants and recombinant osteopontin or CHI3L1 induced phenotypic changes in (i) mesenchymal cells, which turned into B7-H3(+) pericyte-like cells, (ii) endothelial cells, which organized complex tube networks and overexpressed the blood-brain barrier marker BCRP, even in subcutaneous xenografts, and (iii) macrophages, which were polarized towards the M2-like type. CONCLUSIONS: DIPG cells secrete proteins that create an immunosuppressed niche.