To confirm in vivo these findings, and corroborate that tumor growth might be assisted by GBC-PC with immunosuppressive properties, GBM cell proliferation was studied in grafts of co-cultured human RFP-GBM cells and GFP-mouse pericytes (GBM+Pc) grafted into brain cortex of an immunocompetent C57Bl/6 mouse model . GBM tumors from both U373 and U87 cell lines (clonally labelled by RFP expression), were observed in most of the grafts with GBM cells alone (control grafts) or GBM+Pc at 4 and 11 weeks after graft (Figure 4C, 4D). Interestingly, pericytes were not observed when pericytes were grafted alone or together with GBM cells but without prior co-cultured (control grafts). As expected, these control mice showed the same tumor progression than mice grafted with glioblastoma alone (not shown). Increased perivascular infiltration of GBM cells was observed in GBM+Pc grafts (Figure 4E). The GBM tumor mass was larger in the brain of mice xenografted with GBM+Pc than in mice xenografted with GBM cells alone (Figure 4C, 4F). Due to the ovoid-like shape of the tumor mass we measured the tumor diameters in the central section and calculated the volume of the tumoral ovoid: 5.8+/−0.5×106 μm3 in GBM+Pc grafts [n=5] and 2.5+/−0.3×106 μm3 in GBM grafts [n=4] (Figure 4F). Supporting this data, we observed that dividing GBM cells, which appear as spherical strong RFP+ Ki67+ cells (Figure 4G), were more abundant in GBM+Pc grafts than in GBM grafts (Figure 4H, 4I). Relative quantification (counting strong fluorescent RFP cells in the same area of two consecutive sections of GB+PC grafts [90+/−10; n=5] and GBM grafts [30+/−7; n=5]; ImageJ-NHI software) showed that dividing cells were three times more numerous in GBM+Pc than in GBM grafts (Figure 4J). To determine if xenografts of GBM+Pc were able to acquire an anti-inflammatory phenotype in vivo, we analyzed the expression of IL-10 and TGF-β on the tumor/brain edge. Compared to GBM cell grafts (Figure 4K, 4P), we detected an increase of IL-10 and TGF-β expression in GBM+Pc grafts (Figure 4L–4M and 4Q–4TQ, respectively). This expression was particularly high in areas where grafted GBC-PC were identified surrounding blood vessel. The most intense fluorescent areas of Cy5-IL10 and Cy5-TGF-β colocalized with GFP expression corresponding to PC cellular mass (Figure 4N–4O and 4U–4VU, respectively). Quantitative analysis revealed that the expression of IL-10 (by measuring the area of immunopositive particles in equivalent areas of the central section of each xenograft [n=5] using ImageJ-NIH software) and TGF-β (by quatifying luminosity in the same area of the central section of each xenograft [n=5] using Adobe Photoshop software) were significantly higher in GBM+PC grafts than in GBM control grafts (Figure 4W and 4X, respectively). Supporting these data, we detected the appearance of inhibitory PD-1/PDL-1 interactions, which corresponded to perivascular lymphocytic infiltration and pericytes respectively in GBM+Pc grafts (Supplementary Figure 3), and compared it to host pericytes, where we did not find those interactions though perivascular PDL- 1 expression was present (not shown). Interaction between PD-1 receptor in lymphocytes and PDL-1 in GBM progression has been described, but it is not clear if, apart from GBM cells, other cell types may also present PDL-1 in perivascular areas . Our data showed pericytes expressing PDL-1 at the plasma membrane and interacting with PD-1 expressed in perivascular infiltrating T cells. Unspecific detection of high membrane-associated expression of PDL-1 was also detected in perivascular areas that did not correspond to pericytes but to GBM cells [36, 37] (Supplementary Figure 3).